Abstract
We present a Decline Curve Analysis-based algorithm to rapidly determine the rig drilling schedule (effectively the investment timing) for future large offshore gas fields with the aim of sustaining nominated gas delivery. The algorithm generated schedules for multiple platforms and wells, and incorporates constraints due to facilities and pipelines. The resulting production profile is benchmarked against the profile generated by a full-blown reservoir simulation.
The new algorithm takes the gross rate vs time profiles from each well and platform that were generated from standalone reservoir simulation models and the Arps’ empirical model. This well and platform sales gas rate vs time profile will be transformed into sales rate vs cumulative sales gas using relevant CO2 shrinkage factors to deliver gas within sales specifications. The algorithm will seek for contributions from each well/platform to meet the nominated gas delivery based on the current production profile, imposed prioritizations and facility/pipeline constraints. Finally, individual standalone models are coupled into the existing integrated model, and the results are compared against the ones generated by the algorithm.
The algorithm (implemented using Visual Basic) is benchmarked against a full-blown reservoir simulation that couples multiple large gas fields with numerous wells. The wells are grouped into 5 platforms; platform A1, A2, B1, B2, and C1. Each platform contains 4 to 5 wells, which are optimized to fit into a single drilling rig campaign if possible. Drilling rig schedules of all 5 platforms are shown to satisfactorily match (i.e., within the campaign) for both the DCA-based algorithm and full-blown simulation. The results from the algorithm reveal that the total gas delivered (i.e., EUR, estimated ultimate recovery) from each platform is consistent with the original EUR. The blended feed gas CO2 profiles generated from both methods also satisfactorily agree. It can also be seen that the plateau of the gas production ends at about the same time in each case. Based on these results, it can be concluded the DCA-based algorithm is an excellent tool to generate multiple forecasts to select the optimal integrated development strategy for multiple gas fields.
This new algorithm thus reduces the required time to optimize rig drilling schedules from days to hours. This is a valuable tool that can also alert management on investment opportunities and cost optimizations, in a timely manner before final evaluation and decision making.
