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The key in unlocking unconventional reserves is to create massive fracture surface area. During the fracturing treatment, a huge volume of fracturing fluid is pumped to generate fractures and then followed with a large amount of proppant to provide enough conductivity for reservoir fluid to flow to the wellbore. The ultimate proppant distribution in the fracture system directly impacts well productivity and production decline rate. However, it is very challenging to predict how far proppants can go and where they will settle because of the complexity of the fracture system. Therefore, accurate modeling of proppant transport inside the fracture system is critical to enable stimulation optimization. Previous modeling and experimental studies were usually based on simple proppant settling velocity models and limited only to planar fracture cases. To accurately evaluate propped complex fracture systems, which are more common in unconventional reservoirs, advanced proppant transport models are required. In this paper, proppant transport in various fracture geometries is investigated using computational fluid dynamics (CFD) models, in which the interaction between proppant particles and the carrying fluid phase is fully coupled to track proppant movement in the fractures. The planar fracture case is first investigated using a CFD model and benchmarked with results from commercial software. The CFD models are then used to simulate the proppant transport in T-junction and crossing-junction scenarios, which are often seen in unconventional reservoir fracture systems. Parametrical studies are also conducted to better understand how proppant transport is affected by fracture fluid viscosity, proppant density, and fluid injection rates. The results from the proposed CFD models indicate that proppant transport within complex fracture geometries is significantly affected by fracture fluid dynamics and proppant properties. At fracture junctions, turbulent flow regime will develop, which helps proppant propagate to natural fractures. According to the parametrical studies, higher injection rate and light-weight proppant are beneficial in transporting proppant through fracture junctions to reach further in both hydraulic fractures and natural fractures. Proppant transport models developed in this work fully incorporate the interaction between proppant particles and carrying fluid dynamics. This study extends the current understanding of proppant movement in complex fracture geometries and helps optimize hydraulic fracturing design to improve unconventional well production performance.

Gel damage has been identified as the most severe damage effect on gel-fractured well productivity. The reduced effective fracture length and fracture conductivity can significantly decrease the initial production rate and the ultimate recovery, especially for tight reservoirs. In this paper, we conduct numerical studies on fracture cleanup effects, and explore procedures for improving post-fracture production. To effectively simulate fracturing fluid rheology and transportation, a dual continuum method is employed in our model. Gel residue inside the fractures is determined based on gel and breaker reaction dynamics from experimental data. Increased gel concentration in slurry injection, increased breaker concentration during fracture clean-up, and the flowback procedure are simulated to quantify the gel damage phenomenon. Features such as non-Darcy flow through fractures and polymer adsorption/desorption are included in the model. Parametric studies on the gel and breaker concentrations, reservoir properties, and flowback designs are also discussed. Our studies determined that interactions between breaker and gel can be fully integrated into numerical simulations. Tested with a variety of gel and breaker concentrations as well as the reaction dynamics, the numerical model is robust in quantifying the gel damage phenomenon in the fracture. Detailed numerical evaluations of gel damage effects with severe gel-damage zone inside the fracture are demonstrated in this paper. High gel residue concentration is observed at the tip of fractures due to high yield stress during flowback, which reduces the effective fracture drainage. Cleanup efficiency is highly dependent on the gel and breaker interactions. Based on parametric studies, optimized gel/breaker concentrations and sufficient interaction time can significantly reduce the gel residue inside the fracture and provide possibilities of having less gel damage in hydraulic fractures. The results provide new observations about improving well productivity through proper gel/breaker selection and flowback design.

Sustainable high fracture conductivity is a key to successful stimulation. The reduction of hydraulic fracture conductivity due to proppant deformation and crushing is frequently observed. Previous researches are based on laboratory experiments and empirical correlations, which can not fully explain proppant damage in field cases. In this paper, we applied our fully coupled fluid flow and geomechanical model to further understand the proppant pack deformation and crushing. Parametric studies on wellbore and reservoir pressures, formation properties, and proppant biot constant were performed to understand proppant deformation and crushing in different conditions. Additionally, an analytical model for avoiding proppant crushing was developed for fractured wells. Through this research, we found fracture conductivity loss due to deformation and crushing are severer than laboratory results. Large deformation and high probability of crushing were observed near wellbore according to the net pressure. Fast flow back (low bottom hole pressure) would generate large proppant crushed zone. Various reservoir properties as pressure gradient, formation stiffness, and matrix permeability were also investigated. Strong proppant is highly recommended for natural fractures, and hydraulic fracture near well bore especially for tight formations. Small chock size (high BHP) is also recommended during early production. Additionally, a simple analytical model is provided, accoding to the parametrical studies, for operating well without breaking proppant pack.