Pressure Transient Behavior of Fractured Wells in Coalbed Reservoirs

Abstract

Abstract A solution package that describes the pressure transient behavior of coal reservoirs in the presence of a hydraulically fractured well is presented. In the development of the solutions uniform-flux vertical fracture model is employed. The proposed solutions are then extended to describe the pressure transient behavior of composite coalbed reservoirs with a vertical well which may have a vertical fracture. The proposed composite system solution has potential applications when "tailored-pulse" fractures or damage exist around the degasification well. In this study, dual porosity nature of coal seams is modeled by describing micropore structure by spherical matrix elements. The formulation employed includes single-phase laminar gas transport in the natural fracture network (cleat system). Flow within the micropores is described by Fick's law of diffusion. Matrix to fissure flow mechanism is modeled by unsteady-state sorption/diffusion formulation. The equilibrium isotherm which defines the amount of desorbed gas is obtained through Langmuir's theory. The solutions are obtained analytically in the Laplacian space and inverted to the real time domain via Stehfest algorithm. The proposed solutions are tested against the subsets of the problem with known solutions and in each case excellent agreements are observed. Furthermore, the pressure transient data generated by a finite difference coal seam degasification model are used to check the validity of the solutions presented. Sensitivity of the solutions to a wide range of coal seam properties are also included. The solutions presented in this paper equip the well test analysis engineer with a precise tool to analyze the pressure transient behavior of the hydraulically fractured degasification wells. The solution package encompasses infinite. constant pressure and no-flow outer boundary conditions together with constant flow rate specification at the wellbore. Introduction Pressure transient behavior of coal seams differ significantly from conventional gas reservoirs. The existence of the adsorption phenomena together with the dual porosity nature of the coalbed reservoirs call for the development of solutions which are applicable to pressure transient analysis of the coal seam degasification wells. In addition to the gas stored in the pore volumes, coal seams store gas as an adsorbed mono-molecular layer on coal grain surfaces. The amount of gas in adsorbed stage is defined by the equilibrium isotherm. Because of small pore diameter in the coal matrix the flow of desorbed gas from coal grain surface to the cleat system is governed by Fick's law of diffusion. Once gas reaches to the cleat system flow is laminar and governed by Darcy's law. Hydraulic fracturing is an effective well stimulation technique employed in production of oil and gas from damaged wells or wells producing from low permeability reservoirs. The vicinity of a vertical well drilled in a coal seam may be damaged by invasion of the natural fracture network with drilling fluid and cement. The presence of cement over 100 ft from the wellbore has been noted in mine-through operations in coal seams. Hydraulic Fracturing of vertical wells drilled in coal seams is employed to bypass wellbore damage. A hydraulically induced fracture will also be effective in connecting the fracture network to the wellbore, accelerating and extending the pressure drawdown into the reservoir, and reducing production of fines by reducing the pressure drop in the near-wellbore area where the coal is often in a degraded condition. P. 407^