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

This paper evaluates the performance of a standard oil-based mud (OBM) to d rill horizontal wellbores, concentrating on its formation-damage characteri stics and the flow-initiation pressures (FIP's) required for production to flow through the filter cake. For heterogeneous reservoirs, damage is relat ively low in low-permeability rocks, but the FIP is high. Conversely, for h igh-permeability rocks, the FIP is low but formation damage is relatively h igh. If the drawdown pressure available from the reservoir is low, the scen ario exists where inflow will occur predominantly from the higher-permeabil ity 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 lower ing the FIP of OEM filter cakes. Various fluids were screened for their mud -removal performance, which would indicate potentially good OEM "chemical b reakers." Mud parameters such as oil:water (O:W) ratio, base-oil type, and emulsifier content all affected the efficiency of the cleanup fluids. The b est cleanup fluids were used then in a series of core tests to evaluate the ir effectiveness in reducing the filter-cake FIP. Reductions between 25 and 40% were possible, although parameters such as soak time and overbalance p ressure were critical to their success.