Identifying Simple Criteria to Assess the Risk of Mud Loss During Drilling Operations Nearby Faults

In a West Africa offshore oil field, the drilling campaign faces challenges due to numerous normal faults in the overburden and the reservoirs. Crossing these faults with a mud weight too high have led to severe losses that increased the non-productive time and the risk of well control issues. Anticipating mud losses near faults remains challenging, as the conductivity and sealing capacity of the mud in such context remains hard to predict. To optimize the drilling performance in the faulted sections, we built an extensive database by analyzing 232 mud loss events on and nearby faults crossed by 29 wells. We complement the database with the lithology, the stresses acting on the fault planes and the mud weight while drilling, or equivalent circulating density, that generated the losses. 1D mechanical models were done for each well to obtain these stresses, then we used the Mohr-Coulomb failure criteria (MC) with the mud pressure while drilling to examine fault instability. We show that the mud losses are related to the rock contacts at the fault-well intersections and the pressure of drilling mud. Without making prior assumptions on the fault mechanical properties, such as cohesion and friction angle, the latter are directly obtained from the effective-stress state with mud pressure during losses that clearly align on a Mohr-Coulomb type failure line. The inverted mechanical properties of the fault are compared to the intact rock properties by applying the same friction angle, but by degrading the cohesion by a constant factor. Lithologies at fault contacts, like shale/sandstone or sandstone/sandstone, are more prone to significant mud losses compared to shale/shale contacts where the MC failure criteria is met. Losses occur in 68% of fault contacts involving sandstones, compared to 50% in shale-on-shale contacts. Moreover, we observed that high mud pressure used to drill development wells were more prone to significant mud losses. From this analysis, we developed a simple, yet robust, method using the Mohr-Coulomb failure criterion to predict fault reactivation by mud pressure during drilling operations. It refines mud weight strategies and well design to address faults and proactively improves drilling performance for upcoming wells.