Early development of Eagle Ford Shale (EFS) indicated the reservoir was relatively sweet, typically producing H2S in low concentrations (<1%). However in McMullen Co. TX, wells with high concentration (>4%) are found. Mapping raw untreated H2S gas concentration shows a direct correlation to salt domes and subsequent deep faulting. The enigma has been high H2S wells offset by low H2S wells, not associated with salt domes or faulting. Micro-seismic and re-processed seismic data revealed that deep faults do intersect these wellbores. This additional data indicates that deep faulting into the Edwards creates a conduit for the H2S to enter the EFS. Mapping of these deep features allows for the prediction of areas with high H2S.
Initially, long range plans were made with sweet EFS oil in mind. Encountering wells with large concentrations of H2S in an otherwise sweet field has the potential to lead to operating inefficiencies and higher OPEX and CAPEX as treatment solutions are brought into place after the fact. The best solutions take time to implement and advance warning of high H2S is critical in minimizing the financial impact.
A model was generated, based on the expected H2S concentration and production forecast, that is capable of directing the long term drilling and completions strategy as well as to provide expectations for use in the construction of facilities and selection of H2S treatment options. The drilling and completion strategy minimized the amount of H2S that will be encountered and the optimization of facilities reduces operating inefficiencies and OPEX and CAPEX outlays.
Building on that correlation, deep features were used to predict high H2S wells and high H2S was used to predict and locate deep features not previously identified. Mapping of these deep features allows for the prediction of areas with high H2S and has lead to a change in drilling and completion strategies by avoiding features associated with high H2S.