An Evaluation of Geomechanical Properties of Potential Shale Gas Reservoirs in the Lower Indus Basin, Pakistan

Pakistan has been facing a growing energy crisis for the last decade, and the government is seeking new horizons for enhancing oil and gas production to reduce the gap between supply and demand. Although several shales of the Indus Basin in Pakistan are known source rocks for conventional hydrocarbon reservoirs, data currently available to assess their potential as shale gas reservoirs are somewhat limited. The objective of this research was to investigate, assess and improve methods for geomechanical characterization of shales using standard datasets of the type available (in the public domain) for Indus Basin shales. In this research, six shales which are known to be source rocks in the Indus Basin, Pakistan, were evaluated for their shale gas potential by comparison against several of the most active shale gas plays in North America. The comparison included available geological, geochemical, petrophysical and elastic properties, and concluded that all of the Pakistani shales investigated are promising regarding their shale gas potential. However, more petrophysical and geomechanical data are required before conclusions about these shales can be made with greater confidence. In light of this, the remainder of the research conducted in this project focused on applying (and improving) advanced interpretation techniques on two of the prospective Lower Indus shales deemed to have the best available (public domain) data. The interpretation of geomechanical properties generally requires knowledge of sonic shear wave velocity (Vs). Given that Vs measurement is commonly omitted from routine geophysical logging suites, many investigators have developed empirical models and rock physics models of varying form and complexity for the estimation of Vs using available well log and/or core analysis data. This study evaluated various relationships in the literature for the estimation of shear wave velocity applied to sandy shale and shale intervals of the Lower Goru Formation, Lower Indus Basin, for which two wells with Vs data were available. The results reveal that some empirical models can be effective for estimating Vs, but only when the model coefficients are adjusted by calibrating to site-specific Vp and Vs data. A modification to rock physics modeling developed for this type of work demonstrated that the use of Biot’s model (rather than Gassmann’s model) for fluid substitution improved model performance for Vs estimation in gas-saturated sandy shale and shale of the Lower Goru Formation. The rock physics-based model offers the advantage of being useful in settings where only Vp data are available for model calibration, and it is suggested that the rock physics model should be reliable when applied to a broader range of saturations and lithologies in the Lower Goru Formation. The next phase this work involved characterization of a shale interval in the Early Cretaceous-age Sembar Formation, Lower Indus Basin of Pakistan, using only readily available data. A workflow was developed for the estimation and mapping of geomechanical properties using logs from multiple wells and relevant post-stack seismic reflection data. Mineralogy data from well cuttings, core testing results for elastic properties and hydraulic fracturing test data were utilized to constrain the values of the properties estimated from geophysical data. The following results obtained at the well-scale suggest that the Sembar Shale is favorable for development: high gas saturation, good porosity (up to 10%), moderate quantity of thermally mature organic matter (2% - 4% TOC), a number of brittle intervals separated by thicker intervals that fall slightly below the brittle-ductile threshold, and a strike-slip stress regime. At the scale of the study area, robust statistical techniques were used to invert seismic stacks and develop a 3D mechanical earth model. This model shows a trend of increasing shale brittleness towards the northeastern portion of the study area, hence suggesting that this area might be most prospective for initial shale gas development. The final phase of this research involved the assessment and improvement of techniques for estimating mechanical properties using drill cuttings, which serve as the only available basis for laboratory testing when core samples are unavailable. Microindentation testing was selected for this work based on the literature review. Experimental techniques developed or improved in this work include: embedding multiple cuttings into an epoxy puck to facilitate sample preparation, mineralogical analysis, and testing of a large number of sampling points; progressive re-saturation to restore cuttings to in-situ moisture conditions; selection of optimal indentation force; assessment of sample anisotropy; brittleness assessment based on indentation morphology; (and a statistical / rock physics framework for estimating macroscopic properties from extensive testing of samples with variable mineralogy). Limitations of this testing method are discussed, as are recommendations for future research