--> Novel Data From a Routine Analysis: Determining Pore Compressibility From Mercury Injection Capillary Pressure Analyses of Fine-Grained Reservoir Rocks

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Novel Data From a Routine Analysis: Determining Pore Compressibility From Mercury Injection Capillary Pressure Analyses of Fine-Grained Reservoir Rocks

Abstract

Calculations for determining sample compressibility were developed for mercury injection capillary pressure (MICP) analyses of organic-rich mudrocks from the Duvernay Formation, Alberta. Within the literature, MICP is commonly used to study the pore throat distributions (PTD) of rocks and has been applied, somewhat blindly, to analyze the PTD of fine-grained reservoir rocks, such as mudrocks. For most mudrock samples, a predictable uptake of mercury into the sample cell is evident in MICP data at high intrusion pressures (up to 10,000 PSI) which occurs prior to intrusion of the pore structure. The predictable uptake of mercury is attributed to the compression of the sample particles and cannot be accounted for by blank corrections. Compression of the sample prior to intrusion of the pore structure may alter the PTD significantly and by an unknown amount. Calculations were developed within this study to compute the bulk volume compressibility of the sample and by assuming a value for grain compressibility, the pore volume compressibility. Once the pore volume compressibility is determined from MICP, the porosity of the sample at stress can be determined. Since mudrock porosities are commonly measured at ambient stress conditions, the calculation of a porosity value at stress represents an important step towards determining rock properties more representative of reservoir stress conditions. Sample compressibility is correlated to the composition of mudrocks and in the absence of geomechanical data, MICP analyses of mudrocks may help to determine the compositional controls on rock strength under compression. For Duvernay mudrocks, compression is negatively correlated with increasing quartz and feldspar content and shows no correlation to total clay or total organic carbon content. In addition to determining the sample compressibility, the volume of mercury intruded into the sample cell due to sample compression can be corrected from the incremental PTD curve yielding a compression-corrected PTD. The corrections, equations and workflows developed for this study can lead to more consistent MICP results when applied to characterize the pore structure of mudrocks, as well as generate novel data, such as pore volume compressibility and porosity at stress.