A Case Study on the Optimal Design of the Horizontal Wellbore Trajectory for Hydraulic Fracturing in Nong’an Oil Shale

Ying Zhu,Ying Zhang,Dongbin Pan,Han Zhang,Shuai Gao,Chen Chen,Lianghao Zhai

doi:10.3390/en13010286

Abstract

A horizontal well with hydraulic fractures is key to forming a fracture network in oil shale for the generated hydrocarbon flows. By considering the influence of anisotropic strength, a prediction model is proposed for fracture initiation by studying different fracture initiation modes (FIMs) in oil shale: failure of the intact rock matrix and of the bedding planes. Through a case study on Nong’an oil shale, the influences of wellbore trajectory and bedding planes on the fracture initiation pressure (FIP), location (FIL), and FIM were analyzed and the induced changes in wellbore trajectory design were concluded. The preferred angle between the wellbore axis and the minimum horizontal principal stress was the same of 90° or 270°, when the lowest required FIP corresponded to the failure of the intact rock matrix. However, when the angle corresponded to the failure of the bedding planes, the preferred direction of the wellbore axis was away from the fixed direction and not corresponding to the lowest required FIP due to the fracture morphology. The error between the theoretical and experimental results ranges from 7% to 9%. This research provides a framework for the design of horizontal wellbore trajectories in oil shale, for easier fracture initiation and more complex fracture networks.

Highlights

Hydraulic fracturing in the horizontal well is a promising approach to form the fracture network for the generated hydrocarbon flow in the in-situ retorting process [1]
Based on the hydraulic fracturing model [13], which was proposed for the prediction of longitudinal fracture initiation, the fracture initiation models [14,15,16,17] for different fracture patterns were improved with regard to the tensile strength, fluid seepage and fracture toughness
According to the coordinate system settings of Lee et al [42], we introduced the bedding plane coordinate system (BCS) and polar coordinate system (PCS)

Summary

Introduction

Hydraulic fracturing in the horizontal well is a promising approach to form the fracture network for the generated hydrocarbon flow in the in-situ retorting process [1]. Liu et al [20] analysed the fracture initiation mechanism using the tensile failure criterion and determined the laws influencing wellbore location and perforation parameters. These studies were carried out based on the improvement in the fracture initiation model, but it was assumed that the rock was an elastic isotropic medium. This is contrary to the fact that the real rock is anisotropic, as the existence of bedding planes and laminations for sedimentary rocks, foliation and cleavage for metamorphic rocks, and even natural fractures will result in anisotropic strength [25]. Since oil shale is a typical sedimentary rock [9], it is required to establish the fracture initiation model by considering the influence of the anisotropic strength

Methods

Results

Conclusion