Abstract

Sustained casing pressure is a serious problem that is prevalent in most of the oil producing regions of the world. Annular pressure can be a significant safety hazard and, on a number of occasions, has resulted in blowouts. Sustained casing pressure results from the migration of fluids in the annulus. The most common path for migration of fluids is through channels in the annular cement. To safely and economically eliminate sustained casing pressure on a well in the Gulf of Mexico, W&T Offshore, Inc. utilized an injectable pressure-activated sealant technology to seal channels in the annular cement of their well and eliminate the casing pressure. The mechanical integrity of the well was restored, saving over $1,000,000 compared to a conventional rig workover.

Introduction

Migration of fluids through the annuli of wellbores can result in a condition known as "sustained casing pressure" or "SCP". SCP is pressure that rebuilds in the annulus after being bled down.1

With age, the integrity of all wellbores deteriorate. Cracks and fissures develop in the annular cement due to a number of factors related to cement composition, thermal stress, hydraulic stress, compaction, wellbore tubulars, and the downhole environment. The most significant cause of sustained casing pressure in the outer casing strings is a poor cement bond that results in the development of cracks and annular channels.2 The cracks and microannulus channels through the cement provide a path for high-pressure fluids to migrate from deeper strata to low-pressure strata or to the surface.

If left uncontrolled, SCP represents an ongoing safety hazard and can cause serious or immediate harm or damage to human life, the marine and coastal environment, and property.3 A significant flow of high-pressure fluids to a low-pressure strata results in an underground blowout. A significant flow of high-pressure fluids to the surface results in an irreducible casing pressure at the wellhead and the potential for catastrophic failure of wellbore integrity.

SCP is a pervasive problem for the oil and gas industry. According to the records of the Minerals Management Service ("MMS") of the United States Department of the Interior, SCP affects over 8,000 wells in the Gulf of Mexico.4

Conventional Remediation Risks

The conventional remedy for outer casing SCP is to perform an expensive and risky workover of the well using a rig. In the past, the industry has been reluctant to cure SCP problems on most wells based on a cost/benefit analysis of the relative risks. A conventional rig workover is a dangerous operation. Personnel can be injured or killed. Equipment can be damaged or destroyed. Blowouts or spills pose a significant environmental risk. The costs and risks of the conventional rig workover solution exceed the costs and risks associated with the current sustained casing pressure practices.5

The rig workover procedure requires removal of the tubing and injection or squeezing cement in an attempt to block the cracks and channels through the annular cement. Depending on the location, porosity and permeability of the cracks and channels, the cement squeeze may or may not be successful in sealing the paths for the migration of the fluid through the annulus. A cement squeeze is a costly procedure with a questionable probability of success.

Cost-Effective Alternative

As an alternative to a rig workover, a safe, cost-effective sealant process has been developed that eliminates the SCP by sealing the annular channels that provide the paths for the migration of the fluid through the annulus. Tests and actual job histories have shown that this sealant can be injected into the annular channels even after attempted injection with normal mud / cement mixtures have failed.

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