The objective of this paper is to highlight the techniques used during a recent workover program to alleviate sustained casing pressure in a Gulf of Mexico field. Discussion will include cement and workover fluid programs, hole preparation before milling and cementing operations, and improved milling procedures.
The field was drilled during the 1980s and sustained casing pressure has been prevalent in some cases previous to initial completion operations. In two rig workover programs in 1989, and again in 1990, a total of nine temporary abandonments were attempted. These programs resulted in limited success until the most recent workover program beginning in 1999. In addition, a non-rig program was running concurrently with the most recent rig operations in an attempt to avoid rig intervention when possible. However, this system will not be discussed in this paper except as it applies to specific operations.
All indications are that the workover program was successful even though there was some need for remedial rig work on three wells while operations were ongoing. A review of past attempts, relying on internal correspondence and drilling reports, will be made and compared to the present 1999 rig operations program using first hand knowledge of the operations and decisions.
Varying magnitudes of sustained casing pressure (SCP) exist in some of the fields of the Gulf of Mexico. The sources of this pressure vary along with the particular affected casing string in the well. Casing pressure does occur from thermal expansion of annular fluid in some high rate wells. However, once a well is flowing at steady state conditions, the pressure from all casing strings should bleed through a needle valve to and remain at atmospheric conditions. If the casing pressure builds up when the valve is closed, then the casing exhibits SCP. Regulations contained in the Mineral Management Service (MMS) 30 CFR 250.517 stipulate that the casing and tubing annuli should be monitored for pressure buildup and the MMS should be notified if SCP is observed. Departures from 30 CFR 250.517 do exist for low risk cases as long as proper monitoring and reporting is in place.
The MMS is the Federal Government's regulatory agency that manages the natural resources on the nation's outer continental shelf. The guidelines it imposes act to maximize the recovery of mineral resources from federal lands while preserving the environment. The MMS regulations contained in 30 CFR 250 seek to maintain a safe work environment in offshore oil and gas operations, and adherence to these guidelines spurred the rig workover program covered in this paper.
The goal of the workover project was to address the most persistent cases of SCP. Some of the wells had been previously worked over to eliminate SCP, but the pressure had returned. Future drilling from the platform was planned and taken into consideration while the workover program was being designed. With continued use of the platform, worker safety became the primary concern because it could be manned for many years. Addressing the SCP directly and safely while still allowing for possible future platform utility became the design driver.
The objective of the 1999 workover program was to address SCP in the field with a consistent and effective method. Techniques were developed by analyzing the successes and failures of past operations and applying aggressive remediation programs tailored to individual wellbores. Discussion will include improved design guidelines in hole preparation before milling and cementing operations, improved milling procedures, and application of a latex cement slurry.