METHOD AND APPARATUS FOR STABILIZING GAS/LIQUID FLOW IN A VERTICAL CONDUIT

Abstract

A method and apparatus for stabilizing gas/liquid flow in a vertical conduit by utilizing one or more flow stabilizing devices positioned inside the conduit along a structural support tube. The flow stabilizing devices are configured and dimensioned to accelerate the velocity of the flowing two-phase fluid mixture in the conduit to sustain upward liquid flow.

Claims

1. A vertical production tubing for conveying a flowing two-phase fluid mixture, comprising of gas-liquid fluids, the tubing having an inner surface and upstream and downstream ends, the tubing comprising: one or more flow stabilizing devices positioned in the tubing along a structural support tube, the one or more flow stabilizing devices configured and dimensioned to accelerate the velocity of the flowing two-phase fluid mixture in the tubing to sustain upward liquid flow, wherein at least one of the one or more flow stabilizing devices includes: a first portion facing upstream, wherein the distal end of the first portion forms a first apex that faces upstream; a second portion facing downstream, wherein the distal end of the second portion form a second apex that faces downstream; a passageway extending from the first apex through the second apex; the structural support tube attached directly to the second apex and extending axially from the second apex, wherein the structural support tube is disposed entirely within the tube; and wherein each of the flow stabilizing devices is dimensioned and configured so that the exterior surface of the flow stabilizing device does not touch the adjacent inner wall of the tubing, wherein the exterior surface of the first portion forms a solid surface that is configured to block and deflect the path of the flowing two-phase fluid mixture; wherein a first portion of the mixture flows through the passageway; and wherein a second portion of the mixture flows around the flow stabilizing device.

2. The vertical production tubing of claim 1 comprising a plurality of flow stabilizing devices in predetermined spaced-apart relation.

3. The vertical production tubing of claim 2, wherein at least one of the flow stabilizing devices has a configuration that is different than the other or others of the flow stabilizing devices.

4. The vertical production tubing of claim 1, wherein at least one flow stabilizing device is symmetrical about the central axis extending from the first end portion to the second end portion.

5. The vertical production tubing of claim 1, wherein the momentum transfer is varied by adjusting the flow acceleration ratio : where $=\frac{D_p^2}{D_{h_1}^2-D_t^2}$ at the flow stabilizing device throat where the cross-sectional area presented to the fluid flow is the minimum.

6. The vertical production tubing of claim 5, wherein the flow acceleration ratio is in the range:
1.25.

7. The vertical production tubing of claim 1, wherein the flow stabilizing device diameter ratio : where $=\frac{D_t}{D_{h_1}}$ should be in the range from 0 to for the flow stabilizing device to generate the desired blockage to the flow section area in the vertical column.

8. The vertical production tubing of claim 1, wherein diameter D.sub.t of flow stabilizing device supporting tube is preferably between:
0.2 inch<D.sub.t<0.5 inch.

9. The vertical production tubing of claim 1, wherein the discharge coefficient C is defined as the ratio of the actual flow rate to the ideal flow rate, which is calculated as: $C=\frac{Q_{\mathrm{actual}}}{Q_{\mathrm{ideal}}}=Q_{\mathrm{actual}}/\left(\frac{}{4}.Math.D_p^2.Math.\sqrt{\frac{2.Math..Math..Math.P\left(-1\right)}{\left(+1\right)}}\right)$ $\mathrm{where}.Math..Math.=\frac{D_p^2}{D_{h_1}^2}.Math..Math.\mathrm{since}.Math..Math.D_t=0.$

10. The vertical production tubing of claim 1, wherein the structural support tube and the at least one flow stabilizing device are formed as a unitary structure.

11. The vertical production tubing of claim 1, further comprising a motion actuator attached to an inner surface of the at least one flow stabilizing device for adjusting the diameter of the exterior surface.

12. The vertical production tubing of claim 1, further comprising at least one opening positioned in a side wall of the flow stabilizing device for passing an injected gas or a liquid down through the structural support tube and out through the at least one opening in order to help stabilize the liquid film at the wall of the tubing.

13. A flow stabilizing device positioned in a vertical production tubing to sustain upward liquid flow, the flow stabilizing device comprising: a first portion facing upstream, wherein the distal end of the first portion forms a first apex that faces upstream; a second portion facing downstream, wherein the distal end of the second portion form a second apex that faces downstream; a passageway extending from the first apex through the second apex; the structural support tube attached directly to the second apex and extending axially from the second apex, wherein the structural support tube is disposed entirely within the tube; and wherein each of the flow stabilizing devices is dimensioned and configured so that the exterior surface of the flow stabilizing device does not touch the adjacent inner wall of the tubing, wherein the exterior surface of the first portion forms a solid surface that is configured to block and deflect the path of the flowing two-phase fluid mixture; wherein a first portion of the mixture flows through the passageway; and wherein a second portion of the mixture flows around the flow stabilizing device.

14. A method of accelerating the velocity of a gas/liquid flow in a vertical production tubing, the tubing having an inner surface, the method comprising: providing a flow stabilizing device positioned centrally in the tubing along a structural support tube, wherein the structural support tube is disposed entirely within the tubing; the flow stabilizing device including a central passageway configured and dimensioned to receive a portion of the gas/liquid flow and wherein a portion of the gas/liquid flow is directed around the flow stabilizing device; wherein the flow stabilizing device is dimensioned and configured so that the exterior surface of the flow stabilizing device does not touch the adjacent inner wall of the tubing.

15. The method of claim 14, further comprising: providing the flow stabilizing device with at least one opening positioned in a side wall of the flow stabilizing device; injecting a gas or a liquid down through the structural support tube and out through the at least one opening in order to help stabilize the liquid film at the wall of the tubing

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention will be described in further detail below and with reference to the attached drawings in which the same or similar elements are referred to by the same reference numerals, and in which:

[0024] FIG. 1 is a longitudinal cross-sectional view of a vertical well tubing inside which a flow stabilizing device of the present invention is positioned;

[0025] FIG. 2 is an enlarged view of section 2-2 of FIG. 1 schematically illustrating the splitting of the ascending gas/liquid flow inside the vertical well tubing between a gas rich flow through the center of the flow stabilizing device and a gas/liquid annular flow around the flow stabilizing device, according to one embodiment of the present invention;

[0026] FIGS. 3A, 3B, and 3C illustrate alternative embodiments of internal mechanisms for adjusting the diameter of a flow stabilizing device;

[0027] FIG. 4 illustrates a prior art non-welded spring bow centralizer for centering the flow stabilizing device;

[0028] FIGS. 5A and 5B illustrate an alternative embodiment wherein one or more openings are positioned in the side wall of the flow stabilizing device;

[0029] FIGS. 6A, 6B, 6C, and 6D are a series of rear perspective, side, rear, and front view, respectively, of one embodiment of a flow stabilizing device;

[0030] FIG. 7 is a longitudinal side view of a flow stabilizing device positioned inside a vertical well tubing and marked with references for dimensioning;

[0031] FIG. 8A is longitudinal cross-section view of another embodiment of a flow stabilizing device of the present invention;

[0032] FIG. 8B is longitudinal cross-section view of another embodiment of a flow stabilizing device of the present invention;

[0033] FIGS. 9A, 9B, and 9C are schematic illustrations of various geometries used in a fluid flow simulation test;

[0034] FIG. 10 is a table providing geometry data for a comparative computational fluid dynamics study conducted using the geometries illustrated in FIGS. 6A, 6B, and 6C in a 4 inch gas producing well;

[0035] FIGS. 11A, 11B, and 11C are schematic illustrations of fluid flow simulation results using the geometries illustrated in FIGS. 6A, 6B, and 6C and showing the liquid film stabilization effect induced by the flow stabilizing device;

[0036] FIG. 12 is a table providing computational fluid dynamics results using the geometries illustrated in FIGS. 9A, 9B, and 9C and showing gas production improvement with the flow stabilizing device;

[0037] FIG. 13 is a graph comparing tube size variation against gas production.

DETAILED DESCRIPTION OF THE INVENTION

[0038] The aim of the present invention is to produce or sustain a desired two-phase annular flow regime to transport liquid and gas upward inside vertical pipes of circular cross-section by incorporating one or several flow stabilizing device configurations in the pipe. The flow stabilizing device configuration of the present invention is useful for the purpose of accelerating the flow in vertical pipes at critical locations where additional momentum is needed to sustain the upward liquid flow. The flow stabilizing device configuration can be applied in production columns, and natural gas producing wells with liquid loading problems, to enhance liquid lifting in gas wells.

[0039] FIGS. 1 and 2 illustrate a longitudinal cross-sectional view of a vertical well tubing 40 inside which a flow stabilizing device 20 of the present invention is positioned. Within the upstream end of the well tubing 40, an ascending gas/liquid flow F is present and results from tapping into a reservoir 30 that produces a gas, liquid condensate and water mixture 31. The flow stabilizing device 20 includes a first portion 21 which is positioned towards the upstream end which terminates at a first apex 22. The flow stabilizing device includes a second portion 23 which is positioned towards the downstream end which terminates at a second apex 24. The first portion 21 and second portion 23 are joined along an intersection 25. In a preferred embodiment, the first portion 21 and second portion 23 are conically-shaped, although other shapes can be used. A central passageway extends from the first apex 22 through the second apex 23. A structural support tube 10 is attached directly to the second apex 24. Preferably, the diameter of the central passageway and the structural support tube 10 are substantially equal.

[0040] Thanks to its converging-diverging geometry, the flow stabilizing device 20 increases the gas flow velocity, which increases momentum transfer to the liquid phase. The momentum transfer is varied by adjusting the flow acceleration ratio : [0041] where

[00001]$\begin{array}{cc}=\frac{D_p^2}{D_{h_1}^2-D_t^2}& \left(1\right)\end{array}$

at the flow stabilizing device throat where the cross-sectional area presented to the fluid flow is the minimum.

[0042] The flow acceleration ratio should preferably be in the range:

1.25(2)

for the flow stabilizing device to perform stabilization of vertical gas-liquid flows. An optimum acceleration ratio is dependent on the flow regime and the physical properties of the gas and liquid phases.

[0043] The flow stabilizing device diameter ratio : [0044] where

[00002]$\begin{array}{cc}=\frac{D_t}{D_{h_1}}& \left(3\right)\end{array}$

should be in the range from 0 to for the flow stabilizing device to generate the desired blockage to the flow section area in the vertical column.

[0045] The flow stabilizing device 20 is supported and held in position inside the production column by a supporting center tube 10. The diameter D.sub.t of flow stabilizing device supporting tube 10 is preferably between:

0.2 inch<D.sub.t<0.5 inch(4)

[0046] For a typical gas producing well that is completed with 4 inch or 5 inch diameter production tubing 40, a 1-inch diameter or larger supporting tube 10 will be preferred because it is commercially available as a standard coiled tubing, which offers a lower cost option for manufacturing and installing the flow stabilizing device 20.

[0047] The flow stabilizing device 20 is preferably fabricated from a rigid corrosion resistant material. The material can be a metal, sleeved metal or non-metallic. A pneumatic, hydraulic or electrical mechanism is used to adjust the vertical location of flow stabilizing device 20 and fix the position the flow stabilizing device at the desired height in the production column 40.

[0048] In another embodiment of the flow stabilizing device, the cross-sectional area of the annular passageway or throat is adjustable via an upward/downward sliding mechanism to change D.sub.h1 and thus control momentum and liquid entrainment as required by the flow conditions. For example, a motion actuator can operate via electrical, hydraulic, or pneumatic operation.

[0049] For example, FIG. 3A illustrates a motion actuator 50 that is joined to an attachment member 52 via a connector 51. The attachment member 52 is joined to an inner surface of the flow stabilizing device. The motion actuator 50 controls the movement of the attachment member 52 along the direction 53. The position of the intersection 25 can therefore be controlled by the motion actuator wherein the intersection 25 can be a hinged or flexible connection point at the intersections of two rigid arms. In this embodiment, two or more sets of rigid arms are symmetrically distributed radially along the inner surface of the flow stabilizing device to create a framework for adjusting the diameter of the flow stabilizing device.

[0050] In another embodiment of the flow stabilizing device, the outer surface of the flow stabilizing device is flexible allowing, with an internal mechanism, the adjustment of the throat gap to change D.sub.h1 and thus control momentum and liquid entrainment as required by the flow conditions. For example, FIG. 3B illustrates a motion actuator 60 that is joined to an attachment member 62 via a connector 61. The attachment member 62 is joined to an inner surface of the flow stabilizing device. The motion actuator 60 controls the movement of the attachment member 62 along the direction 63. The position of the intersection 25 can therefore be controlled by the motion actuator. In this embodiment, two or more sets of attachment members 63 are symmetrically distributed radially along the inner surface of the flow stabilizing device to create a framework for adjusting the diameter of the flow stabilizing device.

[0051] In another embodiment of the flow stabilizing device, FIG. 3C illustrates a motion actuator 70 that is joined to an attachment member 72 via a connector 71. The actuator 70 and the attachment member 72 are separately joined to an inner surface of the flow stabilizing device via rigid arms. The motion actuator 70 controls the movement of the attachment member 72 along the direction 73, which in turn controls the position of the rigid arms. The position of the intersection 25 can therefore be controlled by the motion actuator. In this embodiment, two or more sets of rigid arms are symmetrically distributed radially along the inner surface of the flow stabilizing device to create a framework for adjusting the diameter of the flow stabilizing device.

[0052] With reference to FIGS. 3A-3C, the diameter of the flow stabilizing device can be adjusted starting from the position illustrated in solid lines, with the alternative positions being illustrated in dashed lines in either side of the starting position. It should be understood that the various positions illustrated are exemplary only, and any alternate position can be achieved according to the configuration of the motion actuator. In another embodiment, a flexible ring can form the framework of the intersection 25, in order to maintain the generally circular cross-section of the outermost diameter of the flow stabilizing device.

[0053] The flow stabilizing device can be centered in the production column 40 by any means such as centering mechanical device located on the flow stabilizing device supporting tube 10 or by means of ribs or centering winglets attached to some of the cone inserts forming the flow stabilizing device. For example, FIG. 4 illustrates a prior art non-welded spring bow centralizer for centering the flow stabilizing device.

[0054] In another embodiment, the flow stabilizing device 20 is used in combination with a known pressure control device (not shown) to split the ascending flow inside the column into a gas rich flow F.sub.1 through a central tube at the opening formed at the first apex 22 of the flow stabilizing device with diameter D.sub.t in FIG. 2, and a gas+liquid annular flow F.sub.2 around the flow stabilizing device. The pressure control device can be any prior art system capable of keeping a constant static pressure difference between the center tube and the annular flow section, at the top of the production column, or the well head. This embodiment is named the Hollow Flow Stabilizing Device or HFSD. The foreseen advantage of the HFSD is a way to optimize the production process by continuously adjusting the static pressure difference to maximize the total gas production rate.

[0055] In another embodiment, illustrated in FIGS. 5A and 5B, the flow stabilizing device is used in combination with a pressurizing system capable of injecting a gas or a liquid 28 down through the structural support tube 10 into the production column in order to help stabilize the liquid film at the wall of the production column 40. In this embodiment, the flow stabilizing device can be equipped with holes or slots 26 distributed in any part of the cone inserts or above the cone inserts in the center tube supporting them. This embodiment is referred to as the Injection Flow Stabilizing Device or IFSD. The foreseen advantage of the IFSD is the assisted flow stabilizing effect by means of pressurized gas injection (added gas lift effect resulting in enhanced liquid film stabilization at the column wall) and/or chemical additive injection (low dosage liquid film stabilization agent or viscosifier or surfactant). In this embodiment, a plug 27 is positioned at the first apex 22 of the flow stabilizing device to direct the injected gas or liquid 28 through the openings 26.

[0056] FIG. 6 shows different views of an embodiment of the flow stabilizing device.

[0057] FIG. 7 is a longitudinal side view of a flow stabilizing device positioned inside a vertical well tubing and marked with references for dimensioning. The actual dimensions for the flow stabilizing device will depend on the existing tubing dimensions and will be specifically selected to accommodate the production lining. There is a wide range of tubing sizes in the oil and gas industry suited for application of the present invention. For example, standard production tubing sizes for oil and gas production range from as small as inch to as large as 5 inches or more.

[0058] In alternative embodiments, the flow stabilizing device is composed of more than two cone inserts arranged along their supporting center tube 10. FIGS. 8A and 8B show two non-limiting examples of such embodiments.

[0059] As shown in FIGS. 9-12, the inventors have conducted Computational Fluid Dynamics (CFD) simulation work in order to prove the effectiveness of the flow stabilizing device of the present invention and to optimize the flow stabilizing device design for the purpose of stabilizing the liquid film at the inner wall of the production column, in the range of flowrates and Gas Volume Fraction (GVF) of interest to the practical application.

[0060] In a first stage, the inventors' research work had the objective to model the annular vertical two-phase flow in an open tubular (see FIG. 9C) and the same tubular configuration equipped with a velocity string (see FIG. 9B) in order to understand the different flow structures in these geometries and establish a set of reference data points quantifying the performances of the prior art technique. According to FIG. 9B, a coiled tubing velocity string is a smaller diameter tube inserted down the production tubing to raise the production gas velocity above the critical velocity required to lift liquid and unload the gas well. Then the CFD model was then run with a reverse-venturi or Flow Stabilizing Device (see FIG. 9A) according to the present invention to assess its impact on the annular flow regime and compare the flow characteristics with the reference data sets. The three geometries tested with CFD are schematically illustrated by FIG. 9A (flow stabilizing device attached to support tube), 9B (velocity string), and 9C (open tubular configuration).

[0061] Typically, gas production inside wells is operating in the annular gas-liquid flow regime. The destabilization of the annular flow regime occurs with a decrease in the gas flow rate. FIGS. 10, 11 and 12 show the main results of a comparative CFD case study inside a 4 inch tubing producing gas with 1% vol condensate. In this example, the two-phase flow annular regime was modeled inside the open tubing (see FIG. 9C), the coiled tubing velocity string (see FIG. 9B) and with the flow stabilizing device mounted on coiled tubing (see FIG. 9A), successively, these geometries having the dimensions reported in the table shown in FIG. 10, with a vertical tubing length of 20 meters. The flow conditions were selected to obtain annular flow regime in each case. However, the average gas velocities are chosen identical in the velocity string and the flow stabilizing device experiments in order to obtain comparative results shown in the table of FIG. 12.

[0062] The plots shown in FIGS. 11A, 11B, and 11C show the liquid film thickness. In the velocity string case, shown in FIG. 11B, the flow regime is initially churn-turbulent in the downhole region and then is transitioning to annular.

[0063] FIG. 12 displays the calculated averaged produced gas velocities and the corresponding pressure loss coefficients thus providing evidence of the advantage of the present invention over the velocity string technique (see FIG. 9B). In FIG. 12, the discharge coefficient C is defined as the ratio of the actual flow rate to the ideal flow rate, which is calculated as:

[00003]$\begin{array}{cc}C=\frac{Q_{\mathrm{actual}}}{Q_{\mathrm{ideal}}}=Q_{\mathrm{actual}}/\left(\frac{}{4}.Math.D_p^2.Math.\sqrt{\frac{2.Math..Math..Math.P\left(-1\right)}{\left(+1\right)}}\right).Math..Math.\mathrm{where}.Math..Math.=\frac{D_p^2}{D_{h_1}^2}.Math..Math.\mathrm{since}.Math..Math.D_t=0& \left(5\right)\end{array}$

[0064] The flow stabilizing device extends the gas well operability range by increasing the gas velocity through a minor reduction in the cross-sectional flow area, and operating the well in the stable annular flow regime thereby stabilizing the annular film and preventing film reversal. The gradual restriction of the inverse venturi geometry minimizes pressure drop due to the flow stabilizing device and optimizes the pressure recovery downstream of the flow stabilizing device. A benefit of the flow stabilizing device is the ability to sustain higher flow rates due to the pressure recovery that is achieved with the flow stabilizing device geometry. The gradual restriction of the inverse venturi geometry minimizes pressure drop due to the flow stabilizing device and optimizes the pressure recovery downstream of the flow stabilizing device. The annular area available for flow with the flow stabilizing device, or in combination with the central tube with the HFSD, is prescribed to achieve an annular flow regime and stable liquid film. For example, a 4 inch tubing well configured with an flow stabilizing device would allow a 400% increase in gas production compared to a coiled tubing velocity string to sustain an equivalent annular flow regime as shown in the example given in FIGS. 9-12.

[0065] The method and apparatus of the present invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be defined by the claims that follow.