Summary During the last few decades, several laboratory investigations and field studies have been conducted in an attempt to find solutions to the problem of gas migration after primary or remedial cement jobs. This article reviews the primary or remedial cement jobs. This article reviews the general findings of previous investigators and offers an updated explanation of the mechanism of gas migration. Results of our laboratory studies show that "mobility" of the fluids in the pore spaces during the early life of the cement, particularly after the cement structure becomes load-bearing at a given hydrostatic pressure, is the main factor that must be controlled to minimize gas migration within the cement lattice. We also show that fluid-loss control alone, though helpful, is not sufficient to stop gas migration. This paper also describes an "impermeable" cement system developed by applying the principles outlined in the laboratory investigation. This impermeable cement has been used in field applications in areas where severe gas migration problems have been experienced after cementing. Thus, gas migration through the cement has been prevented when this new approach is used. Several of prevented when this new approach is used. Several of these case histories are presented and discussed. Introduction Literature Review. For many years the petroleum industry has recognized the problem of gas invasion of wellbores after cementing. In the early 1960's, Evans and Carter showed the importance of the condition of the pipe surface (roughness and wettability) in obtaining an pipe surface (roughness and wettability) in obtaining an effective bond at the casing/cement and cement/formation interfaces. In 1964, Bearden et al. introduced a special mechanical device that could be attached to the casing to control interzonal communication. The device consisted of a sealed ring of deformable rubber molded between two steel flanges, one of them movable. In 1966, Scott and Brace reported that primary cementing was improved by running resin-coated casing through completion intervals. The first published attempt to explain the problem of gas communication by means other than leakage at the casing/cement and cement/formation interfaces was presented by Carter and Slagle in 1970. The concept of presented by Carter and Slagle in 1970. The concept of the "inability of the cement column to effectively transmit full hydrostatic pressure" was formally introduced to the industry in that paper. In 1974, Stone and Christian used laboratory scale models to show that when the gas pressure was higher than the hydrostatic pressure after the cement had taken an initial set, a channel would form and gas would continue to migrate even after decreasing the formation gas pressure. In their recommendations, the authors brought up the need for good mud and cement placement practices as well as for the usage of cement placement practices as well as for the usage of cement slurries with good fluid-loss control and short setting times. The industry as a whole had been very aware of the need for proper displacement of the cement slurry to achieve good primary cement jobs. Even as early as 1948, Howard and Clark dealt extensively with the factors to be considered for proper casing cementing. Following the steps of previous investigators, Christian et al. in 1975 wrote a paper emphasizing the need to use cement slurries with good fluid-loss control to prevent gas migration. Their research indicated that prevent gas migration. Their research indicated that premature dehydration of cement slurries, resulting from premature dehydration of cement slurries, resulting from lack of fluid-loss control, may be the primary cause of gas communication. They proposed that fluid-loss additives effectively tie up the water required for hydration of cement and slowly release the water during the entire hydration process, as well as minimize the ability of fluids to flow through the cement porosity. In 1976, Garcia and Clarks ran a series of experiments and reported that annular gas influx was seen to occur if cement fluid-loss or uneven slurry setting occurred high in the hole such that hydrostatic head communication no longer existed between the bottom of the hole and the mud column above the set cement point. They indicated that while the cement slurry remained fluid, gas flow between zones was controlled. However, sometime after the cement set, gas flow began. Cook and Cunningham in 1977 presented an improved method for evaluating the fluid-loss requirements necessary to obtain successful liner or casing cementing jobs. They recommended the use of maximum fluid-loss control in cement slurries when cementing across zones of varying pressure to minimize gas leakage, since increased fluid-loss control resulted in less gas invasion and lower cement permeability. Another way to improve gas migration control, as reported in the literature, is to use expanding cements to promote better bonding at the casing/cement and promote better bonding at the casing/cement and cement/formation interfaces. One of the most recent papers dealing with this subject was presented by Griffin et al. in 1979; they discuss an expanding cement system that can provide superior bonding and zone isolation. A paper containing a series of practical techniques to control gas migration was written by Levine et al. in 1979. A graphical technique was introduced that predicts the potential of annular gas flow after cementing. Also in 1979, Tinsley et al. introduced, for the first time, a new cement system intended primarily to control gas migration at the cement/formation interface. JPT P. 1041

A method to scale down well parameters to laboratory conditions for more realistic testing of cement recipes to be used to control wellbore invasion/migration of formation fluids after cementing is introduced. The scale-down procedure proposed here addresses a worst-case scenario. It assumes that the offending gas zone (source of the invading gas) has enough permeability, thickness, and gas volume to invade fully and to pressure-charge the cemented annulus (cement column), if conditions allow. The method is demonstrated in conjunction with the use of a certain laboratory gas-flow apparatus. This general scale-down procedure can be used with other test equipment.An imaginary set of well conditiojls is used to illustrate the mechanics of the use of the technique. Tests run with the procedure and the computer-monitored gas-migration test cell are described. A preliminary set of criteria for selecting a cement recipe for a given well is presented. A field case illustrates the use of the procedure.

The petroleum industry has recognized the problem of gas flow after cementing for more than two decades. Several laboratory investigations and field studies have been conducted in an attempt to solve the gas migration problem. However, most of the previous field practices designed to control the gas migration problem have been either unsuccessful or only partially successful. A study of the cause and a laboratory-developed solution for controlling the gas migration problem were presented in SPE 11207. The present paper summarizes the field results obtained during an eighteen-month period using an "impermeable cement" system to control the gas migration problem. Results of 84 cement jobs performed in Texas, Oklahoma, New Mexico and Louisiana are summarized, and unique field cases are discussed in detail. All four major types of cementing jobs have been performed using the "impermeable cement" system and an overall success ratio of over 90% was obtained.

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