Pore Volume Compressibility in Shale - Revisited

Abstract This study presents a novel method to estimated pore volume compressibility of shale samples based on mercury injection test data. We revisit our previous study (SPE-185059-PA) for more realistic estimation of pore volume compressibility for shale samples. We present a mathematical model to determine accessible porosity and pore compressibility as a function of pressure using Mercury Injection Capillary Pressure (MICP) data. During MICP testing in a typical shale sample, the rock sample experiences conformance, compression, and intrusion stages as effective pressure increases. By evaluating compression stage, we calculate bulk compressibility. Further by introducing a system of equations, bulk compressibility is decomposed to estimate accessible pore and grain compressibility separately. Different from our previous model, in this study grain compressibility is calculated based on weight average of mineralogy determined from Fourier-transform infrared spectroscopy (FTIR) experiments. Moreover, bulk compressibility obtained from MICP data is compared with the values calculated from ultrasonic velocity measurements. Samples from both Haynesville shale plays are used to perform our study and validate the hypothesis. Our results indicate that pore compressibility values are higher than anticipated, where calculated values are in the range of 1E-5 1/psi for shale samples at lower pressure. When pressure reaches to 8000 psi, pore compressibility reduces to the range of 1E-6 1/psi in most of the cases. Moreover, when compared with ultrasonic velocity measurements, results indicate that bulk compressibility obtained from MICP is overestimated at lower pressures and slightly underestimated at higher pressures. The outcome of the paper changes the industry's take on prediction of the reservoir performance, especially the rock compaction mechanism. This study suggests that production owing to rock compaction can be much greater than what has often regarded, which can change the performance evaluation on a great number of reservoirs in terms of economic feasibility.