The Upper Triassic Xujiahe formation is very tight formation located in highly tectonically stressed zones in the Western Sichuan Basin of China. The second and fourth members (abbreviated as T3X2 and T3X4, respectively) of the Xujiahe formation are gas-bearing tight sandstones. Bedding planes and natural fractures with low deviations are developed in the sandstones. This tectonically active in-situ stress state and the low angle natural fractures make the hydraulic fracture to propagate either horizontally or vertically but with small fracture height, which are unfavorable for enhancing gas production.
Multidisciplinary data from the field and experiments were used to determine the in-situ stress state. Minifrac test data were analyzed to determine the minimum horizontal stress. The FMI logging and stress polygon method were used to constrain the maximum horizontal stresses. The differential strain analysis was also performed to measure the principal stresses. A series of laboratory tests were conducted to determine the influences of in-situ stresses and natural fractures on hydraulic fracture propagations. The cohesions and internal friction angles of the bedding cracks were obtained by direct shearing tests using the downhole cores of T3X2. To imitate preexisting low angle fractures in the laboratory tests, man-made fractures were cut and then cemented in the outcrop rock samples, and the true tri-axial fracturing experiments were conducted in these samples to analyze the effects of in-situ stress state on the hydraulic fracture propagation in the tight sandstone.
The following are the results obtained in this paper: (1) The T3X2 reservoirs are in the strike-slip faulting stress state, which is beneficial to generate vertical hydraulic fractures; while the T3X4 reservoirs are close to the reverse faulting stress state where the hydraulic fracture height might be lower than that in the T3X2 reservoirs; (2) The true tri-axial fracturing experiments under the conditions of the in-situ stress states of the T3X2 and T3X4 reservoirs show that the hydraulic fracture geometry is mainly composed of the vertical fractures accompanying with few horizontal fractures. When the ratio of the overburden stress to the minimum horizontal stress is less than 1.2, hydraulic fracture opens or slides along the low angle natural fracture and may form a limited fracture height as a result; (3) The hydraulic fracturing target should be selected at the intervals that have the lowest horizontal stress, and the intervals having low angle natural fractures should be avoided. The pad fluid with high viscosity guar fluid is recommended to use to initiate the main vertical hydraulic fracture.
This study provides an integrated method and a guide for evaluating and designing the stimulation of the Xujiahe tight gas reservoirs in the Western Sichuan Basin. This method may be applicable to similar reservoirs.