Observing Alterations in Fault Dynamics Following Extended Production and Injection in a Carbonate Reservoir. A Case Study from UAE Offshore

Managing faults in carbonate reservoirs can be feasible with proper handling. Historically, various fault-related issues have been documented; some show no change, while others encounter significant problems like total mud losses, tight spots, stuck pipe, casing failure, washouts, breakouts, and hole collapse. This paper examines the characteristics of faults in their initial state and their behavioral changes after production/injection interference. Seismically driven fault interpretation outlines the fault plane(s) location and dip/strike-slip components. To further reduce fault-associated risks, Allan diagrams help understand potential leakage through juxtaposition. This analysis reveals the initial characteristics of faults, aligned with seismic vintage data. At the well-planning stage, any intersected fault would be characterized using the same vintage data, indicating the depth and throw of the fault(s) but not recent behavioral changes. An example from the study area highlights the necessary predrill evaluation (while the fault was in its initial state) before drilling a producer well (Well-A). Well-A faced no difficulties during drilling nor any unexpected water breakthrough during production. An additional supportive data (e.g., borehole resistivity image) was obtained in Well-A to further define the fault/fracture zone. After several years of production with a consistent water cut, a new well (Well-B) was drilled through the same fault. While drilling near this fault zone, Well-B experienced complete mud losses. Upon Well-B intersecting the fault zone, fractures opened, causing the drilling fluid to choke Well-A, resulting in a complete cessation of its production. After a clean-out procedure, Well-A was revived; however, its water cut increased to over 50%. Faults in the study area display more strike-slip than vertical components, resembling fractures or fracture corridors. The borehole resistivity image shows that most initial condition fractures/faults are closed, except for a few continuous conductive fractures. Although the predrill Allan diagram indicates some leakage through juxtaposition zones, Well-A's water cut remained minimal throughout its production phase. Prolonged production led to partial depletion of the fault zones, reducing the pore pressure gradient from 0.6 psi/ft to 0.5 psi/ft and the frac gradient from 0.8 psi/ft to 0.7 psi/ft. This reduction was significant within the fracture zones. When Well-A was reactivated, the water cut was initially low. This was anticipated because the opened fracture might close. After a few weeks of production, the water cut drastically increased from 10% to 62%. This significant increase in water cut suggests that a large amount of water was being produced along with the oil, likely due to the communication between the reservoirs and the opened fractures. Accurately characterizing faults and fractures during well-planning is essential for optimal lower completion planning, which helps to prevent early water breakthrough and supports a sustainable development plan.