TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA variable block size model is presented to study naturally fractured reservoirs, taking into account continuous uniform, exponential and normal distributions of matrix block sizes. The work is based on the transient matrix flow and is an extension of the analytical model formulated by Cinco, Samaniego and Kuchuk for a finite number of block sizes. Several new groups of the pressure derivative function were developed to study the features of their graphical representation in log-log scales, to determine the matrix block size distribution corresponding to a certain shape of the curves. These groups are a better diagnostic tool than the conventional pressure derivative function, specially for pressure data on the transition period between the fracture storage dominated flow and the matrix pseudosteady-state flow. The solutions are obtained in the Laplace space, involving an integral which is numerically solved.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA procedure is developed to estimate the variability of the block size from pressure drawdown data, applying an optimization technique; in other words, the inverse problem is solved.The confidence of the estimated results is evaluated by applying the confidence limits theory and the procedure is illustrated using data from a simulated pressure drawdown test. On the other hand, the estimated parameters of the variability of the block size were used to obtain results for the pressure derivative function; the agreement with the simulated pressure drawdown data was very good.Finally, a theory was developed and applied to obtain the imbibition flow rate in water wet naturally fractured reservoirs, using the estimated parameters of the variability of the block size, in an oil recovery process by water injection.

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