A New Method To Measure Pore Volume Compressibility of Sandstones
Authors: Sampath K. 
Published: December 2007

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A new method has been developed to measure the pore volume compressibility of dry sandstone samples. The method has been applied to some low-permeability sandstones, and the results are presented along with description of the measurement technique.

Introduction

Extensive research has shown that the permeability of low-permeability sandstones is affected significantly by confining pressure. The porosity of these sandstones is another petrophysical parameter that is of interest in formation evaluation and reserve calculations. The common method reported in the literature for measuring pore volume reduction in sandstones requires the monitoring of fluid expelled from a liquid-saturated core sample as effective confining pressure is increased. A different method, applicable to dry samples, has been developed and applied successfully to measure the pore volume reduction in some low-permeability sandstones. The method can be thought of as a modification of the classical USBM technique.The low-permeability sandstones investigated are from the Mapco MINI RBU 11 - 17F (Section 17, T10S, R20E) and the Uinta basin, UT. The samples range from a depth of 8,234 to 8,366 ft (2509.8 to 2549 m). They are petrographically similar to other low-permeability petrographically similar to other low-permeability sandstones of the Late Cretaceous age from the region. They are fine to very fine grained, moderately well sorted, and rich in detrital quartz and rock fragments. Detailed mineralogical description of the samples has been presented elsewhere. presented elsewhere.

Experimental Procedure

Core plugs 2 in. (5 cm) long and 1 in. (2.5 cm) in diameter were used in the experiments. Samples were dried at 158 to 176 deg. F (70 to 80 deg. C) under vacuum for 2 days before measurement. A schematic of the experimental setup is shown in Fig. 1. Pore volume reduction was measured directly with a calibrated positive-displacement pump in conjunction with pressure positive-displacement pump in conjunction with pressure monitoring instruments. A dry sample was placed in the core holder, and a nominal confining pressure of 200 psi (1.4 MPa) was imposed on the sample. Nitrogen gas then was expanded into the core at a pressure of 80 psi (0.55 MPa). The position of the piston in the positive displacement pump was altered until the pressure was increased back to 80 psi (0.55 MPa). Confining pressure then was increased in steps to 5,000 psi (34.5 MPa). As confining pressure was increased, pore volume reduction of the sample resulted in an increase in the pore pressure. The position of the piston was adjusted at each stage to maintain pressure constant at its initial value, and the change in volume was measured directly. The use of a strip-chart recorder in conjunction with a pressure transducer enabled visual ascertainment of equilibrium. The time required for equilibrium was generally on the order of a few minutes. Thus, the time needed for each measurement was modest. In addition, the proper choice of transducer range enabled operation at pressure conditions so that the error introduced by instrumentation was minimized. A pressure transducer (0.1 % accuracy, as specified by the manufacturer) with a full-scale reading of 80 psi (0.55 MPa) was used. Since pore pressure was maintained at full-scale value, there was a possible 0.1 % error in pressure readings.In addition to this experimental technique, pore volume reduction also was measured by monitoring the amount of water expelled from a water-saturated rock as confining pressure was increased. The experimental setup was similar to that described by Hall, where a microburet was used to measure the water expelled. All experiments were conducted at ambient temperatures [73 deg. F (23 +/- 0.1 deg. C)].

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