Maximizing Energy at Coalface for Coalbed-Methane Fracturing

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 114974, "Maximizing Energy at Coalface for Coalbed-Methane- Fracturing Operations," by Abbas Mahdi, SPE, Schlumberger; Mike Yu, SPE, EnCana; Doug Pipchuk, SPE, Craig Wasson, SPE, Jim Nguy, SPE, and Nathan Kathol, Schlumberger, originally prepared for the 2008 CIPC/ SPE Gas Technology Symposium Joint Conference, Calgary, 16–19 June. The paper has not been peer reviewed. Nitrogen coiled-tubing (CT) fracturing is the predominant method for completing and stimulating dry-coalbed-methane (CBM) formations, such as the Horseshoe Canyon in the west-ern Canadian sedimentary basin. Currently, energy that can be delivered to the coalface of these dry-CBM wells has been limited by the frictional pressure loss through CT. The full-length paper discusses the development of a technique that releases a large-volume pressure pulse downhole during the fracturing process to create an order-of-magnitude change in available energy at the fracture face. Introduction The Horseshoe Canyon dry-coal formations have been exploited commercially since late 2000. The Horseshoe Canyon CBM play is a unique CBM play, in that it consists of a dry underpressured coal, which does not require dewatering before production. The coal consists of multiple thin seams, ranging from 10 to 30 seams per well, that are treated individually. Coal-seam thick-ness ranges from 0.1 to 2.5 m. The production rates from these CBM wells dictate that an efficient drilling and completion model be adopted to be economic. The wells are drilled with CT drilling rigs, cased, and cemented. Wireline perforating crews perforate the zones of interest in a rigless operation. The individual coal seams then are stimulated by injection of dry nitrogen at high rates through a CT reel equipped with a fracturing isolation tool. The dry coal has not responded well to any other form of stimulation. To optimize the completion process, high-rate nitrogen-pumping units were developed to reduce the footprint and cost and to increase efficiency of the operation to the point where one and a half to two wells can be stimulated per day. Since the introduction of that change, various optimization initiatives have been attempted. The optimization of nitrogen-pumping rates and pumped volumes has been the focus of these attempts. High rates have been limited by the inside diameter of the CT string. Several CT sizes have been used for this application. Economics and logistics have limited the CT size for CT fracturing to 2 7/8 in. for deeper wells and 3 1/4 in. for shallow wells. The migration to bigger pipe did pro-vide some reduction in frictional pressure loss, but effective pressure at the coalface still was not transported from the surface efficiently.

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