Well Integrity Assurance for Subsalt, Near-Salt Deepwater Gulf of Mexico Reservoirs

P-215

Program
This project is part of the Natural Gas and Oil Technology Partnership, Oil and Gas Recovery Technology. The program partners the National Laboratories with petroleum industry organizations to develop advanced technologies for improved natural gas and oil recovery.

Project Goal
The goal of this Joint Industry Project (JIP) is to identify, quantify, and mitigate potential well integrity issues associated with subsalt and near-salt deepwater Gulf of Mexico (GoM) reservoirs.

Performer
Sandia National Laboratory (SNL)
Albuquerque, NM

Project Results
The work conducted in this project modeled the timing and magnitude of salt forces on well casings. Computer models were developed that enabled understanding and prediction of the complex geomechanical behavior of subsurface formations. From this work, identification of optimal well paths and locations of potential borehole instability could be determined.

The improved casing design developed in this project has been applied in two of the five largest oil fields ever discovered in the deepwater GoM-Thunder Horse North and Mad Dog fields operated by BP America with multiple partners. The more-efficient well casing design implemented in Thunder Horse North field resulted in a well construction cost reduction of over $30 million. Applied to other potential wells in the GoM, the improved understanding of well casing design is expected to significantly reduce drilling and completion costs.

Benefits
The cost of drilling and completing a single well in the deepwater GoM can be as much as $50-100 million. Integral to the successful economic development of the area is that the lifetime of a well spans 10-20 years. This research has enabled development of a more-efficient casing design for oil and gas well construction in the GoM that reduces the risk of well failures. Such innovative technology will help to produce efficiently the vast remaining discovered and undiscovered oil and gas resources in the GoM.

Significant cost savings result from efficient casing designs. The appropriate design specifies what must be done for well casing stability but also what operations do not add to the stability or longevity of the well and can be omitted. For example, the analyses conducted in this project show that it is not always necessary to cement the casing/borehole annulus through the salt because the subsequent uniform loading is insufficient to substantially deform the casing. This poses no threat to drilling operations or impingement on the inner casing string in the long term and results in considerable cost savings. However, if hole quality is poor, a cemented annulus is necessary, as the cement effectively transforms the potentially nonuniform loading situation into one of uniform loading.

Other significant benefits can accrue from quantifying the magnitude and timing of salt loading. Difficult cementing jobs and liner tiebacks can be omitted, and a more aggressive well design adopted. The simplified well design and the elimination of potentially troublesome operations leads to millions of dollars in cost savings in individual wells.

Background
The deepwater GoM is the most active deepwater region in the world and provides some of the greatest challenges in scope and opportunity for the petroleum industry. The region is estimated to contain undiscovered recoverable resources of nearly 30 billion barrels of oil. By 2005, as much as 67 percent of the daily oil production and 26 percent of the daily gas production in the gulf will come from deepwater fields.

Huge formations of salt, thousands of feet thick, underlie much of the deepwater areas-and these salt formations deform plastically due to forces caused by sand and shale formations. The complex salt tectonics, coupled with the extreme water depths (as great as 10,000 feet) and reservoir depths, necessitate high development costs, and innovative technology is required to bring these fields on stream. Difficulties, such as collapsing a well during drilling, can be encountered near salt formations because of the changing stresses associated with salt deformation. Then, after a well penetrating a salt formation is cased and completed, the slow movement of salt over the field lifetime may cause premature failure of the casing through shears and twisting.

Although the behavior of salt is well described from a geologic standpoint, our knowledge is poor of the influence of salt deformation at both the well and reservoir scale (both temporally and spatially). However, the nature of the deformation occurring during field life is considered more likely to be detrimental than beneficial. In subsalt reservoirs in which the salt is laterally extensive and in close vertical proximity to the reservoir formations, there may a tendency for the salt to flow laterally to fill "subsidence bowls" formed by compaction of the reservoir interval. This lateral movement could jeopardize the integrity of well casings drilled through the deforming salt because of anisotropic loading and induced shears at the bounding formation interfaces.

Assuring the integrity of subsalt wells in the deepwater of the GoM throughout the fields life is a major drilling engineering challenge. The consequences of well failures may result in billions of dollars in remedial costs and lost production. On the other hand, the costs associated with overly conservative well design are significant, which motivates systematic analyses of casing loading for scenarios of interest.

Simplified hole-closure and casing-design guidelines for salt, many developed for the Western U.S. Overthrust Belt, are not appropriate for the relatively pure, slow-moving halite found along the Gulf Coast.

Project Summary
The research was designed to enhance the understanding of the stress fields associated with salt diapirs and the forces that subsurface salt bodies exert on oil and gas wells. This work provides the basic research to develop technologies that counter the effects of salt deformation. The project is designed as a JIP, with cofunding by DOE and an industry consortium.

The study assessed the timing and magnitude of salt loading on well casings, including the behavior of reservoir rocks during oil production, and on in-situ stresses in formations adjacent massive salt diapirs. Laboratory experiments were conducted to constrain the constitutive behavior of deepwater GoM salts and to compare their behavior with that of salts encountered during oil and gas exploration and production in the U.S. Western Overthrust Belt and in the North Sea. Computer models were developed to understand and predict the complex geomechanical behavior of subsurface formations and salt diapirs during oil production.

The computer modeling increased understanding of the geomechanical considerations needed to drill through or next to massive salt sections, by identifying optimal well paths and locations of potential borehole instability. The study also identified forces on well casings over the field lifetime, resulting in a modified design for well construction. The project performer examined the role of cement between the casing and wellbore and made recommendations on when and how cement should be used to alleviate casing stresses resulting from salt flow that could lead to failure.

Current Status (October 2005)
The project is complete.

Project Start: April 12, 2001
Project End: April 11, 2005

Anticipated DOE Contribution: $590,000
Performer Contribution: $1,175,000 (67% of total)

Contact Information
NETL - Paul West (paul.west@netl.doe.gov or 918-699-2035)
SNL -Joanne Fredrich (fredrich@sandia.gov or 505-844-2096)