Foamed cement systems, consisting of 35% to 45% nitrogen by volume, have been pumped during the past several years in a number of different applications throughout the continental United States including the Appalachian Basin. Candidates for foamed cement include wells that have low breakdown pressures (lost circulation), those that require a stage tool to obtain required fillup and air-drilled holes where high viscosity slurries in narrow annulus create excessive pressures that contribute to weak zones thieving cement.1
The Devonian Shale exhibits all the above criteria in areas of West Virginia, so was selected as a field study for purposes of developing technology while improving the percentage of "successful" cement jobs in the field. The exact definition of success may vary from area to area, operator to operator, so for purposes of this paper it will be defined as obtaining consistent cement fillup in the annulus and subsequently fracture stimulating through the cement sheath with no operational or production problems due to high break down pressures. Other evaluation criteria of successful cement jobs such as economics and bond logs must be analyzed on operator/field basis and not a regional basis.
Operationally, significant strides have been made in mixing, pumping and maintaining a stable foamed cement in the field. Early jobs may have failed due to nitrogen bubbles becoming too large by interconnecting and migrating through the base cement slurry. Those jobs resulted in, at best, inconsistent slurry density throughout the column and at worst nitrogen returning to surface in the annulus causing complete job failures resulting in remedial squeeze work.
Recent improvements in maintaining foam stability have allowed foamed slurry samples to be retained at the surface during pumping operations. Precise job monitoring of rates, pressures, temperatures and slurry densities have established quality control assurances at the surface so that downhole variables can be better determined.
The state of the art of foamed cement systems pumped in the Appalachian Basin today is vastly superior to that of the past. What was once a good idea in theory is now an even better idea in practice.