3.2.4. Highlights of Important Results

The Western Gas Sands Program was designed to accelerate the production of domestic gas resources. It was directed specifically at the development of new and improved techniques for recovering gas from low-permeability gas reservoirs in western basins that at the time of initiation of the program could not be economically produced. The purpose of the program was to encourage and supplement industry efforts to develop technology and demonstrate the feasibility of producing from tight reservoirs.

The key contributions resulting from the WGS Program include:

• Collection and analysis of core material from a variety of location to provide data on the fundamental reservoir properties of tight sandstones that had previously been lacking,
• Development of new equipment and procedures for measuring tight sandstone rock properties,
• Detailed characterization of the geology and tight sand gas resource potential of five Rocky Mountain basins, that provided a basis for subsequent decision-making related to both R&D and resource development investments,
• Creation of an extremely well-characterized reservoir laboratory at the MWX site, that allowed for experiments that contributed critical insights towards answering questions related to the continuity of tight sands, the importance of natural fracture systems, the importance of in situ stress to hydraulic fracture design, and the size and shape of created hydraulic fractures compared to their design dimensions, and
• Development of geologic and production predictive models that enhanced efforts to find and economically produce natural gas from tight gas sands.

• Advanced Tight Gas Core Analysis Technology. Specialized equipment and techniques were developed at IGT, Core Labs, and New Mexico Tech to fully characterize the reservoir properties of very low permeability rocks. Of particular importance were methods that were developed to test rocks under both in situ stress and water saturation conditions and capillary pressure measurements to aid in the understanding of the important two-phase flow mechanisms (water and gas). These techniques are now routinely used for low-permeability core testing and are available commercially.
  
  
• Reservoir Characterization Methodology. The core samples in the three closely spaced wells, along with the careful assessment of numerous surface outcrops of these same reservoir rocks, provided the information necessary to develop methods for quantifying the size of sandstone lenses in various depositional environments. This methodology, which is now used by numerous companies working in the tightsand basins, provides critical information needed for resource assessment, reserves calculations, fracture design, and well spacing in these fields.
  
  
• Naturally Fractured Core Analysis. A process for analyzing fractured core has been transferred to numerous companies and is routinely used by industry and techniques for identification of coring induced fractures is widely used. Information from stress sensitivity testing of naturally fractured cores is now used in many reservoir and fracture models.
  
  
• Stress Testing and Applications. MWX was the site where a methodology for microfracture stress testing through perforations using down-hole shut-in was fully developed and where anelastic strain recovery and circumferential velocity anisotropy were validated. Micro-fracture stress testing is a testing procedure now used routinely by companies throughout the world and anelastic strain recovery is a commercial service available through Halliburton. Altered stress fields due to fractures and other down-hole processes are now calculated and considered in many applications such as re-fracturing and in-fill drilling. The mechanism defined for the formation of regional fractures is being applied towards exploration strategies in many basins.
  
  
• Advanced Tight Gas Log Analysis. By running multiple logging suites (including experimental logs such as Mobil’s televiewer, Amoco’s and Schlumberger’s longspaced sonic logs, and Schlumberger’s dipmeter), and coupling the results with the results of core analysis, new correlations were developed that more accurately predicted reservoir properties. The data was made available to all of the commercial logging companies.
  
  
• Extreme Overbalanced Perforating. While attempting to develop methods for effectively connecting to and testing the reservoir, the first extreme overbalanced perforation operations were conceived and performed at MWX. Extreme Overbalanced Perforating is a service performed by Halliburton and used routinely.
  
  
• Deviated or Horizontal Drilling in Fractured Reservoirs. The Slant Hole Completion Test resulted in a number of recommendations for using deviated well bores to exploit fractured reservoirs. This approach is now widely applied throughout the U.S.

• The first successful micro-seismic monitoring tests in tight gas reservoirs were conducted at MWX and showed that fractures grew out of zone proportional to the stress contrasts and that fracture lengths were considerably shorter than designed. The M-Site testing validated the accuracy of down-hole micro-seismic monitoring for real-time mapping of hydraulic fracture growth and established the accuracy of the technique, the interpretation of the data, and the technology needed to acquire and process the data. Micro-seismic monitoring is now considered the most accurate method of imaging fracture growth and is now a globally-available commercial service.
• Downhole tiltmeters were first used for fracture monitoring during these tests and this technology is now a commercial service (Pinnacle Technologies) available to map fracture height and length.
• Important insights into the mechanisms of fracturing were developed at M-Site and are now being used in fracture models. Some of these are the development of multistranded fracture systems as a routine part of fracturing, the identification of additional fracture height containment in highly layered reservoir systems, the measurement of a large fluid lag region around the fracture tip, the formation of secondary and T-shaped fractures, and the variability in fracture development with different fluid systems.

• The Mineback Experiments at the Nevada Test Site provided the first observational evidence of fracture behavior in situ. These tests showed that in situ stress contrasts were the primary feature controlling fracture height growth, that modulus contrasts had little effect on height growth, that natural fractures caused considerable offsetting and branching of fractures, and that stress changes across faults and interfaces could stop fractures.

• Significant effort expended in developing hydraulic fracture diagnostic technology that could map the created fractures helped to advance surface tiltmeter technology for fracture mapping. Surface tiltmeters are currently used for monitoring fractures as part of a successful commercial service (Pinnacle Technologies).
• Drilling technology was advanced by breakthroughs in PDC bits through work done at Sandia National Labs. This bit has proven to be fast and rugged in many of the rock types in western gas sands basins and PDC bits are commonly used throughout the industry.

• The program in resource assessment provided the first comprehensive scientific quantification and characterization of a vast new resource, removing any question that pursuing the difficult technical challenges of enabling large-scale tight gas production was clearly worthwhile, both for industry and government. Second, in tandem with the other efforts in the WGS Program, it highlighted the concept and importance of basin-center gas formations, providing a rationale for the unique off-structure exploration and development techniques that would enable production from overpressured, low-permeability reservoirs.
  
  As a result of this work, industry began not only to appreciate the volumes of gas present, but could also begin to see a way in which it could be produced. As a result, tight gas began to be widely recognized as a key part of the nation’s resource base. Most notably, the USGS, which had previously excluded tight gas in their national resource assessment, included the category for the first time in 1995. Key outcomes of the effort include:
  • Industry now knows how the resource is distributed in each basin, and which plays hold the most potential.
  • Industry has more information on the typical porosities, water saturations, temperatures, and pressures of the rocks containing the resources.
  • Industry now understands the unique qualities of basin-centered, or “continuous” deposits, and recognizes the need for new perspectives on appraisal methods tailored for conventional resources.