Should We Worry About Our Field Development Planning due to Compaction Acceleration?

Abstract

Reservoir A is a mature carbonate gas-cap field that has been depleting gas for a long time. Reducing the compression pressure can increase the gas recovery. But this can also speed up the reservoir compaction due to higher withdrawals, which can cause subsidence, permeability reduction, wellbore instability, completion integrity, etc. To prevent this, history-matching exercises with Assisted History Matching (AHM) technique are used to investigate the minimum pressure that would start shrinking the porosity. AHM is a way to find the best values of the uncertain parameters that minimize the mismatch between the simulated and observed data. Objective functions (OF) are used to match the production and pressure within a small defined tolerance. The reservoirs are split into 6 sectors for easier analysis. After doing AHM exercises, the best cases are chosen and various cross-plots are made to see how OF and history-matching variables are related. The cases with the strongest relationships are checked to see if modifying the rock compaction can be the most likely option to improve the history-matching. In the initial attempt with AHM, the simulated pressure in the southern sector consistently surpasses the actual pressure, whereas the opposite is true for the northern sector, especially in recent times. This discrepancy remains even after adjusting factors such as permeability, fluid contact, reservoir pore volume, and transmissibility during the history-matching process. The variation in pressure might be due to different compaction rates across the gas cap zone. Utilizing two separate rock compaction tables that transition from fast to slow from north to south could potentially improve the pressure matching. Future reservoir development, either through infill wells or field compression projects aimed at reducing abandonment pressure, may negatively impact the safety and economy of both sub-surface and surface facilities. To mitigate these risks, it is essential to avoid excessive production withdrawal, particularly in regions where a faster rate of compaction could pose a threat. Compaction acceleration can affect the future development plan of reservoir A, by reducing the recovery factor, increasing the risk of reservoir subsidence, well issues, altering the optimal design of the new wells and the compression system. The study with AHM can help to evaluate how serious the compaction problem is for reservoir A, and it can recommend the need to initiate more sophisticated geomechanics modelling if it is essential.