Experimental and Theoretical Studies of Drilling Fluid Imbibition into Shale Reservoirs

Abstract Drilling fluid imbibition can severely damage oil and gas pay zones, especially shale reservoirs, due to its induced clay swelling effect. The objective of this work is to present the results of experimental and theoretical studies of drilling-fluid imbibition into the shale reservoirs and factors affecting the process. Water imbibition depth was measured on dry cores from four shale gas/oil reservoirs namely TMS, EFS, MS, and GRS, using a Krüss DSA 100S with Krüss’ Advance software. The time-dependent droplet volume, together with the contact area and shale porosity, were used to calculate the imbibition depth. An analytical model was developed to upscale the tested fluid imbibition data to field level by considering both the spontaneous imbibition driven by capillary pressure and the flow resistance by viscous force. A sensitivity analysis was performed to identify key factors affecting the water imbibition depth. The differences of water imbibition into the four shale samples were analyzed based on the tested and model-upscaled data. The initial water-contact angles on the TMS, EFS, MS and GRS core surfaces were measured to be 36.62°, 66.68°, 52.78° and 84.73°, respectively. The contact angle and thus volume of liquid droplet were found to change with time due to fluid imbibition into the core samples and evaporation. The time-dependent imbibition depths in the four shale samples vary significantly due to different shale compositions. The analytical model for fluid imbibition was slightly tuned to match the tested imbibition processes. This tuning procedure is needed to account for the uncertainties in determination of model parameters such as shale permeability and pore size. The result of time-upscaling shows that the 1-month water imbibition depths for the TMS, EFS, MS and GRS are 2.61 cm, 1.59 cm 0.89 cm and 0.16 cm, respectively. These low values suggest that the direct impact of water imbibition into shale matrix on formation damage in shale gas/oil reservoirs is insignificant in the practical scales of space and time. However, the imbibition-induced clay-swelling and shale cracks may significantly damage formation permeability and more importantly cause drilling complications. This work provides a novel method for measuring fluid imbibition into ultra-low permeability reservoirs such as shale formations. The measured data, together with the developed analytical imbibition model, allows for identifying key factors affecting fluid imbibition and thus formation damage due to drilling fluid filtration.