Abstract
This paper presents a three-stage integrated workflow using thermal/ geomechanical and thermal/ chemical simulation models to prevent reservoir souring during high concentration sulphate water injection. Reservoir souring takes place when sulphate reducing bacteria (SRB) is activated by cold injection water and gains access to high concentrations of sulphate along with volatile fatty acids (VFA) and/ or hydrocarbons (BTEX) as nutrients.
In the first stage a thermal/ chemical reservoir simulation model is history matched to quantify the changes in nutrient distribution around the injector. Earlier in the life of the well high volumes of low sulphate content water injection washes away the VFA/ BTEX in the near wellbore area creating a nutrient depleted zone for SRB.
In the second stage, a detailed wellbore model is coupled with a thermal/ geomechanical reservoir simulator and history matched to predict the downhole injection water temperature. Because the size of the cooled region changes with injection induced fractures extending from the wellbore into the formation, a thermal/ geomechanical simulation model is preferred.
The final stage of the workflow takes place later in the life of the injector when injection water has high sulphate concentrations. During this stage, the size of the formation cooling around the wellbore is constrained within the nutrient depleted zone with dynamic injection rate optimization to stop bacterial activity and accordingly reservoir souring.
Case studies from deepwater Gulf of Mexico reservoirs demonstrate how the workflow has been successfully operationalized to eliminate reservoir souring during high concentration sulphate water injection. Four years after the initiation of high sulphate concentration water injection, lack of sour gas production from the fields points to the robustness of the method. As suggested by the simulation models SRB activity is not promoted if cooling is only limited to the nutrient depleted zone.
This method helps quantify an upper limit for the sulphate concentration in injection water that will not cause reservoir souring. Chemical injections and/ or adjustments to the existing facility design is not required.
