METHOD AND SYSTEM FOR STEPWISE VARYING FLUID FLOW IN A WELL

Abstract: A method for stepwise varying fluid flow in a well utilizes a plurality of downhole valves which have orifices of different sizes in the valve bodies, so that fluid flow can be varied by partially closing one or more valves. The method may be used for testing gas wells such that the wellbore storage delays caused by gas compression and/or expansion in the production tubing are minimized.

Description: METHOD AND SYSTEM FOR STEPWISE VARYING FLUID FLOW IN A WELL Background of the Invention The invention relates to a method and system for stepwise varying fluid flow in a well. Generally fluid flow in a well is controlled by a production choke at the wellhead whereas downhole safety valves and/or test valves are installed which generally can be switched between a fully open and a fully closed position. Such dual position downhole valves can be actuated by spring mechanisms that can be triggered by acoustic or hydraulic pulses so that they do not require a wear prone electric motor and electrical power supply and control conduits, which are difficult to install and remove, in particular in a complex multilateral well. Currently gas wells are tested by flowing the well at three or four different flow rates and calculating the so-called total skin at each flowrate . Said total skin is an accumulation of the mechanical skin (permeability reduction due to mud invasion, partial penetration effects and other relatively constant factors) and the rate-dependant (or non-Darcy) skin (which is due to turbulence or non-Darcy flow effects) . From the flowrate data gathered the well test procedure the total skin is plotted as a function of flow rate and the rate dependant skin is calculated as the slope of a line fit through the data. In this procedure, each flow period must reach either Pseudo Steady State (PSS) which is known as "stabilized" flow in the gas well testing literature of the 1950' s through the 1970' s, or Infinite Acting Radial Flow (IARF) . Because the time required to reach IARF is much shorter than the time required to reach PSS, the most economic method is to flow the well until sufficient IARF data has been gathered, and then change the rate to the next flow rate in the test program. Details of gas well test procedures at different flow rates to determine the effect of the mechanical and rate dependant skin are disclosed in the handbook "Modern Well Test Analysis, A Computer Aided Approach", Second Edition; author R N Home, issued in 1995 by Petroway, Inc. If the rate change from one flow rate to the next is imposed by the production choke valve at the surface, the high compressibility of the gas and the large volume of tubing between the Total Depth (TD) of the well and the surface choke results in a long "wellbore storage" or "after flow" period prior to the IARF period. In order to reduce the wellbore storage period downhole test valves exist to achieve pressure build-ups (in which the rate is changed to a rate of zero) with minimal wellbore storage. These test valves have been very successful and are widely used now. Examples of these remotely actuable dual position test valves are the electronic downhole shut-in tool marketed by Halliburton Energy Services, Inc. under the tradename ETUX and the IRIS (a Schlumberger trademark) dual-valve tool marketed by Schlumberger Technical Services Inc. The IRIS dual-valve uses annulus pressure levels to transmit signals from surface to switch the valve between the open and closed position and the ETUX valve incorporates a downhole computer to perform a preprogrammed test procedure. With these known downhole valve assemblies and variable production choke it may still take significant time to reach IARF and allow the production to stabilize before a next rate change can be applied, in particular in deep wells and if skin effects are high. In a typical 3000 m deep gas well the time required to reach IARF could take 20 hours and the next rate change should not be imposed until about 100 hours. Therefore the well test procedures take significant amount of time during which produced gas may be flared and significant personnel and materials expenses are made, in particular if the gas is produced offshore or in a remote area. The present invention aims to alleviate these problems associated with the prior art test methods and to provide a method and system for stepwise varying fluid flow in a well such that the time required for testing the well is reduced. The present invention also aims to provide a method and system for stepwise varying fluid flow in a well which can not only be operated in well test procedures but which permanently or temporarily operates downhole to adjust the flowrate from different well branches into a main wellbore of a multilateral well. Summary of the Invention In the method according to the invention fluid flow in a well is varied stepwise by means of a downhole valve assembly that can be switched between a first position in which fluid flow is permitted and a second position in which fluid flow is inhibited, wherein the valve assembly comprises a plurality of valves which each comprise an orifice through which a limited flow of fluid is allowed when the valve is in the second position thereof. Preferably the valves comprise orifices that form different flow restrictions and wherein the method is used to determine the properties of the rate-dependent skin around the inflow region of a gas production well and wherein during a selected period of time a first valve is in the second position and each of the valves are in the first position thereof, which period is followed by another selected period of time during which a second valve is in the second position and each of the other valves is in the first position thereof. In such case it is also preferred that the valve assembly comprises at least three valves and wherein during a first period of time a first valve, which has an orifice that forms a larger flow restriction than the other valves is maintained in the second position and the other valves are in the first position thereof, which period of time is followed by a second period during which a second valve which has an orifice that forms a smaller flow restriction than the first valve, but larger flow restriction than the other valve or valves, is in the second position and the other valves are in the first position and which second period of time is followed by a third period of time during which a third valve which has an orifice that forms a smaller flow restriction than the orifices of the first and second valves is in the second position and the first and second valves are in the first position . Generally said first, second and third periods of time have a substantially similar duration which is sufficient to reach IARF. For most gas reservoirs said duration will be in the range between 0.5 and 50 hours, and typically between 1 and 20 hours. The invention also relates to a system for stepwise varying fluid flow in a well. The system comprises a downhole valve assembly that can be switched between a first position in which fluid flow is permitted and a second position in which fluid flow is inhibited, wherein the valve assembly comprises a plurality of valves which each comprise an orifice through which a limited flow of fluid is allowed when the valve is in the second position thereof . Preferably the valve assembly is retrievably mounted in a production tubing such that the valves can be inserted into and retrieved from the well through the interior of the tubing whilst the production tubing remains in place. This can be accomplished by mounting the valves on a common carrier body which can be lowered into the well through the production tubing and which is provided with a sealing ring assembly which can be landed in a nipple to seal off an annular space between carrier body and the tubing. It is observed that US patent No. 5,447,201 discloses the use of a permanently installed downhole valve, which can be maintained in any partly open position, but such a valve is wear prone and requires the use of electric or hydraulic power cables which involves a complex and expensive well infrastructure. In the method and system according to the present . invention the downhole chokes can be operated wireless, for example by using a time- controlled or fluid pulse actuation mechanism, and be installed inside an existing tubing using a slickline, which does not comprise electric or hydraulic conduits. Brief Description of the Drawings The invention will be described in more detail, and by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a valve assembly according to the present invention at the bottom of a production tubing inside a gas production well; Fig. 2 shows schematically how the gas production rate is influenced by sequentially positioning one of the four valves of the assembly shown in Fig. 1 in its first, substantially closed, position; and Fig. 3 shows how with the valve assembly according to the invention IARF can be reached within about 10 hours, whereas with a conventional surface choke it would take up to about 100 hours to reach IARF. Referring to Fig. 1 there is shown the lower portion of a gas production well 1, which traverses an underground gas bearing formation 2. The well 1 comprises a well casing 3, which is cemented within the wellbore and which is equipped with a series of perforations 4 through which gas enters the wellbore as is illustrated by arrows 5. A production tubing 6 is suspended in the well 1 and sealingly anchored near its lower end to the well casing 3 by a production packer 7. A valve assembly 8 comprising a set of four ball valves 8A, 8B, 8C and 8D. Each of the valves 8A-D is shown in the first, open, position, wherein the central bores 9 in the valve bodies 10 are aligned with each other and with a central bore 11 in a valve carrier body 12. The valve carrier body 12 is sealingly and removably secured inside the production tubing 6 by landing a shoulder 13 in a nipple and is suspended from a reeling drum at the wellhead (not shown) by means of a wireline 14. If desired the wireline 14 may be equipped with electrical or fibre optical, power and/or communication conduits to power the valve bodies 8A-8D. Alternatively the wireline 14 is only a hoisting cable, cq. slickline, and the valve bodies are powered by springs and/or a downhole electronic system which is powered by the gas flow and/or a downhole electrical battery (not shown) . The valve bodies 10 are, apart from the central bore 9, also equipped with orifices 15A-D, which intersect the central bore 9 of the relevant valve body 10 in an orthogonal direction. The orifices 15A-D have a cylindrical shape, the orifice 15A of the uppermost valve 8A has the largest diameter, whereas the orifice 15D of the lowermost valve 8D has the smallest diameter. The orifice 15B of the valve 8B has a larger diameter than the diameter of the orifices 15C and D of the valves 8C and 8D, but a smaller diameter than the orifice 15A of the uppermost valve 8A. When the gas well 1 is to be tested the valve assembly 8 is lowered through the interior of the production tubing 6 by paying out the wireline 14 until the valve assembly 8 is located close to the lower end of the tubing 6. Then the valve assembly is sealingly secured inside the production tubing 6 by landing the shoulder 13 in the nipple. Then gas production is allowed to start by opening the surface choke (not shown) at the wellhead whilst the valves 8A-D of the valve assembly 8 are in the first, open, position as is illustrated in Fig. 1. Then during a first phase I of the well test cycle the first, lowermost valve 8D is positioned in the second position thereof, whilst the other valves are maintained in the first, open, position. As illustrated in Fig. 2 said first phase I continues for about 10 hours, which is sufficient for the well 1 to reach IARF as illustrated in Fig. 3. The example is based on a computer simulation of the effects of a downhole test choke assembly in which the following assumptions were made: h: 30 m = thickness of the gas bearing formation k: 1.0 md = formation permeability S: 15 = skin μ g : 0.0198 cp. = gas viscosity C g : .2E ~ 5kPa " l = gas compressibility Tubing Diameter: 9 cm Well Depth: 3000 m Test Tool Depth: 2900 m In the diagram shown in Fig. 2 it is assumed that the diameter of the orifice 15A of the first valve is 3 cm and that the diameters of the orifices 15B, 15C and 15D of the second, third and fourth valves 8B, 8C and 8D are 2.5; 2 and 1.3 cm. The diagram shown in Fig. 2 indicates that under the above conditions the downhole pressure p during the first phase I increases and that the gas flow rate Q j decreases. Fig. 3 shows that in the example shown as a result of the pressure measurement with a pressure sensor at the wellhead and the absence of a gas column with a length of 2900 m and a volume of 1/4.π.0.09 2 .2900=18.4 m 3 the response of the well 1 with a downhole choke as represented by pressure curve 20 is much quicker than if the well production would be reduced with a surface choke as represented by pressure curve 21 and that accordingly the time required to reach IARF is reduced from 100 hours to 10 hours. Referring again to Figs. 1 and 2, when the well has reached IARF after about 10 hours the second test phase II is commenced by placing the fourth, lowermost valve 8D again in the first, open, position and simultaneously placing the third valve 8C in the second position whilst the first and second valves 8A and 8B are maintained in their first, open, position. The diameter of the orifice 15C is 2 cm, which is larger than that of the fourth valve 8D so that during the second phase I the downhole pressure p j decreases whereas the gas production Q increases. When IARF is reached after about 10 hours the second test phase II is terminated and the third test phase III is commenced by placing the second valve 8B in the second position and placing the other valves 8A, 8C and 8D in the first, open position. Since the orifice 15B of the third valve 8C is 2.5 cm, which is larger than the diameters of the orifices of the third and fourth valves 8C and D, the downhole pressure Pm decreases, whereas the gas production rate Qm further increases. When IARF is reached again after about 10 hours the third test phase III is terminated and the fourth test phase IV is commenced by placing the first valve 8A in the second position and placing the second, third and fourth valves 8B, 8C and 8D in the first, open, position. Since the orifice 15D in the valve body 10 of the fourth valve 8D is 3 cm, which is larger than the diameters of the orifices 15A-D of the other valves 8A-C, the downhole pressure pjv will further decrease, whereas the gas production rate will further increase. When IARF is again reached after about 10 hours the fourth test phase IV is terminated and a downhole valve is closed so that the gas flow Q σ is interrupted and the pressure P 0 increases until it reaches the reservoir pressure and then the valve assembly 8 is retrieved to surface . As can be seen in Fig. 2 the four phases I-IV of the total well test cycle take in total about 40 hours. If the well would have been equipped with a traditional surface choke and a single downhole valve that would be closed to achieve pressure build up when the position of the surface choke is changed the total test cycle would have taken about 400 hours. Thus, the test time is reduced by about 90% by using the valve assembly 8 according to the invention.

Claims: C L A I M S 1. A method for stepwise varying fluid flow in a well by means of a downhole valve assembly that can be switched between a first position in which fluid flow is permitted and a second position in which fluid flow is inhibited, wherein the valve assembly comprises a plurality of valves which each comprise an orifice through which a limited flow of fluid is allowed when the valve is in the second position thereof. 2. The method of claim 1, wherein the valves comprise orifices that form different flow restrictions and wherein the method is used to determine the properties of the rate-dependent skin around the inflow region of a gas production well and wherein during a selected period of time a first valve is in the second position and each of the valves are in the first position thereof, which period is followed by another selected period of time during which a second valve is in the second position and each of the other valves is in the first position thereof. 3. The method of claim 2, wherein the valve assembly comprises at least three valves and wherein during a first period of time a first valve, which has an orifice that forms a larger flow restriction than the other valves is maintained in the second position and the other valves are in the first position thereof, which period of time is followed by a second period during which a second valve which has an orifice that forms a smaller flow restriction than the first valve, but larger flow restriction than the other valve or valves, is in the second position and the other valves are in the first position and which second period of time is followed by a third period of time during which a third valve which has an orifice that forms a smaller flow restriction than the orifices of the first and second valves is in the second position and the first and second valves are in the first position. 4. The method of claim 3, wherein said first, second and third periods of time have a substantially equal duration which is between 0.5 and 50 hours. 5. The method of claim 4, wherein said duration is between 1 and 20 hours. 6. A system for stepwise varying fluid flow in a well, the system comprising a downhole valve assembly that can be switched between a first position in which fluid flow is permitted and a second position in which fluid flow is inhibited, wherein the valve assembly comprises a plurality of valves which each comprise an orifice through which a limited flow of fluid is allowed when the valve is in the second position thereof. 7. The system of claim 6, wherein the valves are mounted on a common carrier body which can be lowered into the well through the production tubing and which is provided with a shoulder that can be landed in a nipple to seal off an annular space between carrier body and the tubing. 8. The system of claim 7, wherein the valves have openings in which valve bodies are rotatably arranged, which openings form a longitudinal fluid passageway through the centre of the carrier body. 9. The system of claim 8, wherein the valves are ball valves having each a first orifice and a second orifice which is oriented in a direction orthogonal to the first orifice and which is smaller than the first orifice. 10. The system of claim 9, wherein the carrier body carries at least four valves which each have a second orifice that forms a flow restriction different from the flow restrictions formed by the second orifices of the other valves.

Inventors: BERGREN FRANK EDWARD, BERGREN, FRANK, EDWARD
Applicants: BERGREN FRANK EDWARD, SHELL CANADA LTD, SHELL INT RESEARCH, BERGREN, FRANK, EDWARD, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 2001
Main classification: E21B34
IPC codes: E21B34