ABSTRACT

At a five year old producing project in the deep water of the Gulf of Mexico, a sea water flood has been proposed. One of the consequences of such a flood is the souring and compaction potential of the reservoir after the seawater is injected. To assess the effect of souring on the casing already in place modified NACE tensile tests were conducted. The test was modified specific to the environment and stress situation projected for the project. The modified test provided a conservative evaluation of sulfide stress cracking susceptibility of the casing in the specific environment souring conditions. The specific objective of these tests was to determine the applicability of CYP-110 casing in high strain, H2S concentrations, and pH that may be encountered during the water flood. A casing alloy was tested in twelve separate test environments that included four H2S concentrations (0.03, 0.30, 0.60, and 3.0 psia H2S, balance N2), three strain levels (4, 6, and 8%), pH 7.4 at 130°F for 30 days.

INTRODUCTION

In evaluating a waterflood in the Gulf of Mexico, the use of raw seawater is an option. If the water flood is undertaken to improve ultimate hydrocarbon recovery and alleviate formation compaction due to depletion there is a high likelihood of reservoir souring. The strains that formation compaction could put on the casing were estimated to be as high as 12%. Additionally, pumping seawater into the formation could result in a nearly neutral pH of 7.4, but various levels of H2S could be generated. The following is an estimate of the H2S concentrations estimated for souring conditions.

The formation temperature and aqueous environment were estimated at 130°F and pH of 7.4, respectively. A review of the casing in place at the project that would be affected by reservoir souring found that controlled yield, (CY) P-110 was the predominant casing in place. Hence, this work was targeted to determine the effect of H2S at different partial pressures on this alloy at high strains that would be generated by the compaction forces.

A review of the data available on sulfide stress cracking (SSC) for CYP-110 due to the combined effect of nearly neutral pH, variable H2S content and high strain found that no information was available. (Ref 1-6). Most of the published work is limited to strains less than 0.2% that are found when specimens are tested at 80-90% of yield. Consequently, an experimental program addressing the variables of concern was undertaken. The strains in this experimental evaluation were in the plastic regime.

EXPERIMENTAL PROCEDURES

Materials and Test Samples

A section of 7 5/8" OD x 1/2" wall seamless CYP-110 was obtained that was similar to casing that has been installed in the project. Standard NACE tensile specimens (4" long w/l" gage X 0.25" diameter) were machined in a longitudinal orientation from the midwall of the casing.

Pre-Straining

Applied stress requirements for a standard NACE tensile test typically use load level or percent of yield as loading criteria (i.e. 80% or 100% of yield). However, for this work instead of stress as the load variable, applied strain was the parameter used, 4%, 6%, or 8%. As noted in Figure 1, a typical yield curve for the CYP-110, all three of these applied strain targets are well beyond the yield point and in the plastic deformation zone. It was not possible to attain these elevated strain requirements using the standard NACE tensile stressing procedure as described in NACE-TM0177 - Method A. Instead, a pre-straining technique using computerized servo-hydraulic equipment and a calibrated strain measuring extensometer was developed. In this technique, each specimen was stressed to a level slightly

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