The Dry Coal Anomaly-The Horseshoe Canyon Formation of Alberta, Canada

Abstract

Abstract The Horseshoe Canyon coal formation in Alberta, Canada is an anomaly for coals. It is dry, for the most part producing no formation water. Therefore, no dewatering is required. In fact, it is severely damaged by water influx. Normal permeability injection tests and conventional hydraulic fracturing techniques, even foams, have been unsuccessful in this environment. The best part of this reservoir has an areal extent in excess of 12,000 square miles containing 1–2 BCF per square mile of recoverable gas.Commercial development of this resource is a very appealing prospect. Numbers of coal seams vary from 5 to 30 per well, spread out over 600 – 1,300 ft. Individual seam thicknesses vary from 1 to 13 ft. Most seams are discontinuous over a large area. The better part of the formation has permeability in the range of 1 – 100 md. This makes it difficult to avoid damaging the formation during drilling and cementing operations, hence some type of stimulation is required. Nitrogen hydraulic fracturing is the only stimulation process that has had any success in this formation to date. The treatments involve pumping nitrogen at high rates, without proppant, through coiled tubing (CT) and a selective cup-type packer (SCP), isolating each coal seam while treating it. Bottomhole treating gradients vary greatly with an average of approximately 2.2 psi/ft. Whether these nitrogen treatments are in fact fracturing the formation or just having the damage flushed out has been a matter for speculation. Over 1,600 wells have been treated in this way and more continue to be added on an everyday basis. This paper describes the formation characteristics and geology of the Horseshoe Canyon coal and presents case histories of hydraulic fracturing treatments performed including production results. Introduction Geology Alberta, Canada has a huge potential for coalbed methane (CBM) gas production. The lower half of the province has six main coal-bearing formations: the Mannville, Horseshoe Canyon, Belly River, Scollard, Kootenay, and Luscar. The areal extent of the formations is shown in Fig. 1; several of the formations overlap each other. The Horseshoe Canyon (HSC) formation lies in the lower center of the province, the thicker portion of it having a length on the order of 250 miles (400 km) and an approximate average width of 50 miles (80 km), for an areal extent of over 12,000 mi[2] (32 000 km[2]). The Alberta Geological Survey (AGS) has done extensive research into the various coal formations in Alberta. The following paragraphs and a number of figures contain information they have made public.[1] The more detailed information is from the central portion of the Horseshoe Canyon geographical area, east of Red Deer, specifically Townships 39 and 40 and Ranges 22 and 23 west of the 4th meridian, and represents some of the better coal pay. The geological cross-section (Fig. 2) illustrates the depth relationships of the coal-bearing formations to each other, as well as in general. The stratigraphic chart (Fig. 3) illustrates the age relationships of the formations and defines many of the coal zones within those formations. The geological cross-section shows how the Upper Cretaceous and younger formations are at their deepest level in the west in the foothills of the Rocky Mountains and trend upward to the east until they outcrop at ground level. Several coal mines are located near the outcrops of each coal zone, allowing some study of the characteristics of each of these particular coals. Fig. 4 highlights the net coal thicknesses of the lower Horseshoe Canyon[2] with contours up to a thickness of greater than 39 ft (12 m).As is evident, even the best part of the pay is still a very large area.