Impact of Fracture Roughness on Fines Migration and Fracture Aperture Growth in Calcareous Shale Rocks During Acidized Core Floods

Shales are impermeable rocks that need to be hydraulically fractured using an acidic hydraulic fracturing fluid (HFF) to enable commercial production. The fracturing process injects HFF at a high pressure to break the rock and create a series of high-flow fractures with rough surfaces; new fractures are generated along the direction of minimum horizontal stress and old cement-filled fractures are reopened. In the process, the oxic HFF physically and chemically alters the fracture surface and creates a “reaction-altered zone”. Recent work has shown the depth of reaction penetration to be minute and that most of the changes occur on the fracture surface. To better characterize the impact of fracture roughness on fracture aperture growth, we take a coupled experimental-modeling approach. Two carbonate-rich Wolfcamp shale samples with calcite-filled fractures are cored. One sample is cut through the center to create a smooth fracture and the second is fractured along the calcite-filled fracture to generate a rough fracture. The topography and mineral distribution of the fracture surface is analyzed before a reactive core-flood with an equilibrated acidic brine is conducted. The effluent is collected and analyzed in a optical emission spectrometer. Pre- and post-flood µCT scans are conducted to log the change in the fracture aperture over time. Preliminary results show that calcite dissolution is the main chemical reaction occurring on the fracture surface. Due to this, the fracture roughness has reduced and the largest change in the fracture aperture is observed nearer to the inlet. The majority of the fracture surface still contains Ca but new “growths” of Si are visible, which have been potentially generated by the chipping of the Ca mineral. The smaller calcite-filled fractures, which are deeper inside the rock away from the fracture surface, show loss of Ca due to the acid reaction indicating deeper penetration of the acidic brine. This work shows the potential impact of fracture roughness on the mechanism of fracture evolution during acidized core floods. A higher fracture roughness shows more fines migration and an overall larger change in fracture aperture during the injection.