Modeling fluid imbibition for hydraulic fracturing and its implications on unconventional hydrocarbon recovery

Abstract

The deliverability of a hydraulically fractured well relies on three segments: fluid flow from matrix to the contact surface between fracture and matrix media, fluid-rock-ion interactions at the fracture-matrix interface, and conductivity of fracture network in the created stimulated reservoir volume (SRV). Thus, any fluids and salt exchange through matrix-fracture interface is critical and worth detailed investigations. Furthermore, fluid imbibition as an important recovery mechanism for fractured reservoirs has been extensively studied, but the focuses were mostly given to capillarity. However, in this dissertation, several contributing fluid flow mechanisms have been identified based on the unique rock characteristics and subsurface conditions of unconventional formations, including capillarity, diffusion, osmosis, and adsorption. To investigate the impact of the fluid imbibition process on post-treatment well performance and oil recovery, comprehensive modeling efforts are made. In this dissertation, the derivation of a matrix fluid imbibition model is presented, which captures various relevant physical mechanisms. Specifically, the rock matrix is conceptualized as a combination of semi-permeable membrane-like components and non-membrane components and the osmosis membrane efficiency term is redefined to properly simulate osmosis effect and rock-ion interactions. Moreover, to handle the sub-normal water saturation conditions in some unconventional formations, the Freundlich (1909) water isotherm model is coupled with the van Genuchten (1980) capillary pressure model to simulate the fluid flow in dehydrated rocks. Furthermore, this matrix imbibition model is incorporated into a comprehensive reservoir-scale dual-porosity numerical model to simulate the mass transfer through fracture-matrix interface controlled by the near-fracture fluid-rock-ion interactions. The conventional features of the reservoir model have been validated using analytical and numerical approaches. The matrix model with the osmosis effect was used to match the pressure and concentration responses from a laboratory chemical osmosis experiment. Numerical studies on a hydraulically fractured well using the developed model have been performed to simulate the dynamic processes during fracturing, the following well shut-in (soaking), and flowback/production, assuming no physical damages. The simulation results through imbibition mechanism assessments indicated that adding osmosis effect showed an enhanced fluid imbibition process and improvement in well production performance compared to that of base model, which only considers capillarity effect. Moreover, the oil production was significantly improved and the amount of recovered fracturing fluids was greatly reduced due to adsorption effect. All four models: base model, base model with osmosis effect, base model with adsorption effect, and complete model with combined effects, showed low fracturing fluid recoveries and this suggested that the low flowback recovery reported in field could be partially explained by fluid imbibition. A case study of varying osmosis membrane efficiencies from 0% to 50% showed a moderate increase in total recovered oil. The second case study on extending the (shut-in) soaking time from one hour to one month boosted the recovered oil by 19% and reduced cumulative produced water by 85%. However, the incremental oil recovery and reduction in cumulative produced water decreased with longer soaking time, which suggested there may exist an optimum shut-in time. Additionally, the discussions on soaking encourage rethinking the role of hydraulic fracturing fluids, which is not only to create highly conductive fracture network as an efficient flow pathway but also to displace the stored hydrocarbon in the matrix to feed the fracture network if given sufficient time. Finally, a case study on refracturing existing hydraulically fractured wells showed promising results with additional oil recovery due to added new fracture-matrix surface areas with greater fluid imbibition.