A novel technique has been developed for gas migration control in shallow wells. It uses a limited volume of a right angle set cement followed by a low fluid loss tail slurry. This technique reduces significantly the well completion cost and eliminates expensive remedial treatments when gas leakage to the surface was identified after the primary cement jobs.
A comparison of post treatment gas leakage potentials for jobs using just a right angle set cement and jobs using this novel technique was made by evaluating the reduction of Formation and Hydrostatic factors.
This paper describes in detail the process, the recommended job design and execution procedures, the desired cement systems and their properties. A summary of the job result to date and two case histories are also provided.
References and illustrations at end of paper.
High pressure gas formations at depths as shallow as 50 m have been encountered in Southeastern Alberta and LIoydminster areas in Canada. These formations are often wet even before the first casing string is run. This causes gas channeling of the surface easing after normal well completion and hazards such as ground water contamination and rig fires.
Because of the shallow depths and the corresponding low ambient and formation temperatures (10 to 20 °C), preventive methods and cementing practices that have proven to be successful in deeper and higher temperatures zones could not be applied.1,2 Gas channels in the set cements are created when gas enters the slurry before cement is fully set or when severe chemical contraction takes place after the cement is set3. During the cement placement, application of 2 to 3 KPa/m excess pressure gradient above the gas pore pressure gradient will prevent gas entry into the cement slurry. After the placement, the gas entry can still be impeded if the hydrostatic pressure of the cement column above the gas zone exceeds the pore pressure in the gas zone.
The problem is that this hydrostatic pressure decreases quite drastically during the transition time in which the cement slurry changes physically from liquid to solid. The pressure drop can be more rapid if the cement fluid loss is excessive. When the hydrostatic head of the cement column falls below gas zone pore pressure, the invasion of gas into the unset cement becomes inevitable.
Several techniques4 have been implemented to overcome the early loss of the hydrostatic pressure. Some successes were achieved by hesitating the displacement of the slurry over a 2 to 3 hour period. However, the increasing slurry viscosity and friction pressure made the fluid placement very difficult to predict and control.
A well-known idea is to reduce the transition time. For this purpose, the RAS (Right Angle Set), also called the quick set, cements5 were used. RAS cements are expensive and yield relatively low early compressive and bond strengths which could cause gas leakage to the surface after rapid drill out of the surface casing, as is normal in shallow well areas.
