Experimental investigation of propagation mechanisms and fracture morphology for coalbed methane reservoirs

Chi Ai,Jun Zhang,Dan Jia,Wen-Jing Tan,Xiao-Xuan Li

doi:10.1007/s12182-018-0252-z

Abstract

Fracture propagation mechanisms in coalbed methane (CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. “Step-like” fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.

Highlights

Hydraulic fracturing (Ma et al 2014, 2017a; Taleghani et al 2016; Wang et al 2018; Zhang 2014), is a very important technology and has been successfully applied to the industrialized development of coalbed methane (CBM) to improve low coalbed permeability (Alexis et al 2015; Qin et al 2018; Wu et al 2018)
The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. ‘‘Step-like’’ fractures were formed in most experiments due to the existence of discontinuous butt cleats
The mechanical properties of similar materials were tested, and the results indicated that the average compressive strength, tensile strength and elastic modulus were 12.2, 0.63, and 5431 MPa, which were extremely close to the specimen mechanical parameters in the tests

Summary

Introduction

Hydraulic fracturing (Ma et al 2014, 2017a; Taleghani et al 2016; Wang et al 2018; Zhang 2014), is a very important technology and has been successfully applied to the industrialized development of coalbed methane (CBM) to improve low coalbed permeability (Alexis et al 2015; Qin et al 2018; Wu et al 2018). Different from singlejointed rock, such as shale, the cleat system developed in coal can generate complex and diverse fractures. College of Geoscience and Technology, China University of Petroleum (East China), Qingdao 266580, Shandong, China seam fracturing based on ideal hypotheses can hardly reflect the true regime of fracture propagation. Fracturing experiments in laboratory can simulate the initiation and propagation process in CBM reservoirs under real stress conditions, which can reveal complex fracture propagation mechanisms for CBM reservoirs. The uniquely developed internal cleat system (Abass et al 1990) makes a great difference in hydraulic fracture propagation mechanisms between CBM reservoirs and other conventional oil and gas reservoirs. Deng et al (2016) investigated the effects of notch angle, notch

Methods

Results

Conclusion