Well cementing is a process used in penetrating subterranean formations that produce oil and gas. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set, i.e., harden into a solid mass, to thereby attach the string of pipe to the walls of the well bore and seal the annulus. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
One problem commonly encountered during primary cementing is the presence of one or more permeable zones in the subterranean formation. Such permeable zones result in the loss of at least a portion of the cement slurry to the subterranean formation as the slurry is being pumped down through the casing and up through the annulus. Due to such loss, an insufficient amount of the slurry passes above the permeable zones to fill the annulus from top to bottom. Further, dehydration of the cement slurry may occur, compromising the strength of the cement that forms in the annulus. The permeable zones may be, for example, depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the cement slurry, or combinations thereof. In some cases, the weak zones may contain pre-existing fractures that expand under the hydrostatic pressure of the cement slurry.
Traditional methods of overcoming the above described problem include sealing the permeable zones using thixotropic cements, non-aqueous dispersions of clays, sodium silicate solutions in combination with calcium salt sweeps, or well fluids containing inert platelets such as mica or Cellophane™. Unfortunately, the use of some of these materials can contaminate fresh water in the subterranean formation. Such fresh water is often encountered when drilling for oil or gas and may be a source of water for animals and humans. Another limitation of these materials is that they either fail to adequately plug or are slow to form plugs in permeable zones that expand in size due to, for example, the hydrostatic pressure of the cement slurry. A need therefore exists to develop a method of inhibiting a cement slurry from migrating through permeable zones in a subterranean formation even if those zones expand and without risking the contamination of water in the formation.