A TRANSIENT PRESSURE STUDY ON THE INFLUENCE OF BLOCK SIZE FOR DRAWDOWN TESTS IN NATURALLY FRACTURED RESERVOIRS

Block size has been concluded to be one of the main parameters of a naturally fractured reservoir. In onephase flow, it controls the transition from the early fracture dominated stages of production to asymptotic composite (fracture plus matrix) behavior. Classical well test analysis in terms of a semilogarithmic graph of pressure vs time, has shown that drawdown pressures are not sensitive enough to the variation of block size method. This work presents the results of a study aimed at characterizing the naturally fractured reservoir by means of the transition period behavior of the pressure derivative function wD D ṕ t . Based on the numerical Laplace inversion results, it was observed that the pressure derivative function for the different block sizes can be correlated through a horizontal displacement of the D t axis, when multiplied by the ratio 2 2 ma ma ) h h ( , where hma1 and hma 2 are two block sizes satisfying hma 2 > hma1 . A discussion of previous results by Braester analyzed by means of the radial flow semilog graph of pressure vs time is also included. INTRODUCTION An important portion of the hydrocarbon and most geothermal reservoirs are contained in naturally fractured reservoirs (NFR ́s). Optimizing the exploitation of these reservoirs requires a complete description of the formation. A reliable characterization of a reservoir can be achieved through an integrated approach, combining data from geophysics, geology, well logging, well tests, core and fluid analysis, and well flow rates, which allows the estimation of reservoir geometry, reserves, and flow characteristics of the formation, among other factors. Well testing provides a useful tool to find the reservoirflow parameters and to detect and evaluate heterogeneities that affect the flow process in the formation. A NFR is created under highly complex subsurface conditions. Fig. 1.a shows a portion of the rock in these types of systems, that may include several elements, such as fractures and matrix, and in sedimentary formations, vugs. Reservoir fluids (and heat) are contained in both fractures and rock matrix; usually fractures act as channels to yield high well-flow rates. Reservoir studies must carefully consider these heterogeneities because they significantly affect the energy recovery. Because of the importance of NFR ́s, many publications are available that provide en understanding of the behavior of these systems. Currently, important advances in well test analysis allow a more accurate characterization of these systems, based on new flow models that better account for heterogeneities of fractured reservoirs. Block size has been concluded to be one of the main parameters of a NFR. In one phase flow it controls the transition from the early fracture dominated stages of production, to asymptotic composite (fracture plus matrix) behavior. Among other methods, block size can be estimated through well logging (Najurieta, 1980), well test analysis (Cinco Ley and Samaniego, 1982; Da Prat, 1990) and tracer tests (Ramírez et al., 1995). Classical analysis in terms of a semilogarithmic graph of pressure vs time, has shown that drawdown pressures are not sensitive enough to the variation of the block size (Braester, 1984). In 1984 Braester published an important paper which thoroughly addresses this type of transient pressure analysis. The author also presents drawdown solutions for variable block size conditions. The main conclusion of this paper based on numerical solutions to the flow problem in NFR ́s, with ordered fractures and blocks, computed through the Galerkin method, was already stated in this paragraph. The purposes of this paper is to present the results of a study aimed at characterizing NFR by means of the transition period behavior of the pressure derivative