Abstract

This paper presents a method to characterize the strength of water drive in oil reservoirs. A parameter called characteristic coning time (tcc) is defined for the purpose. The tcc is proportional to oil zone porosity, oil viscosity, total compressibility of the oil zone, oil and water-oilcontact (WOC) zone thickness, and inversely proportional to the vertical permeability of the WOC zone. It represents the relative resistance of the oil zone to water coning tendency. The value of the tcc can be determined from pressure transient test data analysis. On the basis of case studies, it has been found that the value of the t ccis less than 6 hours for a bottom-water drive of medium strength, and 8 to 50 hours for strong to weak edge-water drive oil reservoirs. It is this characteristic coning time (tcc), rather than vertical permeability alone, that should be used for oil recovery computations in development of water-drive oil reservoirs with horizontal wells.

Introduction

A large number of developed oil fields in the world have water drive. Oil recovery under bottom water presence is characterized by the low efficiency of recovery owing to high water cut. Water cones largely dictate oil recovery in a typical bottom-water situation. Major parameters influencing coning include mobility ratio, producing rate, completion interval, and reservoir anisotropy.

Several techniques have been used by the industry to prevent water breakthrough due to coning. These methods include: keeping production rates below the critical value, perforating the well with the optimum interval fraction, 1 creating a water-blocking zone around the well by injecting cross-linking polymers or gels, 2,3,4 downhole water-sink5, and downhole separation technology. 6 Successfulness of all these methods depends on the strength of the bottom water drive.

Horizontal wells have been used to ease the water coning problems in many fields in the past decade. Use of horizontal wells to improve oil recovery for thin oil columns affected by active water support has been demonstrated in various fields. 7,8 Oil recovery with horizontal well development depends on horizontal well length, withdrawal rate, and well acreage. 9 These parameters should be designed based on the strength of the bottom-water drive.

Vertical permeability is a big concern in water coning systems. This permeability can be estimated on the basis of data from pressure transient tests in water coning reservoirs. The vertical permeability can be better determined from vertical interference test. 10 However, the vertical permeability alone does not represent the strength of bottom water drive. Other factors, including pay zone thickness, should also be considered. A lumped parameter accounting for all these reservoir natures is required to characterize the strength of water drive. This paper presents a better approach to characterization of water drive in oil reservoirs based on pressure transient data.

METHOD DESCRIPTION

It can be assumed that, in water-drive oil reservoirs, horizontal flow dominates in the oil zone and vertical flow exists in the water-oil-contact (WOC) zone. When a radial flow prevails in the oil zone, the pressure in the oil zone is supported by the vertical flow in the WOC zone.

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