Numerical Analysis for Dynamic Propagation and Intersection of Hydraulic Fractures and Pre-existing Natural Fractures Involving the Sensitivity Factors: Orientation, Spacing, Length, and Persistence

Abstract

Pre-existing natural fractures in a fractured reservoir will intersect with the hydraulic fractures and propagate dynamically during the hydrofracturing process, which has a significant impact on the morphology of the fracture network and subsequent gas production. If the representative properties of the natural fractures cannot be accurately extracted and appropriate numerical models are not established to describe the propagation and intersection behaviors of the fractures, it is difficult to determine the fracturing scheme for naturally fractured reservoirs and to obtain the expected fracture network and gas production. In this study, the combined finite element-discrete element method and discrete fracture network model were used to simulate the hydrofracturing process of naturally fractured reservoirs by controlling different sensitivity factors (orientation, spacing, length, and persistence) of natural fractures. To investigate the sensitivity factors, typical numerical cases were carefully designed and established, and the results of the dynamic propagation and intersection of hydraulic fractures and pre-existing natural fractures were derived. Two typical types of fracture network morphologies are detected when hydraulic fractures intersect the natural fractures: center-type (intersection of hydraulic fractures and the crossed cluster of the natural fractures) and edge-type (intersection of hydraulic fractures and the edge of the natural fractures). The quantitative results of the length and volume of the fracture networks and gas production in enhanced permeability fractured reservoirs were analyzed. The mechanisms by which the sensitivities of natural fractures affect and control the optimal fracturing behaviors are well understood; the conditions of small fracture spacing, large fracture length, or small fracture persistence of natural fractures are conducive for hydraulic fractures to intersect with the natural fractures and form a complex center-type fracture network and improve gas production.