Previous work on annular gas flow has shown that the behavior of cement between its fluid and set states is the controlling factor that may allow gas entry. This transition phase of cement previously has not been recognized in slurry design since its importance was not understood fully and since test procedures for its definition had not been presented or established. To predict the occurrence of annular gas flow and to design cement slurries capable of helping to prevent annular gas flow, it is necessary to define the slurry characteristics at the beginning and end of this transition period as well as the length of time of the transition state. Test techniques have been developed to study the start of this transition period. Additional tests have been conducted to define the condition of cement required to prevent gas entry. The test techniques developed to define this transition period of a cement slurry are described. Numerous job variables such as pumping time, placement time, slurry composition, and circulating temperature and pressure were investigated to evaluate their influence on transition time. A method for using transition time and static gel strength (SGS) development data to help predict annular gas flow and to evaluate annular gas flow control materials is discussed.
Annular gas flow (also called gas leakage) refers to the flow or migration of gas in a cemented casing/borehole annulus. Annular gas flow has long been recognized as causing severe problems, including communication between producing zones, flow into shallow sands, and gas flow to the surface. Gas flow back to the surface in as little as 30 minutes after completing primary cementing has been reported, but interzone communication may not be evident until weeks or even months after completion of the well. Some minor interzone gas flow problems can be lived with (usually with some sacrifice in production), but control usually is demanded, and stopping gas leakage after it occurs always means expensive remedial work.
Understanding the mechanics of gas flow through a cement column and the prediction of this gas flow requires additional knowledge of the downhole cement behavior. In addition to the usual slurry properties, well data and formation pressure, the prediction method uses a parameter called transition time. Briefly, the transition time is the period during which the slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. The transition time starts when the slurry develops enough SGS to restrict transmission of full hydrostatic pressure and ends when the cement develops enough solid characteristics to control percolation of gas through the cement column. With this definition of transmission time, annular gas flow can be prevented if the pressure in the cement adjacent to the high-pressure gas zone can be maintained at a value greater than or equal to the gas reservoir pressure until the end of the transition time.