A NEW METHOD FOR FRACRABILITY EVALUATION IN DEEP AND TIGHT SANDSTONE RESERVOIRS

Fracrability evaluation is the basis for the optimization of fracturing level and the evaluation of productivity after fracturing. The tight sandstone reservoirs in block 4 in central Junggar Basin are buried deep and poor in physical property, and it is urgent to improve the industrial productivity by fracturing. Therefore, the fracture property of the Jurassic tight sandstone of Dong 2 well north area is studied as an example. Based on the experimental data of the rock triaxial test, the fitting formula of fracture energy density and elastic modulus of tight sandstone is established. Brittleness index of rocks at different depths in the study area is determined by mineral composition method and elastic modulus-Poisson ratio method.The fracture development index of different depths in the study area is determined by the criterion of rock fracture. Fracture energy density is used to characterize the fracture toughness of tight sandstone, and the fracture development index is used to characterize the development degree of natural fractures. Considering the influence of rock brittleness, fracture toughness, in-situ stress and development degree of natural fractures, the weight of each parameter is calculated by analytic hierarchy process, and a new quantitative fracrability evaluation of deep and tight sandstone reservoirs is established. The results show that when the fracrability index is greater than 0.55, the reservoir can be fractured well, when the fracrability index is between 0.50~0.55, the fractured reservoir is moderate, and when the fracrability index is less than 0.50, the fractured reservoir is poor. In the study area, the optimal fracturing horizons of Well D7 are 4145~4160 m, 4470~4480 m, 5290~5330 m, Well D8 are 5120~5330 m, 5350~5365 m, and Well D701 are 3900~3910 m, 4430~4440 m, 4455~4465 m, 5125~5135 m.