GB2311312A

GB2311312A - Well system

Info

Publication number: GB2311312A
Application number: GB9605784A
Authority: GB
Inventors: Allan Cassells Sharp
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-03-19
Filing date: 1996-03-19
Publication date: 1997-09-24
Anticipated expiration: 2016-03-19
Also published as: GB2311312B; GB9605784D0

Abstract

A method and apparatus are provided for enabling the simultaneous but separate production of hydrocarbons from selected sectors of a reservoir or of a contiguous reservoir in a single well, or in the simultaneous production of the hydrocarbons from the reservoir and water injection into an aquifer thereunder in a single well. Each of the produced hydrocarbon streams or both the produced hydrocarbons and injected water flow through the Xmas tree. A tubular string 8 is hung from a hanger which is landed on a profile disposed internally in a central section of the tree body while a tieback casing 13 is hung from a hanger which is landed on a profile disposed internally in a lower section of the tree body. The tubular string 8 is used either for conveying hydrocarbons to the surface or for conveying injection water down from surface into the aquifer. A casing string 10 is provided with a packer 9 near its downhole extremity and with perforations 4 into the aquifer. The tieback casing 13 runs to a sealing receptacle 12 at the top of the casing 10 or may, alternatively, have a packer set near its downhole extremity. The casing 10 is additionally perforated below the sealing receptacle at a selected position 5 in the hydrocarbon reservoir above the lower packer position. In both the tubular string and casing string may be sited safety valves which close the well system in the event of an emergency.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the simultaneous injection of water into an aquifer beneath a hydrocarbon reservoir and the production of fluids therefrom through one single drilled well completed with casing and tubing.

2. Description of the Prior Art Economic circumstances and trends have obliged companies and technologists involved in the exploration for and production of hydrocarbons to seek sound methods of cost reduction. One major cost is that incurred in the very drilling of the wells themselves through which hydrocarbons are produced, or water injected to maintain the reservoir pressure that drives hydrocarbons towards the surface.

Prior art methods for reservoir exploitation require drilling of a number of wells of which there are two kinds; production wells, through which hydrocarbons are conveyed to surface for subsequent processing, marketing and sale; and injection wells through which suitably treated water is forced under pressure into the aquifer supporting the oil reservoir thereby maintaining the reservoir pressure as hydrocarbons are being removed. Production wells are often subsequently converted to injection wells as reservoir conditions change in the course of exploitation.

The importance of maintaining reservoir pressure is that it maximises the yield of hydrocarbons.

Should this pressure be allowed to fall through short-sighted overproduction, flow rates to surface shall be reduced and gas shall evolve from the liquids. These gases are more mobile than the heavier and more valuable medium hydrocarbons and shall be produced preferentially to the liquids which may be left in the reservoir.

SUMMARY OF THE INVENTION A method and apparatus are provided for enabling the simultaneous injection of treated water into an aquifer and production of the hydrocarbons from the reservoir immediately above. A Xmas tree is provided to support two tubular strings running from set positions in the well to allow conveyance of fluids as the well conditions and operations require. The inner string, known as the "tubing", runs from the Xmas tree to a cemented casing strmg, kno vn as a "liner", which is entirely set downhole.

This tubing is provided with a packer near its downhole extremity further down from which are perforations'through the liner into the aquifer or, alternatively, into another or even the same reservoir. The tubing is for conveying injection water down from surface and into the aquifer, or hydrocarbon out of another or same reservoir to surface. The outer stnng ,known as "tieback casing", is provided to run from the Xmas tree at surface to a sealing receptacle at the top of the liner or may, alternatively, have a packer set near its downhole extremity. The liner is additionally perforated below the sealing receptacle at a selected position in the hydrocarbon reservoir above the lower packer position. This outer string conveys hydrocarbons from the reservoir to surface and through the Xmas tree to process plant. In both tubing and tieback casing, and the surface casing string immediately larger than the tieback casing, may be sited safety valves which close the well system in the event of an emergency.

Henceforth in this document, the section void inside the inner string shall be termed the "bore"; the section void between the inner string and the outer string shall be termed the "active annulus"; and the sectional void between the outer string and the well casing shall be termed the "passive annulus".

BRIEF DESCRIPTION OF THE DRAWINGS The novel features of this invention are set against the background of its applications in the detailed description which follows and in conjunction with accompanying drawings wherein: Figure 1 depicts a sectional elevation of a well drilled through a hydrocarbon reservoir into the aquifer immediately beneath.

Figure 2 depicts a producer-injector downhole completion highlighting all principal technical and geological features.

Figure 3 depicts the Xmas tree and associated equipment which enables simultaneous production and injection operations.

Figure 4 depicts the configuration of equipment for drilling operations through the Xmas tree.

DETAILED DESCRIPTION OF THE PREFERRED EMBODINIENT Figure I is a sectional elevation of a well I drilled through a hydrocarbon reservoir 2 into the aquifer 3 immediately beneath. The well has been shown as deviated as such producer-injector wells shall typically seek to maximise the distance between the injection perforations and production perforations to prevent or minimise the effect known as "coning", a phenomenon where water is attracted to production perforations thereby permaturely increasing the water cut of produced fluids. The deviation, profile. and perforation positions shall be established by reservoir experts using geological and petrophysical data.

Injection perforations 4 allow entry under applied pressure of injection water into the aquifer beneath the hydrocarbon reservoir. Production perforations 5 in the oil-bearing formation permit inward flow of hydrocarbon into the active annulus and up to the Xmas tree. For brevity. the illustration depicts an offshore platform 6 as the origin of the well.The oil-water contact 7 is the interface between the hydrocarbon reservoir and the aquifer.

Figure 2 is a depiction of a producer-injector completion. Injection tubing string 8 extends from the Xmas tree (not shown) through packer 9 into the area of the well in the proximity of the injection perforations 4. The packer ensures that injected and produced fluids are isolated from each other. Further up the well are production perforations 5 through which hydrocarbons enter the active annulus. The positions of both injection and production perforations shall be dependent upon factors pertaining to each individual well particularly, location of oil-water contact 7, angle of deviation of well, location of faults and fractures,and permeability of formation. A downhole liner string 10 extends from the bottom of the well to a position typically 100-200 metres measured length above the shoe of the surface casing.

The liner string, which is fully encased in cement 11, has a sealing receptacle 12 located at and attached to its upper end. Into this receptacle is run a tieback casing string 13, the downhole extremity of which seals in the receptacle. The sealing receptacle may accommodate movement of tieback casing due to thermal and pressure effects whilst maintaining a fully functioning seal. An annular safety valve 14, installed, if required, in the surface casing 15 at a position above the level of cement, provides protection for the passive annulus in which pressure may change should the tieback casing-downhole liner combination develop a leak. Annular safety valve 16, installed, if required, in the tieback casing, ensures that hydrocarbon production ceases in the event of an emergency because it closes the active annulus. Sub-surface safety valve 17 installed, if required. in the tubing string also closes in the event of an emergency and prevents injection of water into the aquifer or. alternatively, halts production of hydrocarbons from a hydrocarbon reservoir.

Figure 3 depicts the Xmas tree arrangement for a completed well. The Xmas tree block 18 has two profiles, lower and upper, into which are landed production tieback-casing hanger 19 and injection-tubing hanger 20 respectively. The hangers have their associated seals 21 and 22 to ensure isolation of conveyed fluids. In the injection tubing hanger is installed a plug 23 which is removed in the event of workover. Crown plugs 24 are installed in dedicated profiles in the upper end of the tree block. Both the upper and lower external ends of the tree block have profiles machined to enable mechanical connection to a blowout preventer (BOP) 25 and the wellhead 26 respectively. Valves are essential elements of the system.

Mechanically attached to, or integral to, the tree block are valve blocks 27 and 28 allowing and controlling flow of production fluids up or injection fluids down the tubing bore and production fluids up the active annulus.

Figure 4 depicts the situation during drilling operations through the Xmas tree for the section of hole for the liner. A bowl protector 29, which is removed after completion of all drilling, protects all sealing surfaces for the subsequently installed hangers. Prior art methods have installed the wellhead and surface casing strings.

An example of a casing program to which this invention is applicable would be a surface string of 103/4 in. x 9-5/8 in. (273mm x244.5mm)to 13,000 feet (c. 4000 metres) measured depth, a downhole liner of 7 in. (177.8mm) from 12700ft to 17,000ft measured depth (c. 3900 metres to 5000 metres) and a tieback string of 7.5/8 in.(193.7mm). The injection tubing of 5 in. diameter (127mm) would run to 16,500fit measured depth (c. 4800 metres). It is probable that the well would be highly deviated and the liner program could be adjusted to 7 in. x 5-1/2 in.(l77.8mm x 139.7mm) with the packer being set immediately above the crossover. The 5-1/2 section would be the conduit for injection fluid from the 5 in.(l27mm) tubing. The perforation program for a highly deviated well may require the injection perforations to be undertaken by coiled tubing.

This present invention provides several advantages over the prior art. It is current practice that dedicated injection wells be drilled to support a hydrocarbon reservoir where a suitable aquifer is present. There are several examples where short-sighted and precipitous production of reservoirs has reduced the reservoir pressure below viability with subsequent costly measures taken to re-pressurise through water injection. This invention, if employed at the outset of a reservoir development program, ensures maintenance of reservoir pressure with no additional drilling costs. This invention is applicable to those reservoirs where water injection is feasible and desirable on the basis of geological and petrophysical factors. For example, a field development program envisaging twelve production wells and eight injection wells would, using the production-injection (PI) system, only require twelve wells. The cost savings in the drilling program are in the order of 40% for such a field. The production-injection Xmas tree enables implementation of PI-wells, and is indispensable to the minimisation of capital and operating costs.

Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. Examples of such modifications include, inter alia; removing the upper shoulder in the Xmas tree and landing the tubing hanger on the tieback casing hanger, splitting the tree block into two separate spools for hanging equipment. It is therefore contemplated that the appended claims will cover any such modifications or embodiments that fall within the scope of the invention.

Claims

I claim 1. An apparatus for simultaneous water injection and hydrocarbon production in a single drilled well for employment onshore, from offshore platforms, or subsea, comprising in combination: a production-injection Xmas tree set on and mechanically attached to the prior art wellhead; and a dedicated completion program for simultaneous injection and production.

2. The apparatus according to claim 1 wherein the Xmas tree comprises: a profile disposed internally in the lower section of the Xmas tree onto which is landed, secured and sealed a casing hanger, through which attached casing string shall pass produced fluids from the hydrocarbon reservoir; a profile disposed internally in the central section of the Xmas tree onto which is landed, secured and sealed a tubing hanger through which attached tubing string shall pass downhole injection fluids or, from downhole, shall pass produced fluids; and two valve blocks each externally attached by mechanical means to the Xmas tree at the principal bores through the wall of the Xmas tree; one to the void immediately above the upper hanger and below the crown plug(s) - the upper branch - through which pass injection fluids going downhole, or produced fluids from downhole to processing; and the other - the lower branch - to the void between the two respective hangers through which pass produced fluids from the reservoir.

3. The apparatus according to claim 1 wherein the completion program comprises; a production-tieback casing string run from the lower hanger in the Xmas tree to a tieback receptacle effecting a seal from the tieback casing to the liner; a liner set in cement through the hydrocarbon-bearing formation to a suitable position in the supporting aquifer thereunder; a tubing string run from the upper hanger in the Xmas tree to a packer set between the upper and lower perforations; and a packer to isolate the liner and tieback casing annulus i.e. the active annulus, from the tubing bore; with perforations through the liner into the hydrocarbon reservoir and separate perforations through the liner into the aquifer, both sets of perforations crucially being distinct and separate through isolation by the packer; 4. A method for reservoir exploitation comprising the steps beyond the prior art methods of: (a) perforating the liner in two distinct zones, these being in the hydrocarbon reservoir and in the supporting aquifer, at suitable periods in the well development; then (b)running and setting production-tieback casing into sealing receptacle at top of liner to obtain sealed conduit from reservoir to Xmas tree; then (c) running and setting tubing and packer into liner below the upper perforations to obtain a sealed conduit for injection fluids in the aquifer; then (d) producing hydrocarbons up the active annulus and the tubing bore simultaneously or selectively or, injecting treated water down the tubing bore and producing hydrocarbons up the active annulus simultaneously or selectively.

Amendments to the claims have been filed as follows 1. A Xmas tree for setting on, and mechanical attachment to, a wellhead when used in the simultaneous but separate production of hydrocarbons from selected sectors of a reservoir,or of a contiguous reservoir, in a single drilled well, or in the simultaneous production of hydrocarbons from said reservoir and water injection into an aquifer thereunder in a single drilled well, characterised by each of the produced hydrocarbon streams or both the produced hydrocarbons and injected water flowing through the Xmas tree, said Xmas tree being of surface or subsea types and having intemal profiles for reception of hanging and sealing equipment for tubing strings.
2. The Xmas tree according to claim 1 wherein a profile is disposed internally in the lower section of the Xmas tree body onto which profile is landed, secured, and sealed a hanger for an outer tubing string.
3. The Xmas tree according to claim 1 wherein a profile is disposed intemally in the central section of the Xmas tree body onto which profile is landed, secured, and sealed a hanger for an inner tubing string.
4. The Xmas tree according to claim 1 wherein a profile is disposed internally in the upper section of the Xmas tree body onto which profile is landed, secured, and sealed one or more plugs or capping mechanisms.
5. The Xmas tree according to claim 3 wherein the inner tubing hanger is landed upon the previously installed outer tubing hanger.
6. The Xmas tree according to claim 2 wherein the outer tubing hanger hangs off a tubing string which is tied into a tieback receptacle sited at the upper extremity of a pre-installed liner.
7. The Xmas tree according to claim 1 wherein two separate valve systems are attached to, or integral to, the Xmas tree body.
8. The Xmas tree according to claim 7 wherein the upper valve block provides fluids with access to, or egress from, the void between the aforementioned plug(s) and the aforementioned inner tubing hanger and attached tubing string extending downhole.
9. The Xmas tree according to claim 7 wherein the lower valve block provides fluids with access to, or egress from, the void between the aforementioned inner tubing hanger and outer tubing hanger, and the annulus formed by the tubing strings attached respectively thereto.
10. The Xmas tree according to claim 1 in which the Xmas tree block is split into separate spools for hanging and sealing equipment.
Il. A method for simultaneous but separate production of hydrocarbons from sectors of a reservoir in a single drilled well; and for simultaneous or singular water injection into underlying aquifer and hydrocarbon production from said reservoir, also in a single drilled well, possibly the same well as the former after conversion of the lower sector from hydrocarbon production to water injection; comprising the following steps beyond the prior art of: a) drilling a targetted deviated primary well bore through a hydrocarbon reservoir.

b) obtaining the desired angle of deviation from the vertical, and continuing to drill primary well bore to sectors of reservoir offset from the first sector destined for production.

c) drilling further through the reservoir into the underlying aquifer.

d) running a liner in the wellbore and cementing in situ.

e) running and installing a tubing string which ties into a tieback receptacle sited at the upper extremity of the liner; the tubing being hung from its hanger installed in the lower profile of the Xmas tree.

f) perforating the liner at intervals as required by development program etc. said perforations being set at intervals in the reservoir.

g) running and installing a second tubing string and setting the packer below the existing perforations, this second tubing string being hung from the tubing hanger installed in the central profile of the Xmas tree.

h) further perforating in the liner section below the packer, these perforations being preferably sited in the underlying aquifer or just above the oil-water contact.
12. The method according to claim 11 wherein hydrocarbons are produced up the annulus formed by the inner and outer tubing strings with produced fluids flowing through the lower valve system.
13. The method according to claim 11 wherein hydrocarbons are produced up the tubing string with produced fluids flowing through the upper valve system.
14. The method according to claim 11 wherein injection fluids are pumped down the inner tubing string and injected into the aquifer.
15. The method according to claim Ii comprising simultaneously the methods of claim 12 and claim 13.
16. The method according to claim 11 comprising simultaneously the methods of claim 12 and claim 14.
17. The method according to claim 11 wherein the chronological order of events e) to h) is subject to variation.