An Integrated Geomechanical Approach in Mud Loss Pressure Prediction in a Giant Oil Field in Offshore Abu Dhabi

Abstract

In a giant oil field in offshore Abu Dhabi, the intermediate hole section has suffered from borehole instability in highly laminated shale, mud loss in leaky carbonate and high reservoir pressures. Appropriate control of equivalent circulation density (ECD) is a key for safer drilling to the target reservoir. The present study aimed at analyzing the mud loss pressure in the leaky carbonate which is the allowable limit of ECD in the hole section. The study investigated spatial distribution of mud losses and variation in ECD at mud loss based on review of drilling operation in nearly 560 boreholes. The geomechanical models were built in five key locations. The area in which vuggy and fractured carbonates are distributed was interpreted based on the seismic interpretation. The mud loss pressure in the carbonate formation was analyzed based on the comparison of formation breakdown pressure and the minimum in-situ stress (S3) with ECD applied in drilling. The study proposed a map of mud loss pressure in the leaky carbonate in the six regional divisions in the studied field. The P90 and P50 values of the mud loss pressure in each region were analyzed by using the geomechanical models. The P90 value was analyzed by comparing ECD at mud loss with the magnitude of S3 in each region. Mud losses are highly likely to be prevented by controlling ECD under the P90 value in each of the regions. The P50 value was estimated by analytical solution which considers the effect of natural stress cage due to stress redistribution and thermal stress due to cooling effect by drilling fluid in near-borehole wall area. The P50 value was represented in a polar plot since regional variation in the rock mechanical characteristics and in-situ stresses and borehole direction were considered. The map functions as a guide for the design of mud weight in the ongoing drilling campaign. The P90 value is applied to the standard design. The P50 value is referred to as the upper limit in the case that excessive borehole instability in the shale or kick in the reservoirs is expected by applying the P90 value. If ECD above the P50 value is expected, additional casing should be considered to secure the loss zone before penetrating collapsing shale and high-pressure reservoirs.