Estimation of Optimal Frac Design Parameters for Asymmetric Hydraulic Fractures as a Result of Interacting Hydraulic and Natural Fractures - Application to the Eagle Ford
Authors: Paryani M. , Poludasu S. , Sia D. , Bachir A. , Ouenes A. 
Published: April 2016

Estimation of permeability in the Stimulated Reservoir Volume (SRV) is a vital input in any completion optimization workflow. One method to estimate the stimulated permeability in the SRV is to couple geomechanical modeling of the interaction between hydraulic and natural fractures with hydraulic fracture mechanics commonly used to design frac jobs. The proposed approach starts by deriving strain resulting from the integration of geological, geophysical and geomechanical modeling of interacting hydraulic and natural fractures. A unique feature of this approach is its ability to predict microseismicity, thus confirming the validity of the input natural fracture model and the geomechanical approach used to evaluate its interaction with the hydraulic fractures. The optimum validated geomechanical asymmetric half-lengths are then estimated from the derived strain map. These estimated geomechanical half lengths are used as a constraint in a frac design model which is able to incorporate this information and optimize stage treatments according to the variable SRV. The frac design parameters then need to be adjusted in order to approximately match the geomechanical half-lengths provided by the strain map.

A new analytical asymmetric frac design model is developed, validated with existing commercial frac design software, and used in this study. The new asymmetric analytical frac design model is a pseudo 3D model that accounts for the variation in height in an iterative approach along with considering the asymmetric half lengths due to the lateral stress gradients in a heterogeneous reservoir. The new asymmetric analytical frac design model was compared to existing commercial frac design software and was found to provide similar estimations of frac heights but in a fraction of the time needed to run the commercial frac design software. The ability to combine these models and simultaneously solve for the optimum fracture height is provided by the constraints of the geomechanical half lengths derived from the strain map. In order to guide the engineer designing a frac job an optimum selection of the design parameters to get the target fracture geometry, this paper also presents a parametric analysis using experimental design of various fracing parameters used in our asymmetric hydraulic fracture model.

In this study, the workflow was successfully applied to a complex Eagle Ford well. The frac design tool optimizes important parameters such as the injection rate, fluid viscosity, proppant type, proppant size, proppant specific gravity and leak-off coefficient in order to honor the interaction of natural and hydraulic fractures accounted for in geomechanics. The frac design model also provides vital information such as the proppant schedule to be pumped and the variation of propped length, width, and net pressure as a function of time. The results of this workflow are the fracture conductivity and proppant concentration along the fracture length and their interpolation between the stages so they can be exported to any reservoir simulator. 2-level fractional factorial design was used to screen for statistically significant parameters that affected fracture geometry in the considered Eagle Ford well. The use of experimental design drastically reduces the simulations necesary to evaluate 15 frac design parameters from 215 to 24, a 1000-fold decrease.

Using an analytical frac design model is fast—its half lengths already validated with microseismic data, efficient, scientific, and derived from principles of mass balance, fluid momentum, pressure-width relations and applied with appropriate initial and boundary conditions. A systematic and quick assessment of treatment parameters to match the desired half lengths provided by the geomechanical simulation results in an accurate frac design model. The key results provided by the frac design model help in long-term planning of operations necessary for optimal well completions and field development, accomplished in a fraction of the time currently required by other software and workflows.