Oil-based muds for reservoir drilling: Their performance and cleanup characteristics

This paper evaluates the performance of a standard oil-based mud (OBM) to drill horizontal wellbores, concentrating on its formation-damage characteristics and the flow-initiation pressures (FIP's) required for production to flow through the filter cake. For heterogeneous reservoirs, damage is relatively low in low-permeability rocks, but the FIP is high. Conversely, for high-permeability rocks, the FIP is low but formation damage is relatively high. If the drawdown pressure available from the reservoir is low, the scenario exists where inflow will occur predominantly from the higher-permeability formations, which could be damaged badly, but little inflow will occur from relatively undamaged lower-permeability rocks. In terms of maximizing production, this is obviously a less-than-optimal scenario. Evaluations of cleanup fluids were conducted to gauge their effect on lowering the FIP of OEM filter cakes. Various fluids were screened for their mud-removal performance, which would indicate potentially good OEM "chemical breakers." Mud parameters such as oil:water (O:W) ratio, base-oil type, and emulsifier content all affected the efficiency of the cleanup fluids. The best cleanup fluids were used then in a series of core tests to evaluate their effectiveness in reducing the filter-cake FIP. Reductions between 25 and 40% were possible, although parameters such as soak time and overbalance pressure were critical to their success.