WO1999025952A1

WO1999025952A1 - Repressurization of oilfield reservoir for enhanced hydrocarbon recovery

Info

Publication number: WO1999025952A1
Application number: PCT/US1998/023754
Authority: WO
Inventors: Joseph Francis Donovan
Original assignee: Baker Hughes Incorporated
Priority date: 1997-11-15
Filing date: 1998-11-04
Publication date: 1999-05-27

Abstract

An enhanced hydrocarbon recovery technique is presented wherein energy (that is, stored pressure) from naturally occurring high pressure fluid reservoirs (24, 26) (typically natural gas reservoirs) located above or below a hydrocarbon reservoir to be produced (18) are used to supply a pressure source for driving oil or other hydrocarbons to the surface. This is accomplished by accessing the high pressure reservoir and directing the accessed high pressure fluid into a downhole pressure pathway to drive the oil from the oil reservoir to the surface.

Description

REPRESSURIZATION OF OILFIELD RESERVOIR FOR ENHANCED HYDROCARBON RECOVERY

Field of the Invention:

This invention relates generally to the field of oil and gas production techniques. More particularly, this invention relates to a technique for enhancing the recovery of hydrocarbons from a well using naturally occurring high pressure fluid reservoirs for repressurizing a hydrocarbon reservoir.

Background of the Invention:

As is well known, oil and other hydrocarbons are produced from a well by achieving a flow of the hydrocarbons from downhole to the surface, typically through production tubing This flow of oil occurs due to a driving force which pushes or forces the oil from downhole to the surface. In general, the oil is driven to the surface either by high pressure associated with the downhole oil reservoir (a so-called depletion drive reservoir) or by high pressure downhole water sources (a so-called water driven reservoir). In either the depletion drive or water driven reservoirs, the recovery of

produced oil decreases with time. That is, as oil is produced at the surface, the downhole pressure steadily decreases until the pressure of the downhole oil reservoir is equal to the surface pressure. Of course, at that point, the oil remaining downhole will no longer flow to the surface due to the lack of an appropriate pushing force (i.e.,

pressure differential). Indeed, many oilfields leave over sixty-five (65%) percent of their oil in place in

the ground due to the aforementioned lack of reservoir pressure available to remove

the oil from the downhole formation In an effort to alleviate this problem, well known

enhanced recovery techniques include injecting fluids from the surface downhole (via

an injection well) The injected fluids typically comprise pressurized natural gas or

treated water or steam An example of an injection well of this type is disclosed in

U S Patent 4, 199,028 wherein enhanced recovery of hydrocarbons is accomplished

through the re-introduction (i e , injection) of natural gas containing acquifiers

While injection wells do offer enhanced recovery of oil reservoirs, such wells

suffer from certain drawbacks and deficiencies including high cost, lack of

compatibility between the driving and driven fluids, and safety concerns

Summary of the Invention

The above-discussed and other drawbacks and deficiencies of the prior art are

overcome or alleviated by the improved enhanced recovery technique of the present

invention In accordance with the present invention, energy (that is, stored pressure)

from naturally occurring high pressure fluid reservoirs (typically natural gas reservoirs)

located above or below the hydrocarbon reservoir to be produced are used to supply a

pressure source for driving oil or other hydrocarbons to the surface This is

accomplished by accessing the high pressure reservoir and directing the accessed high

pressure fluid into a downhole pressure pathway to drive the oil from the oil reservoir

to the surface In general, the present invention achieves this downhole pressure

pathway by using at least two boreholes (i.e , a "production" borehole and a "self- injecting" borehole) both of which are open to the oil reservoir and only one of which (the "self-injecting" borehole) is open to the high pressure fluid reservoir. In this way, high pressure fluid is directed from the high pressure fluid reservoir through the self- injecting borehole to the oil reservoir whereupon the high pressure fluid will drive the oil from the oil reservoir into the production borehole and then to the surface.

The present invention may be utilized in a wide variety of wellbore configurations including wellbores associated with separate wells and wellbores associated with multilateral wells.

The present invention offers significant features and advantages over prior art enhanced recovery techniques. The present invention takes advantage of the energy residing in the numerous, naturally occurring high pressure reservoirs which are not typically produced due to poor economics and uses this energy to repressurize a subpressurized oil reservoir so that more oil is recovered and produced from this reservoir. In addition, the enhanced recovery technique of this invention will be of lower cost relative to conventional injection wells and will not have the safety and fluid compatibility issues associated with injection wells.

The above-discussed and other features and advantages of the present invention will be appreciated and understood by those sl illed in the art from the following description and drawings. Brief Description of the Drawings

Referring now to the drawings wherein like elements are numbered alike in the

several FIGURES

FIGURE 1 is a schematic view of the enhanced hydrocarbon recovery

technique of the present invention employed with two wells,

FIGURE 2 is a schematic view of the enhanced hydrocarbon recovery

technique of the present invention employed in a multi-lateral well, and

FIGURE 3 is a graph of production rate versus time for a conventional

depletion drive reservoir and a well system employing the enhanced recovery technique

of the present invention

Description of the Preferred Embodiment

Referring first to FIGURE 1, an oil and gas well field is shown for producing

hydrocarbons This field includes a production well 10 comprised of a borehole 12

lined with casing 14 A portion of the casing 14 has been perforated in a known

manner to produce perforations 16 for the purpose of producing hydrocarbons from a

reservoir 18 These hydrocarbons are produced by flowing through perforation 16 and

then into production tubing 20 and upward to the surface as indicated by the arrow 22

As is well known, when reservoir 18 is initially produced, the reservoir is under

significant pressure which forces or drives the hydrocarbons from the reservoir into the

well and onward through the production tubing 20 However, as the hydrocarbons are

produced, the pressure in the reservoir 18 steadily decreases until the pressure

equalizes with the atmospheric pressure, (that is, there is no more pressure differential or Dp) so that the hydrocarbons from reservoir 18 cease flowing into well 10 and

therefore can no longer be produced.

In accordance with the present invention, underground, naturally occurring

high pressure fluid reservoirs such as shown at 24 and 26 are tapped or accessed and

are directed through an appropriate pathway so as to provide repressurization to oil

reservoir 18 which in turn will drive additional hydrocarbons from reservoir 18 into

production well 10. Typically, the naturally occurring high pressure fluid in reservoirs

24 and 26 will consist of natural gas or other trapped hydrocarbon gases. It will be

appreciated however, that the high pressure fluid may also be entrapped steam or other

fluids. While the pathway for directing the naturally occurring high pressure fluids to

the hydrocarbon reservoir 18 may be constructed in a variety of manners, in the

FIGURE 1 embodiment, a second well 28 is formed and appropriately cased with

casing 30. Casing 30 is then perforated to define perforations 32 and 34 which allow

the high pressure driving fluid from reservoirs 24 and 26 to escape therefrom and enter

well 28. It will be appreciated that in order for the high pressure fluids to be directed

in a pathway for providing repressurization to hydrocarbon reservoir 18, well 28 must

be capped (such as shown by the valve 36) to avoid any escape of the high pressure

fluids to the surface. In this way, the high pressure fluid will be driven to a low

pressure area which is defined by the hydrocarbon reservoir 18 (which of course will

have been perforated as is shown at 38). The high pressure fluid (natural gas) from

reservoirs 24 and 26 will apply a driving force (indicated by the gas phase 40 atop

reservoir 18) and thereafter drive the hydrocarbons from reservoir 18 into production

well 10. In effect, the hydrocarbon reservoir 18 will have been repressurized by accessing the high pressure fluid (natural gas) which is naturally occurring in the

ground so as to form a pressure loop between the high pressure reservoir, the

hydrocarbon reservoir to be produced and the production well

In contrast to the surface injection methods associated with prior art enhanced

recovery systems, the present invention can be viewed as a self-injected system where

natural gas (or other high pressure fluid) already present downhole is self-injected into

an appropriate hydrocarbon reservoir so as to repressurize that hydrocarbon reservoir

and enhance recovery.

The present invention may also be associated with a variety of known

monitoring and control instrumentation and sensors including those monitoring and

control systems disclosed in U S Patent No 5,597,042, which is assigned to the

assignee hereof and incorporated herein by reference As an example, as shown in

FIGURE 1 , a downhole temperature sensor 42, pressure sensor 44 and flow sensor 46

is associated with the self-injecting borehole 28 to monitor parameters related to the

flow of the high pressure fluid into the hydrocarbon reservoir 18 Information from

sensors 42, 44 and 46 can be sent to a surface or sub-surface monitoring and/or

control system 48 either by a cable 50 or through an appropriate wireless

communications means (such as acoustic telemetry through the casing 30) By

communication of relevant parameters to the monitoring system 48, the high pressure

fluid flow can be adjusted automatically or manually, as appropriate. In addition,

important information relative to downhole conditions can be determined For

example, monitoring pressure/flow into the hydrocarbon reservoir versus pressure/flow

out of the reservoir can determine the volume of the reservoir Referring now to FIGURE 2, an alternative embodiment of the present invention is shown in conjunction with a lateral or multilateral well 52 In the FIGURE 2 embodiment, well 52 includes well casing 54 and production tubing 56 which is positioned above a production zone for producing hydrocarbons from reservoir 58 Perforations 60 are provided to access hydrocarbons from reservoir 58 and allow for production of hydrocarbons (such as oil) through production tubing 56 and then to the surface as indicated by arrows 62 In accordance with the present invention, a lateral borehole 64 is drilled from primary borehole 52 Lateral borehole 64 will intersect both the hydrocarbon reservoir 58 and one or more naturally occurring high pressure fluid reservoirs 60 Typically, lateral 64 will be cased and appropriate perforations 66 and 68 provided so as to access high pressure fluid 60 and hydrocarbon reservoir 58, respectively It will be appreciated that in order to establish the closed pressure loop and direct the high pressure fluid from reservoir 60 to hydrocarbon reservoir 58 along an appropriate pressure pathway defined by lateral 64, lateral 64 needs to be shut-in from primary borehole 52 (such as shown by valve 70) so that the appropriate pathway is achieved Of course, high pressure fluid from reservoir 60 will seek the path of least resistance (that is, the lower pressure area) so that the higher pressure fluid will flow to reservoir 58 and provide a driving force to the hydrocarbons residing in reservoir 58 from reservoir 60 forcing such hydrocarbons into the producing zone of production well 58 and then into the production tubing 56 to the surface Of course, well 52 may

include any number of lateral boreholes which may intersect one or more of the same

or other high pressure (natural gas) reservoirs Well 52 may also be associated with a

control and monitoring system of the type discussed with regard to FIGURE 1 FIGURE 3 is a graph of hydrocarbon production rate versus time comparing prior art depletion drive reservoirs with a well constructed in accordance with the present invention. As shown in FIGURE 3, in a prior art depletion drive reservoir, the rate of production will increase steadily from the time the hydrocarbon is first produced until the time that the pressure in the reservoir being produced equalizes atmospheric pressure. Thus, the prior art not only leads to a situation where production totally ceases when there is an equalization in pressure, but the prior art also leads to a situation wherein the rate of production steadily decreases with time leading to inefficiencies and much higher operating costs. In contrast, and in accordance with the present invention, by tapping or accessing a high pressure fluid reservoir such as is normally located in and around production wells, the production rate of hydrocarbons may be maintained at a steady state rate since there will be a continuous driving force against the reservoir until the naturally occurring high pressure fluid has been depleted. In a preferred embodiment, multiple high pressure gas reservoirs can be accessed in sequential time periods so that the reservoir will have a constant driving force until such time that either the hydrocarbon reservoir is depleted or there are no longer any naturally occurring high pressure fluid deposits which are accessible. Referring to FIGURE 3, it can be seen that in accordance with the present invention, not only is the production rate maintained at a steady state and high rate, but the time of producing a particular

hydrocarbon reservoir is substantially decreased leading to improved efficiency and

lower cost operations. More specifically, the method of this invention allows for

accelerated production of the hydrocarbons in that the total hydrocarbons normally recovered in a depletion drive well is recovered much earlier at a time Ti (using the present invention) rather than a later time T₂ (using conventional depletion drive techniques). In addition, the method of this invention allows for incremental (that is, increased) recovery (assuming "A" equals "B") above that of a normal depletion well 5 of hydrocarbons "C" by supplying pressure from another zone as discussed in detail above.

The present invention is particularly important when considering the fact that many of the naturally occurring reservoirs such as shown at 24, 26 and 60 in FIGURES 1 and 2 are not normally accessed or tapped since many are too small to be i o commercially viable and/or there are many regions in the world where natural gas (the primary constituent of the naturally high pressure reservoirs) is not an energy source which is used in that particular geographic location. The present invention thus provides maximum recovery of a particular hydrocarbon reservoir at a lower cost and over a decreased time period. This maximized recovery is accomplished without the

15 high cost associated with injection wells as well as the compatibility issues associated with the fluids being injected by the injection wells and the safety concerns that are associated with injection wells.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit

20 and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

Claims

What is claimed is

CL.AIM 1 A method of recovering hydrocarbons from a downhole hydrocarbon reservoir, comprising accessing at least one downhole pressurized fluid reservoir, and 5 directing pressurized fluid from the pressurized fluid reservoir along a first downhole pathway to the downhole hydrocarbon reservoir wherein the hydrocarbons from the hydrocarbon reservoir are driven by the pressurized fluid to the surface along a second downhole pathway

l o CLAIM 2 The method of claim 1 wherein said pressurized fluid comprises natural gas

CLAIM 3 The method of claim 1 wherein said first downhole pathway comprises a borehole in a first well, and 15 said second downhole pathway comprises a borehole in a second well

CLAIM 4 The method of claim 1 wherein: said first and second downhole pathways comprise boreholes in a multilateral well

20 CLAIM 5 The method of claim 1 including

sensing at least one parameter associated with said pressurized fluid reservoir,

and

performing at least one of monitoring and control of said hydrocarbon recovery

in response to said sensed parameter

CLAIM 6 The method of claim 5 wherein

the sensed parameters are selected from the group consisting of downhole

temperature, pressure and flow

CLAIM 7 The method of claim 5 including determining the volume of the hydrocarbon reservoir based on said sensed

parameter

CLAIM 8 The method of claim 1 including multiple downhole pressurized fluid

reservoirs and including

accessing said multiple reservoirs

CLAIM 9 The method of claim 8 including

sequentially accessing each of said multiple reservoirs so as to maintain a

preselected driving force against said downhole hydrocarbon reservoir CLAIM 10 A system for recovering hydrocarbons from a downhole hydrocarbon reservoir, comprising a first downhole pathway which directs pressurized fluid from a downhole pressurized fluid reservoir to the downhole hydrocarbon reservoir, and 5 a second pathway communicating between the downhole hydrocarbon reservoir and the surface wherein hydrocarbons from the hydrocarbon reservoir are driven by the pressurized fluid to the surface along the second pathway

CLAIM 11 The system of claim 10 wherein l o said pressurized fluid comprises natural gas

CLAIM 12 The system of claim 10 wherein said first downhole pathway comprises a borehole in a first well, and said second downhole pathway comprises a borehole in a second well

15

CLAIM 13 The system of claim 10 wherein said first and second downhole pathways comprise boreholes in a multilateral well

CLAIM 14. The system of claim 10 including: at least one sensor sensing at least one parameter associated with said pressurized fluid reservoir; and a monitoring and control system associated with said sensor for monitoring and controlling hydrocarbon recovery in response to said sensed parameter.

CL.AIM 15. The system of claim 14 wherein: the sensed parameters are selected from the group consisting of downhole temperature, pressure and flow.