A novel technology for enhanced coal seam gas recovery by graded proppant injection.

Abstract

Coal bed methane (CBM) is one of the world’s fastest growing unconventional gas resources and offers the potential for much cleaner power than from traditional coal. However, low productivity index in coal bed methane reservoirs places them on the margin of economic efficiency. One of the key technological hurdles affecting the productivity index in CBM reservoirs is the extremely low permeability of coal’s natural cleat and fracture system. Thus, development of new techniques for enhancing coal cleat permeability is essential for cost-effective gas production from CBM reservoirs. The hydraulic fracturing is the most widely used CBM well stimulation method; however, the hydraulic fracturing is often restricted by the environmental regulations. Besides, the available injection power may not be sufficient to fracture the well. The way around this problem is stimulation of a natural cleat system keeping the reservoir pressure below the fracturing pressure. The main objective of this study is to develop a new well stimulation technology utilizing graded proppant injection to allow sequential filling of both distant and near-well fractures. This mechanism leads to a significant enhancement of permeability and, therefore, improved well productivity. Mathematical modelling and experimental studies are conducted for stimulation of natural cleat system in coal bed methane reservoirs. The aim of this work is to determine an optimum injection schedule, i.e. the timely dependencies of the injected proppant size and concentration that avoids fracture closure during production stage and provides minimum hydraulic resistance in the system of fractures plugged by proppant particles. The laboratory tests on one dimensional injection of different size particles into coal cores have been conducted under different effective stress conditions. Calculations of electrostatic interactions result in determining the physicochemical conditions, favourable for particle-particle and particle-coal repulsion. The repulsion prevents: particle attachment to the coal surface, particle agglomeration and consequent formation damage due to external and internal cake formation. Particle placement with low-salinity water, which promotes the repulsion, improves the coal permeability. A laboratory-based mathematical model is developed to describe the proppantfree water injection stage; capture kinetics of proppant particles in the natural fractures and calculation of an optimal injection schedule. The analytical model is derived for exponential stress-permeability relationship and accounting for permeability variation outside the stimulated zone. Field case studies show that the productivity index can be significantly increased by applying the stimulation technology developed in this thesis. The sensitivity analysis of well index shows that the most influential parameters are the stimulated zone size, injection pressure and the cleat system compressibility. The above laboratory study, mathematical modelling and the field-scale predictions allow recommending the developed technology of graded proppant injection for improving gas recovery from Coal bed methane reservoirs.