Sustained Casing Pressure in Offshore Producing Wells

Abstract A large number of producing wells in the OCS develop undesirable and sometimes potentially dangerous sustained pressure on one or more casing strings of completed wells. This paper examines the severity and frequency of the occurrence of sustained casing pressure in the offshore Gulf of Mexico area. Possible causes for this problem are discussed and case histories of remediation techniques being tried by offshore operators are presented. Introduction The invention of portland cement by Joseph Aspdin has allowed major advances in our civilization because of its low cost, strength, and ability to set under water. It has been modified and used by the oil and gas industry since the early 1900's as the primary means of sealing the area between the open borehole and the casing placed in the well. Shown in Figure 1 is a typical well completion showing the placement of cement to seal off the interior of various casing strings from the subsurface formations exposed by the drill bit. Ideally, the well of Figure 1 should show pressure only on the production tubing. Gauges on all of the casing strings should read zero if:the well is allowed to come to a steady-state flowing condition, andthe effect of any liquid pressurization due to heating of the casing and completion fluids by the produced fluids is allowed to bleed off by opening a needle valve. Only a small volume of fluid generally has to be bled in order for the casing pressure to fall to atmospheric pressure if the pressure was caused by thermal expansion effects. If the needle valve is closed and the well remains at the same steady-state condition, then the casing pressure should remain at zero. If the casing pressure returns when the needle valve is closed, then the casing is said to exhibit sustained casing pressure (SCP). In some cases the pressure can reach dangerously high values. The Minerals Management is concerned about wells on the Outer Continental Shelf (OCS) that exhibit significant sustained casing pressure because of its responsibility for worker safety and environmental protection as mandated by congress. At present, any amount of sustained casing pressure seen on one or more casing strings of a well (excluding drive pipe and structural casing) is viewed as significant enough to trigger notification of MMS. Structural and drive pipe are excluded because it is recognized that gas of biogenic origin is sometimes encountered in the shallow sediments and can cause insignificant pressures on the drive and structural casing. SCP also triggers a requirement that records of the casing pressures observed be kept available for inspection in the operator's field office. Regulations under 30 CFR 250.517 state that the lessee shall immediately notify the MMS District Supervisor if sustained casing pressure is observed on a well. A written record of notification must be placed in the operator's SCP records by close of business the next working day after the SCP is discovered. If the well is felt to be in an unsafe condition, the district supervisor can order that remedial actions be taken. However, provisions are made for a departure from 30 CFR 250.517 to be obtained.