Hydraulic fracturing is the most effective technique used in the oil industry for economical production of hydrocarbon from very low permeability reservoirs. Recent experimental studies have indicated a change in hydraulic fractures (HF) conductivity as the result of the interactions between fracturing fluid and shale matrix. Clay swelling is one of the well-known undesirable interactions of this kind. If clay swelling occurs on the surface of hydraulic fractures, it can cause a major damage to the overall performance of the reservoir. Thus, a detailed understanding of clay stability issue is essential for fracturing fluid selection and operation planning.
Clay swelling induced conductivity damage is primarily a function of rock mineralogy, fracturing fluid concentration, and formation brine salinity. Thus, various levels of clay-water interaction are expected in different shale formations. In this study, we present a mechanistic approach to model clay swelling in various rock mineralogies including Barnett (clay-rich), Eagle Ford (Calcite-rich), and Marcellus shales. Subsequently, we investigate the production loss due to clay swelling in a realistic complex hydraulic fracture network. We used UTCOMP_IPhreeqc, a coupled multi-phase reactive- transport simulator developed at The University of Texas at Austin, to comprehensively model this process. Expansion of double diffusive layer was assumed to be the main clay swelling mechanism. Surface complexation and ion exchange reactions were considered to capture the ion diffusion into the double diffusive layer. In each time step of the simulation, the volume expansion of clay materials exposed on the fracture surface was then used to modify the fracture aperture. In order to evaluate the performance of the complex hydraulic fracture network after clay swelling damage, the Embedded Discrete Fracture Model (EDFM) was applied.
The simulation results indicate that the degree of clay swelling varies in different shale formations. Based on the clay content and the mineralogies that were considered in this work, a significant expansion in double diffusive layer was observed for the Barnett shale when fresh water was injected. However, this effect was much lower in Eagle Ford and Marcellus shales. Similarly, the production loss in the hydraulic fracture network was substantial in the Barnett example. The contribution of the fractures far from the producing well in gas production was negligible after clay swelling damage. The pressure depletion profiles clearly showed the adverse impact of conductivity damage on production performance. The presented approach provides the capability to accurately model the clay stability and to approximate its impact on transport properties of hydraulic fractures.