Rigorous Estimation of the Initial Conditions of Flowback Using a Coupled Hydraulic-Fracture/Dynamic-Drainage-Area Leakoff Model Constrained by Laboratory Geomechanical Data

Abstract

Summary The application of rate-transient-analysis (RTA) concepts to flowback data gathered from multifractured horizontal wells (MFHWs) completed in tight/shale reservoirs has recently been proposed as an independent method for quantitatively evaluating hydraulic-fracture volume/conductivity. However, the initial fluid pressures and saturation in the fracture network and adjacent reservoir matrix are generally unknown at the start of flowback, creating significant uncertainty in the quantitative analysis of flowback data. In this study, we present a semianalytical flow model, coupled with a hydraulic-fracture (fracture) model and constrained with laboratory-based geomechanical data, for evaluating the initial conditions of flowback. In previous work, a semianalytical model based on the dynamic-drainage-area (DDA) concept was used to simulate water-based fluid leakoff from an MFHW into a tight oil reservoir (Montney Formation, western Canada), with minimal mobile water, during and after fracturing operations. The model assumed that each fracturing stage can be represented by a primary hydraulic fracture (PHF) containing the majority of the proppant, and an adjacent nonstimulated reservoir (NSR) or enhanced fracture region (EFR), which is an area of elevated permeability in the reservoir caused by the stimulation treatment. Each region was represented by a single-porosity system. The DDA propagation speed within the PHF during the stimulation treatment was constrained through using a simple analytical fracture model. Although this approach was considered novel, several improvements and additional laboratory constraints were considered necessary to yield more accurate predictions of initial flowback conditions. In the current work, the modeling approach described previously was improved by representing the EFR with a dual-porosity system; fully coupling the fracture model (used for PHF creation and propagation) with the DDA model for fluid-leakoff simulation into the EFR and adding a proppant-transport model; and modeling the shut-in period. Finally, to ensure that model geomechanics were properly constrained, a comprehensive suite of previously gathered laboratory data was used. Laboratory-derived propped (PHF) and unpropped (EFR) fracture-permeability/conductivity data as a function of pore pressure, as well as fracture-compressibility data, were used as constraints for the model. It should be noted that our model assumes that fracture closure has no effect on the pressure/saturation of the PHF/EFR/matrix. The improved model was reapplied to the tight oil field case and yielded more realistic estimates of initial flowback conditions, enabling more confident history matching of flowback data. The results of this study will be important to those petroleum engineers interested in quantitative analysis of flowback data to accurately obtain fracture properties by ensuring proper model creation.

A semi‐analytical model for capturing dynamic behavior of hydraulic fractures during flowback period in tight oil reservoir

“…The fracture conductivity derived by Clarkson is ~292 mD ft, and the fracture conductivity derived by Jia is ~161 mD ft. 4,8 Both are bigger than the fracture conductivity extracted by the new coupled model considering the fracture closure, as shown in Figure 19. Therefore, in order to match the water production, a larger fracture conductivity is used resulting in a shorter fracture half-length captured by the flowback model without considering fracture closure, while our result is similar to the fracture half-length derived by flowback model proposed by Zhang et al, 10 where laboratory-based petrophysical and geomechanical inputs were used to constrain the flowback history-matching process.…”

“…Therefore, in order to recreate initial condition, a new coupled model considering the fracturing fluid leak-off for the shut-in and flowback period will be proposed in our future work. 10 The initial fracture half-length and the fluid pressure within the fracture at the end of hydraulic fracturing will be set as the initial condition of the new coupled model.…”

“…The interpreted fracture half-length is 1010 ft, which is greater than the 450 ft derived by Clarkson and 725 ft derived by Jia et al, 4,8 but comparable with 1043 ft derived by Zhang. 10 The reason may be that the constant fracture conductivity without consideration of fracture closure is used in Clarkson and Jia's flowback model. The fracture conductivity derived by Clarkson is ~292 mD ft, and the fracture conductivity derived by Jia is ~161 mD ft. 4,8 Both are bigger than the fracture conductivity extracted by the new coupled model considering the fracture closure, as shown in Figure 19.…”

“…Many two-phase (oil + water) flow semi-analytical models [7][8][9] have been presented to capture the fracture key parameters (eg, effective fracture length and fracture permeability) by history-matching the flowback data. The initial condition of the flowback can be accurately considered by coupling hydraulic fracture model, 10 and the semi-analytical model also can be used in complex fracture network by the star-delta transformation used to solve the interplay flow of fracture intersection. 11 However, current flowback models always ignored the fracture closure or simply treated the process of fracture closing as stress-sensitive fracture permeability and porosity, which cannot accurately describe the fracture closure because of the inaccurate effective stress calculation.…”

Experimental and computational evaluation of cyclic solvent injection in fractured tight hydrocarbon reservoirs

“…fracturing under stress) better compared to the conventional methods (e.g., Brazilian test) in which the fracture is created artificially outside of the coreholder using uniaxial stress. This approach has enabled the measurement of tight rock unpropped/propped fracture permeability/conductivity 23 , and fracture compressibility 24 under "in-situ" stress and as a function of effective stress.…”