Borehole stressed packer inflation system

Abstract: A method for determining a proper inflation pressure for an elongated inflatable packer to effect a positive contact stress seal of an elastomer packer element with a borehole wall in a wellbore traversing earth formations. The temperature differential for each layer in a radial plane is determined. The final contact stress, the finite inflation pressure required to inflate the packer element and the temperature differential are functionally interrelated to one another to obtain a final contact stress of the packer element with a borehole wall.

Description: This invention relates to a method of effecting a seal between an elongate inflatable packer and a borehole well and is particularly suited for use in a wellbore traversing earth formations and having a temperature differential defined between a disturbed temperature condition of the wellbore and an undisturbed, or normal in-situ, temperature condition thereof, for example in situations where liquid circulation in the wellbore disturbs normal in-situ temperatures along the wellbore and where the disturbed temperatures are offset or different relative to a normal in-situ temperature profile of the wellbore as a function of depth when the wellbore is in a quiescent, or undisturbed, state. In drilling a borehole, the borehole can have the same general diameter from the ground surface to total depth (TD). However, most boreholes have an upper section with a relatively large diameter extending from the earth's surface to a first depth point. After the upper section is drilled a tubular steel pipe is located in the upper section. The annulus between the steel pipe and the upper section of the borehole is filled with liquid cement which subsequently sets or hardens in the annulus and supports the liner in place in the borehole. After the cementing operation is completed, any cement left in the pipe is usually drilled out. The first steel pipe extending from the earth's surface through the upper section is called "surface casing". Thereafter, another section or depth of borehole with a smaller diameter is drilled to the next desired depth and a steel pipe located in the drilled section of borehole. While a steel pipe can extend from the earth's surface to the total depth (TD) of the borehole, it is also common to hang the upper end of a steel pipe by means of a liner hanger in the lower end of the next above steel pipe. The second and additional lengths of pipe in a borehole are sometimes referred to as "liner". After hanging a liner in a drilled section of borehole, the liner is cemented in the borehole, i.e. the annulus between the liner and the borehole is filled with liquid cement which thereafter hardens to support the liner and provide a fluid seal with respect to the liner and also with respect to the borehole. Liners can be installed in successive drilled depth intervals of a wellbore, each with smaller diameters and each cemented in place. In any instance where a liner is suspended in a wellbore, there are sections of the casing and of the liner and of adjacent liner sections which are coextensive with another. Figuratively speaking, a wellbore has telescopically arranged tubular members (liners), each cemented in place in the borehole. Between the lower end of an upper liner and the upper end of a lower liner there is an overlapping of the upper and lower liners and cement is located in the overlap sections. After the liners have been located through the strata of interest, the well is completed. In the completion of the well using a compression type packer, typically a production tubing with a compression type production packer is lowered into the wellbore and disposed or located in a liner just above the formations containing hydrocarbons. The production packer has an elastomer packer element which is actually compressed to expand radially and seal off the cross-section of the wellbore by virtue of the compressive forces in the packer element. Next, a perforating device is positioned in the liner below the packer at the strata of interest. The perforating device is used to develop perforations through the liner which extend into cemented annulus between the liner and the earth formations. Thereafter, hydrocarbons from the formations are produced into the wellbore through the perforations and through the production tubing to the earth's surface. Typically, in the production of hydrocarbons there is a pressure differential across the packer element and heat energy is applied to the packer element. The heat energy comes from downhole temperature conditions of the hydrocarbons which are higher than ground surface temperature conditions. In summary, a packer element of a compression packer used in the well completion is composed of rubber or an elastomer product which is highly compressed to span the annular gap between the liner and the production tubing and is compressed to exert sufficient contact pressure with the wellbore to provide a fluid tight seal. In time, the downhole temperature and differential pressure across the packer element can cause the packer element to deteriorate and consequently leak. In other instances in the life of a production well gas migration or leakage is a particularly significant problem which can occur when fluids migrate along the cemented overlapped sections of a liner and borehole. Any downhole fluid leak outside the production system is undesirable and requires a remedial operation to prevent the leak from continuing. Some completions use an inflatable packer in-preference to a compression packer. Some operations also prefer to use an inflatable packer to isolate areas of a wellbore where fluid leaks occur. US-A-3918522, cited during examination of this application, discloses a method of completing a well using an inflatable packer to effect a seal with a borehole wall, the packer having a central tubular mandrel and an elastomeric packer element mounted on said mandrel in sleeved relation thereto, the packer element being subject to inflation by a liquid to a finite inflation pressure to produce a radial expansion of the packer element. An inflatable packer typically includes an annular elastomer element (up to about 40 feet in length) on a central steel tubular member which extends therethrough. In use, the inflatable packer is disposed in a borehole on a string of production pipe and is located at the desired location in the borehole. The packer element is adapted to receive a medium, e.g. a cement slurry or a liquid ("mud") under pressure to inflate and to compress the packer element between the inflation liquid and the wellbore. A valving system in the packer is used to access the cement slurry or mud under pressure in the attached string of tubing to the interior of the elastomer packer element. The inflating pressure of the inflating liquid medium must be such that after the inflating medium is trapped in the packer element, the packer element maintains a positive seal with respect to the borehole wall. A positive seal is provided by a pressure inside the packer element which exceeds the pressure in the formations in the wellbore. Inflatable packers seal extremely well in open boreholes. Heretofore, use of an inflatable packer to provide a gas tight seal in a smooth walled liner to bypass fluid leaks has not been reliable, for example because there has been no reliable way to determine what the inflation pressure for the packer should be in order to obtain the desired packer seal in a liner. Too much pressure in an inflatable packer can over stress a liner or burst the inflatable packer element while too little pressure will not provide a proper packer seal. In some instances, even a fully inflated packer element at maximum inflation pressure will not obtain a gas tight seal in a liner. In another form of well completion, in hard earth formations, inflatable packers on a production string of pipe can be spaced apart by a section of pipe and inflated to straddle a production zone so that a liner is not required. Such a process is described in US-A-4 440 225 issued April 3, 1984. US-A-4 440 225 recognizes that a cement inflated packer can leak under pressure because cement shrinkage in the packer, upon curing, can produce a micro-annulus gap which permits fluid migration. The solution in the patent for cement shrinkage is to algebraically sum the radial elastic compression of the mandrel, the radial elastic compression of the packer element and the radial elastic compression of the formation so that this sum exceeds the radial shrinkage of the cement element upon curing by an amount sufficient that the sealing pressure exceeds the formation pore pressure after the cement is set or cured. In the present state of technology, it has been discovered that the US-A-4 440 225 method sometimes over stresses the earth formations and can sometimes result in gas leaks. While having great utility, the method lacks preciseness in predetermining the effectiveness of an inflatable packer seal. Also, the method does not deal with completions where the inflation diameter of an inflatable packer used in a borehole extending below a liner is a factor in the operations. During and after a well completion, some well operations such as acidizing or fracturing develop a downhole temperature effect on the wellbore elements and can cause fluid leakage. The net effect of a considerable number of wellbore completion and remedial operations is to temporarily change the temperatures along the wellbore from a normal in-situ temperature condition along the wellbore. At any given level in a wellbore, the temperature change may be an increase or decrease of the temperature condition relative to the normal in-situ temperature depending upon the operations conducted. What happens then is that an inflatable well packer, which includes metal, elastomer and an inflation liquid is normally set in a stressed condition in a metal liner or overlapped sections of liners, which are at a different temperature condition than the normal in-situ temperature conditions. After the operations are concluded and the wellbore returns to its normal in-situ temperature, this change in temperature changes the dimensions of the well packer which affects the stressed condition of the packer. In the case of a cement filled packer, the decrease in volume when the cement cures also affects the stressed condition. These changes in temperature and cement volume can reduce the stressed condition of the packer to a failure mode where the packer leaks after the temperature returns to an in-situ temperature condition. In its simplest form, a wellbore packing system comprises an inflatable packer in an initial inflated condition in the wellbore and the surrounding rock formation. It therefore includes a layer of steel (packer mandrel), a fluid slurry layer of cement, a layer of elastomer and the surrounding rock formation. The layers are at successively greater radial distances from the centerline of the borehole in a horizontal plane and have wall thicknesses defined between inner and outer radii from a center line. Because completion operations in the wellbore alter temperatures along the length of the wellbore, the temperatures of various layers located below a given crossover depth in the wellbore will be below the normal temperatures of the various layers after the wellbore returns to an undisturbed temperature. Above the given crossover depth in the wellbore, the temperatures of the various layers will be higher than the normal temperatures after the wellbore returns to an undisturbed temperature. When the packer is in the wellbore, the temperature of the liquid cement slurry is introduced at a lower temperature than the temperature of the rock formation and also lower than any mud or control liquid in the wellbore. After the packer element is inflated, in the initial condition of the inflated packer, the cement slurry under pressure induces a certain strain energy in each of the more or less concentrically radially spaced layers of steel, cement, elastomer and rock. Strain energy is basically defined as the mechanical energy stored up in stressed material. Stress within the elastic limit is implied; therefore, the strain energy is equal to the work done by the external forces in producing the stress and is recoverable. Stated more generally, strain energy is the applied force and displacement including change in radial thickness of the layers of the packing system under the applied pressure. When the inflation pressure is trapped at a fixed pressure in the inflatable packer, the liquid cement slurry cures and converts to a solid layer of cement. The solid layer of cement has a reduced wall thickness compared to the liquid cement slurry because of the volumetric shrinkage of the cement. This results in a packer condition where the cured cement layer loses some of its strain energy which decreases the overall strain energy of the packing layer system and reduces the contact sealing force of the packer element with the borehole wall. However, in time, the wellbore temperature will increase (or decrease) after the packer to the in-situ undisturbed temperature which will principally increase (or decrease) the strain energy in the packer element which reestablishes an increased (or decreased) overall strain energy of the packing layer system. An object of the invention is to obtain a positive final contact stress between the wellbore and the elastomeric layer. Accordingly, the invention provides a method of effecting a seal between an elongate inflatable packer and a borehole wall in a wellbore traversing earth formations, the wellbore having a temperature differential defined between a disturbed temperature condition of the wellbore and an undisturbed temperature condition thereof; and the packer having a central tubular mandrel and an elastomeric packer element mounted on said mandrel in sleeved relation thereto; said packer element being subject to inflation by a liquid to a finite inflation pressure to produce a radial expansion of said packer element such that a positive final contact stress is obtained at the interface between the packer element and the borehole wall and provides said seal with respect to the borehole wall after the wellbore returns to the undisturbed condition thereof; said mandrel, said packer element and said liquid being radial layers between a centerline of the borehole and the borehole wall; and parameters of said inflation pressure, said contact stress and said temperature differential being interrelated by aximetric plane strain equations for radial stress and radial displacement in a radial plane; said method including the steps of: selecting a depth in said wellbore for inflation of said packer element; using two parameters selected from said inflation pressure parameter, said temperature differential parameter and said contact stress parameter for said selected depth together with established physical parameters of said layers, determining the other of the inflation pressure parameter, the temperature differential parameter and the contact stress parameter, adjusting at least one of the parameters as required and obtaining a positive value for said contact stress parameter using the aximetric plane strain equations for radial stress and radial displacement in the radial plane; and running the packer into the wellbore and inflating the packer element at said selected depth at the determined or selected inflation pressure to obtain said positive contact stress at said selected depth. The contact stress parameter may be determined by matching common stress values at interfaces of said layers for each interface of said layers and utilizing the temperature differential parameter and the inflation pressure parameter for the packer element; and adjusting a thickness parameter of the packer element with respect to the other parameters to obtain said positive value for said contact stress parameter on the borehole wall. The contact stress parameter at the borehole wall may be determined by matching common stress values at interfaces of said layers for each interface between said layers including the outermost layer with said earth formation; and for each layer, adjusting its temperature differential parameter between said disturbed and undisturbed temperature conditions by adjusting the disturbed temperature condition parameter to an adjusted value thereof necessary to obtain a positive value for the contact stress parameter on the borehole wall. The disturbed temperature condition parameter may be adjusted by reducing a temperature parameter of the liquid. The liquid may be a cement slurry which hardens over time and undergoes a volume contraction. The liquid may be a drilling mud which hardens over time and undergoes volume contraction. The plane strain equations used may be limited to those for radial stress and radial displacement of said layers. At said selected depth in said wellbore there may be a larger diameter bore which is located below a smaller diameter bore and the packer may be run through the smaller diameter bore and inflated in the larger diameter bore. The mandrel and the packer element may be sized to optimize the inside diameter of the mandrel relative to the inside diameter of the smaller diameter bore, the optimum wall thickness of said packer element being determined from said plane strain equations so as to obtain said positive contact stress. In order that the invention may be well understood, various embodiments thereof, which are given by way of example only, will be described with reference to the accompanying drawings, in which: FIG. 1 is a vertical sectional view of a wellbore in which a known inflatable straddle packer is installed; FIG. 2 is a fragmentary section of a wellbore showing in cross-section a known inflatable packer suspended from a tubing string in the wellbore; FIG. 3 is a view similar to FIG. 2 but showing the packer in its inflated sealing condition; FIG. 4 illustrates various fluid pressures acting on the inflated packer of FIG. 3; FIG. 5 is a graphical plot of borehole temperature versus depth for a typical wellbore; FIG. 6 is a fragmentary view in longitudinal cross section of an inflatable packer in a wellbore; FIG. 7 is a fragmentary view in transverse cross section of the inflatable packer of FIG. 6; FIG. 8 is a view similar to FIG. 6 but showing radial dimensions and thicknesses of some components of the packer; FIG. 9 is a view similar to FIG. 7 but showing the packer in inflated sealing condition in the wellbore; FIG. 10 is a view similar to FIG. 9 but showing shrinkage and dimensional changes of components of the inflated packer which are induced by temperature changes; FIG. 11 is a view in partial longitudinal cross-section showing radial components of a multilayered liner system in a wellbore; FIG. 12 is a view on partial horizontal cross-section of FIG. 11; FIG. 13 is a schematic plot of temperature distribution as a function of radii of layers of a packer in a wellbore; FIG. 14 is a schematic plot of various sizes of inflatable packers to illustrate the expansion characteristics as a function of differential inflation pressure; FIG. 15 is a temperature profile generated by a Drill program. Referring now to Figure 1, a wellbore is schematically illustrated with a borehole section 10 extending from the ground surface 12 to a first depth point 14 and with a tubular metal casing 16 cemented in place by an annulus of cement 17. An adjacent borehole section 18 extends from the first depth point 14 to a lower depth point 20. A tubular metal liner 22 is hung by a conventional liner hanger 24 in the lower end of the casing 16 and is cemented in place with an annulus of cement 25. An adjacent borehole section 27 extends from the depth point 20 to a lower bottom at the Total Depth "TD" (not shown in FIG. 1). The borehole section 27 goes through earth formations and an inflatable straddle packer 26 which typically comprises a pair of inflatable packers mounted in a longitudinally spaced relationship on a single mandrel is connected by a production tubing 28 to the earth's surface. In drilling the borehole sections and in cementing operations, liquids are circulated in the borehole which change the in-situ undisturbed temperatures along the length of the borehole as a function of time and circulation rate. The change in temperatures will be discussed later in more detail. In another type of completion in lieu of a straddle packer, a single inflatable packer can be utilized with a perforating gun to produce fluids through the packer. Such a system is shown in US-A-3 918 522 (Reissue No. 30,711). For various reasons it may be desirable to utilize a single inflatable packer or straddle inflatable packer in either an open borehole section or a liner. For background information, a typical inflatable packer as schematically shown in FIG. 2 consists of a central tubular steel member or mandrel 30 which is coupled to upper and lower subs 31, 32 where the upper sub connects to a string of pipe or tubing 33 and the lower sub 32 connects to an extension pipe 34. Normally a plugseat (not shown) is located below the packer to retain pressure for inflating a packer member, or element, 35. The tubular elastomer, or elastomeric, packer element 35 coextensively extends along the mandrel 30 in sleeved relation thereto and is attached to the upper and lower subs 31 and 32. The upper sub 31 has a valve system 37 which controls access of liquid to the interior of the packer element 35 between the mandrel 30 and an inner wall of the packer element 35. The valve system usually has a knock off plug or valve control 39 which is activated to admit liquid to the interior of the packer member 35 when a plug is seated in the plug seat at lower end of the mandrel 30. Usually the liquid is required to have a preselected threshold pressure to commence inflation of the packer element 35. Anti-extension or reinforcing elements 36 are located in the end well of the packer element 35 for bridging support when the packer element 35 is inflated. After the desired inflation occurs and the packer element 35 is inflated as shown in FIG. 3, the valve system 37 traps the pressurized liquid at its final inflation pressure in the interior of the packer element 35. The inflating liquid is usually a cement slurry which, after inflation, cures and hardens into a solid, but mud inflation liquids are also used. In the process of curing the cement, heat is generated and upon curing, the volume of cement shrinks. The elastomer element 35 is under compression to maintain a seal with respect to the borehole wall. Excess cement is reversed out or drilled out in a conventional manner. A typical length for a packer member includes lengths up to forty feet. Further details and features of inflatable packers are shown and described in US-A-4 420 159 (Re 32,345). In an inflatable packer, the outer diameter, or O.D., of the uninflated packer relative to the inner diameter, or I.D., of the liner is a factor in determining the inflation pressure limits. Typically, a clearance gap of 3/8" to 1/2" exists between the I.D. of the liner and the O.D. of the packer. Thus, the smallest I.D. in a liner controls the size of the packer, however, the wellbore diameter where the packer is inflated below the liner can be considerably larger in diameter than the smallest I.D. in the liner. Inflatable packers can be inflated to up to five times their un-inflated diameter. However, as the diameter of a wellbore increases relative to a given size of well packer, the amount of differential inflation pressure that can safely be used will decrease. This is illustrated in FIG. 14 for various sizes of inflatable packer. As an example in FIG. 14, a 4 ½ " O.D. packer can be used in a 10" I.D. wellbore below a liner but the inflation pressure as shown at point A in FIG. 14 will be kept low to avoid rupture of the packer so that wall contact pressure will be low. Conversely, such a 4 ½ " O.D. packer when placed in a 6" I.D. well hole below a liner can utilize substantially higher inflation pressures (as shown at point B in FIG. 14) without rupture. In the two cases described, the contact sealing pressure will be substantially different. Acceptable contact sealing pressures between the inflated packer and wellbore are generally about 500 psi above pore (formation) pressure in open hole wellbores and about 1000 psi above pore pressure in cased wellbores. The object therefore is to inflate a packer with a safe differential inflation pressure and to obtain adequate contact sealing pressures. In selecting an optimum differential inflation pressure for an inflatable packer, in most instances, a consideration must be made of one or more of the following factors: 1) fracture pressure of the formation when in a hard hard rock formation; 2) break down pressure of the formation when in a soft rock formation; 3) internal yield pressure of the outermost liner for packer installations inside a liner; 4) internal yield pressure of the innermost liner (normally most critical near the surface); 5) collapse pressures of the liner or liners and the packer mandrel (normally most critical in the packer mandrel); 6) maximum recommended "Differential Inflation Pressure" for the elastomer packer relative to the size of the borehole. If the total inflation pressure for the packer exceeds the fracture pressure of a hard rock formation or a friable formation, small fractures may be initiated along the packer and formation interface. In the worst case, these fractures or cracks can form a fluid communication path along the length of the packer element ("seal" length). In order to prevent this from occurring, the installation must be planned to insure that the total inflation pressure does not exceed the fracture pressure of the surrounding rock formation. If the total inflation pressure for a packer exceeds the breakdown pressure of a soft rock formation, the formation may experience breakdown and cannot hold inflation pressure above this value. When this occurs, surface volumetric data may erroneously indicate an enlarged well hole. This is common occurrence in Gulf Coast wells where the annular pressure during the primary cement job may be near the formation breakdown pressure but does not result in failure of the installation as the soft rock formations do not crack. For installations where packers are run inside a liner or casing pipe, the internal yield pressure of the pipe must not be exceeded by the inflation pressure of the packer. In an open hole installation, the pressures that act on a liquid filled Inflatable Formation Packer 40 are illustrated in FIG. 4. The inflation pressure P 1 in the elastomer packer element 41 is the same as the inflation pressure PT 1 in packer mandrel 42. The mid-section of the packer element 41 is an unsupported elastomeric portion which is adapted to conform to wellbore irregularities (including washouts) in wellbores up to 5 times the O.D. of the uninflated packer element and which compresses under the effect of pressure. Since the mid-section of the packer element is not constrained, the inflation pressure P 1 acts directly on the wall of the wellbore although pore pressure of fluids within the formation offsets a portion of the packer pressure on the wall. The net pressure acting on the wall of the formation (inflation pressure - pore pressure) is defined as a "seal" load. Since the seal load is the net force or load, acting radially against the contact area of the wellbore, the seal load acts to restore the stress that is lost in the formation when the hole is drilled and is equal to the effective radial stress. From the foregoing it can be appreciated that the maximum differential pressure that can be safely applied to the center of the element is a function of fracture pressure which is independent of hole size up to 5 times the O.D. of the uninflated packer element ("run-in" diameter). Each end section of the inflatable element 40 can have pliant petal-shaped metal support reinforcements 43 which are embedded in the end section of the elastomer element 41 and are enclosed in the adjacent sub 31 or 32 (see Figs 1 and 2). The metal reinforcements 43 are of sufficient length to extend between the packer mandrel 42 and the wall of the wellbore when the packer is inflated. The pressure that acts on the end sections is the differential pressure between the inside of the elastomer element (pressure P 1 ) and the annulus pressure (pressure P s1 , or pressure below, downhole of the packer 40 and pressure P s2 , or pressure above, uphole of packer 40). This differential pressure is called the differential inflation pressure. The strength of the end sections is selected as a function of the annular area in a radial cross-section of the annulus and the wellbore geometry. Generally, the smaller the annular area, the stronger the end section. If the pressure in the annulus either above or below the inflatable element 41 is increased to a pressure that is greater than the initial inflation pressure, the deformable end sections are designed to transfer this pressure to the inflation fluid within the packer. This self energizing feature maintains the annular seal in cases where treating or injection pressures in the wellbore exceed initial inflation pressure of the packer, and the packer was inflated with mud. However, this increased inflation pressure will then increase the differential inflation pressure on the opposed end assembly 43. If this exceeds the strength of the end assembly, the packer element will be damaged. It is important to note that the elastomer element of a cement inflated Formation Packer also self energizes in a manner likened to an elongate packer element in a compression type packer. In this case, the sealing capability is limited only by the strength or elasticity of the formation independent of hole size. As discussed above, the maximum differential pressure that a liquid filled inflatable packer can safely hold is primarily a function of hole size (annular area) in relation to packer size. However, other factors such as borehole geometry, hole deviation, centralization, and temperature changes during well treatments can also induce non-uniform and excessive stresses on an end assembly of the packer element. Referring now to FIG.5, where the wellbore traverses earth formations from the earth's surface (ground zero "0" depth) to a total depth (TD), the earth formations, the liners and the cement in the borehole in a quiescent undisturbed state will have a more or less uniform temperature gradient 45 from an ambient temperature value t 1 , at "O" depth (ground surface) to an elevated or higher temperature value t 2 at a total depth TD. A quiescent undisturbed state is herein defined as that state where the wellbore temperature gradient is at a normal in-situ temperature undisturbed by any operations in the wellbore. Liquids which are circulated in the wellbore during drilling, cementing and other operations can and do cause a temperature disturbance or temperature change along the wellbore where the in-situ undisturbed temperature values are changed by the circulation of the liquids which cause a heat transfer to or from the earth formations. A circulating liquid in the well changes the temperature values along the length of the wellbore as a function of depth, time and circulation rate so that a more or less uniform disturbed temperature gradient 46 is produced which has a higher temperature value t 3 than the temperature value t 1 at "O" depth and a lower temperature value t 4 than the in-situ undisturbed temperature value t 2 at the depth TD. The plot of the disturbed temperature gradient 46 will intersect the plot of the undisturbed temperature gradient 45 at some depth point 47 in the wellbore. Below the cross over temperature depth point 47, the wellbore will generally be at a lower temperature than it would normally be in its quiescent undisturbed state. Above the cross-over temperature depth point 47, the wellbore will generally be at a higher temperature than it would normally be in its quiescent undisturbed state. It will be appreciated that a number of factors are involved in the temperature change and that, in some operations, the downhole TD temperature can approach ambient surface temperature because of the heat transfer mechanism of the circulating fluids and fluids used in the operation. In FIG.6, a fragmentary view of an inflatable packer in an uninflated condition is shown in longitudinal cross-section and in Fig.7, a partial transverse cross-section of the packer in a wellbore is illustrated. In the illustration of FIGS.6 and 7, the central mandrel 50 of the inflatable packer supports an elastomer and inflatable packer element 52 of the type herein described. The outer wall surface 54 of the packer element 52 is spaced by an annular gap 55 from the interior wall 56 of a borehole traverses earth formations 58. As shown schematically in FIG.8, the central mandrel 50 of the packer has an inner radius R 1 and a wall thickness W 1 . The packer element 52 has an inner radius R 2 and a wall thickness W 2 . In FIG.9, a fragmentary transverse cross-section of the inflatable packer illustrates the inflation of the packer element 52 into contact with the borehole wall 56 with a cement slurry 59a at a time prior to curing the cement slurry 59a. At this time, the cement slurry 59b is also in the bore 60 of the central mandrel 50 and is at a same pressure P n as the inflation pressure P l of the cement slurry 59a in the inflatable packer element 52. With respect to temperature effects, the temperature change in the central mandrel 50 and elastomer packer, or seal, element 52 is minimal when the packer is first disposed in the wellbore to its desired location because the equipment has a relatively large mass and is introduced both at the surface ambient conditions. The cement slurry is also normally introduced at surface ambient conditions. If desired, however, the cement slurry can be reduced in temperature at the surface or mixed with ice to reduce its temperature. Prior to inflating the packer seal element 52 there is a hydrostatic or mud pressure P m in the borehole. Since the pressure P T in the mandrel 50 is equal to the pressure P m , there is no differential pressure to affect the wall thickness W 1 of the mandrel 50. However, when the cement slurry 59a is introduced under the pressure P c , the slurry 59a compresses the packer seal element 52 and reduces its wall thickness to a thickness less than the wall dimension W 2 and the compressive force in the elastomer element 52 seals the element 52 against the borehole wall 56 (See FIG. 9). At the selected final inflation pressure of the packer, the inflating medium at a pressure P c is trapped within the packer element 52 by a conventional valve system (not shown) in the inflatable packer. Thereafter, the pressure P c in the central member 50 is released to reduce to an ambient pressure value. At this time, there is a pressure differential across the wall of the central mandrel 50 which radially compresses the central member 50 inwardly towards a central axis 61. As described before, the inflation pressure of the packer develops strain energy in the mandrel 50, the cement slurry 59a, the packer seal element 52 and the surrounding rock formation. Thereafter, the cement slurry 59a cures to a solid form and generally changes bulk volume (changing the layer thickness) as shown in exaggerated form by 59b in FIG. 10. This results in a change of strain energy in the packing system. In time, however, the strain energy in the system will again change because the temperature in the central mandrel 50, the hardened cement slurry 59b and the packer element 52 will increase (or decrease) to the in-situ undisturbed temperature at the depth of the packer element in the wellbore. The change in temperature in all of these elements causes a change in the radial dimensions (thickness) principally in the elastomer packer seal and mandrel elements which increases (or decreases) the strain energy in the system. The effect of temperature change is greatest on the elastomer packer element. The strain energy increases when the packer is located below the cross-over temperature depth point 47 illustrated in FIG. 5 and decreases when the packer is located above the cross-over temperature depth point 47. In either case, if the packer seal element 52 lacks the desired final strain energy (is not sufficiently compressed) after all of the elements at the packer location return to an undisturbed temperature, the shrinkage and dimensional changes of the cement and the elastomer packer element can produce an annular gap 62 (exaggerated for illustration) between the elastomer seal element 52 and the borehole wall 56 or a lack of sufficient pressure to maintain a seal. In a method hereinafter described, a precise inflation pressure to obtain a desired contact pressure, or stress, can be determined so that the gap 60 or a loss of seal with the borehole wall 56 to permit a leak does not occur and a sufficient desired contact pressure remains in the packer seal element 52 to maintain a seal without borehole fluid leakage even after the packer components in the borehole return to their undisturbed temperature values. It can be appreciated that in a wellbore, a given cross-section of wellbore at a given depth can have an infinite variety of configurations. As shown in FIGS. 11 and 12, a given cross-section of a wellbore can include an inner tubular member or liner 64 located within an outer tubular member or liner 66 with a cemented annulus 67 between the liners 64, 66 and a cemented annulus 69 between the outer liner 66 and the borehole wall 70. Thus, within the wellbore, there can be a number of different layers or materials at a given cross-section as just described. In practicing the method, the first step is to obtain the quiescent or in-situ undisturbed temperature in a wellbore as a function of depth. This can be done with a conventional temperature sensor or probe which can sense temperature along the wellbore as a function of depth. This temperature data as a function of depth can be plotted or recorded. Alternatively, a program such as "WT-DRILL" (available from Enertech Engineering & Research Co., Houston, Texas) can be used at the time a well completion is in progress. In the WT-DRILL program, well data is input for a number of parameters for various well operations and procedures. Data input includes the total depth of the wellbore, the various bore sizes of the surface bore, the intermediate bores, and the production bores. The outside diameters (O.D.), inside diameters (I.D.), weight (WT) of suspended liners in pounds/foot and the depth at the base of each liner is input data. If other well characteristics are involved, the data can include for deviated wells, the kick-off depth or depths and total well depth. For offshore wells, the data can include the mudline depth, the air gap, the O.D. of the riser pipe, and the temperature of the seawater above the mudline, riser insulation thickness and K values (btu/hr-Ft-F). Input of well geometry data can include ambient surface temperature and static total depth temperature. In addition, undisturbed temperature at given depths can be obtained from prior well logs and used as a data input. The Mud Pit Geometry in terms of the number of tanks, volume data and mud stirrer power can also be utilized. The mud pit data can be used to calculate mud inlet temperature and heat added by mud stirrers can be related to the horsepower size of the stirrers. Drilling information or the number of days to drill the last section, the total rotating hours, start depth, ending depth and mud circulation rate are input data. The drill string data of the bit size, bit type, nozzle sizes or flow area, the O.D., I.D. and length of dill pipe (DP), the DP and collars are input data. The mud properties of density, plastic viscosity and yield point are input data. Post Drilling Operations includes data of logging time, circulation time before logging, trip time for running into the hole, circulation rate, circulation time, circulation depth, trip time to pull out of the hole. Cementing data includes pipe run time, circulation time, circulation rate, slurry pump rate, slurry inlet temperature, displacement pump rate and wait on cement time. Also included are cement properties such as density, viscometer readings, test temperature and bulk contraction (shrinkage). Further included are lead spacer specification of volume, circulation rate, inlet temperature, density, plastic viscosity and yield point. Thermal properties of cement and soil such as density, heat capacity and conductivity are input. The time of travel of a drill pipe or a logging tool are data inputs. All of the foregoing parameters for obtaining a temperature profile are described in "A Guide For Using WT-Drill", (1990) Enertech Computing Corp., Houston, Texas. The disturbed temperature as a function of depth can be determined from the WT-Drill Program just prior to running the inflatable packer. As discussed above, the packer element 52 when run in the wellbore will be inflated with a cement slurry 59a, which is then pressured to establish a contact stress between the packer element 52 and the wall 56 of the wellbore. A successful sealing application of this packer 52 in the wellbore depends upon the contact stress remaining after cement shrinkage and after temperature changes occur when the wellbore returns to a quiescent undisturbed state. In order to predict with some certainty the final wellbore contact stress, a thermal profile of the wellbore prior to inflating the packer is utilized with the inflation pressure for the packer in a horizontal plane strain determination to obtain a value for the contact stress after the wellbore returns to an undisturbed state or condition. In some instances it will be determined that the packer cannot obtain the desired results thus predetermining that a failure will occur. When the contact stress as thus determined is insufficient or inadequate for effecting a seal, then the inflation pressures or the packer parameters can be adjusted to utilize sufficient inflation pressure or to design the right packer for the operation. In all instances, the stresses are etablished for future reference values. The residual contact stress is determined by a stress analysis of the mandrel 50, the cement 59b, the elastomer 52 and the formation 58. The stress analysis is based on the radial strains in the layered components of the packing system as taken in a horizontal, or transverse, plane where the radial strains are fairly symmetric about the central axis of the mandrel 50. In elastic strain analysis, a plane strain axi-symmetric, or aximetric, solution of static equilibrium equations with respect to temperature changes is stated as follows: σ r ( R ) = aE Δ T R 1-ν +2λ C 1 where: R o - outside radius (in) R i - inside radius (in) u(R) - radial displacement (in) σ r (R) - radial stress (psi) σ  (R) - hoop stress (psi) σ z (R) - axial stress (psi) E - Young's modulus (psi) ν - Poisson's ratio G - Shear modulus, 2G - E/ (1+ν), (psi) λ - Lame's constant, λ = 2G ν/(1-2ν), (psi) a - coefficient of linear thermal expansion (1/F) ΔT - temperature change (F) and is a function of R with respect to RdR C 1 , C 2 - constants determined by boundary conditions ξ - is a symbol for R for notational purposes R - any radius between R o and R i In one aspect of the method, the hoop stress (Equation 3) and axial stress (Equation 4) are not considered significant factors in determining the sealing effects of an inflatable packer after the wellbore returns to its in-situ undisturbed conditions. Considering Equations (1) & (2) then for radial displacement and radial stress it can be seen that each layer at a given horizontal plane in a wellbore has two unknown coefficients C 1 and C 2 . By way of reference and explanation, FIG. 13 is a partial schematic diagram or a packer in a wellbore illustrating a center line CL and radially outwardly arranged layers of steel, cement, elastomer and earth formations. Overlaid on the FIG. 13 illustration is a temperature graph illustrating increasing temperatures along he vertical CL axis from a formation temperature T F to a wellbore temperature T H . At a medial radial location in the steel liner, there is a temperature T S which is lower than the temperature T H . A median radial location in the cement has a temperature T C which is lower than the temperature T S . A median radial location in the elastomer has a temperature T R which is lower than the temperature T C . At some radial distance into the formation beyond the elastomer seal, an undisturbed formation temperature T F exists. With a disturbed condition in the wellbore, the temperature of the components defines a gradient from a location at the center of the wellbore to a location in the formation temperature T F . As the illustration in FIG. 13 shows, the various layers are defined between radii as follows: steel layer between R SI and R SO cement layer between R SI and R CO elastomer layer between R EI and R EO and where the following inside radii and outside radii are equal. R SO = R CI R CO = R EI R EO = R HI At the depth location illustrated in FIG. 13, a temperature gradient occurs between a radius location in the formation where the temperature T F is at the undisturbed formation temperature and a center line location in the wellbore where the temperature T H is at the wellbore temperature. The shape of the gradient is largely a function of the properties of the formations and can be almost linear. All of the parameters of Equations (1) & (2) are predetermined for each layer of the system so that the only unknowns for each layer are the coefficients C 1 and C 2 . By definition, the coefficient C 1 and C 2 for the interface between the steel and cement are equal, the coefficients C 1 and C 2 for the interface between the cement and the elastomer are equal and the coefficients C 1 and C 2 for the interface between the elastomer and the borehole wall are equal. In other words, the stress at one edge of one layer wall is equal to the stress at the edge of an adjacent layer wall. In the fundamental analysis then, there are two equations (1) and (2) for the steel layer and two equations (1) and (2) for the cement layer which total four equations and two unknown coefficients. The equations can be solved by Gauss elimination or block tridiagonals. In the solution, a desired inflation pressure is selected and the associated contact sealing pressure is determined. Material Properties The solution of the above stress formula requires a determination of the elastic properties of several diverse materials in the layers. Steel properties do not vary greatly and are relatively easy to obtain: Common reported values are: Values selected for use Young's modulus: E = 28-32 x 10 6 psi 30 x 10 6 Poisson's ratio: v = 0.26-0.29 .29 Thermal expansion: a = 5.5-7.1 x 10 -6 /F 6.9 x 10 -6 Rock or formation properties are considerably more varied and some properties are more difficult to find, such as the thermal expansion coefficients for different materials: Values associated with representative formation materials include the following: Limestone: Young's modulus: E = 73-87 x 10 5 psi Poisson's ratio: v = 0.23-0.26 Thermal expansion: a = 3.1-10.0 x 10 -5 /F Sandstone: Young's modulus: E = 15-30 x 10 5 psi Poisson's ratio: v = 0.16-0.19 Thermal expansion: a = 3.1 -7.4 x 10 -6 /F Values selected for use: Shale: Young's modulus: E = 14-36 x 10 5 psi 30 x 105 Poisson's ratio: v = 0.15-0.20 .18 Thermal expansion: a = 3.1-10.0 x 10 -6 /F 3 x 10 -6 Cement properties vary with composition. The following values are considered nominal: Values selected for use: Young's modulus: E = 10-20 x 10 5 psi 15 x 10 5 Poisson's ratio: v = 0.15-0.20 .20 Thermal expansion: a = 6.0-11. x 10 -6 /F 6.0 x 10 -6 The volume change of the cement layer due to cement hydration and curing is needed for the analysis, and is one of the critical factors in determining the residual contact stress between the packer and the formation. A study by Chenevert [entitled "Shrinkage Properties of Cement", SPE 16654, SPE 62nd Annual Technical Conference and Exhibition, Dallas, Texas (1987)] indicates a wide variation in cement shrinkage because of different water and inert solids content. It appears that a shrinkage of about 2% is the minimum that can be achieved. Cement producing this minimum shrinkage can be used in the practice of this invention for optimum results. In any event, with the packer and cement parameters, the thickness of the cement annulus after curing can be predetermined. Elastomer properties are critical in stress analysis because the elastomer is the most compliant material and it is least sensitive to cement shrinkage. High compliance, or low elastic modulus means small stress changes for small strains. However, because rubber has the largest coefficient of thermal expansion, it is most sensitive to temperature changes. The following values for nitrile rubber were estimated from various research sources. Value selected for use Young's modulus: E = 400 800 psi 640 Poisson's ratio: v = 0.49932-9.499356 .49934 Thermal expansion: a = 6.0-13. x 10 -5 /F 13. x 10 -5 The Poisson's ratio of .49934 and the Young's modulus of 640 psi implies a bulk modulus for the nitrile rubber of about 162,000 psi. The range for bulk modulus for nitrile rubber is 150,000 to 350,000 psi. EXAMPLES OF ESTIMATED CONTACT STRESSES GENERATED BY AN INFLATABLE PACKER The formation contact stresses for certain wells was determined using the following assumptions: Rubber Elastic Modulus = 400 psi Rubber Poisson Ratio = .49934 Cement Shrinkage = 2% The following example for practicing the method is in a well based on a frac pressure of 4400 psi, a packer depth of 3=8600 ft., and bottom hole pressures of 3000 psi. The inflation pressure of the inflatable packer was rated to safely exceed hydrostatic by 1400 psi. At this point then, a selection of inflation pressure was made. The value of 1350 psi (slightly less than 1400 psi) was used as a selected inflation pressure increment. At the depth where inflation of the packer is intended, the temperature differential is +10°F (below the temperature cross-over depth point). The 7" packer has an 8.035" O.D. and Well #1 has an 8½" wellbore. The following are layer characteristics for the mandrel, the cement, the elastomer, and the earth formation (rock) in an inflated condition. WELL #1 8 1/2 " I.D. LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) YOUNGS MODULUS (PSI) POISSONS RATIO COEF LIN THERM EXPNSN (1/F) Mandrel 3.09 3.50 30.00E+6 .290 6.900E-6 Cement 3.50 3.72 15.00E+5 .200 6.000E-6 Elastomer 3.72 4.25 400. .49934 1.300E-4 Rock 4.25 30.00E+5 .180 3.000E-7 The temperature differential ΔT for the various layers at the desired depth is obtained from a WT-Drill program. Utilizing Equations (1) & (2) above with the ΔT determinations and an inflation pressure of 1350 psi above hydrostatic pressure, gave the following stress results for the various layers while the cement is still liquid: (a) INCREMENTAL TOTAL LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) Mandrel 3.09 3.50 1350. 1350. 4350. 4350. Cement 3.50 3.72 1350. 1350. 4350. 4350. Elastomer 3.72 4.25 1350. 1349. 4350. 4349. Rock 4.25 * 1349. * 4349. * Next utilizing Equations (1) and (2) above with the ΔT determinations and assuming the condition when inflation pressure is trapped in the packer and the pressure in the string of tubing is adjusted to hydrostatic pressure, and using a cement volume change upon curing equal to -.0200 ft3/ft3, the stress in the layers calculated at the time the packer cement has hardened is: (b) INCREMENTAL TOTAL LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) Mandrel 3.09 3.50 0. 1708. 3000. 4708. Cement 3.50 3.72 1708. 1020. 4708. 4020. Elastomer 3.72 4.25 1020. 1020. 4020. 4020. Rock 4.25 * 1020. * 4020. * It can be seen that the contact stress of the elastomer is at 1020 psi. If the desired contact sealing force is 1000 psi or more, then this is sufficient sealing contact pressure and the packer can be run in the wellbore and inflated to 1350 psi above the hydrostatic pressure of 3000 psi with a resultant ultimate contact stress of 1020 psi. To determine the contact force after the wellbore returns to an undisturbed temperature condition, the Equations (1) & (2) are solved for the in-situ temperature. In the example, the temperature increase is 10°F. The results are: (c) INCREMENTAL TOTAL LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) Mandrel 3.09 3.50 0. 1937. 3000. 4937. Cement 3.50 3.72 1937. 1243. 4937. 4243. Elastomer 3.72 4.25 1243. 1243. 4243. 4243. Rock 4.25 * 1243. * 4243. * The temperature increase of 10°F at the location of the packer illustrates that higher contact stresses are obtained in the elastomer layer at the higher undisturbed temperature. If the location of the packer was above the cross-over depth point and the undisturbed temperature was 10°F lower than the disturbed temperature then the results would decrease the stress in the elastomer below 1020 psi (see "b" above) because of the temperature contraction of the elastomer. The above results show in that a 1000 psi contact stress can be achieved for the 7" packer in the 8-1/2" hole. A temperature increase of 10°F in the undisturbed in-situ temperature adds about 200 psi to the results which illustrates the effect of temperature on contact stress. As discussed heretofore, there are two unknown boundary constants C 1 and C 2 for each layer of material. The stress analysis of the packer to formation assembly (radial layers of materials) is determined by matching boundary conditions at the inside of the mandrel, at the interfaces between layer components and at the outside radius of the wellbore. There are two load cases considered in the above packer analysis, the packer inflation pressure and the packer contact stress with the wellbore after the cement sets. In the packer inflation problem, the boundary conditions used are: 1. the radial pressure at the inside radius of the mandrel is at the inflation pressure. 2. the radial pressure at the outside radius of the mandrel is the inflation pressure. 3. the cement is considered a fluid at the inflation pressure, so the stress formulas are not used. 4. the radial pressure at the inside radius of the elastomer element is at the inflation pressure. 5. for open hole applications, the displacement and radial stress at the outside radius of the elastomer element match the displacement and radial stress at the inside radius of the wellbores. The displacement of the formation at infinity is zero. 6. for packer inflation pressure inside casing or liners, the displacement and radial stress at the outside of the elastomer element match the displacement and radial stress of the casing. Outside the casing may be more cemented casings, a fluid filled annulus, or formation. Between all solids, cement, steel, or formation, the displacement and radial stress must be continuous. For a fluid filled annulus, the fluid pressure must be applied to the outside radius of the last casing. Analysis of the packer after the cement sets differs only in the treatment of the cement. In this case the cement is considered a solid, so that the following boundary conditions are used: 1. The displacement and radial stress at the outside radius of the mandrel match the displacement and radial stress at the inside radius of the cement. 2. The displacement and radial stress at the outside radius of the cement match the displacement and radial stress at the inside radius of the rubber. The set of boundary conditions forms a block tridiagonal set of equations with unknown constants C 1 and C 2 for each layer of material. The boundary conditions are solved using a block tridiagonal algorithm. After the cement sets, the temperature change is utilized to determine the contact stress when the wellbore returns to an undisturbed temperature condition. In the above example, it is established that the selected inflation pressure is a function of the ultimate contact stress. Thus, the analysis process can be used so that for a selected inflation pressure, the ultimate contact stress can be determined before the packer is used in a wellbore. Therefore, it is predetermined that the packer will obtain a sufficient contact stress after the well returns to an undisturbed condition. Alternatively, a desired contact stress can be selected and the inflation pressure necessary to achieve the selected contact stress can be determined. This permits the operator to safely limit contact pressures by controlling the inflation pressure. This also predetermines if the inflation pressure is within the capabilities of the packer. Stated another way, the maximum inflation pressure obtains a maximum contact stress. However, because of wellbore conditions, the maximum inflation pressure for the packer in a given borehole may be insufficient to obtain a satisfactory contact stress so that the inflatable packer would be unproductive and expensive. Similarly, for a desired contact stress it can be determined that the packer would be ruptured or otherwise exceed its rated limits. Similarly, in casing or liners which have weaknesses, a precise inflation pressure to obtain a precise contact stress can be determined and utilized. In the foregoing explanation of the method, only equations (1) and (2) were employed as a fundamental example where the z axis and hoop stress are effectively valued at zero. This is a solution based upon isotropic cement contraction in which the change in wall thickness is greater than actually encountered which provides a safety factor. The effect of plane strain cement contraction can best be understood by consideration of the following examples: CASE 1 This packer is a 7" nominal (8-1/8" O.D.) packer LAYER PROPERTY SUMMARY LAYER INSIDE DIAMETER (IN) OUTSIDE DIAMETER (IN) YOUNGS MODULUS (PSI) POISSONS RATIO COEF LIN THERM EXPNSN (1/F) Mandrel 6.28 7.00 30.00E+6 0.29000 6.900E-6 Cement 7.00 9.11 15.00E+5 0.20000 6.000E-6 Elastomer 9.11 10.00 640. 0.49934 1.300E-4 Rock 10.00 * 30.00E+5 0.18000 3.000E-7 The following differential temperature profile was used RADIUS (IN) TEMPERATURE (F) 3.40 5.00 4.50 5.00 10.00 5.00 100.00 0.00 Utilizing Equations (1) & (2) above with the ΔT determinations and a packer inflation pressure of 1000 psi above pore pressure of 2000 psi, gives the following stress results for the various layers while the cement is still liquid: (a) LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INCREMENTAL INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) TOTAL INSIDE STRESS (PSI) TOTAL OUTSIDE STRESS (PSI) Mandrel 3.14 3.50 1000. 1000. 3000. 3000. Cement 3.50 4.55 1000. 1000. 3000. 3000. Elastomer 4.55 5.00 1000. 966. 3000. 2966. Rock 5.00 * 966. * 2966. * Next utilizing Equations (1) and (2) above with the ΔT determinations and assuming the condition when inflation pressure is trapped in the packer and in the string of tubing is adjusted to hydrostatic pressure, and using a cement volume change upon curing equal to -.0200 ft3/ft3, the stress in the layers calculated at the time the packer cement has hardened and the wellbore has returned to its original undisturbed temperature (+5°F) is: LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INCREMENTAL INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) TOTAL INSIDE STRESS (PSI) TOTAL OUTSIDE STRESS (PSI) Mandrel 3.14 3.50 0. 1538. 2000. 3538. Cement 3.50 4.55 1538. -973. 3538. 1027. Elastomer 4.55 5.00 -973. 1007. 1027. 993. Rock 5.00 * -1007. * 993. * It can be seen that the contact stress of the elastomer is at -1007 psi which means there is a seal load failure because the cement volume contraction (wall thickness) decreased more than the expansion effect on the elastomer due to the temperature change. With the above Case 1, the wall thickness of the uninflated elastomer element was 0.5625 inches. CASE 2 The effect of increasing the wall thickness of the above discussed elastomer element of Case 1 is illustrated by the following case which has the same parameters except that the uninflated packer wall thickness is increased to 0.875 inches: However, the stress in the layers calculated at the time the packer cement has hardened with a +5°F temperature change is: LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INCREMENTAL INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) TOTAL INSIDE STRESS (PSI) TOTAL OUTSIDE STRESS (PSI) Mandrel 3.14 3.50 0. 1868. 2000. 3868. Cement 3.50 4.12 1868. 431. 3868. 2431. Elastomer 4.12 5.00 431. 400. 2431. 2400. Rock 5.00 * 400. * 2400. * This illustrates that by proper selection of a wall thickness of the elastomer element a positive seal load can be obtained where a common sized packer element would fail. CASE 3 The effect of a higher temperature differential can be shown in the following instance which is the same parameters as Case 1 but using a 10°F temperature differential. The stress in the layers calculated at the time the packet cement has hardened is: LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INCREMENTAL INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) TOTAL INSIDE STRESS (PSI) TOTAL OUTSIDE STRESS (PSI) Mandrel 3.14 3.50 0. 2079. 2000. 4079. Cement 3.50 4.12 2079. 636. 4079.. 2636. Elastomer 4.12 5.00 636. 605. 2636. 2605. Rock 5.00 * 605. * 2605. * It can be seen that an increase of 10°F causes the final seal load to increase to 605 psi. One of the ways to obtain an increase of 10°F is to decrease the temperature of the elastomer element by reducing its temperature prior to installation. In effect then the temperature differential would be great enough to effect a positive seal when the temperature returned to normal for the well. CASE 4 In the following case, various parameters utilize in the WT-Drill Program and temperature changes are as follows: Referring to FIG. 15, a plot of temperature vs. depth shows the undisturbed temperature gradient 80 and the disturbed temperature gradient 81. The cross-over depth point 82 is at 5000 feet and at 8107 feet the temperature differential is about 38°F. In Table I, it can be seen that at 8107 feet the "fluid" or cement temperature from the WT-Drill Program is 124°F as compared to 160°F for the undisturbed temperature. This is a 41°F temperature differential between the inflation fluid and the undisturbed temperature at that depth. RESULTS AFTER SLURRY PLACEMENT WELLBORE TEMPERATURES, F DEPTH FLUID STRING ANNULUS CASING CASING CASING UNDIST. 0. 80. 83. 91. 91. 88. 86. 80. 591. 84. 87. 95. 93. 64. 46. 46. 611. 85. 88. 95. 95. 78. 67. 45. 2000. 95. 97. 103. 103. 91. 71. 3200. 103. 105. 110. 110. 103. 94. 4000. 108. 110. 114. 114. 112. 109. 5200. 114. 116. 120. 120. 122. 125. 6000. 118. 119. 122. 122. 128. 136. 7200. 122. 123. 125. 125. 136. 153. 8000. 124. 125. 125. 164. 8107. 124. 125. 125. 165. THE LAYER PROPERTY SUMMARY IS : LAYER INSIDE DIAMETER (IN) OUTSIDE DIAMETER (IN) YOUNG MODULUS (PSI) POISSONS RATIO COEF LIN THERM EXPNSN (1/F) Mandrel 4.28 5.00 30.00E+6 0.29000 6.900E-6 Cement 5.00 5.59 15.00E+5 0.20000 6.000E-6 Elastomer 5.59 6.50 640. 0.49934 1.300E-4 Rock 6.50 * 20.00E+5 0.18000 3.000E-7 THE TEMPERATURE DIFFERENTIAL IS: RADIUS (IN) TEMPERATURE (F) 2.32 38.10 2.69 38.90 3.81 31.80 5.01 24.51 6.21 19.36 7.41 15.69 8.60 13.06 9.80 11.11 11.00 9.65 13.00 8.39 27.97 1.49 60.20 0.04 129.56 0.00 278.81 0.00 600.00 0.00 The temperature differential ΔT for the various layers at the desired depth obtained from a WT Drill program and utilizing equations(1) & (2) above with the ΔT determination and a packer inflation pressure of 1000 psi above a pore pressure of 5380 psi, gives the following stress results for the various layers while the cement is still liquid: LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INCREMENTAL INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) TOTAL INSIDE STRESS (PSI) TOTAL OUTSIDE STRESS (PSI) Mandrel 2.14 2.50 1000. 1000. 6380. 6380. Cement 2.50 2.80 1000. 1000. 6380. 6380. Elastomer 2.80 3.25 1000. 984. 6380. 6364. Rock 3.25 * 984. * 6364. * Next utilizing Equations (1) and (2) above with the ΔT determinations and assuming the condition when inflation pressure is trapped in the packer and in the string of tubing is adjusted to hydrostatic pressure, and using a cement volume change upon curing equal to -.0200 ft3/ft3, the stress in the layers calculated at the time the packer cement has hardened is: LAYER INSIDE RADIUS (IN) OUTSIDE RADIUS (IN) INCREMENTAL INSIDE STRESS (PSI) OUTSIDE STRESS (PSI) TOTAL INSIDE STRESS (PSI) TOTAL OUTSIDE STRESS (PSI) Mandrel 2.14 2.50 0. 2777. 5380. 8157. Cement 2.50 2.80 2777. 1698. 8157. 7078. Elastomer 2.80 3.25 1698. 1683. 7078. 7063. Rock 3.25 * 1683. * 7063. * It can be seen that the seal load increases dramatically with increasing temperature of 38.1°F. It will be appreciated that the foregoing process can be refined to determine the axial, radial and hoop cement shrinkage strains on an independent basis so that any combination can be used. In cement. the relationship for stresses and strains for general cement shrinkage is given by: E (ε R +δ r ) = σ r -γ (σ z + σ  ) E (ε  +δ  ) = σ  -γ (σ r + σ z ) E (ε z +δ z ) = σ z -γ (σ r + σ  ) where: ε r - strain in the radial direction ε  - strain in the hoop direction ε z - strain in the axial direction δ r - cement volume decrease in the radial direction δ  - cement volume decrease in the hoop direction δ z - cement volume decrease in the hoop direction σ r - stress in the radial direction (psi) σ  - stress in the hoop direction (psi) σ z - stress in the axial direction (psi) E - Young's modulus (psi) γ - Poisson's ration where δ r is the shrinkage in the r direction, δ  is the shrinkage in the hoop direction, and δ z is the shrinkage in the z direction. The total volume change is: Δγ/γ = -δ r -δ  -δ z The radial strain only case is then a special case of this general model (δ  =δ z =0), The cement shrinkage option may be used to allow the cement to shrink only in the radial direction within the packer. The anticipated effect of this application is to decrease the radial compressive stress on the mandrel due to cement shrinkage. For example, if the cement is assumed to fail in the hoop direction, the hoop contraction should be set to zero. The effect of cement shrinkage may be decreased due to axial movement of the cement during setting. In plane strain, the axial shrinkage affects the radial and hoop stresses through the Poisson effect. If axial movement is allowed (not plane strain), the axial shrinkage has no effect on the radial and hoop stresses. For this reason, the effect of the axial cement shrinkage is removed from the calculations. In the above-mentioned methods, it is recognized for the first time that the temperature effects in a wellbore disturbed by drilling or other fluid transfer mechanisms in the wellbore can significantly affect the downhole sealing efficiency of an inflatable packer when the borehole temperatures reconvert to an in-situ undisturbed temperature condition or to operational conditions of the well. As will be apparent from the foregoing, in a packing system which includes the use of an inflatable packer in a wellbore, the packer provides more or less concentric layers which include an inner layer of the packer tubular element, or mandrel, a layer of cement and a layer of an elastomer sealing element which, in a simple system, engages the wall of the wellbore. The packing system also includes the surrounding rock formations. In more complex systems, the wellbore can further be provided with additional nested liner elements and cement layers extending radially outward from a central axis of the borehole and substantially concentrically arranged. The temperature profile of the wellbore is determined for the undisturbed in-situ state and for the disturbed state prior to use of the inflatable packer. Then at the desired depth location for the inflatable packer and in a horizontal plane, the temperature difference between the disturbed state and undisturbed state of each layer is determined. Next, the intended inflation pressure for the inflatable packer is selected and utilized with the temperature differences between disturbed borehole temperatures and undisturbed borehole temperatures in the equations for the elastic strain and radial displacement for each of the layers using known borehole and drilling parameters to ascertain and obtain a positive contact stress of the elastomer element with the wall of the borehole after the borehole returns to undisturbed in-situ temperatures. Alternatively, the desired contact stress with a borehole can be selected and utilized with the temperature difference between undisturbed borehole temperatures and disturbed borehole temperatures in the equations for elastic strain and radial displacement for each of the layers using known borehole and drilling parameters to ascertain the inflation pressure necessary in an inflatable packer to obtain the desired contact stresses. Alternatively, for a desired contact stress with a borehole and a selected inflation pressure it can be determined what temperature differential is required to obtain the desired contact stress. Then the temperature of the packer system can be adjusted to produce the necessary operation differences. The proportioning of the packer element necessary to obtain a positive seal can also be determined. A general form of the strain equation for radial displacement of a layer element is: and for radial stress (or pressure) is Where the symbols A, X, Y and Z are established parameter values for the materials of the layer, R is a radius value, and ΔT is the temperature difference between the disturbed state and the undisturbed state at the location for the layer in question. In particular, by use of temperature data of the environmental elements, or layers, as taken in a horizontal plane in a wellbore where the inflatable packer will be set, inflation pressures of the packer element relative to the contact sealing forces can be determined so that the integrity of the seal of such inflated packer element will be positive when the environmental elements of the wellbore return to the quiescent or undisturbed in-situ temperature state. In practice then, the contact stress exerted on the borehole wall by the elastomer layer can be predetermined and the wall thicknesses of the layers can be optimized by preselection to obtain predicted contact stress in a wellbore as a function of inflation pressure and the utility of a packer to obtain a desired result can be predetermined. It will be apparent to those skilled in the art that various changes may be made in the method without departing from the scope of the claims.

Claims: A method of effecting a seal between an elongate inflatable packer (40) and a borehole wall (56) in a wellbore traversing earth formations (58), the wellbore having a temperature differential defined between a disturbed temperature condition of the wellbore and an undisturbed temperature condition thereof; and the packer (40) having a central tubular mandrel (30, 42, 50) and an elastomeric packer element (35, 41, 52) mounted on said mandrel (30, 42, 50) in sleeved relation thereto; said packer element (35, 41, 52) being subject to inflation by a liquid to a finite inflation pressure to produce a radial expansion of said packer element (35, 41, 52) such that a positive final contact stress is obtained at the interface between the packer element (35, 41, 52) and the borehole wall (56) and provides said seal with respect to the borehole wall (56) after the wellbore returns to the undisturbed condition thereof; said mandrel (30, 42, 50), said packer element (35, 41, 52) and said liquid (59a) being radial layers between a centerline (61) of the borehole and the borehole wall (56); and parameters of said inflation pressure, said contact stress and said temperature differential being interrelated by aximetric plane strain equations for radial stress (σ r ) and radial displacement (u) in a radial plane; said method including the steps of: selecting a depth in said wellbore for inflation of said packer element (35, 41, 52); using two parameters selected from said inflation pressure parameter, said temperature differential parameter (ΔT) and said contact stress parameter (σ) for said selected depth together with established physical parameters of said layers, determining the other of the inflation pressure parameter, the temperature differential parameter (ΔT) and the contact stress parameter (σ), adjusting at least one of the parameters as required and obtaining a positive value for said contact stress parameter using the aximetric plane strain equations for radial stress (σ r ) and radial displacement (u) in the radial plane; and running the packer (40) into the wellbore and inflating the packer element (35, 41, 52) at said selected depth at the determined or selected inflation pressure to obtain said positive contact stress at said selected depth. A method as set forth in claim 1 wherein the contact stress parameter (σ) is determined by matching common stress values at interfaces of said layers for each interface of said layers and utilizing the temperature differential parameter (ΔT) and the inflation pressure parameter for the packer element (35, 41, 52); and adjusting a thickness parameter of the packer element with respect to the other parameters to obtain said positive value for said contact stress parameter on the borehole wall. A method as set forth in claim 1 wherein the contact stress parameter (σ) at the borehole wall is determined by matching common stress values at interfaces of said layers for each interface between said layers including the outermost layer with said earth formation; and for each layer, adjusting its temperature differential parameter (ΔT) between said disturbed and undisturbed temperature conditions by adjusting the disturbed temperature condition parameter to an adjusted value thereof necessary to obtain a positive value for the contact stress parameter on the borehole wall. A method as set forth in claim 3 wherein said disturbed temperature condition parameter is adjusted by reducing a temperature parameter of the liquid (59a). A method as set forth in any one of the preceding claims wherein the liquid (59a) is a cement slurry which hardens over time and undergoes a volume contraction. A method as set forth in any one of claims 1 to 4 wherein the liquid (59a) is a drilling mud which hardens over time and undergoes volume contraction. A method as set forth in any one of the preceding claims wherein the plane strain equations used are limited to those for radial stress and radial displacement of said layers. A method as set forth in any one of the preceding claims wherein at said selected depth in said wellbore is a larger diameter bore which is located below a smaller diameter bore and the packer (40) is run through the smaller diameter bore and inflated in the larger diameter bore. A method as set forth in claim 8, including sizing said mandrel (30, 42, 50) and said packer element (35, 41, 52) to optimize the inside diameter of the mandrel relative to the inside diameter of the smaller diameter bore, the optimum wall thickness of said packer element being determined from said plane strain equations so as to obtain said positive contact stress.

Inventors: BROOKS ROBERT T, WOOD EDWARD T, BROOKS, ROBERT T., WOOD, EDWARD T., BROOKS, ROBERT T. CTC INTERNATIONAL, INC.
Applicants: CTC INT CORP, CTC INTERNATIONAL CORPORATION
Application year: 1993
Main classification: E21B33
IPC codes: E21B47, E21B33, E21B49