07122-44

Goal
The goal of this project is to increase gas recovery from tight gas formations in the Uinta Basin, Utah by analyzing and modeling natural fracture networks and their interactions with hydraulic fractures, mainly in Mesaverde Group reservoir rocks. The plan is to incorporate natural and hydraulic fracture data into a multiphase, discrete-fracture fluid flow model that simulates gas production from these reservoirs. The results will be used to optimize gas production from selected tight gas formations in the basin.

Performers
University of Utah, Salt Lake City, UT 84112
Utah Geological Survey, Salt Lake City, UT 84114
Golder Associates, Redmond, WA 98052
Utah State University, Logan, UT 84322
Itasca Houston, Inc., Houston, TX 77042

Background
Mapping and characterizing natural fracture systems and understanding their interactions with induced fractures is critical for optimizing gas production from tight reservoirs. Tight reservoirs have ultra-low matrix permeability, so natural fractures are needed to provide pathways for gas to flow through the rock. Hydraulic fracturing treatments that reactivate and intersect the natural fracture network can improve gas flow and provide connections between the natural fracture network and the well bore. These connections can make the difference between a dry well and one that produces economic quantities of gas.

This project focuses mainly on tight Mesaverde Group reservoirs in the Uinta Basin, which have permeabilities ranging from 0.01 to 0.1 millidarcies. The U.S. Geological Survey estimates that 5 to 14 TCF of undiscovered gas remains in these reservoirs -- a significant resource. Successful exploitation of this resource depends on how well the natural fracture systems are developed and understood, and how effectively they are connected to the production system by hydraulic fracturing.

The project team will utilize available log, core, outcrop, and seismic data to create a comprehensive natural fracture map of targeted reservoir formations. Geomechanical tools will be used to represent hydraulic fracture propagation patterns and characteristics. The natural and induced fractures will be incorporated into an existing reservoir simulator developed at the University of Utah. Finally, gas production and drainage potential of the fracture network will be evaluated.

University of Utah will lead the project and provide its in-house finite element code for modeling and predicting production from tight gas reservoirs. Utah State University and Utah Geological Survey will contribute to geological and geomechanical aspects of the project. Golder and Associates will be providing Fracman and FRED software for developing the static fracture model, and Itasca will contribute to modeling the growth of hydraulic fractures in Uinta Basin reservoirs. All project partners will contribute to technology transfer throughout the duration of the project.

Deliverables for this project will include a series of reports on the various tasks as they are completed and a final report integrating the results of the project and providing guidelines for its application elsewhere.

Potential Impacts
If successful, this project could bring increased quantities of Uinta Basin tight gas to market. The Mesaverde Total Petroleum System in this basin has significant potential, with an estimated 5 to 14 TCF of undiscovered gas remaining in the ground. If only a modest, 10% increase in gas recovery resulted from the application of the knowledge developed from this research, it would mean added gas reserves on the order of 1 TCF.

This project is expected to result in better tools for understanding hydraulic fracture propagation, which in turn will lead to more efficient stimulation treatments, not only for the Uinta Basin, but for tight gas reservoirs in other basins as well. The cumulative benefit would be determined by the total number of fields where the technology is ultimately applied. The national economic benefit from any incremental increase in domestic gas production would be an increase in tax revenue, royalties and regional economic benefits.

Accomplishments
Discrete-fracture network (DFN) simulations were performed on tight gas reservoirs to evaluate the impact of fracture properties. Geomechanical simulations and reservoir simulations on generic models were also performed. The team prepared a presentation for the Review of the Onshore Unconventional Program of the work completed thus far.

A preliminary reconnaissance of all the identified outcrop locales that will be used to study to collect additional data for our DFN models was completed. The team constructed well log cross sections to correlate the Mesaverde Group in Greater Natural Buttes field to the outcrops of the Mesaverde in the Vernal area where Utah State University will be studying fractures in the exposed Mesaverde

The core of the Sego and Castlegate Sandstones of the Mesaverde Group from the NBU 253 well in section 10 of the study area was described. The core is housed at the Utah Geological Survey Core Research Center in Salt Lake City, Utah. Lithology and bedding features were described, and each fracture in the core was described, measured, and photographed.

The team finalized the formulation for geomechanics and developed an algorithm and code. Several issues were identified during the debugging of the code and a modified formulation that is more consistent with the existing simulator framework is being developed.

Current Status
Greater Natural Buttes (GNB) gas field which covers about 400 square miles in T. 8 S. to T. 12 S., and R. 18 E. to R. 24 E. (Salt Lake Baseline and Meridian) Uintah County, Utah is the subject of study in this project. The project team has selected section 10 (T. 9 S., R. 21 E.) in the north-central portion of GNB for detailed modeling, after series of discussions with the operator, Anadarko. Geologic evaluation consisted of log and core evaluation and detailed fieldwork including outcrop and surface fracture analysis. An example cross section assembled is shown in Figure 1.

Figure 1. Detailed cross-section in Section 10 of the Greater Natural Buttes field.

The geologic team constructed an initial static fracture model. A wider array of data in the field is being used to refine this geologic description of fractures.

The geomechanical program 3-DEC from Itasca was used to study the influence of natural fractures on hydraulic fracturing. Results of a sample simulation are shown in Figure 2.

Figure 2. Pore pressure distribution profile at the end of hydraulic stimulation treatment (plan view) showing influence of the fracture network. In the first quadrant where natural fractures are closely spaced, injection fluid tends to follow the paths of least resistance, and no longer results in a simple elliptical pressure diffusion front as assumed by analytical models.

The static fracture models created by the geologic team are being used to study the propagation of hydraulic fracture in the domain populated by natural fractures. The team is also adding the geomechanical simulation capability in the current reservoir simulation framework. In this task, the project team will run coupled flow simulations for the selected study sites, using the University of Utah simulator for complex subsurface processes. The simulation results will be used to evaluate the gas production potential of a given tight gas formation, using a realistic geometry of natural fractures, hydraulic fractures, and the well bore.

Technology Transfer. Throughout the duration of the project, results will be made available via publications, presentations, yearly workshops, and a dedicated web site to be housed at the University of Utah and at the Utah Geological Survey. The following presentation was made. SPE 127888: "Modeling Fluid Invasion and Hydraulic Fracture Propagation in a Naturally Fractured Rock, a Three Dimensional Approach", J. McLennan, D. Tran, N. Zhao, S. Thakur, and M. Deo, University of Utah; I. Gil, and B. Damjanac, Itasca, Society of Petroleum Engineers, This paper was prepared for presentation at the 2010 SPE International Symposium and Exhibition on Formation Damage Control held in Lafayette, Louisiana, USA, 10–12 February 2010.

Presentations are planned at the 2010 AAPG Meeting and the 2010 Annual SPE Meeting.

Project Start: September 2, 2008
Project End: December 31, 2010

DOE Contribution: $1,068,863
Performer Contribution: $267,216

Contact Information:
RPSEA – Charlotte Schroeder (cschroeder@rpsea.org or 281-690-5506)
NETL – Virginia Weyland (Virginia.Weyland@netl.doe.gov or 281-494-2517)
University of Utah – Milind D. Deo (milind.deo@utah.edu or 801 581-7629)

Additional Information

Preliminary Field Analysis [PDF]

State-of-the-Art Report [PDF] - January 2009