Effect of coal maturity on CO2-based hydraulic fracturing process in coal seam gas reservoirs

Abstract

Hydraulic fracturing of deep coal seams is challenging due to both the complex processes involved in fracturing and the typically poorly defined characteristics such as natural cleat system, mineral-maceral distribution and strength parameters of the subsurface. This study evaluates the effectiveness of fracturing using liquid CO2 as the propellant through observations of break-down pressures and the form of the induced fracture network in various ranked coals. Coal ranks are defined through a rigorous proximate analysis to determine the moisture, volatile matter, ash and fixed carbon contents of each coal type fractured. Fracturing experiments were conducted on 38 mm × 76 mm core samples, under fixed stress, temperature conditions (i.e. σ3 = 6 MPa, σ1 = 8 MPa and T = 25 °C). Break-down pressures are observed to increase with increasing coal maturity. Increasing rank or maturity identifies that the coal has been subjected to progressively higher pressures and temperatures, has gained proportionately higher strength and thus exhibits a higher break-down pressure. No direct relationship is observed between volatile matter content and either strength or break-down pressure. The colocation of acoustic emission (AE) hypocenters and mineral grain boundaries delineated by micro-CT imaging indicate preferred pathways for the propagation of fractures induced by liquid CO2. Stiffness contrasts between mineral phases result in stress concentrations and localized weakness at grain-grain boundaries. The complex mineral distribution in coal accentuates such heterogeneity of weakness and may be the key feature promoting the evolution of a well distributed rather than localized fracture network. For low rank coal, hydraulic fracturing is least effective, as the fracturing process does not create a significant fracture network to enhance the permeability. This may result, since low rank coals are intrinsically weak due to their low carbon content and high moisture content allowing extensive fracturing to develop at only very low break-down pressures – minimizing damage. These observations emphasize the sensitivity of break-down pressures and the resulting complexity of fracturing to pressurization rates and coal rank – inferring important controls on these parameters for the safe and effective use, when fracturing with CO2 as the propellant.