EXPERIMENTAL ASSESSMENT OF PETROPHYSICAL CHARACTERISTICS OF CORE SAMPLES FROM HAYNESVILLE AND PEARSALL FORMATIONS

This study assesses reservoir characteristics of Haynesville and Pearsall Formations in Texas. With an estimated recoverable gas of about 75 trillion cubic feet (TCF), the Upper Jurassic Haynesville Shale Formation is one of continual interest to the oil and gas industry. On the other hand, Pearsall Shale is a gas-bearing lower Cretaceous formation still at the development stage. Based on various complementary tests such as XRD, pyrolysis, vacuum saturation, N2 gas physisorption, mercury intrusion porosimetry (MIP), wettability, and imbibition, this study tests and compares reservoir properties of core samples for these two shales. The upper Haynesville is clay-dominated with a porosity of 6.5-7%, the middle layer is carbonate mixed mudstone with a porosity of ~5.5%, and the lower layer is carbonate-dominated with a porosity of ~2%. These porosity values from vacuum saturation tests are consistent with MIP-based results of 4.2 to 5.9%. TOC values vary from 0.85–3.91 wt.%, and pyrolysis-derived maturity is 1.41 to 3.48 VRe%. Spontaneous fluid imbibition into Haynesville shale exhibits a connectivity slope, based on log (imbibed mass)-log (time) scales, close to ¼ for deionized (DI) water, consistent with contact angle results of mixed-wet (45-52^o for DI water) but extreme oil-wet nature (<detection limit of 3^o for n-decane). MIP analyses show that the median pore-throat diameters for Haynesville shale are dependent on lithology. 60% of the pore volumes have pore-throats above 100 nm, and permeability from 0.2 to 145 µdarcy. The upper part of the Pearsall Formation is silicate-dominated, where middle and lower layers are carbonated-dominated. The average porosity of 2.5% determined by vacuum saturation and DI water immersion is larger than MIP based porosity of ~0.6%. TOC values vary from 0.84 to 3.9%, and pyrolysis-based maturity is 0.98 to 1.16 VRe%. Pearsall shale MIP analyses show that the median pore-throat diameters lie between 0.01-3 µm and 70% of the pore throats by volume is greater than 100 nm, with the permeability of about 0.2 to 1.11 µdarcy. The overall findings from these experimental approaches provide an improved understanding of nanopore structure and mixed-wet characteristics of Haynesville and Pearsall Shale.