Mechanistic Modeling of Clay Swelling in Hydraulic–Fractures Network

Abstract

Listen

Translate article

SummaryHydraulic 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-fracture (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 the HF, it can cause major damage to the overall performance of the fracture network. Thus, a detailed understanding of the 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 composition, 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 caused by clay swelling in a realistic complex HF network. We used UTCOMP-IPhreeqc, a coupled multiphase reactive-transport simulator developed at the University of Texas at Austin, to comprehensively model this process. The ion hydration and the expansion of electrostatic double layer (EDL) were assumed to be the main clay-swelling mechanisms. Surface complexation and ion-exchange reactions were considered to capture the ion diffusion into the electrostatic double layer expansion. In each timestep of the simulation, the calculated volume expansion of clay materials exposed on the fracture surface was used to modify the fracture aperture. To evaluate the performance of a complex HF network after clay-swelling damage, the embedded discrete fracture model (EDFM) was applied.The simulation results indicated that the degree of clay swelling varies in different shale formations. On the basis of the clay content and the mineralogies that were considered in this work, a significant expansion in electrostatic double layer expansion 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 HF network was substantial in the Barnett example. The contribution of the fractures far from the producing well in gas production significantly decreased after clay-swelling damage. The pressure-depletion profiles clearly showed the adverse impact of conductivity damage on fracture-network performance. The presented approach provides the capability to mechanistically model the clay stability and to approximate its impact on transport properties of HFs.