A Parametric Study of Horizontal and Multilateral Wells in Coalbed-Methane Reservoirs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 96018, "A Parametric Study on the Benefits of Drilling Horizontal and Multilateral Wells in Coalbed-Methane Reservoirs," by N. Maricic, SPE, Chevron Corp., and S.D. Mohaghegh, SPE, and E. Artun, SPE, West Virginia U., prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October. The full-length paper attempts to identify the most appropriate drilling pat-terns for coalbed-methane (CBM) reservoirs of different thickness and with different characteristics. The comparison yardstick for this study is net present value (NPV). In this way, drilling cost is accounted for when comparing different horizontal and multilateral configurations. Dual-lateral, trilateral, quadlateral, and fishbone configurations were compared. Total horizontal well length as well as spacing between laterals (SBL) was studied. Introduction CBM Background. Coal represents an unusual reservoir rock because of its highly complex reservoir characteristics. Unlike conventional sandstone reservoirs where gas is found in a free state within the pore structure of the rock, the methane gas is adsorbed onto the internal structure of the coal, which allows a significant amount of gas to be stored in the coal rock. A coal-seam system is a naturally fractured heterogeneous reservoir characterized by macropores (the fracture system, also known as the cleat system) and micropores (coal-matrix system). In general, the coal cleat system is orthogonal, with one direction cross-cutting. CBM production depends on fracture spacing and fracture interconnection. If the cleat system is not sufficiently developed, gas production can be very difficult. This occurs as a result of the low porosity and permeability values of the matrix, making it almost impossible for gas to move from the matrix into the fractures. At the beginning, the coal system is in equilibrium and typically, water must be produced continuously from coal seams to reduce the reservoir pressure and release the gas. Gas from the coal can be produced only after initial system dewatering and lowering the reservoir pressure. The dewatering process can take anywhere from a few days to several months and depends on CBM-well configuration. Generally, water production declines until the gas rate reaches the peak value. This "time-to-peak-gas" is a critical parameter because gas production begins declining soon after reaching the peak. Horizontal Wells. One of the main advantages of horizontal wells is that well direction, shape, and position can be controlled. With horizontal wells, an almost ideal well position can be determined and achieved with respect to principal permeability directions of the coal. Proper well positioning as well as borehole length drilled perpendicular to the main fracture system of the coal can contribute significantly to draining large areas.