Multiwell Experiment Site Research Articles

The Multiwell Experiment - A Field Laboratory in Tight Gas Sandstone Reservoirs

This paper reports on the U.S. DOE's Multiwell Experiment (MWX), a field laboratory aimed at improved characterization and gas production from low-permeability reservoirs typified by the Mesaverde Group in western Colorado. A broad spectrum of activities was conducted over 8 years at a site containing three closely spaced (< 225 ft (< 68 m)), deep (7,500 to 8,350 ft (2300 to 2550 m)) wells. The results yield insights and contributions into the technology of gas production from this resource.

Thermal history of the multi-well experiment (MWX) site, Piceance Creek Basin Northwestern Colorado, derived from fission-track analysis

Fission-track analysis (FTA) of apatite and zircon from 19 depth intervals in two drill holes at the MWX site in the Piceance Creek Basin, Colorado, is used to determine the burial and subsequent cooling history of the Upper Cretaceous Mesaverde Group and the Paleocene Wasatch Formation. The fission-track data, as well as available temperature, vitrinite reflectance, and geological information, indicate that the sampled sediments attained maximum burial at approximately 10 Ma, with maximum temperatures in the 150–200°C range. After 10 Ma the sediments began to cool during erosion related to the downcutting of the Colorado River, which lies just to the north of the MWX site. The heat flow in this area has remained relatively constant for the past 10 Ma.

Determining the minimum in situ stress from hydraulic fracturing through perforations

Hydraulic fracture stress measurements have been performed through perforations at depths from 1310 to 2470 m at the U.S. Department of Energy's Multiwell Experiment site. The results of over 60 stress tests conducted through perforations have shown that small-volume hydraulic fractures generally provide an accurate, reproducible measurement of the minimum in situ stress. However, unusual behaviour can occur in some tests and techniques to evaluate the behaviour are suggested. Stress results show that the stress distribution is dependent on lithology at this site; mudstones, shales and other non-reservoir rocks generally have a near-lithostatic stress, while sandstones have a considerable lower minimum stress value.

Fracture Characteristics and Reservoir Behavior of Stress- Sensitive Fracture Systems in Flat-Lying Lenticular Formations

Summary A model is presented that suggests that regional fracture systems commonly control permeability in flat-tying reservoirs. Such fractures are distributed in a continuum of sizes and occur in subparallel, en echelon patterns. Few high-angle, "orthogonal" fractures exist because this system patterns. Few high-angle, "orthogonal" fractures exist because this system is created by high pore pressures and relatively low differential horizontal (tectonic) stresses rather than by significant structural deformation. Interfracture communication occurs primarily at infrequent, low-angle intersections of fractures. Vertical continuity of such fractures through a reservoir commonly is limited by the numerous lithologic discontinuities inherent in nonmarine sandstones. This type of fracture system has been documented in Mesaverde rocks in the Rulison field of the Piceance Creek basin, northwestern Colorado, by studies of 4,300 ft [1310 m] of core from the U.S. DOE's three Multiwell Experiment (MWX) wells and by studies of the excellent nearby outcrops. Well test results and geologic data from core and outcrop support the model. The described natural fracture system has a significant effect on production and stimulation. Introduction Efforts to stimulate production of natural gas from tight, lenticular reservoirs of the western U.S. [including massive hydraulic fracturing (MHF) and nuclear stimulations] have not been notably successful because of the complexity of these reservoirs. While speculation on the cause of poor well performance has focused on such phenomena as fracturing out of zone, poorly log-defined reservoir properties (particularly with respect to gas in place), or high water saturations, few attempts have been made to examine the results in relation to specific flow mechanisms, especially natural fractures. As this paper describes, the results of experiments conducted at the U.S. DOE's MWX site have shown that narrow, stress-sensitive. natural fracture systems have a significant effect on well testing, stimulation, and production activities. Yet the presence of these fractures might not be apparent unless detailed core analysis and well tests are performed. Although natural fractures are important production mechanisms in low-permeability reservoirs, the natural production mechanisms in low-permeability reservoirs, the natural fracture systems through most of the Mesaverde at the MWX site are not well interconnected. Rather, they are subparallel and poorly connected vertically and laterally. This paper describes the unconventional natural fracture system that exists in the Mesaverde for mation at the MWX site and proposes that similar systems are common in other flat-lying, lenticular reservoirs. It also describes the supporting core, outcrop, and well-test data and discusses the effects of such fracture systems on completion and production activities. The model is subject to refinement, but we are confident that. in most respects. it corresponds to the subsurface reservoir characteristics and conditions at the MWX site. These results have important implications for reservoirs that have not been structurally deformed and for lenticular reservoirs in particular. The technology for stimulating these reservoirs will be particular. The technology for stimulating these reservoirs will be ineffective without an understanding of the interaction of stimulation treatments with natural fracture systems. The MWX The MWX project consists of three wells that penetrate and test the Late Cretaceous Mesaverde formation in the east-central part of the Piceance Creek basin. northwestern Colorado. The wells are arranged in a triangle with legs of 140 to 180 ft 143 to 55 m] at the surface, varying to 110 to 215 ft [34 to 66 m] at depth. Some 4,300 ft [1310 m] of core was taken from the Mesaverde formation in these three wells, a third of it oriented. The project is a field laboratory, combining geology, engineering, geophysics, and a number of analytical techniques to characterize low-permeability, lenticular, natural-gas reservoirs. The excellent data on reservoir characteristics are provided by abundant core. Complete log suites, close well spacing, and an extensive well-testing program. plus detailed studies of nearby outcrops of the target formation, have been combined to provide a well-controlled model of subsurface reservoir fracture networks.

Crosswell Logging for Acoustic Impedance

Crosswell seismic data are recorded by placing the seismic source in one well and receivers in a second well to measure physical properties between the two wells. When reflected seismic energy is extracted from the recorded seismograms, the resultant picture is a function of acoustic impedance (density times velocity) between the wells. Previous crosswell seismic studies have predominantly used direct waves and measure only seismic velocities by tomographic techniques. Synthetic crosswell seismic data are generated by a computer to illustrate the concepts and procedures used for crosswell impedance logging in this study. Such synthetics are useful for investigating how the experiment should work, but real data reveal how the experiment does work. Crosswell seismic data recorded at the Multiwell Experiment (MWX) site in western Colorado revealed a few surprises when compared with synthetic data. Real data consist of a lot more than simple direct arrivals and reflections. There are numerous converted waves, borehole (tube) waves, and general noise. Source directivity proves to be a very limiting problem, because energy is concentrated in a horizontal plane perpendicular to the borehole. The final product of crosswell logging for acoustic impedance is a seismogram similar to the classic CDP seismogram recorded on themore » surface for exploration. The crosswell data in this paper are in the kilohertz range as opposed to surface data, which are generally below 100 cycles/sec (100 Hz). Seismic data in the kilohertz range can theoretically resolve layers as thin as 2 ft (0.6 m) vertically. Actual resolution indicated is about 5 to 10 ft (1.5 to 3 m).« less

Tectonism, Subsidence, and Fracturing of Mesaverde Reservoirs in the Piceance Basin, Colorado: ABSTRACT

Cretaceous strata in the Piceance basin of northwestern Colorado were subjected to several phases of tectonic, burial, and uplift stress. However, extensive data on fractures in core from the US Department of Energy's Multiwell Experiment (MWX) wells suggest that most of the fractures in the Cretaceous strata in this part of the basin originated during one episode of stress. Time-depth relationships, fracture orientations, and fluid inclusion analyses all indicate that fracturing occurred about 35-40 m.y. during Laramide west-northwest horizontal compression. Most Cretaceous rocks at the MWX site contain a single set of west-northwest extension fractures. Isochore calculations indicate trapping pressures around 325 bars for fluid inclusions in 16 samples of quartz and calcite mineralization from the fractures, suggesting that mineralization occurred in a pressure-temperature regime compatible with the reconstructed burial depths of 10,000-12,000 ft. Younger episodes of stress are recorded in both the post-Cretaceous strata and in the less deeply buried Cretaceous strata near the basin boundaries. However, the effects of these younger stresses are not evident in deeply buried Mesaverde reservoirs at the MWX site in the east-central part of the basin. Moreover, if the orientation of the horizontal compressive stress rotated significantly during the late Laramide, as suggested more » by some authors, its effects are not apparent in the Cretaceous formations examined. Most measurements of ancient and present-day stress in Mesaverde strata at the MWX site indicate only west-northwest Laramide compression. « less