Research on an effective permeability prediction method for shale reservoirs based on imbibition experiments

Permeability is a critical parameter for effective shale gas extraction. Shale reservoirs are dense, and fluid can hardly flow through shale rock under pressure, greatly limiting the permeability, which is commonly obtained by the steady-state core analysis method. At the same time, shale reservoirs contain abundant clay minerals and micro- and nanopores, resulting in favorable spontaneous imbibition ability. At present, most studies for predicting the absolute rock permeability are based on the imbibition process, while the combined influences of the dynamic capillary pressure, slip effect, and osmotic pressure have been considered in few effective permeability prediction studies. First, an imbibition experiment of an undersaturated shale core was conducted to obtain the spontaneous imbibition coefficient of shale. Furthermore, considering the influences of the slip effect, osmotic pressure, tortuosity, and dynamic capillary pressure, an effective permeability prediction method was established based on the imbibition coefficient of shale reservoirs. Combined with previous experimental data, the permeability calculated by this model is close to the experimental data, and the maximum error is less than 11.6%, verifying the effectiveness of the model. The permeability of the lower Silurian Longmaxi Formation shale in the Changning area, Sichuan, can be obtained by using this method. The results showed the following: (1) the calculated result error with the model results of the literature is 1.43 to 3.57%, which shows good contrast. The established effective permeability prediction method considers more comprehensive factors, requires fewer parameters, involves a clear calculation process, accurately meets the production demand, and agrees better with the real formation conditions. (2) The effects of the osmotic pressure, tortuosity, and slip length on the effective permeability successively decrease. Ignoring the osmotic pressure and slip length could lead to a higher effective permeability, while ignoring the tortuosity could lead to a low effective permeability. (3) The effective permeability of reservoirs can be obtained by using this method with different working fluids, and reservoir damage can be rapidly evaluated, which has guiding significance for selecting reasonable working fluids. The method provides a new way to evaluate the effective permeability of shale reservoirs effectively and quickly.