Numerical reactive flow transport simulation on core samples during acid fracturing in carbonaceous shale

Abstract

Acid fracturing also referred to as fracture acidising, is an application of hydraulic fracturing in which fluid is mixed with an acid. Recently, special attention has been paid to carbonaceous shale reservoirs as new unconventional energy resources. In this study, geochemical and hydraulic behaviour of the fracture acidising was investigated through numerical simulations. A geochemical model was constructed based on the rock composition and the simulated initial state of the formation before acid fracturing. Then, changes in the pH versus time in the form of pore volume injected (PVI) was modelled using an explicit finite difference algorithm.For the hydraulic phase, a fractured core sample model was built and calibrated using the experimental results for a carbonate sample with the same characteristics. Then the model was modified for a carbonaceous shale sample, and the results were examined considering influential parameters such as injection rate, acid concentration, and temperature. For fractured samples, lower injection rate decreases the required pore volume injected (PVI). Simulation results also showed that an increase in acid content reduces breakthrough PVI up to a certain acid concentration. The temperature of injected fluid did not affect acidising efficiency to a great extent. Finally, statistical analysis was carried out to amend some implications and indirect measurements for experiments. The results of this benchmark simulation can be used to improve acid fracturing design for field case studies and to optimise effective parameters to enhance the productivity in unconventional carbonaceous shale reservoirs.