Pore Shape Factors in Shale: Calculation and Impact Evaluation on Fluid Imbibition

Abstract

Abstract In shale gas reservoirs, the fluid imbibition was quite common during well drilling and stimulation, and it also related with engineering designs such as the borehole instability control and hydraulic fracturing operation, which fluid spontaneous imbibition into shales became a hot issues. In current fluid imbibition models, the pore shape was usually assumed to be circular tube, whereas scanning electron microscope (SEM) images analysis showed that the pore shapes in shale derived a lot from the ideal circular pores. One of the approaches to correct the deviation between the pore shapes and the ideal circular pores was to introduce a pore shape factor into the imbibition model. This paper provided a model to calculate the shape factors for specific geometric pores, and then based on the shape and proportion analyses of the inorganic and organic pores in shale, a comprehensive pore shape factor can be obtained. For tested two shale samples, the comprehensive pore shape factor was 0.54 and 0.61. Taking the values into the fluid imbibition model, it showed that the predicted curves accord quite well with the experimental data, and if the pore shape factors were ignored, there would present an overestimation of 37.0% and 27.9%, respectively. It seemed that calculating the pore shape factor accurately and involving it in the imbibition model would be very essential when dealing with the experimental fluid imbibition data or conducted field fluid imbibition volume predictions.