Formation damage mechanisms associated with drilling and completion fluids for deepwater reservoirs

Immense oil and gas reserves exist in the deepwater of the South China Sea, and the first priority for implementing successful drilling and completion operations without reservoir damage is to understand the damaging mechanisms to the reservoir formation. With the use of deepwater reservoir cores from the western South China Sea, the mineral composition, microstructure, porosity, and permeability characteristics of the reservoir formation were analyzed. With the use of core flow tests, potential fluid sensitivity was analyzed to determine how and which fluids can damage the reservoir formation during drilling and completion. The results show that the reservoir formation is composed of unconsolidated siltstone with a clay mineral content of 6-19%, and has high porosity and permeability with large pore throats. Sensitivity damage due to the increasing flow rate is strong, and an unusual salinity sensitivity exists with a critical salinity at 40,000-45,000 mg/L. Moderate alkali fluids sensitivity damage exists, with water sensitivity being weak. The main mechanisms of formation damage by drilling and completion fluids were analyzed as following: plugging of pore throats by solid invasion occurs because the sizes of the solid materials in the drilling fluids are just ideal for the formation of an external filter. Rock-fluid incompatibilities occur due to the increasing flow rate and salinity of the fluids because the formation rock is unconsolidated and the fines will easily detach and migrate, plugging the pore throats. Therefore, effective temporary plugging of the pore throats with a large diameter is important for protecting the reservoir. This study provides basic data for the design and implementation of reservoir protection measures in deepwater oil and gas exploration.