DE-FC26-06NT42955

Goal
The goal of this project is to develop methods and tools that can enable operators to design, optimize, and implement energized fracture treatments in a systematic way. The simulator that will result from this work would significantly expand the use and cost-effectiveness of energized fracs and improve their design and implementation in tight gas sands.

Performer
University of Texas-Austin, Austin, TX

Results
In the process of creating an energized fracture model, a new pseudo 3-D model was formulated and a code is being written. This new model will be used as a base so that additions can be made for the effects of an energized fluid. The new code assumes a constant fracture height and a simplified (PKN) model for fracture mechanics. The proppant transport equations are solved in the same manner as a fully 3-D model. An important additional capability of the new version is that it now capable of handling injection into different zones and is able to predict fracture size and proppant transport of multiple fractures. Fluid temperature in the fracture will affect the stresses of the fracture and change the fluid rheology, therefore, effecting fracture propagation and proppant placement. An energy balance was formulated and implemented so that temperature in the fracture can be determined. Two important corrections in the heat conduction term have been added. One correction is the effect of leak-off (cooling the surrounding reservoir), and the other is for the thermal resistance of the leaked-off fluid around the fracture.

Benefits
By adding thermal and compositional capabilities to 3-D hydraulic fracture models, operators will be able to design and optimize energized fracture stimulation treatments in a systematic way. Such improvements will help operators better develop underpressurized reservoirs, many of which are water-sensitive and will require energized treatments to be produced effectively. The resulting improvement in development approach will result in fewer unnecessary wells being drilled and fewer completion or recompletion attempts being required to achieve a satisfactory level of natural gas production, reducing costs. In addition to the energy supply and economic benefits flowing from a more cost-effective way to boost domestic energy supply, the reduction in the number of wells and overall activity through improved efficiency also means less impact on the environment.

Background
A significant portion of U.S. natural gas production comes from unconventional gas resources such as tight gas sands. Tight gas sands account for 58 percent of the total proved natural gas reserves in the United States.

As many of these tight gas sand basins mature, an increasing number of wells are being drilled or completed into nearly depleted reservoirs. This includes infill wells, recompletions, and field-extension wells. When these activities are carried out, the reservoir pressures encountered are not as high as the initial reservoir pressures. In these situations, where pressure drawdowns can be less than 2,000 psi, significant reductions in well productivity are observed, often due to water blocking and insufficient clean-up of fracture-fluid residues. In addition, many tight gas sand reservoirs display water sensitivity—owing to high clay content—and readily imbibe water due both to very high capillary pressures and low initial water saturations.

This sensitivity to water-based fracturing fluids means that a large proportion of the re-fracs and infill well fracturing operations in many U.S. tight sand basins will most likely be conducted using energized fracture stimulation technology. This approach avoids many of the problems listed above by “energizing” the fracturing fluid through the addition of carbon dioxide or nitrogen. None of the 3-D hydraulic fracture models available today for simulating and designing hydraulic fracture treatments have the capability to adequately simulate energized fracture treatments. These models are not capable of accounting for the changes in fluid composition and phase behavior during injection and flowback after the energized fracture treatments.

Summary
Researchers have formulated a simpler pseudo 3-D hydraulic fracturing model with the idea that it will be more accessible than the fully 3-D fracture model currently in use. Run times for this are expected to be 10 to 100 times less than the fully 3-D model (UTFRAC-3D) when compositional and phase behavior effects of energized fluids are implemented. The energy balance to solve for fracture temperature has been applied to the new model, and two heat conduction correction factors have been added. Formulation and partial coding of simpler pseudo 3-D fracture model. Formulation and coding of a energy balance to calculate the temperature profile in the fracture. Correction of energy balance for leaf-off effects resulting in a better understanding of fluid temperature in the fracture.

Two methods for phase behavior have been implemented into the fracturing code. In the first method K-values are estimated to calculate phase solubility. The second method uses the Peng-Robinson equation of state. Compressible, multi-component fluids can now be modeled in the fracturing code.

Current Status (February 2008)
The phase behavior is being implemented into the code.

Project Start: October 1, 2006
Project End: September 30, 2009

DOE Contribution: $694,366
Performer Contribution: $785,414 (53 percent of total)

Contact Information
NETL – Virginia Weyland (virginia.weyland@netl.doe.gov or (918) 699-2041)
UT-Austin – Dr. Mukul Sharma (msharma@mail.utexas.edu or 512-471-3257)

Publications
Colwell, D.A.F., O'Brien, C.G.J., and Gates, T.D., "Evolution of Completion Practices in the Wild River Tight Gas Field," SPE 89719, presented at the SPE Annual Technical Conference and Exhibition, September 26-29, 2004, Houston, TX.

Gabris, S.J., and Taylor III, J.L., “The Utility of CO2 as an Energizing Component for Fracturing Fluids,” SPE 13794, SPE Production Engineering, September 1986, pp. 351-358.

Warnock Jr., W.E., Harris, P.C., King, D.S.,“Successful Field Application of CO2-Foam Fracturing Fluids in the Arkansas-Louisiana-Texas Region,” SPE 11932, Journal of Petroleum Technology, 1985, pp. 80-88.

Valko P., and Economides, M.J., “Volume Equalized Constitutive Equations for Foamed Polymer Solutions,” Journal of Rheology, 36(6), August 1992, pp.1033-1055.

Tulissi, M.G., May, R.E., “A Comparison of Results of Three Different CO2 Energized Frac Fluids: A Case History,” SPE 75681, presented at the SPE Gas Technology Symposium, April 30–May 2, 2002, Calgary, AB, Canada.

Mazza, R.L., “Liquid-Free CO2/Sand Stimulations: An Overlooked Technology—Production Update,” SPE 72383, presented at the SPE Eastern Regional Meeting held in Canton, OH, October 17-19, 2001.

Antoci, J.C., Briggiler, N.J., and Chadwick, J.A., “Crosslinked Methanol: Analysis of a Successful Experience in Fracturing Gas Wells,” SPE 69585, presented at the SPE Latin America and Caribbean Petroleum Engineering Conference, March 25-28, 2001, Buenos Aires, Argentina.

Harris, P.C.,“Dynamic Fluid-Loss Characteristics of CO2-Foam Fracturing Fluids,” SPE Production Engineering, May 1987, pp. 89-94.

Energized treatments using carbon dioxide can be more cost effective than conventional gel treatments(1).
1. D.A.F. Colwell, C.G.J. O'Brien, and T.D. Gates, "Evolution of Completion Practices in the Wild River Tight Gas Field" SPE 89719, presented at the SPE Annual Technical Conference and Exhibition, 26-29 September, 2004, Houston, Texas.