2. Description of the Prior Art Economic circumstances and trends have obliged companies and technologists involved in the exploration for and production of petroleum to seek sound methods of cost reduction. One such major cost is that incurred in the very drilling of the wells themselves through which petroleum is produced, or water injected to maintain the reservoir pressure that drives petroleum 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 though which suitably treated water is forced under pressure into the aquifer supporting the oil reservoir thereby maintaining the reservoir pressure as petroleum is 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.

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 an oil-bearing 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 EMBODIMENT Figure 1 is a sectional elevation of a well 1 drilled through an oil-bearing 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 ingress under applied pressure of injection water into the aquifer beneath the petroleum reservoir. Production perforations 5 in the oil-bearing formation permit inward flow of petroleum 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 dependant upon factors pertaining to each individual well particularly; location of oil-water contact 7, angle of deviation of well, location of faults and fractures, 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 tubing string 13, the downhole extremity of which seals in the receptacle. The sealing receptacle may accommodate movement of tieback tubing 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 tubing-downhole liner combination develop a leak. Annular safety valve 16 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 tubing 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 after the production tieback-casing hanger is set in the Xmas tree lower bowl. A bowl protector 29, which is removed after completion of all drilling, protects the 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 10-3/4 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 16,500ft measured depth (c.

4800 metres). It is probable that the well would be highly deviated and the liner program could be adjusted to 7-5/8 in. x 5 in. (193.7mm x 127mm) with the packer being set immediately above the crossover. The 5 in. section would be the conduit for injection fluid from the 5 in. (127mm) tubing, the well now being of the monobore type for operational advantage.

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 development program, ensures maintenance of reservoir pressure at 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, where a field development program envisaged twelve production wells and eight injection wells would, using the PI-system, only require twelve wells. The cost savings in the drilling program are in the order of 40% for such a field.

The 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 tubing 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. It is further contemplated; i) that injection fluids may be injected directly into the hydrocarbon reservoir instead of the supporting aquifer substantially where the permeability of the aquifer formation is very low relative to that of the reservoir formation, ii) that gas lift operations may be effected by including suitable mandrels in the tieback tubing string allowing gas to enter the active annulus from the hitherto passive annulus, and iii) that suitable sub-surface pumps using the injection fluid as the drive fluid passing through to the aquifer, could be employed to enhance the production of hydrocarbons.