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

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 coalseams, 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 ultralowdensity cement slurries with density of 0.95 to 1.20 g/cm(3) 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 fieldapplication 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.