Abstract
In this work, a series of intensive laboratory tests are conducted to measure the material constituents, mechanical properties, and to examine macro-micro-failure modes of various types of rocks from tight gas reservoirs in the Da Qing oilfield in China. A set of key parameters are experimentally determined, including porosity, mineralogical compositions, microstructure, Young’s modulus, Poisson’s ratio, triaxial compressive strength, as well as macro- and micro-morphology failure modes. The relationships of these parameters are systematically analyzed, and the effects of the material constituents and microstructure characteristics such as cementation type, porosity, and mineral composition on rock mechanical properties are revealed as well as the patterns of micro- and macro-failures in types of rocks are investigated. The result shows that the micro-failure mainly exhibits features of transgranular and intergranular porous polymer fracture, and the macro-failure modes are mainly three types: shear-dominated, mixed shear–tensile and mixed tensile–shear. The mixed tensile–shear failure has mainly tensile fractures with branch fractures crossing each other, which forms a complex system fracture network. These findings are of importance for “sweet pot” evaluations, wellbore stability analysis, and hydraulic fracturing design for oil and gas production in tight gas reservoirs.
Highlights
Tight gas reservoirs are generally defined as matrix permeability of less than 0.1 mD, matrix porosity of less than 10%, gas saturation of less than 60%, and water saturation of greater than 40% (Clarkson et al, 2012; Holditch, 2006; Walls, 1982)
A large number of laboratory tests have been performed to measure the mechanical characteristics for granites, marbles, and sandstones at room temperature, of which triaxial compress experiments, X-ray diffraction (XRD), scanning electron microscope (SEM), and thin section observations were carried out for rocks of the tight gas reservoir in Da Qing oilfield, and stress–strain curves, strength and deformation characteristics, fracture modes, mineral composition, and porosity were obtained in a given pressure and temperature environment
The microstructure, porosity, mineral compositions, mechanical properties, and macromicro-failure modes have been experimentally studied for various types of rock in tight gas reservoirs in the Da Qing oilfield in China
Summary
Tight gas reservoirs are generally defined as matrix permeability of less than 0.1 mD, matrix porosity of less than 10%, gas saturation of less than 60%, and water saturation of greater than 40% (Clarkson et al, 2012; Holditch, 2006; Walls, 1982). (Chen et al, 2017; Haimson, 2006) Material constituents such as porosity, mineral composition, grain size, and microstructure are the most important factors that influence the rock strength and deformability. A denser (less porosity) or a finer (smaller grain size) texture generally resulted in higher strength (Hatzor and Palchik, 1997, 1998) While such relations vary significantly as a result of the natural diverse irregularity, and complex microstructure of rocks, there are no universal relations which can be used for even a same type of rocks. It is revealed that the material constituents and regularities affect the mechanical properties of rocks in tight gas reservoirs significantly
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