(596c) Optimization of Hydraulic Fracturing In Horizontal Wells for Unconventional Gas Production

Massive hydraulic fracturing of horizontal wells has made the recovery of natural gas from unconventional gas reservoirs both technically and economically feasible. However, the design of hydraulic fracturing relies on extensive what-if analysis using commercial simulators.  The entire design process is fairly demanding, and may lead to designs that are far from optimal.  In this work, we present a computer-aided approach that facilitates engineers to efficiently converge to solutions that are at or close to the optimum. 

 The proposed optimization objective is the net present value (NPV) for horizontal well drilled in the direction of the minimum horizontal stress, which allows multiple transverse fractures.  Primary decision variables are the number of transverse fractures in a horizontal well;  horizontal well length; fracturing fluid viscosity; proppant concentration;  injection rate of fracturing fluid; and injection time.  Key constraints are fracture growth control, operational constraints, and geometric constraints.  Critical reservoir properties (e.g. permeability, porosity, and stress orientation) needed for optimization is assumed to be available from a variety of sources (e.g., seismic, prior production logs).  The LMN model for hydraulic fracturing propagation by Lietard et. al.¹ which addresses the transverse case is used. The productivity of multifractured horizontal wells is computed by modifying the analytical method presented by Guo et. al.² and compared with Soliman et. al.³ for consistency.  The optimization algorithm used is NLPQLY developed by Schittkowski⁴.  Probabilistic optimization algorithms are also used for comparison.

The proposed approach is used to design horizontal well geometry and stimulations for a number of test cases, based on numerical simulations.  Results are compared with other available case studies in literature.  The proposed approach demonstrates significant increase in NPV over other available methods. The simulations indicate good numerical efficiency and ability of the approach to be used as a decision making tool for unconventional gas field development.  Work is currently underway to integrate this tool with vertical well hydraulic fracture optimization.

References:

1. Lietard, O., Maniere J., and Norris M.: Modeling of Transverse Hydraulic Fracturing, paper SPE 106251, Hydraulic Fracturing Technology Conference, College Station (January 29-31, 2007).
2. Guo, B., and Schechter, D.S. (1997), A Simple and Rigorous Mathematical Model for Estimating Inflow Performance of Wells Intersecting Long Fractures., SPE 38104 presented at the 1997 SPE Asia Pacific Oil and Gas Conference and Exhibition, Kuala Lumpur, Malaysia.
3. Soliman, M. Y., Hunt, J. L., and Azari, M (1996): Fracturing Horizontal Wells in Gas reservoirs, in SPE 35260, presented at the Gas Technology Symposium, edited, Calgary, Canada.
4. Schittkowski, K. (2009), NLPQLY: An Easy-To-Use Fortran Implementation of a Sequential Quadratic Programming Algorithm - User's Guide, Report, Department of Computer Science, University of Bayreuth.