Experimental investigation of the effect of imbibition on shale permeability during hydraulic fracturing
Abstract Hydraulic fracturing technology is widely applied in shale reservoirs to significantly increase production. However, when many operators report a large percentage of the fracturing fluid is not recovered, it is unclear how the remaining fracturing fluid affects the shale formation. It is believed that the unrecovered fracturing fluid could be imbibed by shale matrix, micro fractures, and surfaces of fractures that are already separated. This paper is to investigate the influence of imbibition on the matrix permeability, micro fracture permeability, and fracture permeability. It is the first time to correlate permeability change with shale imbibition, and provide quantitative results of increase and decrease in permeability due to imbibition process in shale during hydraulic fracturing. This paper uses the pressure build up method to measure permeability of the shale sample, and applies the under weighing approach to do the imbibition experiment. The Niobrara, Horn River, and Woodford shale formations are the source of the samples in the experiment. The experimental results show that the imbibed fracturing fluid will damage and seriously reduce the matrix permeability of the shale sample. When the sample imbibes more fluid, the matrix permeability is reduced greater. Imbibition also decreases the fracture permeability of open fractures, but decrease is less than the reduction of matrix permeability. Moreover, there is a lubrication effect that can reopen micro fractures on shale samples and stimulate and increase the micro fracture permeability during imbibition. Permeability is a criterion that determines the long term production from a formation. By studying the permeability change caused by imbibition during hydraulic fracturing stimulation, this paper presents a new observation that imbibition in shale can not only damage, but also potentially stimulate the shale formation by increasing permeability due to open closed or sealed natural fractures.