On the Economic Optimization of the Fracturing of Coal Seam Reservoirs

Abstract

Abstract The production of natural gas from coal seam reservoirs follows a succession of stages that are well established and understood. Salient components are the execution of hydraulic fractures, dewatering of both native and introduced water, and the subsequent production of what it is almost entirely methane. Most coal seams are of relatively small net pay thickness and in many cases also of low permeability, i.e. 2 md or less. Produced gas comes from a combination of both free gas in the coal porosity and, especially, desorbing gas. Because of large heterogeneities and major differences among coal seam reservoirs, the hydraulic fracturing is rarely uneventful with a large number of issues to be resolved, such as fluid selection or the mass of proppant. Most of these decisions are local and often conclusion cannot be drawn from elsewhere. Frequently, a well is subjected to multiple stages of hydraulic fracture treatments. Production performance characteristics also vary considerably. The interaction between permeability, natural cleat networks and natural and artificial fractures is often complicated and difficult to predict. This results in different periods of post-treatment de-watering, the volume of water that is produced and the onset of predominant natural gas production. We have constructed a realistic physical and economic model where the NPV criterion is used to identify successful or potentially uneconomic candidates. We forecast fractured well performance using the Unified Fracture Design (UFD) approach and we account for time of de-watering and the cost of water management. Charged against the net present value of the revenue are the costs of fracturing and well completion. Five wells with different Langmuir isotherm parameters were considered for the NPV parametric studies. The parametric studies include a range of reservoir permeabilities, porosities, proppant masses, and fracture heights. The results show the window of attractive prospects and delineate the unattractive prospects which can be considerable.