ABSTRACT

Initial production in the deep water Gulf of Mexico has been underway for nearly 10 years. Secondary recovery methods are now in the planning stage. One of these methods is seawater injection. A number of operators in the North Sea have already started using this technique. Consideration of a water flood brings on the possibility of reservoir souring. For the Gulf of Mexico, a review of the materials of construction for existing wells found that well casing and completions were designed for sweet service, consequently a fitness for purpose testing qualification approach was undertaken. A fitness for purpose approach addressed ~20 ppm H2S in a condensed water phase (1% sodium chloride) and a number of alloys including P110, Q125, 8630M, 4130M. NACE TM 0177 test methods were utilized: uniaxial tests, double cantilever beam (DCB) fracture toughness test and bent beam tests, also known as Sc. Results and discussion are presented.

INTRODUCTION

A waterflood is planned for some of the reservoirs of the in the Gulf of Mexico (GoM). The waterflood plans to utilize seawater injection with a nitrate injection system in conjunction with oxygen removal. It is unclear how effective the nitrate injection system will be to control souring. In the event that the chemical injection system is not effective in microbial activity control, there is the potential for H2S evolution. A number of reservoir studies havebeen conducted and have found that the most likely H2S concentration (95% confidence) will build up to 10 ppm. Since most of the well casing and completions currently in place were designed for sweet service, they were not in compliance with NACE Standard MRO 175 Material Requirements Sulfide Stress Cracking (SSC) Resistant Metallic Materials for Oilfield Equipment. (Ref 1) Hence fitness for purpose (FFP) testing was undertaken using the guidelines for such testing as described in ISO 15156. (Ref 2). The wells that will be affected by this waterflood are found in the deepwater GoM in ~ 3000 ft (1000 m) of water primarily produced through tension leg platforms (TLP).

To determine the test parameters the following well information was generated in Table 1.

Table 1. Well pressures and temperatures.

For H2S service, a material of construction review of the well completion equipment was undertaken. Alloys used for casing that are vulnerable to sour service were identified. It was possible to make a compilation of the casing material test reports (MTR?s) for the wells that would be affected by waterflood production. A similar review of the production equipment was undertaken. Alloys that were not in compliance with NACE standard MRO 175 were identified and a fitness for purpose test program was designed. The restrictions for materials for sour service of MRO 175 (Ref 1) apply

When the partial pressure of H2S in a wet (water as a liquid) gas phase of a gas, gas condensate, or crude oil system is equal to or exceeds 0.05 psia.

Although the H2S concentration is not expected to exceed 10 ppm, the fitness for purpose testing was undertaken at twice the 10-ppm level, namely, 20 ppm. Service conditions for either shut-in and flowing conditions were reviewed and testing conditions were established. The scenario for sour service that was of concern is that which may occur during shut-in conditions if a tubing leak occurs, and sour gas may leak into the annulus, it would then expose the top of the casing to sour gas and condensed water. For the GoM, wells considered here, the temperatures of concern are room temperature (found at the top of a platform) and 40°F (4°C) subsea at seafloor .

For qualification of materials for sour service materials, the MRO175 docume

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