Abstract:

Log and seismic data indicate that most shale formations have strong heterogeneity. Conventional analytical and semi-analytical fracture models are not enough to simulate the complex fracture propagation in these highly heterogeneous formations. Without considering the intrinsic heterogeneity, predicted morphology of hydraulic fracture may be biased and misleading in optimizing the completion strategy. In this paper, a fully coupling fluid flow and geomechanics hydraulic fracture simulator based on dual-lattice Discrete Element Method (DEM) is used to predict the hydraulic fracture propagation in heterogeneous reservoir. The heterogeneity of rock is simulated by assigning different material force constant and critical strain to different particles and is adjusted by conditioning to the measured data and observed geological features. Based on proposed model, the effects of heterogeneity at different scale on micromechanical behavior and induced macroscopic fractures are examined. From the numerical results, the microcrack will be more inclined to form at the grain weaker interface. The conventional simulator with homogeneous assumption is not applicable for highly heterogeneous shale formation.

Introduction

Hydraulic fracturing is a well-stimulation technique, which creates fractures in rock formations through the injection of hydraulically pressurized fluid. Since the 1950s, about 70% of gas wells and 50% of oil wells have been hydraulically fractured [1]. Wide and successful applications of horizontal wells and hydraulic fracturing are the key reasons leading to the exponentially growing of tight oil and shale gas production. Therefore, understanding the hydraulic fracture propagation in complex unconventional reservoir plays a crucial role in optimizing the stimulation strategy and maximizing the created contacting surface.

However, there are several challenges in precisely predicting and controlling the induced fracture geometry [2]. One significant difficulty is that rock is a heterogeneous material containing many natural weaknesses, including pores, grain boundaries, and preexisting fractures [3]. Microseismic monitoring, production data, log, and seismic data confirm that the reservoir formation has strong lateral heterogeneity, which is a key impact factor of rock's mechanical behaviors. During the hydraulic fracturing process, these pre-existing weaknesses can induce microcracks or microfractures, which can in turn change the flow capability of the rock [4], [5]. For example, the Bakken formation is a layered heterogeneous reservoir, which is separated into upper, middle, lower and three forks. And even in one layer, the rock mineralogy varies with depth and location. Thus, without considering the intrinsic heterogeneity, the predicted morphology of hydraulic fracture may be biased and misleading in guiding the horizontal well completion strategy.

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