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DOE's Unconventional Gas Research Programs 1976-1995
SOURCE: U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Region
3.2.2. Program Design and Overview of Major Projects
The Western Gas Sands (WGS) Program was directed toward the development of new
and improved techniques for recovering gas from low-permeability (tight) gas reservoirs
that (at that time) could not be economically produced.
2. Develop and implement techniques for determining physical and chemical properties of the reservoirs,
3. Determine appropriate stimulation technology, and
4. Combine these elements to assess potential gas reserves and demonstrate economic productivity.
Generic Research focused on fundamental geoscience research on natural fractures, sedimetology and geomechanics related to tight sands. METC in-house research in this area centered on the analysis of production systems in low-permeability, fractured reservoirs.
Production Research encompassed field research activities: the multiwell site (MWX), drilling and coring of a slant-hole, and mineback experiments.
In the second phase, the results of the Generic and Production Research were combined by METC to enhance the ability to predict tight gas sand reservoir performance, which in turn was used to guide field development and production strategy.
These strategies, when applied to the estimates of potential reserves produced by the Geologic Research element of the program, resulted in the ultimate product of the WGS Program, more reliable estimates of producible gas reserves.
The WGS Program included more than a dozen individual research projects spread over the 1977 through 1992 time frame. The most significant of these projects, listed in rough chronological order, are briefly outlined below along with several related projects that either pre-dated or continued efforts initiated by the WGS Program.
Single-Well Test Program
The Single Well Test Program predated the Department of Energy and the WGS Program. Begun in 1974 and continued through 1981, this program involved government/industry cost-shared programs to evaluate Massive Hydraulic Fracture stimulation of various tight-gas reservoirs.
These stimulations had been somewhat effective in blanket reservoirs, but poor results were typical in lenticular reservoirs.
However, the reasons for these poor results (e.g., poor log evaluation, ineffective stimulation sizes, damaged reservoirs, very small lenses, etc.) were not understood.
The results of these tests provided part of the rationale for the subsequent Multiwell Experiment (MWX) testing program that was an important part of the WGS Program.
A total of 13 wells were stimulated in 4 basins under the Single-Well Test Program. Roughly half of the fracturing operations resulted in a significant increase in gas production.
The Coring Program was started in 1978 with the objective of providing rock samples from various western basins for study and testing by various laboratories, the USGS, and industry partners.
At the time this program was initiated, there was very little information on the low permeability reservoir rocks that comprised “tight gas sand” formations. Studies were carried out that included mineralogy, petrology, reservoir properties and mechanical properties of these reservoir rocks.
These tests provided samples for laboratory work to explore issues associated with clays, acidization, permeability damage from drilling and fracturing fluids, fluid compatibility, proppant embedment and many other factors, and helped provide the basic data needed to evaluate subsequent stimulation and production tests.
A total of 2,980 feet of core was taken from 12 wells in 4 basins between 1978 and 1981. Core material was distributed to the USGS, Sandia National Labs, Institute of Gas Technology and Texas A&M University for analysis. The coring and logging activities were coordinated with the Gas Research Institute.
The Multiwell Experiment (MWX)
The Multiwell Experiment (MWX), conducted from 1981to 1988, was a field laboratory established in the Piceance basin in western Colorado. The MWX was aimed at the dual objective of: 1) improving the ability to characterize tight sands reservoirs and 2) improving the ability to fracture stimulate (and thereby enhance production from) these reservoirs.
All prior tight gas sands experiments had been conducted at well spacings of 640 or 320 acres (roughly 5300 to 3700 feet between wells). This meant that the wells were too widely spaced to permit proper evaluation of the continuity in these lenticular reservoirs or to assess production/stimulation characteristics through interference testing or via various crosswell monitoring and evaluation technologies (e.g., downhole fracture diagnostics, crosswell seismic, etc.).
The MWX experiment was designed to provide a closely spaced three-well pattern (110 to 215 feet between well bores) to provide for the effective evaluation of the reservoirs and assessment of hydraulic fracture growth, and ultimately, to the understanding of tight gas sand production mechanisms.
Key features of the MWX program were the coring, logging, testing and fracturing programs conducted during the eight years of site operation. Over 4,000 ft of core were obtained during the drilling of the three wells and nearly 1,000 ft of that core was oriented
Approximately 60 ft of pressure core were also obtained in order to accurately compare core and well log water saturations. In addition to reservoir properties, the core samples provided samples for all other testing programs at the MWX site. Very importantly, the core analysis that was carried out emphasized not only the pay zones themselves but the abutting formations as well.
The core samples and fracture logs, in particular the oriented core and televiewer data, allowed for a unique evaluation of the in situ natural fracture system and the geologic/tectonic factors that produced the fractures (including the mechanism for the formation of regional fracture systems that are common in western tight gas basins).
The well testing program in conjunction with the core reservoir properties and natural fracture data defined for the first time the true nature of tight gas sand reservoirs in the western US basins.
An attempt was made at MWX to run every log available at the time (including experimental logs such as Mobil’s televiewer, Amoco’s and Schlumberger’s long-spaced sonic logs, and Schlumberger’s dipmeter). One of the very first crosswell tomogram surveys, performed at the MWX site, illustrated the degree of reservoir continuity between the wells.
The stress testing program at MWX was undoubtedly the most comprehensive stress test program ever completed in an oil and gas reservoir, including 63 microfrac measurements, anelastic strain recovery, differential strain curve analysis, circumferential velocity anisotropy, wellbore breakouts, coring induced fractures, basinal calculations, tectonic assessments, and fracture diagnostics for stress azimuths. This study provided the first comprehensive comparison of these techniques and assessment of their accuracy and reliability.
Hydraulic fracture experiments conducted in six intervals highlighted the factors critical to successful stimulation of these reservoirs. The importance of liquid-induced damage to the natural fracture system was shown through careful pre- and post-fracture testing, lab testing, and fluid research performed specifically for these experiments.
The first successful microseismic monitoring tests in tight gas reservoirs were conducted at MWX and showed that fractures grew out-of-zone in a manner proportional to the stress contrasts and that actual fracture lengths were considerably shorter than designed.
Slant Hole Completion Test (SHCT)
The Slant Hole Completion Test (SHCT), conducted from 1990-1993, was a follow on test at the MWX site to exploit the findings about the importance of the natural fracture systems in tight gas sands by using directional drilling to intercept large numbers of fractures.
Although considerable difficulty was encountered in attempting to drill into highly overpressured natural fracture systems, the well was successful in showing that the productivity of a directionally drill well bore in one zone (the Cozzette sandstone) was 10-20 times that of a vertical well completed in the same zone.
Very importantly, core was taken in two deviated well bore intervals (through both lenticular sands and a blanket marine sand) and a number of important findings resulted from the core analysis carried out on these cores.
Multi-Site Experiment (M-Site)
The Multi-Site Experiment (M-Site) was also conducted at the MWX location, but its focus was specifically on the further development of hydraulic fracturing technology.
The M-Site project was a joint effort funded by both DOE and the Gas Research Institute (GRI, now the Gas Technology Institute or GTI). Conducted from 1994 to 1996, after the WGS Program had been curtailed, the objective of this project was to construct a field laboratory that could be used to further develop and validate hydraulic fracture diagnostic technology, assess hydraulic fracturing mechanisms, and improve hydraulic fracturing stimulation models through a more complete physical understanding of the process.
The M-Site testing used two of the MWX wells for injection and monitoring purposes, one newly drilled well for cemented-in-place microseismic and tiltmeter arrays, and two new deviated lateral wells to intercept the fractures, confirm the diagnostics, and directly interrogate the created fractures through flow testing or pressure monitoring.
Comprehensive sets of fracturing experiments were conducted in two intervals (along with a preliminary set of tests in a third interval) in fluvial reservoirs that had been extensively characterized as a result of previous MWX work.
Mineback Stimulation Experiments
The actual shape and character of well bore fractures created by hydraulic stimulation were poorly understood in the late 1970’s because created fractures and the effects of stress contrasts, layered formations, faults, natural fractures and other features of reservoir heterogeneity had never been observed in situ. The mineback stimulation experiments conducted in a tunnel at a Nevada Test Site early in the WGS Program provided the first direct evidence of fracture behavior in situ by creating subsurface
hydraulic fractures near a tunnel and then mining back to intersect them. Hydraulic fracturing experiments were conducted adjacent to an existing tunnel complex at DOE’s Nevada Test Site and the results were directly observed by subsequent mineback through the experimental area.
A proppant distribution fracture experiment revealed a very complex fracture system. Observed fracture lengths were only 5 and 25 ft at the depth of the fracture interval; a significant difference from the design lengths of 175 ft.
The fracture interacted with numerous geologic faults, fractures, and bedding and parting planes. Variations of the in situ stresses and orientations that were found in subsequent testing in this experiment region were determined to be contributing factors.
An experiment was also designed and conducted which examined the behavior of hydraulic fractures at an interface between an ashfall tuff formation and a welded tuff formation.
These formations have significant differences in their elastic moduli, Poisson’s ratios and porosities. However, these differences were found to have no apparent effect on containment, with fracture growth being predominantly vertical. The experiment showed that actual hydraulic fractures were not consistent with expected results based on design models: formations of higher modulus do not contain hydraulic fractures, in situ stresses do; fracture geometry is not rectangular and symmetric; and proppants are not placed in a prescribed manner.
Western Gas Sands Associated R&D
In conjunction with the field testing program, a number of associated research activities were conducted to support the development of improved characterization and extraction technology.
These activities included tool development, modeling, and laboratory studies. Significant effort was expended in developing hydraulic fracture diagnostic technology that could accurately map the fractures created by hydraulic stimulation.
Starting in 1979, the USGS and subsequently, M. D. Wood & Associates, were funded to develop surface tiltmeter technology for fracture mapping. Sandia National Labs developed the Surface Electrical Potential technique and various borehole diagnostic tools (hydrophone strings and tri-axial geophone receivers) for mapping fractures.
Logging technology was studied at several laboratories in attempts to either improve the accuracy of existing logs in tight reservoirs or to develop new ones. Work included: 1) studies of tight gas sand water resistivity at Texas A&M University, 2) studies of the nuclear magnetic resonance (NMR) properties of tight gas sand rocks and the development of NMR instruments at Las Alamos National Labs, and 3) the modeling and measurement of the dielectric properties of tight reservoir rocks at Sandia National Labs.
Drilling technology that improved the efficiency of drilling through hard rocks was advanced by breakthroughs in polycrystalline diamond compact (PDC) bits at Sandia National Labs. The PDC bit has proven to be fast and rugged in many of the rock types encountered in western U.S. basins and continues to be widely used today. In addition, the Bartlesville Energy Technology Center and Sandia National Labs worked with industry to develop pressure-coring capabilities (including PDC core bits) and noninvasive fluids that could be used with pressure core equipment.
The Bartlesville Energy Technology Center, New Mexico Tech and the Institute of Gas Technology developed a number of core analysis techniques designed specifically for tight gas sands, including ways to measure: cation exchange capacity, porosity, porevolume compressibility, Klinkenberg corrected permeability, effects of stress and water saturation on permeability, capillary pressure, and caprock threshold pressures. Sandia National Laboratories developed techniques for measuring the effective stress law parameters for tight rocks.
Resource Assessment Projects
Reviews of the nation’s marginal gas resource base during the mid-1970s to mid-1980s pointed to the possibility that huge natural gas resource volumes existed in Western basins (Figure 3.2.2).
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TABLE OF CONTENTS
2. GRI Research into Unconventional Gas Resources
3. Structure of the Enhanced Gas Recovery Program (EGR)