Interference Well Testing to Predict Early Water Breakthrough in Naturally Fractured Reservoirs

Abstract

Secondary recovery projects in dual porosity reservoirs with high transmissivity fractures must be carefully managed to avoid early water break-through into producing wells. If water production does begin in an injector/ producer system, the origin of the water channelling must producer system, the origin of the water channelling must be identified so that remedial changes can be made in the injection-production schedule. This paper shows that pressure interference testing between injection and producing wells, is a better method for locating problem injectors than the alternative technique using radioactive tracers. Furthermore, information derived from interference tests can be used to prevent water break-through in waterflood projects. Calculations of the interference response before and after break-through are shown in an example. A simple graphical method is used to illustrate water advancement. Introduction Secondary recovery in naturally fractured reservoirs is more difficult to achieve than in non-fractured reservoirs. The traditional concepts of water-flooding in single porosity reservoirs must be replaced by more advanced porosity reservoirs must be replaced by more advanced methods designed to impede the rapid advance of water towards the producing wells. Techniques currently used in fractured reservoirs are based on the phenomenon of imbibition in which water displaces oil from the matrix by means or capillary forces. It is only possible to accomplish a satisfactory conclureion of a water flood project, in a commercially reasonable time, if specifically determined petrophysical and capillary conditions prevail. Controlling the advance of the water through the fracture network is critical. Slowing down water break-through into the production wells can be achieved by regulating the flow rate of both injected and produced fluids. The most frequently used techniques for observing water advance in conventional reservoirs employ either radioactive tracers in the injected water, or the analysis of fall-off, build-up and interference tests. The use of radioactive tracers has also been applied in fractured reservoirs. However, it is impossible with the tracer method to anticipate where the water will break through. It can be therefore too late to take steps to control and optimize the water flood process. N matter which method is used, the main problem is the lack of reliable data, especially concerning the distribution of transmissibility or the fracture network. In this paper we demonstrate that interference tests have an advantage over tracer method. They can be effectively used for data acquisition and supervision of water advance through the fracture network. Experience has shown that the channelling of water in non-fractured reservoirs can be identified by means of interference tests of short duration, that is, usually less than three days for well-spacings of the order of three hundred metres.