FR2776703A1 - Recovery of fluids from reservoir using pressurized gas to drive them out - Google Patents

Abstract

A fluid contained in a reservoir is removed by means of a gas, where the reservoir has at least one inlet for the gas and at least one outlet for the fluid. The process comprises closing the outlet of the reservoir, sending in gas under pressure via the inlet, and closing the inlet and opening the outlet before receiving the fluid displaced by the gas. The outlet is opened after a time determined to allow the gas to disperse into the reservoir and displace the fluid towards the outlet. To recover the fluid in a continuous manner, the reservoir is supplied with two assemblies, each with a gas inlet and a fluid outlet. The gas is sent in under pressure by one assembly while the fluid is recovered from the other and vice versa.

Description

The present invention relates to an enhanced recovery method

  of a fluid by means of a gas, and more particularly to such a method intended to allow the recovery of fluid hydrocarbons originating from an underground reservoir rock. When an oil field is put into production, the extraction of hydrocarbons can be carried out in two phases. The first phase, called primary recovery, results from the natural drainage of the reservoir under the effect of the pressure difference within the deposit and at the bottom of the production wells which lead to the surface. This pressure difference can decrease depending on the volume

of hydrocarbons produced.

  When the well is no longer eruptive, production can be extended, first by mechanical pumping, either from the surface, or by installing a pump at the bottom of the well or by a process to lighten the column. hydrocarbons by injecting gas at the bottom of the well, a process called "gas lift". The reserves which can be effectively produced by pumping being often relatively low, one very often proceeds to a phase of

  secondary recovery, also called assisted recovery.

  In this second phase, a fluid is injected into the reservoir from the surface which spreads through the reservoir rock, displacing the hydrocarbons towards the well (s)

of production.

  Enhanced oil recovery using

  of a pressurized gas is a commonly used process.

  During the implementation of this type of process, pressurized gas is injected into the tank from the surface. The gas, spreading in the porous rock does not disperse there uniformly, because of the viscosity contrast between the gas and the fluid of the reservoir, but tends to follow a narrow path between the gas injection well and the production well which forms a free path to the surface. This feature results in that often little oil is moved to the production well. The present invention therefore relates to a process for enhanced recovery of a fluid which is simple but

leads to improved performance.

  To meet this object, the present invention provides a method of recovering a fluid contained in a reservoir by means of a gas, said reservoir being provided with an inlet for the gas and an outlet for the fluid, the process comprising the steps of: - closing the outlet of the tank, then sending pressurized gas to the tank through the inlet; - open the outlet, after a determined time allowing the gas to disperse in the tank and thus move the fluid, to collect the fluid

displaced by gas.

  The advantages, as well as other features of the present invention, will become more apparent from the

  reading of the following description, made with reference to

  attached drawings, in which: - Figure 1 is a sectional view of a tank provided with a recovery system used according to the prior art; - Figures 2 and 3 are each a sectional view of a tank provided with a recovery system used according to the invention; and - Figures 4 and 5 are each a schematic perspective view of a recovery system allowing the implementation of the

method according to the invention.

  Figure 1 is a sectional view of a reservoir 10 which, in the example illustrated, is a hydrocarbon reservoir rock. Two wells 12, 14 drilled from the surface (not shown) generally extend horizontally through the illustrated portion of the

tank 10.

  In the hydrocarbon recovery process proposed according to the prior art, gas at a high pressure greater than the hydrostatic pressure of the reservoir was injected into the reservoir from the surface by the well 12. The gas, moving in the porous rock in the reservoir 10, did not disperse there uniformly around the gas outlets formed in the well 12, but tended to follow a narrow path 16 between the well 12 and the well 14 which presented a free path to the surface . The formation of such a narrow path, called "digitation", resulted in little hydrocarbons being displaced by the gas towards the outlet well 14. The yield of such a

  recovery process was therefore weak.

  Figures 2 and 3 are sectional views of a hydrocarbon tank during the implementation of the

  recovery process according to the invention.

  According to the invention, before injecting the gas into the reservoir through the well 12, the well 14, forming a path to the surface, is closed. Then, pressurized gas is sent through the well 12 into the tank 10. Preferably, a hydrocarbon gas from a neighboring tank is used, but other gases can also be used. The gas initially moves in a substantially symmetrical manner around the well 12 as shown in FIG. 2. Then, the injection of the gas is stopped and the well 12 is closed. After a determined time, which can be of the order of several days, the well 14, forming a flow path for the hydrocarbons, is open allowing the hydrocarbons displaced by the gas to rise to the surface. The pressurized gas present in the rock of the reservoir tends to disperse as shown in FIG. 3. Once the flow of the hydrocarbons towards the surface has slowed down, the well 14 is closed before recommencing the injection of the gas under pressure in the tank 10. So the gas pressure in the tank

  is restored after the injection and production phases.

  The use of the recovery process according to the invention with the installation of FIGS. 2 and 3 makes it possible to recover the hydrocarbons on the surface only in a discontinuous manner, the gas injection and

recovery being successive.

  In order to be able to recover the hydrocarbons on the surface in a substantially continuous manner, the recovery process according to the invention can be implemented on a

  installation as shown in Figures 4 and 5.

  As shown in FIG. 4, two layers of reservoir rock 18 and 20 are separated by a layer of

  impermeable rock 21 formed for example by clay.

  Two wells 22, 24, forming the pressurized gas inlets, extend from the surface, each towards an associated layer of reservoir rock 18, 20. Similarly, two wells 26, 28 forming flow paths for the hydrocarbons each extend in one of the reservoir rock layers 18, 20. Thus the reservoir is provided with two assemblies each comprising an inlet for the gas and an outlet for the fluid. These two wells can extend to the surface. However, preferably the two wells 26, 28 meet near the tanks so that a single well 30 rises to the surface. In the case of producing wells, each well 26, 28 is provided with an associated valve 32, 34 which is controlled from the surface so

  to be able to selectively close one or the other well.

  During the implementation of the recovery process, the valve 32 is closed isolating the reservoir rock 18 from the well 30, the well 24 is closed and pressurized gas is sent through the well 22 into the reservoir rock 18. During this time , the valve 34 is open allowing the hydrocarbons coming from the reservoir rock

  to flow through wells 28, then 30 towards the surface.

  This phase is shown in FIG. 4. At the end of this phase, the well 22 is closed, the valve 34 is closed, then pressurized gas is sent through the well 24 into the reservoir rock 20. After the necessary time which is of the order of several days, so that the gas injected during the previous phase spreads into the tank 18, the valve 32 is open allowing the hydrocarbons, which have been displaced in the tank 18 by the gas, to s '' flow through the wells 26, 30 towards the surface. This phase is shown in Figure 5. At the end of this phase the production cycle starts again as described above in

reference to figure 4.

  The use with the installation which has just been described, of the recovery process according to the invention, allows a production of hydrocarbons which is substantially continuous. The inlet and outlet wells may extend through the tank other than horizontally, such as

  shown in the drawings, for example, vertically.

  The oil outlet to the surface can be opened immediately after closing the gas supply. In this case, there is no determined waiting step. The process according to the invention finds a particular application in the recovery of summit oil, more

commonly known as "attic oil".

  RFVFNDpTCATIONR 1 - Method for recovering a fluid contained in a reservoir by means of a gas, said reservoir being provided with at least one inlet for the gas and at least one outlet for the fluid, the method comprising the steps consisting in: - closing the tank outlet, then sending pressurized gas to the tank via the inlet; - close the finish, then open the exit to

  collect the fluid displaced by the gas.

  2- A method according to claim 1 characterized in that the step of opening the outlet takes place after a determined time in order to allow the gas to disperse in the tank and thus to move the fluid towards the outlet. 3 - Method according to claim 1 or 2 characterized in that, in order to be able to recover the fluid in a substantially continuous manner from a reservoir provided with two assemblies each comprising an inlet for the gas and an outlet for the fluid, the step consisting to send gas under pressure by the arrival of an assembly being carried out while the fluid is collected by the outlet of the other assembly, then the step consisting in collecting the fluid displaced by the gas through the outlet of the first assembly taking place while sending pressurized gas to the tank by the arrival of the

second set.