Leakoff and Flowback Experiments on Tight Carbonate Core Plugs

In this paper, we investigate the change in oil effective permeability (⁠$koeff$⁠) caused by fracturing‐fluid (FF) leakoff after hydraulic fracturing (HF) of tight carbonate reservoirs. We perform a series of flooding tests on core plugs with a range of porosity and permeability collected from the Midale tight carbonate formation onshore Canada to simulate FF‐leakoff/flowback processes. First, we clean and saturate the plugs with reservoir brine and oil, and age the plugs in the oil for 14 days under reservoir conditions (P = 172 bar and T = 60°C). Then, we measure $koeff$ before (baseline) and after the leakoff process to evaluate the effects of FF properties, shut‐in duration, and plug properties on regained permeability values.

We found that adding appropriate surfactants in FF not only significantly reduces $koeff$ impairment caused by leakoff, but also improves $koeff$ compared with the original baseline for a low‐permeability carbonate plug. For a plug with relatively high permeability (k_air > 0.13 md), freshwater leakoff reduced $koeff$ by 55% (from 1.57 to 0.7 md) while FF (with surfactants) reduced $koeff$ by only 10%. The observed improvement in regained $koeff$ is primarily because of the reduction of interfacial tension (IFT) by the surfactants (from 26.07 to 5.79 mN/m). The contact‐angle (CA) measurements before and after the flowback process do not show any significant wettability alteration. The results show that for plugs with k_air > 0.13 md, FF leakoff reduces $koeff$ by 5 to 10%, and this range only increases slightly by increasing the shut‐in time from 3 to 14 days. However, for the plug with k_air < 0.09 md, the regained permeability is even higher than the original $koeff$ before the leakoff process. We observed 28.52 and 64.61% increase in $koeff$ after 3‐ and 14‐day shut‐in periods, respectively. This observation is explained by an effective reduction of IFT between the oil and brine in the pore network of the tight plug, which significantly reduces irreducible water saturation (S_wirr) and consequently increases $koeff$⁠. Under such conditions, extending the shut‐in time enhances the mixing between invaded FF and oil/brine initially in the plug, leading to more effective reductions in IFT and consequently S_wirr. Finally, the results show that the regained permeability strongly depends on the permeability, pore structure, and S_wirr of the plugs.