METHOD AND DEVICE TO STABILISE THE PRODUCTION OF OIL WELLS

Abstract: In an oil well producing by means of continuous gas lift a device is installed into the tubing, said device being provided with a first lower portion, which provides a progressive constraint in the area for the passage of the flow coming from the reservoir, a second medium portion, located above the first lower portion, located above the lower portion, which makes said area for the passage of the flow coming from the reservoir to be substantially constant at this point and smaller than the original area of the tubing, and a third uppe portion, located above the second medium portion, which provokes a progressive widening in said area for the passage of the flow coming from the reservoir, until such area for the passage of the flow is again equal to the original area of the tubing. Such device can be located in front of the gas lift valve through which the gas is injected.

Description: METHOD AND DEVICE TO STABILISE THE PRODUCTION OF OIL WELLS FIELD OF THE INVENTION The present invention relates to a method and device to stabilise the production of oil wells. The device may be used with an oil production pipe and is intended to overcome the harmful effects caused to the well by the flow of unstable mixtures produced by certain wells. More particularly, the present invention is preferably related to a device which is used with a flow pipe of an oil well equipped to produce oil by means of gas lift, and to a method for its use. STATE OF THE ART Oil is usually found in accumulations under pressure in the subsoil, in porous and permeable sandstone known as reservoir stones, or else hydrocarbon producing rock formations. Wells are drilled from the surface to drain off such reservoirs so as to communicate the reservoir with processing facilities in the surface, which are assembled to collect and to treat the produced fluids. The wells are bores which traverse several rocking formations. Usually a steel pipe is inserted into such bores, and is called a casing. At least one pipe of smaller diameter is inserted into such casing, trough which fluids from the reservoir flow. Oil is a complex mixture of heavy and light hydrocarbons, comprising from dry gas (methane) to heavy oil. Depending on the features of the reservoir, some components may appear in higher concentration than other. Other substances may also accompany the produced oil, such as water, carbonic gas, hydrogen sulphide gas, salts and sand, only to mention some examples. Depending on the conditions of pressure and temperature, the constituents of the oil may be in the gaseous phase or in the liquid phase. Thus, it is concluded that the fluids that usually flow into an oil well may be defined as a multi-phase multi- component mixture. The flow of the fluids into an oil well, from the reservoir to the surface, can occur as a consequence of the accumulated energy in the reservoir, that is, without the presence of an external source of energy which provokes such production. In such α case it is said that the production of the well is normally flowing, or else it is said that the well is producing by surge. When an external source of energy is used, e.g. a down hole pump, there is then what is called an artificial lift. Among the various known artificial lift methods the continuous gas lift can be noted. In a usual configuration for this method, natural gas at high pressure is injected into an annulus formed between the casing and the pipe trough which the production of fluids from the reservoir flows, which is also named the production string or tubing. Valves known as gas lift valves are located at certain points along the tubing, which control the flow of gas flowing from the annulus to the interior of the tubing. The expansion of such pressurised gas provides the necessary additional energy to allow fluids from the reservoir to flow at a certain flow rate. In some oil wells the flow of fluids into the tubing occurs in an unstable way, that is, there are variations of pressure and flow rate with time, which can even be harmful to the integrity of the well and its associated equipment. There are in the technical literature many citations of severe cases in which unstable flows in oil wells cause a halt in production. Such instabilities are also known as "heading", as it is at the surface, at the well head, where they are more vigorously sensed, and such instability is able to occur in the tubing, in the annulus, or in both. The phenomenon of the instability in the flow of multiphase mixtures is complex, and the causes for such instability are not totally understood. Generally, small disturbances give rise to great variations in the flow rates of the produced oil and the injected gas, as well as in the pressures. Many times such phenomenon is characterised by being cyclical. In the article " These methods can eliminate or control annulus heading, by A. W. Θrupping, C. W.F. LucaeF.D. Vermeulen" (OilάGas Journal, July 30, 1984, p. 192), the authors show that the unstable behaviour of the flow in wells producing by means of continuous gas lift may frequently be attributable to the pressure oscillations in the annulus formed between the tubing and the casing. According to the authors, keeping the pressure constant causes the flow in such wells to stabilise. The control of the injection of gas in wells equipped to produce by means of continuous gas lift is usually made by a gas choke valve, located at the surface, and by another valve located at a certain point in the tubing, which is the gas lift valve. According to Θrupping, Luca and Vermeulen, and some others, the ideal situation is to remove the control from the surface, allowing it to be made only by means of the gas lift valve. The authors also recommend that the gas lift valve be provided with an internal passage comprising a single orifice. However, this is not enough to keep the flow rate constant. The conventional gas lift valves used to control the flow rate of injected gas in wells equipped to produce by means of continuous gas lift are not really valves, although they are designated as valves by the experts and by the manufacturers. Actually they are flow regulators equipped with a small disc provided with a round orifice having a certain diameter. The edges of the orifice are usually sharp or smoothly rounded. Such gas lift "valves" are also provided with a check valve, located downstream of the orifice, so as to preclude an undesirable flow of oil from the tubing to the annulus to occur. When a gas flows throughout a constriction, such as an orifice, and the pressure upstream of the orifice is kept constant, the flow rate of the flowing gas increases as the pressure downstream of the orifice decreases, until, for a certain upstream pressure known as critical pressure, the son c speed of the constriction is achieved. From then on a decrease in the pressure downstream of the constriction will not cause the injected gas flow rate to raise. Thus, there are two dynamic behaviours, or rates of flow, for a valve provided with an orifice. The first can be defined as a sub-critical rate of flow, in which a reduction in the downstream pressure causes a raise in the gas flow rate, and the second can be de ined as a critical rate of flow, in which the gas flow rate is constant, independently of the downstream pressure (considering a constant upstream gas pressure). In use, the pressure upstream of the orifice is basically the pressure of the injection gas existing in the annulus at the position where the gas lift valve is installed, and the pressure downstream of the orifice is basically the pressure of the flow of fluids into the tubing at the position where the gas lift valve is installed. Thus, according to the above technical literature, in a situation where the flow is critical the use of the gas lift valve contributes to stabilise the flow into the well, as in this situation the flow rate of injection gas is constant (assuming that the pressure in the annulus is constant). However, due to the irreversible losses of energy in a gas flow passing through such orifices, deriving basically from the heat, the friction and the sound coming from the extremely turbulent flow of gas under pressure passing through the orifice, there is a necessity for the pressure into the tubing being essentially less than 55% of the existing pressure in the annulus so as a critical flow is achieved. Such differential of pressure is not usually found in most of the actual cases, and consequently the orifice valve operates in a sub-critical rate of flow, the variation in the gas flow caused by the variation of pressure into the tubing contributing to the instability of the flow in the well. The Brazilian patent application PI9300292-0, commonly owned by the applicant and the description of which is herein incorporated for reference, contributed for the solution of the above problem by substituting a venturi for the orifice of sharp edges in the gas lift valves. According to this document, the irreversible losses of energy in the injection gas flow are significantly smaller, and the increasing of the pressure in the dif f usor of the venturi causes a critical flow to be achieved for a pressure in the tubing substantially smaller than 90% of the annulus pressure. T erefore, a critical flow is achieved more easily. Consequently it is easier to keep constant the injection gas flow rate which, as previously mentioned, contributes to stabilise the flow into the tubing. Further, the smaller differential of pressure required by the gas lift valve with a venturi for injecting a certain flow of gas into the tubing provokes a more rational use of the available energy, thereby causing the costs for compressing gas to reduce (for the same oil flow rate), or increasing the income as a consequence of an increase in the production flow rate, be it for increasing the injection gas flow rate or injecting gas at a deeper position in the well. However, in actual situations, the stabilisation of the oil production is not always achieved simply using the gas lift valve with a venturi. Although a critical flow is achieved for tubing pressures higher than those in the situation where a conventional orifice is used, such tubing pressure is still low in severe instability situations. The injection of gas by means of a gas lift valve with a venturi operating at a sub-critical rate of flow is even more harmful to the well than by means of gas lift valve with an orifice, and the instability can eventually augment. The sub-critical rate of flow in a gas lift valve with a venturi occurs in a range of 55% to 100% of the annulus pressure. In a gas lift valve with a venturi such range is reduced for 90% to 100%. Thus, in a gas lift valve with a venturi operating at a sub-critical rate of flow the variation of pressure is about 4.5 times higher than a gas lift valve with an orifice operating at a sub-critical rate of flow. Such features of the gas lift valves with a venturi also makes it difficult to use such valves to inject gas at a deeper location, due to the existing uncertainty for calculations in a multiphase flow. A mistake in the calculation can result in positioning the gas lift valve with a venturi at a location where the injection occurs in a sub-critical rate of flow (highly undesirable) or is even not possible (where the tubing pressure is higher than the annulus pressure). Thus, the use of a gas lift valve with a venturi is not the ultimate solution for all the cases where the well produces with instability. There is then a need for a new solution to overcome the problem of stabilising the production of an oil well, in particular in oil wells producin by means of continuous gas lift. Further, there is a need for a solution which enables the injection of gas at a deeper point in oil wells which produce by means of continuous gas lift. SUMMARY OF THE INVENTION The present invention relates to a method and device to stabilise the production of oil wells, the device intended to be inserted into the tubing of an oil well, which usually comprises: - a wellhead; a casing; a tubing inserted into the casing; a packer inserted and locked into the casing and connected to the tubing next to an oil reservoir, so as to create two discrete regions: - a lower chamber, extending downwardly from the packer to the reservoir; and an upper chamber, or annulus, extending upwardly from the packer to the wellhead. In a first aspect the invention provides an oil well comprising: a tubing for carrying fluids coming from a reservoir to the surface; a body inserted in the tubing, the body comprising: a lower portion progressively causing a decrease in cross-sectional area available for the passage of fluid in the direction of fluid flow; a medium portion located downstream of said lower portion causing a substantially constant cross-sectional area to be available for the passage of fluid, said constant area being smaller than the cross-sectional area of the tubing alone; and an upper portion, located downstream of the medium portion progressively causing an increase in cross-sectional area to be available for the passage of fluid in the direction of fluid flow. In a second aspect the invention provides a body to stabilise the production of oil wells when provided to a tubing for carrying fluids coming from a reservoir; said body comprising: a lower portion which progressively causes a decrease in cross-sectional area available for the passage of fluid in the direction of fluid flow when said body is inserted inside said tubing; α medium portion located adjacent said lower portion which causes a substantially constant cross-sectional area to be available for the passage of fluid when said body is inserted inside said tubing, said constant area being smaller than the cross-sectional area of the tubing alone; and an upper portion located adjacent said medium portion which progressively causes an increase in the cross-sectional area available for the passage of fluid in the direction of fluid flow when said body is inserted inside said tubing. In a third aspect the invention provides a method to stabilise the production of oil wells comprising tubing for carrying to the surface the fluids coming from a reservoir, the method comprising: inserting into the tubing a device comprising: a lower portion progressively causing a reduction in the cross-sectional area for the passage of fluid coming from the reservoir; a medium portion, located above the lower portion, which causes said cross- sectional area for the passage of fluid coming from the reservoir to be substantially constant and smaller than the original area of the tubing; an upper portion, located above the medium portion, which causes a progressive widening in the cross-sectional area for the passage of fluid coming from the reservoir, until such area for the passage of the fluid is again equal to the original area of the tubing; allowing the fluids from the reservoir to flow towards the surface, passing through the device, whereby the flow is accelerated when it passes through said lower portion, and consequently the flow pressure is decreased, the flow passing through said medium portion, and then through said upper portion, where the flow is decelerated, and consequently the flow pressure is increased, the above sequence causing a stabilisation of the flow. In preferred embodiments, the device comprises a body inserted into the tubing which carries to the surface a flow of fluids from the reservoir, the body comprising: α first lower portion which provides α progressive constraint into the tubing, thereby provoking a reduction in the area for the passage of the flow coming from the reservoir; a second medium portion, located above the first lower portion, which makes the area for the passage of the flow coming from the reservoir being substantially constant at this point and smaller than the original area of the tubing; a third upper portion, located above the second medium portion, which provokes a progressive widening in said area for the passage of the flow coming from the reservoir, up till such area for the passage of the flow is again equal to the original area of the tubing. A preferred embodiment of the method comprises the following steps: inserting into a tubing a device comprising a first lower portion, which provides a progressive constriction in the area for the passage of the flow from the reservoir, a second medium portion, located above the first lower portion, which makes the area for the passage of the flow from the reservoir to be substantially constant and smaller than the original internal area of the tubing, and a third upper portion, located above the second medium portion, which provides a progressive widening in the area for the passage of the flow from the reservoir, up till such area for the passage of the flow is again equal to the original area of the tubing; allowing the fluids from the reservoir to flow towards the surface, passing through the zone where the device to stabilise the production is located, the flow being accelerated when passing through the region where the first section is located, whereby the flow pressure decreases, the flow passing then through the region where the second section is located, where the flow pressure is substantially constant, and next the flow passes through the region where the third section is located, the flow being decelerated there, whereby the flow pressure increases, the above sequence causing a stabilisation of the flow. If the oil well is equipped to produce by means of continuous gas lift, a gas lift mandrel should be connected to the tubing and a gas lift valve should be connected to the gas lift mandrel. Gas at a high pressure should be injected at the wellhead in the annulus between the casing and the tubing of the oil well. The gas lift valve should be provided with at least one port through which the high pressure gas of the annulus flows towards the interior of the tubing, and the device to stabilise the production must be inserted into the tubing with its medium portion located in front of the point where the high pressure gas is injected into the tubing. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be now described in more detail, by way of example only, with reference to the attached schematic drawings in which:- Figure 1 is a longitudinal cross-sectional view depicting an oil well equipped to produce by means of continuous gas lift; Figure 2 is a longitudinal cross-sectional view depicting a conventional gas lift mandrel having a venturi type gas lift valve connected to it; Figure 3 is a longitudinal cross-sectional view depicting a side pocket gas lift mandrel having a venturi type gas lift valve connected to its side pocket; Figure 4 is a longitudinal cross-sectional view depicting a conventional gas lift mandrel having a venturi type gas lift valve connected to it, a device to stabilise the production of the present invention being provided into the tubing; Figure 5 is a longitudinal cross-sectional view depicting a detail of Figure 4; Figure 6 is a chart of the pressures into the tubing and the annulus for an oil well provided with a conventional gas lift system; Figure 7 is a chart of the pressures into the tubing and the annulus for an oil well provided with a continuous gas lift system when a device to stabilise the production of the present invention is provided to the tubing; Figure 8 is α longitudinal cross section view depicting a conventional gas lift mandrel having a venturi type gas lift valve connected to it, a device to stabilise the production of the present invention being provided into the tubing; Figure 8A depicts a cross section in the gas lift mandrel of the Figure 8, taken along the line A - A in Figure 8; Figure 9 is a longitudinal cross-sectional view depicting a side pocket gas lift mandrel having a venturi type gas lift valve connected to its side pocket, a device to stabilise the production of the present invention being provided into the tubing; Figure 10 is a longitudinal cross-sectional view depicting a conventional gas lift mandrel having a venturi type gas lift valve connected to it, a device to stabilise the production of the present invention being provided into the tubing; Figure 11 is a longitudinal cross-sectional view depicting a first embodiment of a nipple for use with the device to stabilise the production according to the invention; Figure 11A is a transverse cross-section taken along the line A - A of Figure 11; Figure 12 is a longitudinal cross-sectional view depicting a second embodiment of a nipple for use with the device to stabilise the production according to the invention; Figure 12A is a transverse cross-section taken along the line B - B of Figure 12; Figure 13 is a longitudinal cross section view depicting a third embodiment of a nipple for use with the device to stabilise the production according to the invention. Figure 13 A is a transverse cross-section taken along the line C - C of Figure 13. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a schematic longitudinal cross-sectional view depicting a typical gas lift facility showing an oil well 10 equipped to produce by means of continuous gas lift. The oil well 10 is basically a hole extending through a number of rock formations from the surface to an oil reservoir 1. The oil well 10 is provided with a casing 2, a tubing 3 being inserted into the casing 2. A packer 4 is installed into the oil well 10, next to the reservoir 1, and its function is to create two discrete zones in the oil well 10, a first lower chamber 5, located next to the reservoir, and a second upper chamber or annulus 6, formed between the casing 2 and the tubing 3 , the packer 4 providing a seal between the chambers. At the surface there are f aci I ities used to keep operation of the wel I safe, these facilities known by the experts as wellhead 11. Fluids om the reservoir 1 enter the oil well 10 by means of small orifices 7, which were previously drilled in the casing 2. Next the fluids flow into the tubing 3 up to the wellhead 11, where they are directed to the processing facilities 8, which are schematically depicted in the Figure 1. In a continuous gas lift system gas at a high pressure coming from an external source of high pressure gas 9, schematically shown in Figure 1, is admitted into the annulus 6. The high pressure gas flows into the annulus 6 and passes to the tubing 3 through a gas lift valve connected to a gas lift mandrel 1 which is connected to the tubing 3. The gas mingles with the fluids coming from the reservoir 1, and the resultant mixture is carried to the surface. Although in Figure 1 there is shown a single mandrel 12 for installing a gas lift valve, the oil wells producing by such means are usually provided with a number of mandrels, which are spaced apart along the tubing and which are each equipped with gas lift valves, the gas lift valves being not necessarily of the same type. However, in actual use the injection of gas is made by means of a single gas I ift valve, known as the operating gas lift valve. Some other gas lift valves are also installed in the oil well, but they are used to assist the start-up or to restart the oil well production, and these gas lift valves are known as start-up valves. The oil wells equipped to produce by means of continuous gas lift may have other types of configuration than the configuration shown in the Figure 1. Such oil wells may be onshore or offshore oil wells. The offshore oil wells may be equipped with dry wellheads, usually located at a platform, or wet wellheads, that is, the wellhead is located at the seabed. Moreover, in any of the abovementioned configurations use may be made of a single tubing 3, as shown in Figure 1, or more than one tubing may be used instead (double completion, triple completion, etc.). Whatever is the configuration of an oil well it is able to benefit from the device of the present invention, as the type of configuration of the well will not affect the performance of the device. Therefore, the configuration schematically depicted in Figure 1 suff icies for oil industry experts to understand the operation of the device of the present invention, and it will be quite clear that the device can be used with any tubing, as will be seen from here on. There are two types of gas I if t mandrels, namely the conventional one and the side pocket one. Figure 2 depicts a longitudinal cross section of a conventional gas lift mandrel 12 comprising a body 13, which is a segment of pipe having the same internal diameter as the tubing of the oil well, and a side support 15, to which a gas lift valve 14 is connected. The body 13 is provided with threads at both ends for allowing it to be connected to the tubing 3, whereby the conventional gas lift mandrel 12 is in line with the tubing 3. The gas lift valve 14 is of the type which is provided with a venturi, and it comprises a body 19; an internal chamber 20; a gas intake port 17; a concentric venturi 18 located in the internal chamber 20; a check valve assembly located immediately below the concentric venturi 18, and which in the presently illustrated case is formed by a shutter 21, a seating 22 and a tip 23 provided with an opening 26. The tip 23 is provided with threads at its outer portion, so as to enable the gas lift valve 14 to be connected to the conventional gas lift mandrel 1 by screwing the tip 23 in the support 15, with side supports 16 being provided in the conventional gas lift mandrel 12 for lateral support of the body 19 of the gas lift valve 14. The support 15 is provided with an internal chamber 24, which communicates with an end of the hollow tip 23 of the gas lift valve 14. The other end of the internal chamber 24 of the support 15 is connected to an opening 25 existing in the body 13 of the conventional gas lift mandrel 12. Thus gas at a high pressure from the annulus 6 is able to enter the tubing 3, passing then successively through the concentric venturi 18, through the check valve assembly formed by the shutter 21 and the seat 22, through an opening 26 of the tip 23, through the internal chamber 24 of the support 15 and through the opening 25 in the body 13, entering then into the body 13 of the conventional gas lift mandrel 12. Fluids coming from the reservoir flow upwardly into the segment of the tubing 3 located below the conventional gas lift mandrel 12, in the direction indicated by the arrow F, passing then into the body 13 of the conventional gas lift mandrel 1 . When passing in front of the opening 25 the fluids receive an injection of gas at a high pressure coming from the opening 25, whereby the fluids of the flow mix with the injected high pressure gas, and such mixture in then carried to the surface by means of the segment of the tubing 3 located above the conventional gas lift mandrel 12. Such conventional gas lift mandrel 12 has a disadvantage in that it is necessary to retrieve the entire tubing string to replace the gas lift valve, when it is necessary. Figure 3 depicts a longitudinal cross section of a side pocket gas lift mandrel 30 having a venturi type gas lift valve 14' inserted in a side receptacle 31 of the side pocket 32 of the side pocket gas lift mandrel 30. As with the conventional gas lift mandrel 12 of the Figure 2, the side pocket gas lift mandrel 30 is provided with threads in both ends, so as to allow it to be connected to the tubing 3. The side pocket gas lift mandrel 30 is designed in such a way that a venturi type gas lift valve 14' can be replaced, when necessary, without the need to retrieve the entire tubing 3. Such replacement is made by means of an operation using special tools which are inserted and lowered into the tubing by means of a cable or a wireline, such operation being well known by those skilled in the art. The venturi type gas lift valve 14' is substantially equal to the one which has been described with respect with the conventional gas lift mandrel 12 of Figure 2, except for being provided with a tip 33, distinct from the tip 23 of Figure 2. Therefore, the venturi type gas lift valve 14' will not be described here again, and the same numeral references for its parts will be used as those used with respect to Figure 2. The venturi type gas lift valve 14' is introduced into the receptacle 31 of the side pocket 32, where it is kept under pressure due to the compression exerted by gaskets 34a and 34b, which also provide the necessary seal between the body 19 of the venturi type gas lift valve 14' and the receptacle 31. Gas at a high pressure coming from the annulus 6 enters through openings 35 existing in the side pocket 32 into the small annulus 36 formed between the receptacle 31, the venturi type gas lift valve 14' and the side pocket 32. Such small annulus is kept sealed by the gaskets 34a e 34b. Next the high pressure gas enters into the venturi type gas lift valve 14' , through openings 17, passes through the concentric venturi 18 and the check valve assembly formed by the shutter 21 and the seat 22, enters the internal chamber 37 of the tip 33, and finally it exit through discharge openings 38 located at the lower end of the tip 33, mixing then with the fluids coming from the reservoir 1, as will be seen in the following. Fluids coming from the reservoir flow upwardly into the segment of the tubing 3 located below the side pocket gas lift mandrel 30, in the direction indicated by the arow F in the Figure 3, passing then into the side pocket gas lift mandrel 30. When passing in front of the discharge openings 38 of the tip 33 of the venturi type gas lift valve 14' the fluids receive an injection of gas at a high pressure coming from the discharge openings 38, whereby the fluids of the flow mix with the injected high pressure gas, and such mixture in then carried to the surface by means of the segment of the tubing 3 located above the side pocket gas lift mandrel 30. Considering a fixed diameter of the throat of the venturi 18, the gas flow rate passing through it is a function of the pressures upstream and downstream of the venturi. The pressure upstream of the venturi is the pressure P c of gas existing in the annulus 6, the losses of energy in the openings 17 being not taken into consideration for purposes of simplification of the description. The pressure downstream of the venturi 18 is the pressure P t existing in the tubing 3 immediately after the region where the venturi 18 is located, the losses of energy in the internal passage of the tip 23, 33, and in the discharge openings 25, 38 being not taken into consideration for purposes of simplification of the description. If the pressure P t is higher or equal to the pressure P c , a flow from the annulus 6 to the interior of the tubing 3 will not occur. Note that the flow of fluids from the tubing 3 to the annulus 6 is prevented by the check valve assembly. If the pressure P t is smaller than the pressure P c , a flow from the annulus 6 to the interior of the tubing 3 will occur. Considering the case when the pressure P c is constant, as the pressure P t decreases the gas flow rate will then increase, up to a time when the pressure P t reaches the value of the critical pressure P tcr , when the Sonic speed of gas flow occurs in the throat of the venturi 18. From then on an increase in the flow rate of the injection gas will not occur, even if the pressure P t is reduced. It is supposed that in the venturi type gas lift valve the ratio of the pressure P tcr P c is approximately 0,9. Thus, the pressure P t can be at most 90% of the value of the pressure P c to provoke a constant injection gas flow which tends to stabilise the oil well. The above analysis of the behaviour of the flow rate of the injection gas as a function of the the annulus pressure P c and the tubing pressure P t , applies to both the conventional gas lift mandrel 12 and the side pocket gas lift mandrel 30. As has been seen, the pressure P t can be at most equal to 90% of the value of the pressure P c for creating a critical flow throughout the venturi type gas lift valve so as to produce an injection gas flow rate that is constant (when the pressure P c is constant). The value of the pressure P t must be equal to or smaller than 90% of the value of the pressure P c for creating a constant flow rate of injection gas, which is desirable. However, such condition is not always feasible, and it is desired to provide a device that provokes this condition to always occur. The present invention provides a device and a method which alleviates this and other problems. Figure 4 depicts a first embodiment of the device to stabilise the production of oil wells of the present invention, in the case where a conventional gas lift mandrel 12 is used. In this embodiment the device comprises a central body venturi 40 fixed into the body 13 of α conventional gas lift mandrel, which is connected to the tubing 3. The central body venturi 40 is located in the region where a gas lift valve 14 is installed in a conventional gas lift mandrel 12, in such a way that the opening 25 from which gas at a high pressure is coming from the gas lift valve 14 enables the gas to be injected towards the throat of the said central body venturi 40, as will be seen in more detail later. Figure 5 depicts the central body venturi 40 in more detail. The central body venturi 40 comprises a central aerodynamic element of a round cross section installed into the tubing 3 in such a way that its longitudinal axis is substantially coincident with the longitudinal axis of the body 13 of the conventional gas lift mandrel 12. In the present embodiment fixing rods 41 are used to keep the central body venturi 40 centred into the body 13 of the conventional gas lift mandrel 12, although other fixing elements may be used. The longitudinal cross-section of the central element, which is shown in the Figures 4 and 5, indicates, as in the conventional concentric Venturis, that there can be considered three regions of the central body venturi 40, namely: - a region 'A', where the area of the cross section of the annulus formed between the central body venturi 40 and the internal walls of the body 13 of the conventional gas lift mandrel 12, through which the flow of fluids coming from the reservoir 1 passes, is progressively reduced, thereby resulting in the flow being accelerated and the flow pressure being reduced in the region 'A'; - a region 'B', where the area of the cross section of such annulus is constant; and - a region 'C, where the area of the cross section of such annulus is progressively increased up to the point that it is equal to the area of the cross section of the tubing, thereby resulting in the flow being decelerated and the flow pressure being increased in the region 'C. Making a comparison between the concentric Venturis, it can be said that, for the sake of simplification and clarity, that the region 'A' corresponds to the nozzle, the region 'B' corresponds to the throat, and the region 'C corresponds to the diffusor. Such nomenclature will be used hereon when referring to the three regions of the central body venturi 40. With the central body venturi 40 located into the body 13 of the conventional gas lift mandrel 12, as shown in the Figure 5, the pressure in the opening 25, which is basically the pressure upstream of the venturi 18 of the gas lift valve, is no longer the pressure P t of the flow which is passing through the gas lift mandrel 12. The pressure in the opening 25 instead takes a value P tg , which is the existing pressure at the throat (region 'B') of the central body venturi 40, as the flow is accelerated when passing through the nozzle (region 'A'), as previously explained, and the flow pressure consequently is reduced there. Thus, the pressure P tg is smaller than the pressure P t , and such differential of pressures is a function of the constriction rate, that is, the reduction in the area at the throat (region 'B'). An increase in the flow pressure occurs at the diffusor (region 'C') of the central body venturi 40. The value of the P t " downstream of the central body venturi 40 is a result of the composition of the effect of the irreversible losses of energy in the injection gas flow along the central body venturi 40 and of the effect caused by the introduction of the kinetic energy in the injection gas. The irreversible losses of energy causes a reduction in the flow pressure, and they derive from the friction, from a disturbance at the diffusor (region 'C) introduced by the admission of gas at the throat (region 'B'), and from a disturbance introduced by the fixing rods 41 of the central body venturi 40. On the other hand, part of the kinetic energy of the gas will be converted to pressure, due to the deceleration in the flow at the diffusor (region V). The opening 25 acts then as an ejector. The area of the opening 25 is smaller than the area of the annulus 6, and consequently the average gas speed at the opening 25 is greater than the average gas speed in the annulus 6. In the conventional gas lift system shown in Figure 2 such kinetic energy is actually lost when mixing with the fluids of the flow coming from the reservoir. By using the present invention a great amount of such kinetic energy is recovered, and such recovering can compensate or even exceed the irreversible losses of energy. However, it is not an object of the present invention to propose the use of gas as a motive fluid, as in the artificial lifting method known as jet pumping. The use of the gas in such a manner is very inefficient, as the low specific gravity of the gas determines that gas at a very high pressure and at a very high speed (flow rate) is used into the annulus, which is not desirable in practical use. If the pressure P tg is smaller than the pressure P t , it can be inferred how it enables to keep the gas flow rate constant and equal to the critical flow rate through a venturi type gas lift valve. As has been seen with respect to the system of Figures 2 and 3, the value of the pressure P t must be smaller than 90% of the value of the pressure P c for a critical gas flow rate to occur. Using the present invention, it is the value of the pressure P tg which must be smaller than 90% of the pressure P c . As the value of the pressure P tg is smaller than the value of the pressure P t , due to the effect of the acceleration in the flow provoked by the throat (region 'B') of the central body venturi 40, the pressure P t may reach greater values than those required in the normal situation, where the device of the invention is not used. For this it suf f icies that the central body venturi 40 is shaped with such a throat (region 'B') that provides the desired effect. Another advantage is that the value of the pressure P t may even be greater than the value of the pressure P c . Usually, this would mean no gas injection is possible. However, the present invention enables the injection of gas at a region deeper than those of the gas lift systems where the device of the invention is not used, as in these systems the value of the pressure P c must be greater than the value of the pressure P t . Figure 6 depicts a schematic chart of the pressures into the tubing and into the annulus for an oil well equipped with a conventional gas lift system. The chart shows the behaviour of the pressure according to the depth of the well. The fluids flowing into the tubing must reach the wellhead at a pressure P wh , which is the pressure required for the production facilities to operate. The available pressure at the surface of the gas to be injected into the annulus is P^.. Considering that a venturi type gas lift valve is located into the well at a depth V a , the gas pressure P co into the annulus at this depth is greater than the pressure P f0 of the flow into the tubing. Therefore, gαs is injected by the venturi type gαs lift valve into the tubing at a certain flow rate. In the region below the region where the venturi type gas lift valve is located, the pressure suffers an increase at a rate which is greater than the increase above the venturi type gas lift valve, due to the gas entering the tubing increasing the mass of fluids above the gas lift valve. At a depth L y of the reservoir the pressure of the flow is P^. The differential between the static pressure of the reservoir P r and the pressure P,^, which is also known as drawdown, causes a production of the fluids coming from the reservoir at a certain flow rate. The injection of gas at the depth V e is not possible with a conventional gas lift valve, as the pressure of gas in the annulus is smaller than the pressure of fluids in the tubing. Figure 7 shows a schematic chart of the pressures into the tubing and into the annulus for an oil well equipped to produce by means of continuous gas lift and which makes use of the device to Stabilise the production of oil wells of the present invention. A venturi type gas lift valve is located at the depth V e , just in front of a central body venturi device similar to the one shown in Figures 4 and 5. The pressure of the gas into the annulus is P^, which is smaller than the pressure P^ of the flow into the tubing at a region located immediately below the central body venturi device. As the flow passes through the central body venturi device, the pressure in the annulus between the throat (region 'B') of the central body venturi 40 and the internal walls of the body 13 of the conventional gas lift mandrel 12 is reduced, reaching a value P^ which is smaller than the pressure P^, thereby enabling gas to be admitted into the tubing through the gas lift valve at a certain flow rate. A recovering of pressure occurs at the diffusor (region 'C') of the central body venturi device, and the pressure reaches a value P vo . The flow of fluids continues to flow up to the surface, where the pressure reaches the value P wh required by the processing facilities to operate. At the depth L r of the reservoir the pressure of the flow is P'^, which is usually smaller than the value of the pressure P^ of the conventional situation (Figure 6), thereby inducing the oil well to produce at a greater flow rate. In the embodiment of Figure 5, the gαs is admitted into the throat (region 'B') of the central body venturi 40 by means of a single opening 25, which is not the best way to admit the gas. It is therefore proposed to use a conventional gas lift mandrel in which the single opening 25 in which the injection of gas is made is replaced by a number of openings located in front of the throat (region 'B') of the central body venturi 40. Figure 8 depicts a further embodiment of the present invention, showing an asymmetric body venturi 50 which also has a first convergent section or nozzle, denoted in Figure 8 as Α", a constriction section or throat, denoted in Figure 8 as 'B", and a divergent section or diffusor, denoted in Figure 8 as 'C". The admission of the gas is also made in front of the throat or at the beginning of the diffusor, by means of the discharge opening 25. The asymmetric body venturi 50 is aerodynamical ly shaped and it can vary according to the needs, without departing from the present invention. Figure 8A shows a cross-sectional view of the gas lift mandrel of Figure 8, taken along the line A- A in Figure 8. Figure 9 depicts an embodiment of the device to stabilise the production of oil wells installed in a side pocket gas lift mandrel 30. The device to stabilise the production of oil wells comprises a central body venturi 60, which is equally shaped as the central body venturi 40 of the Figures 4 and 5, which also comprises a central aerodynamic element having a round cross section. The central body venturi 60 is located in the side pocket gas lift mandrel 30, and it is fixed there by means of fixing elements 61, just in front of the region where a gas lift valve 14 is installed into a side receptacle 31 of the side pocket 32 of the side pocket gas lift mandrel 30. The axis of the central body venturi 60 is substantially parallel to the walls of the side pocket gas lift mandrel 30, and it is substantially centred in the region between the left wall of the side pocket gas lift mandrel 30, as shown in Figure 9, and the side receptacle 31. An aerodynamical ly shaped extension 45 is added to the lower part of the housing of the gas lift valve, as shown in Figure 9. The extension 45 is provided with an internal passage 46 having a discharge opening 47. Therefore, the flow of injection gαs coming from the discharge openings 38 of the gas lift valve 14 is directed to the throat of the central body venturi 60. The same effects happen here as were described with respect to the central body venturi 40 of Figures 4 and 5, that is, the flow of fluids coming from the reservoir 1 is accelerated when passing through the region where the central body venturi 60 is installed, provoking there a reduction in the pressure of the flow. Thus, the use of the extension 45 causes the flow of injection gas to be injected just in front of the throat of the central body venturi 60, thereby providing the same effect as that which has occurred with the use of the central body venturi 40 of the Figures 4 and 5, whereby the efficiency of the continuous gas lift is improved. As with in the situation where a conventional gas lift mandrel is used in a continuous gas lift system, the embodiment depicted in Figure 9 can also be made to provide a better distribution of the injection gas or, alternatively, geometrically eccentric central bodies can be used. Those skilled in the art will immediately recognise that it is possible to use a number of variations in the geometric configuration of the device to stabilise the production of oil wells according to the present invention. The optimum dimensions for the central body venturi can be calculated by means of theoretical analysis, experimentation or empiricism. The throat may have a certain length or it can comprise just a very small segment. The fixing elements of the central body venturi should preferably be fixed to the diffusor. As they cause an interference in the flow, the number of fixing elements should be as few as possible, and they should be thin and aerodynamical ly shaped. The device proposed by the present invention preferably makes use of a central body venturi. However, other configurations of Venturis or of nozzles may also be used, providing that the principle of the invention is used, that is, gas is injected at a constriction into the tubing, for example a throat of the central body venturi, which momentarily provokes the pressure of the flow to reduce at that constriction. Figure 10 schematically depicts a conventional gas lift mandrel 12 provided with a concentric body venturi 100, which is provided with a convergent segment or nozzle 101, a segment of constant area or throat 102 and a divergent segment or diffusor 103. Gas is injected into the throat 102 of the concentric body venturi 100, by means of the opening 104, which is in registration with the discharge opening 25 which exits the high pressure gas coming from the gas lift valve 14. Figure 11 schematically depicts an embodiment of a concentric body venturi device 40 into a small tube or nipple 70, which can be set at a desired position into the body of a gas lift mandrel. Such mandrel can be a conventional or a side pocket gas lift mandrel. Thin fixing elements 41 fix the central body venturi 40 to the body 71 of the nipple 70, keeping the central body venturi 40 centred. In the present embodiment three fixing elements are used, although more or less fixing elements can be used, depending on the features of the design. Notice that the fixing elements are fixed to the diffusor of the central body venturi 40. Gas is admitted by means of at least one orifice 80 existing in the body 71 of the nipple 70, which is aligned with the throat of the central body venturi 40. Two packing elements 90, located above and below the intake orifice 80, are intended to make a seal between the nipple 70 and the internal walls of the body of the gas lift mandrel, whereby the fluids from the reservoir are only allowed to pass through the right way into the device. Figure 11 A depicts a cross section view of the nipple 70, taken along the line B-B in Figure 11. Figure 12 schematically depicts a further embodiment of an asymmetric body venturi device 50 into a small tube or nipple 110, which can be set at a desired position inside the body of a gas lift mandrel. Such mandrel can be a conventional or a side pocket gas lift mandrel. The asymmetric body venturi device 50 is fixed to the walls of the body 111 of the nipple 110. Gas is admitted by means of at least one orifice 113 existing in the body 111 of the nipple 110, which is aligned with the throat of the asymmetric body venturi 50. Two packing elements 112 are located above and below the intake orifice 113, intended to make a seal between the nipple 110 and the internal walls of the body of the gαs lift mandrel, whereby the fluids from the reservoir are only allowed to pass through the right way into the device. The Figure 12 A depicts a cross section view of the nipple 110, taken along the line C-C in Figure 12. The Figure 13 schematically depicts a further embodiment of a concentric body venturi device 100 in a small tube or nipple 120, which can be set at a desired position inside the body of a gas lift mandrel. Such mandrel can be a conventional or a side pocket gas lift mandrel. The concentric body venturi device 100 is fixed to the walls of the body 121 of the nipple 120. Gas is admitted by means of at least one orifice 123 existing in the body 121 of the nipple 120, which is aligned with the throat of the concentric body venturi 100. Two packing elements 122, located above and below the intake orifice 123, are intended to make a seal between the nipple 120 and the internal walls of the body of the gas lift mandrel, whereby the fluids from the reservoir are only allowed to pass through the right way into the device. Figure 13 A depicts a cross section view of the nipple 120, taken along the line D-D in Figure 13. The actual installation of a nipple into a gas lift mandrel is an operation well known by the experts, and it will not be described here for the sake of simplification of the description. The device to stabilise the production of oil wells of the present invention may be preferably used with a venturi type gas lift valve. However, such device can also be used with other types of gas lift valves, although not so efficiently, for example, the conventional gas lift valve having an orifice plate with sharp edges. The present invention is mainly directed to oil wells equipped to produce by means of continuous gas lift. However, the device of the present invention can also be used in oil wells which naturally flow but which have a flow of fluids that is unstable. The invention can cause the flow of such oil wells to become stable, using or not the injection of gas in conjunction with the device. Having described the present invention with respect to its preferred embodiments, it should be mentioned that the present invention is not limited to the description heretofore made, being only limited by the scope of the appendant claims. LIST OF COMPONENTS reservoir casing tubing packer lower chamber upper chamber or annulus orifices (in the casing) processing facilities external source of high pressure gas oil well wellhead gas lift mandrel body (of the mandrel) gas lift valve gas lift valve support side support opening venturi body (of the gas lift valve) internal chamber packer seat internal passage internal chamber discharge opening side pocket gas lift mandrel side receptacle side pocket tip opening small annulus internal chamber discharge opening central body venturi fixing elements extension passage discharge opening asymmetric body venturi central body venturi fixing elements nipple body (of the nipple) intake orifice packing elements concentric venturi nozzle throat diffusor opening nipple body (of the nipple) packing elements intake orifice nipple body (of the nipple) packing elements intake orifice

Claims: CLAIMS 1. An oil well comprising: α tubing (3) for carrying fluids coming from a reservoir (1) to the surface; a body (40; 50; 60; 100) inserted in the tubing (3), the body comprising: a lower portion (A) progressively causing a decrease in cross-Sectional area available for the passage of fluid in the direction of fluid flow; a medium portion (B) located downstream of said lower portion causing a substantially constant cross-sectional area to be available for the passage of fluid, said constant area being smaller than the cross-sectiona\ area of the tubing alone; and an upper portion (C), located downstream of the medium portion progressively causing an increase in cross-sectional area to be available for the passage of fluid in the direction of fluid flow. 2. An oil well according to claim 1, further comprising a gas lift valve (14, 14') for injecting gas into said tubing (3). 3. An oil well according to claim 2, wherein said gas lift valve (14; 14') is connected to a gas lift mandrel (12; 30) forming a portion of said tubing (3), said body (40; 50; 60; 100) being provided adjacent the opening (25; 38; 47) of said gas lift mandrel (12; 30). 4. An oil well according to claim 3, wherein said body (40; 50; 60; 100) is provided such that gas is injected into a portion of said tubing (3) in which said medium portion (B) of said body (40; 50; 60; 100) is provided. 5. An oil well according to claim 3 or 4, wherein said gas lift mandrel (12; 30) is a side pocket gas lift mandrel (30) provided with an internal passage (46) having a discharge opening (47). 6. An oil well according to claim 5, wherein said discharge opening (47) is directed to inject gas towards the medium portion (B) of said body (40; 50; 60; 100). 7. An oil well according to any one of claims 2 to 6 wherein said gas lift valve (14; 14') comprises a venturi for the injection of gas. 8. An oil well according to any one of the preceding claims, wherein said body (40; 50; 60; 100) is provided internally to a nipple tubing (70; 110; 120) which is removably provided internally to said tubing (3). 9. An oil well according to claim 8, wherein said nipple tubing (70; 110; 120) comprises an intake orifice (80; 113; 123) aligned with the medium portion (B) of said body (40; 50; 60; 100), said intake orifice allowing the passage of injection gas. 10. An oil well according to claim 9, further comprising at least two packing elements (90; 112; 122), located above and below said intake orifice to make a seal between the nipple tubing (70; 110; 120) and the internal walls of the tubing (3). 11. An oil well according to any one of the preceding claims, wherein said body (40; 50; 60; 100) is a central body venturi (40; 60). 12. An oil well according to any one claims 1 to 10, wherein said body (40; 50; 60; 100) is an asymmetric body venturi (50). 13. An oil well according to any one of claims 1 to 10, wherein said body (40; 50; 60; 100) is a concentric body venturi (100). 14. An oil well according to claim 13, wherein said concentric body venturi (100) is provided with an opening (104) communicating with a discharge opening (25; 38; 47) of a gas lift valve (14; 14'). 15. An oil well according to any one of the preceding claims, further comprising: a well head (11), a casing (2), a packer (4) installed in the annulus (6) between the casing (2) and the tubing (3) to create two discrete regions in the well (10), such regions being a lower chamber (5) located next to the oil reservoir (1) and an annulus (6) between the casing (2) and the tubing (3). 16. A body (40; 50; 60; 100) to stabilise the production of oil wells when provided to a tubing (3) for carrying fluids coming from a reservoir (1); said body comprising: a lower portion (A) which progressively causes a decrease in cross-sectional area available for the passage of fluid in the direction of fluid flow when said (40; 50; 60; 100) body is inserted inside said tubing (3); a medium portion (B) located adjacent said lower portion which causes a substantially constant cross-sectional area to be available for the passage of fluid when said body (40; 50; 60; 100) is inserted inside said tubing (3), said constant area being smaller than the cross-sectional area of the tubing alone; and an upper portion (C) located adjacent said medium portion which progressively causes an increase in the cross-sectional area available for the passage of fluid in the direction of fluid flow when said body (40; 50; 60; 100) is inserted inside said tubing (3). 17. A body according to claim 16, wherein said body (40; 60) is a central body venturi having a lower portion (A) which progressively increases in cross-sectional area up to said medium portion (B) which has a constant cross-sectional area, said upper portion (C) progressively decreasing in cross-sectional area from said medium portion (B). 18. A body according to claim 16, wherein said body (50) is an asymmetric body venturi constructed to abut an inner cylindrical wall. 19. A body according to claim 16, wherein said body (100) is a concentric body venturi constructed to abut an inner cylindrical wall. 20. A body according to claim 19, wherein said body (100) comprises an opening (104) for communication with a discharge opening (25; 38; 47) of a gas lift valve (14; 14')- 21. A device comprising: the body according to any one of claims 16 to 20; a nipple tubing (70; 110; 120) surrounding, and attached to, said body (40; 50; 60; 100), said nipple tubing (70; 110; 120) being insertable into the tubing (3) of an oil well. 22. A device according to claim 21, wherein said nipple tubing (70; 110; 120) further comprises at least one intake orifice (80; 113; 123) facing the medium portion (B) of said body (40; 50; 60; 100) for the passage of injected gas. 23. A device according to claim 22, further comprising at least two packing elements (90; 112; 122) located above and below said at least one intake orifice (80; 113; 123) for making a seal between the nipple tubing (70; 110; 120) and the internal walls of the tubing (3) of an oil well. 24. A method to stabilise the production of oil wells comprising tubing (3) for carrying to the surface the fluids coming from a reservoir (1), the method comprising: inserting into the tubing (3) a device comprising: a lower portion progressively causing a reduction in the cross-sectional area for the passage of fluid coming from the reservoir; a medium portion, located above the lower portion, which causes said cross- sectional area for the passage of fluid coming from the reservoir to be substantially constant and smaller than the original area of the tubing (3); an upper portion, located above the medium portion, which causes a progressive widening in the cross-sectional area for the passage of fluid coming from the reservoir, until such area for the passage of the fluid is again equal to the original area of the tubing; allowing the fluids from the reservoir (1) to flow towards the surface, passing through the device, whereby the flow is accelerated when it passes through said lower portion, and consequently the flow pressure is decreased, the flow passing through said medium portion, and then through said upper portion, where the flow is decelerated, and consequently the flow pressure is increased, the above sequence causing a stabilisation of the flow. 25. A method according to the claim 24, wherein: the oil well (10) is equipped to produce by means of a gas lifting system; a gas lift mandrel (12; 30) forms part of the tubing (3); and a gas lift valve (14; 14') is connected to the gas lift mandrel (12; 30), said gas lift valve being provided with at least one opening (26; 38) through which gas at a high pressure from the annulus (6) between said tubing (3) and a well casing (2) is able to flow towards a discharge opening (25; 38; 47) in the gas lift mandrel (12; 30), so that gas at a high pressure is injected into the tubing (2). 26. A method according to claim 25, wherein the device is aligned with the gas lift valve (14), with said medium portion facing said discharge opening (25; 38; 47) to inject gas at a high pressure into the tubing (3). 27. A method according to the claim 24 or 25, wherein said device is shaped in such a way that the drop of pressure occurring in the region of the medium portion of the device enables a constant gas flow rate to occur throughout the gas lift valve, substantially independently of the pressure into the tubing (3). 28. The use of a body to stabilise the production of oil wells, said body being constructed as claimed in any one of claims 16 to 23. 29. A body to stabilise the production of oil wells constructed and arranged substantially as hereinbefore described with reference to Figures 4 to 13A. 30. A method to stabilise the production of oil wells substantially as hereinbefore described with reference to Figures 4 to 13A. 31. An oil well which uses a device to stabilise its production substantially as hereinbefore described with reference to Figures 4 to 13A.

Inventors: DE ALMEIDA ALCINO RESENDE, DE ALMEIDA, ALCINO, RESENDE
Applicants: PETROLEO BRASILEIRO SA, PETROLEO BRASILEIRO S.A. - PETROBRAS
Application year: 2001
Main classification: E21B43
IPC codes: E21B43