Investigation of the Formation Damage Mechanism Caused by Clay-Free Reservoir Drilling Fluids

To investigate the mechanism of formation damage induced by clay-free reservoir drilling fluids, a clay-free oil reservoir drilling fluid system was selected as the research object. Core displacement experiments and filter cake mechanical property tests were conducted by regulating the filter cake composition and varying the differential pressure during core displacement. The effects of filter cake composition, filter cake strength, and invasion depth on formation damage were then systematically analyzed. The results show that the filter cake composition has a significant effect on formation damage. In particular, the bridging and plugging structure formed by ultrafine calcium carbonate particles at pore throats is the main cause of formation damage. The influence of different components on formation damage follows the order ASP1250 > XCD > ASR-1. As the polymer effect in the drilling fluid system becomes stronger, the filter cake structure gradually changes from particle packing to a polymer network structure. The maximum load of the filter cake increases from approximately 15  N to more than 25 N, while the filtration volume increases from approximately 11–48 mL, indicating a positive correlation between filter cake strength and filtration volume. As the displacement pressure increases from 500 to 1500 psi, the core damage rate increases from 90.58 to 99.64%, indicating that an increased invasion depth aggravates formation contamination. However, its effect is less significant than that of the particulate composition of the filter cake. These findings provide a theoretical basis for optimizing reservoir-protective drilling fluid systems.