Abstract

Reservoir rock permeability is a very important parameter in petroleum production since this parameter plays a leading role in determining reservoir productivity. Despite the knowledge that reservoir rock permeability is reduced to a certain degree by the decline in reservoir pressure (i.e. increase in effective overburden stress), it is a common practice to assume the parameter as constant throughout the reservoir lifetime. Lack of information concerning the relationship between rocks forming the reservoir and the permeability reduction is often considered as a factor behind this malpractice. It is the purpose of this paper to present an alternative method that provides the information.

The method is essentially based on a work presented by Dobrynin (1962) who derived a relationship between pore compressibility and reduction in rock permeability. The permeability reduction is derived as a function of effective overburden pressure. Considering the relationship between acoustic P- and S-wave velocities and pore compressibility, it becomes possible to integrate laboratory acoustic measurements to Dobrynin's work. Conceptually, integration of the two concepts promises an efficient method for predicting permeability reduction in a way that it can be applied to any results of acoustic measurement on dry core samples, such as for determination of rock elastic properties, as long as the test is conducted under various overburden pressures. This integration, as well as laboratory tests for validation serve as the main thrust of the work presented in this paper.

The laboratory works are basically divided into two parts, acoustic and permeability measurements. In conducting them five sandstone and five limestone samples taken from two producing fields in Central Sumatera and North Sumatera, respectively, are tested under various effective overburden pressures. Results of the permeability measurements have shown reasonable agreements between calculated and observed permeability reduction. The overall results have also indicated the possibility of using the method for distinguishing different rock types. The general conclusion that can be derived from the works points out the reliability of the integrated method as an alternative method for estimating reservoir rock permeability reduction due to decline in pore pressure, apart from the possibility of using it through seismic survey data.