Pore Volume Compressibilities of Sandstone Reservoir Rocks

Pore volume compressibilities measured in tile laboratory on core samples are reported for typical reservoir sandstones at reservoir pressures. These compressibilities are different for each reservoir sample and cannot be correlated to porosity. The compressibilities are also functions of pressure. Introduction In recent years several authors have pointed out the importance of including pore volume compressibility of the reservoir rock in certain reservoir engineering calculations. Hall showed that in calculating the hydrocarbon volume of an undersaturated reservoir from the production per unit change in reservoir pressure, the neglect of rock compressibility could, in the extreme case of low porosity rock, lead to results in error by a factor of two. Hawkins and Hobson and Mrosovsky showed that both rock and interstitial water compressibilities must be included to achieve satisfactory accuracy in this calculation. Although rock compressibility is an important reservoir property in these calculations, very few data are available. Hall made compressibility measurements on a few samples in a small pressure range. No other direct pore volume compressibility measurements have been reported. Geertsma discussed the elasticity of reservoir rock but did not present any new data. In this paper, rock compressibilities suitable for reservoir calculations are reported for a variety of sandstones typical of U.S. reservoirs. The measurements were made through a range of pressures which is also typical of reservoirs. Experimental Procedure Plugs 1 in. in diameter and 2- to 3-in. long were diamond drilled, 14 from cores from seven oil-bearing or potentially oil-bearing sandstone, one from a quarried sandstone, and another from a shallow, oil-free, pure orthoquartzite. The plugs were extracted in toluene, vacuum dried and then weighed. The plugs were then saturated with kerosene and reweighed. This gave pore volume. They were then again extracted in toluene, vacuum dried and jacketed in 0.004-in. thick copper foil. A short length of ¼-in high pressure tubing led from the jacket. The jacketed sample was then saturated with kerosene and placed in the hydraulic pressure cell as shown in Fig. 1. Movement of the mercury slug showed the change in pore volume of the sample under a given pressure change. Temperature was about 72° F for all measurements.