Hydraulic fracturing has been widely used in stimulating unconventional reservoirs, including tight carbonate formations, to improve oil and gas production. The stimulation process requires massive volumes of fracturing fluid to crack the formation; however, only a small percentage of the fluid is recovered during the fluid flow back process. Where the remaining fluids reside in the formation is still not clear. This work strives to use microscale radiographic imaging to study the fluid uptake pathways.
The spontaneous imbibition of fracturing fluid into organic-rich carbonate source rock was experimentally investigated on the microscopic scale. A thin-section sample, with a dimension of 22×11×3.3 mm (length × width × thickness), was sliced using a trim saw and attached onto a glass slide with epoxy. This sample was composed of two distinguished zones, an organic matter-rich zone and a calcite-rich zone. Iodide containing aqueous-based fluid was used as the source fluid to treat the thin section rock sample from one end. The spontaneous imbibition test was conducted for 100 minutes with one radiographic image taken per minute to capture the dynamic fluid front.
The iodide containing aqueous-based fluid was used in this study to enhance the contrast in microscale radiographic imaging. Microfractures were observed by radiographic imaging in the thin-section sample with 5.48 μm and 2.79 μm resolutions before the fluid treatment, both in the organic matter-rich zone and the calcite-rich zone. Three fluid uptake features have been revealed in this dynamic fluid imbibition study: (1) the fluid moved very fast and reached the other end of the sample in less than 4 minutes in the microfractures within the organic matter-rich zone; (2) the fluid moved very fast and reached the other end of the sample in less than 4 minutes in microfractures within the calcite-rich zone; and (3) in a calcite-rich matrix, the water front progressed very slowly, to about 120 μm penetration in 100 minutes. The fast water uptake through microfractures in the organic-rich zone and fracture networks in the calcite-rich zone correspond to imbibition-controlled transport in the strongly water-wet pore space. The slow water uptake through the calcite-rich matrix could be a combination of weakly water-wet imbibition and slow flow process due to large viscous flow resistance (or small permeability) in small-sized pores.
During the slick water fracturing process in unconventional reservoirs, only a small percentage of fracturing fluids is generally recovered. Where the remaining fluids reside in the formation is still an open question in the industry. This study experimentally reveals that the imbibition into the rock matrix is much slower than into microfractures. Therefore, it is very likely that natural and induced microfractures are the major areas where the fluids remain after the flow back.