51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: Mechanical stratification of shale complexes is an important feature of tight reservoir that controls both: the pattern of natural tectonic strata-bound fractures and propagation of technological hydraulic fractures. In our study we are looking for factors controlling the mechanical properties of a shale complex and define parameters crucial to distinguish geomechanically consistent units based on core and borehole geophysical data. We describe the hierarchic system of Consistent Lithological Units (CLUs) to ease compilation and extrapolation of analyses at different scales and resolution. Higher level member (CLU3) are traceable from well to well at distances of tens of kilometers across the Baltic Basin. The dependence between mineralogical composition and elastic modulus with emphasis on the role of calcite is analyzed. The results show that carbonates may have had a stronger impact on the Young’s modulus and brittleness index growth than quartz. These observations led us to modify the conventionally used mineralogical brittleness index by incorporation of TOC factor and determination of the calcite placement in the formula numerator. We also compare the intensity of vertical and horizontal fractures with the calculated brittleness index and conclude that the natural fracture density is not a direct function of shale brittleness.
Unconventional shale reservoirs have become significant source of U.S. natural gas in the past decade. Promising geology and mining perspectives prompted oil companies to search for shale gas in Poland. The Ordovician and Silurian shales located in the Baltic Basin have emerged as one of the most promising targets for shale gas exploration. The companies drilled several tens of exploration boreholes which provided wealth of information about petrophysical and mechanical properties of the shales. Increasing demand to understand the structure and behavior of these rocks led us to commence our geomechanical research. Geomechanical study in a shale reservoir is crucial to understand rock behavior during hydraulic fracturing treatment and to solve borehole stability problems.
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