Pore Compressibility In Tight Rocks Using Mercury Injection, Ultrasonic Pulse Tranmission, And Nuclear Magnetic Resonance: A Case Study

Pore volume compressibility (cp) is critical for predicting reservoir performance, estimating production, modeling compaction, and designing effective well completion strategies in unconventional tight rocks. Due to small pore volume, relatively compacted nature and the presence of natural fractures within tight rock matrix, the traditional stress-strain compressibility tests are hardly performed. This study presents three cross-validating methods for evaluating cp: mercury injection capillary pressure (MICP), ultrasonic pulse transmission (UPT), and nuclear magnetic resonance (NMR) under confining pressure. Eight lithologies from the same formation were characterized in this study, including Fourier transform infrared spectroscopy (FTIR) for mineralogy, NMR and Helium expansion for porosity, computed tomography (CT) scanning for rock micro-structure, and LECO® TOC for organic richness. MICP and UPT (indirect methods) were employed to measure bulk compressibility and then pore compressibility is obtained by subtracting the grain compressibility derived from mineralogy. For compressional NMR (direct method), pore compressibility is calculated directly from changes in fluid volume inside the rock matrix as confining stress increases. CT scanning revealed that many samples contained fractures, complicating sample coring, thus limiting the number suitable for NMR and UPT. In contrast, the MICP method proved more versatile, allowing measurements across all lithologies. Hill averaging was applied to estimate grain compressibility, from which pore compressibility was successfully determined for the tight-rock sample set, ranging from 10−6 to 10−5 psi-1. Among the three methods, MICP emerged as the most effective, offering adequate accuracy, simplified sample preparation, reduced turnaround time and cost, and the ability to analyze a larger number of samples. Hence, cp values were determined mainly by MICP measurements while NMR and UPT were used to assess MICP accuracy and precision. MICP validated with NMR and UPT successfully determined the pore compressibility of tight rock samples in this study, overcoming several limitations of traditional strain-based methods. In addition, the method demonstrates strong potential for broader application to other tight reservoirs and unconventional geological formations.