This paper presents both laboratory and field data on the use of spacers and washes during cementing to improve the results of a cement job.
Cementing of oil and gas wells involves the placement of cement through casing into the annular space between the casing and the walls of the borehole. Normally, drilling mud is present in the annulus and must be displaced as the cement enters the annulus. Efficient mud removal is necessary to ensure the quality of the cement job.
Almost all drilling muds and cements are incompatible with one another. Incomplete removal of the mud by the cement can result in high displacement pressures and/or the formation of a channel. These problems can be prevented if the cement and mud are not allowed to come in contact with one another.
By preventing the incompatibility, formation damage due to fracturing of and loss of cement to the formations is avoided. Elimination of channeling results in good zone isolation and thus more efficient stimulation treatments. This also reduces production of fluids from undesirable zones.
The future of an oil or gas well depends on its early life. Among the things occurring in a well's early life which affect its future are the drilling, cementing, and stimulation of the well. Proper application of cement insures good zone isolation and protection of casing. Isolation is necessary to insure that the stimulation treatment is applied only to the zone of interest and to limit production of undesirable well fluids (Fig. 1). If a good cement job is not achieved on the first attempt, successive attempts have less and less chance of success.
To insure a good cement job, the best possible cementing techniques must be employed. Of course, the first element of a good cement job is the cement itself. The cement system must be properly designed for the well conditions.1,2,3 If well conditions will permit, casing hardware such as centralizers and scratchers should be used, especially across any potential zones of interest.4,5
After the casing or liner has been run into the hole, the mud should be circulated at least "bottoms up", or until it is in good shape. This is necessary to reduce the gel strength of the mud so the cement will more easily displace it. Fallowing mud conditioning, the cement must be mixed and placed using proper techniques. For cement to make intimate contact with the formation and pipe, which is necessary to achieve good bonding and good zone isolation, all the mud must be displaced. Pipe movement such as rotation or reciprocation during placement of the cement will greatly aid mud removal.4,5
Removal of mud may be a very difficult task, even with very simple muds, proper mud conditioning, pipe movement, and use of casing hardware. Most muds are incompatible with cement, whether they are water-base muds prepared with clay or polymer, or oil-base muds.
Incompatibility can exist with light weight, as well as high density, muds.