Influence of sedimentary microfacies on mechanical properties of ultradeep reservoir rocks and its application:a case study of Cretaceous in BZ gas field of Kuqa Depression, Tarim Basin

In order to optimize the sweet spot intervals with natural fractures in the well of Cretaceous ultra-deep tight sandstone reservoirs in the Kuqa Depression of Tarim Basin, the difference of reservoir rock mechanical properties of different sedimentary microfacies in BZ gas field was systematically analyzed by using outcrop, rock slice, imaging logging and other data. Then the geometric parameter models of different sedimentary microfacies in related outcrops were combined to propose a new method to optimize the reservoir rock mechanical parameter model, which improved the prediction accuracy of natural fractures around the wellbore (< 200 m). This research indicates that:(1) There are differences in reservoir rock fabrics and rock combinations (including debris content, matrix content, grain sorting, sand ratio, and sandstone-mudstone combination) among various sedimentary microfacies and different parts of the same microfacies, which affects the Poisson's ratio and Young's modulus of the third member of Bashijiqike Formation in BZ gas field. (2) The degree of crack development varies among different sedimentary microfacies of sand bodies, with the sandstone in the underwater branch channel of fan delta front being the most developed, making it easier to form cracks than the inter bay microfacies of fan delta front and the branch channel microfacies of fan delta plain. (3) So we conclude that different microfacies reservoirs have different rock mechanical parameters. Furthermore, a three-dimensional microfacies model was established based on the geometric parameters of sedimentary microfacies in outcrops and was used to optimize sedimentary microfacies models and rock mechanics parameter models around wellbores, which were applied to fracture prediction around wellbores.