Evaluation of shale compressibility from NMR and MICP measurements

Abstract

Compressibility is a fundamental reservoir parameter that is often overlooked in the assessment of unconventional reservoir performance. Compressibility affects the reserve, recovery, and the mechanical properties of reservoir rock. This study focuses on the evaluation of Mercury Injection Capillary Pressure (MICP) data to compute compressibility. Additionally, the MICP approach is compared to measured pore volume compressibility from Nuclear Magnetic Resonance (NMR) data and pore volume compressibility using helium. Compressing shale forces fluids out of the pore space; thus, aiding the recovery of hydrocarbon. Compressibility was studied over a range of confining pressure from 0-5000 psi with NMR and helium. NMR, MICP, and helium compressibilities were calibrated on sandstone and limestone samples. The error in the estimation of pore compressibility among the three methods was less than 30% at higher pressures. A total of 175 samples from Barnett, Woodford, Haynesville, Eagle Ford and Wolfcamp shales were analyzed. Eleven total samples from Eagle Ford (6), Wolfcamp (4), and Haynesville (1) are selected for NMR pore volume compressibility measurements. The range of NMR measured pore compressibility varies from 2* 10-6 to 1.8* 10-4 psi- 1.

This study shows that shale has a significant compressibility, especially at lower confining pressure. Furthermore, the shale compressibility also exhibits a strong pressure dependence which is apparently controlled by microcracks at low pressure. The compressibility measured using NMR agrees with the derived MICP compressibility. No relationship could be established between pore volume compressibility and other measure petrophysical properties.

The experimental results of this study are used to evaluate the effect of compressibility change on production from unconventional resources. Three cases of oil, gas condensate, and gas reservoirs are considered. Results from the simulation indicated that 2 to 25 % underestimation occurs in the prediction of production from oil and gas condensate windows in shale resources when shale compressibility is neglected.