Cement Seal Units Eliminates The Inter-Zonal Communication

Abstract

Abstract SEAL UNITS prevent gas/fluid movement at the interface between the outer surface of the casing and the inside of the cement sheath. In spite of the improved practice of releasing pressure on the casing after the top plug bumps, investigations by different research and development groups confirm that leakage at this interface is very possible.When subjected to differential pressures, often encountered in completing or producing a well, inter-zonal communication at this point will occur, like "channeling" in the remainder of the annulus.This will prevent efficient production of the producing zone or cause stimulation work (fracturing, acdizing, etc) to be ineffective due to uncontrolled fluid migration in the annulus. 1) Introduction There are two possible paths for fluid or Gas movement in the annulus:The interfaces between cement/rock and cement/casing andThe cement matrix. Poor mud removal is normally identified as the major source of communication problems, even poor bonding at the interface can occur even if mud cake or oil films have been completely removed. Mud removal generally can be drastically improved by carefully planning the centralization of the string so that an even annulus is created prior to removing the mud by the cement. But cement adherence to the formation and casing is primarily affected by cement shrinkage and by stress changes induced by downhole variations of pressure and temperature. These changes most often take place inside the casing but can also occur in the formation.For years it was standard operating procedures to keep pressure on the top (shut off/closing) plug after it had bumped following a casing cement job until the cement had reached sufficient compressive strength to prevent movement. When the pressure was released, the casing contracted and a small micro-annulus was formed between the casing and the set cement. When this phenomenon was recognized, steps were taken to eliminate the problem. When the top plug bumped, the pressure was released and casing slacked off providing the float equipment held and no back flow. Therefore, the casing would not be in expanded condition while the cement was solidifying. This of course, substantially reduced the nominal size of any micro-annulus that would have existed. Theoretically, if the above procedures are strictly followed, no micro-annulus should be present. Since the introduction of inflates and swellable rubber, there has been a tendency to move towards these alternative way of fighting the microannulus problem. But that alternative is another escalation to the operators cost of completions and adds considerable complications on the rig site. The handling of long sections of swellable materials on production casing can slow an operation down, so the costs implications are for rig time and materials. The material is considered fragile by some people, comparable to the agruement against the use of centralizers with inflates. It always comes back to the right application of these tools together in a system instead of a jumble of many different components.