The Use of a Novel Spacer and Ultralow-Density Cement System To Control Lost Circulation in Coalbed-Methane Wells

Abstract

Summary Coalbed methane (CBM) is one of the major clean-burning gas resources. Development and usage of CBM can increase the energy supply, improve coal-mine safety, and protect the global environment. The southern Qinshui basin of China is rich in CBM; it is the first area that obtained good results in the commercial development of CBM in China. However, lost circulation is a common problem during cementing through the fractured coal-seams, known as cleats. These cleats form an interconnected fracture network, and this allows cement-slurry loss to the coal-seams during cementing operations. Cleats are often wide enough to accept cement slurry, not just cement-slurry filtrate. Hence, simply reducing slurry density does not ensure a good cement job. Preventing cement-slurry loss to coal-seams while cementing production casing is a major challenge. To solve the lost-circulation problem across coal formations, an innovative hydrophilic-fiber-based spacer fluid and ultralow-density cement slurries with density of 0.95 to 1.20 g/cm3 have been formulated. Laboratory tests have verified the effectiveness of hydrophilic-fiber-based spacer fluid in plugging these cleats of coal seams. The ultralow-density cement slurries exhibit favorable properties. These test results are confirmed by the good field-application results obtained in southern Qinshui basin. Fourteen CBM wells were successfully cemented with the novel spacer fluid and the ultralow-density cement slurries. The combined application of the hydrophilic-fiber-based spacer fluid and the ultralow-density-cement-slurry system is able to successfully resolve the lost-circulation problem in CBM wells, and provides better zonal isolation as well. This paper provides details about the slurry design and field application, and also presents the cement-bond evaluations that verify the conclusions.