Influence of Block Size on the Transition Curve for a Drawdown Test in a Naturally Fractured Reservoir

Abstract

Abstract Block size is considered one of the main parameters of a fractured rock reservoir. In one-phase flow, it controls the transition from the early stages of production to asymptotic behavior, and in two-phase wettingnonwetting displacement it controls the rate of production by imbibition. In nonwetting-wetting displacement it controls the pressure difference between the extremities of the block, which, provided that the threshold pressure is exceeded, permits displacement of the nonwetting fluid from the blocks into the fractures. Warren and Root1 relate the size of the block to the transient pressure variation from the first to the second straight line of the drawdown/buildup test. The numerical solutions presented in this paper for reservoirs with ordered fractures and blocks show that the drawdown pressures are not sensitive enough to the variation in sizes of the block. This means that a drawdown/buildup test cannot yield a solution unique for the size of the block. In most practical cases of the one-phase flow, transition to homogeneous behavior is reached in a relatively short time and not noticed, except in a narrow region around the well. Therefore, in terms of reservoir performance, only the homogeneous behavior is of practical interest and the size of the block is not a relevant parameter of the one-phase flow. It is believed that in two-phase flow the displacing fluid circumvents the blocks in groups rather than singly. If so, the individual size of the block is not a relevant parameter, even in two-phase flow.