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.
This paper is a case study of the stimulation and testing of tight, lenticular sands in the paludal interval of the Mesaverde group in the Piceance basin at DOE's Multiwell Experiment (MWX) site in Colorado. Topics discussed include geologic data, stress test results, well testing, laboratory core studies, stimulation and stimulation analyses, and postfracture operations.
SPE Members Abstract Two stimulation operations have been conducted to date in the paludal zone of the Mesaverde formation in one well, MWX-1, at the DOE's Multiwell Experiment test site in the Piceance Basin near Rifle, Colorado. Problems were encountered in the second stimulation: MWX-1 would not sustain production for several months and post-frac production production for several months and post-frac production was less than pre-frac rates. The laboratory program was expanded to examine these problems and program was expanded to examine these problems and these laboratory studies were integrated with well testing and other data to help explain MWK-1 production behavior. A unique explanation cannot be found production behavior. A unique explanation cannot be found for the failure of MWX-1 to produce; a combination of factors was responsible. Water probably inhibited matrix rock production. A system of naturally occurring microfractures is important in production from the paludal zone and it probably sustained damage by water and fracture fluids. The basic gel degraded slowly because only a small amount of breaker was used. The fracture closure (viscosity break) observed from the volte analysis of the stimulation was not the same as the breakdown of the basic gel. The remedial treatment conducted after the second stimulation was probably too reactive. A list of items has been developed from experience gained both inside and outside the laboratory that shows what work and which procedures should be emphasized or avoided in tight sand stimulations. Introduction Two stimulation operations have been conducted in Well MWX-1 of the Department of Energy's Multiwell Experiment (MWK) in the Piceance Basin near Rifle, Colorado. Zones 3 and 4 of the paludal section of the Mesaverde Formation, shown in Figure 1, were the zones of interest. The first stimulation operation (Phase I, December 1983) consisted of a series of small, unpropped step-rate/flow-back tests and minifracs whose primary goals were fracture diagnostics and containment prediction. The second (Phase II, May 1984) was a larger sandpropped hydraulic fracture treatment with the additional goal of production enhancement. Production rates of 250 MCFD (3080 m /d) were measured from the two zones together during prePhase I testing. No difficulty was encountered in prePhase I testing. No difficulty was encountered in getting the well to flow after periodic cleanup during the winter after the Phase I operations. However maximum flows of only 200 MCFD (5660 m /d) could be sustained just prior to Phase II operations. Clean-up after the Phase II Phase II operations. Clean-up after the Phase II stimulation was difficult and the well was not capable of sustained flow. A remedial breaker treatment was performed a month later (June 1984) without notable improvement. Sustained gas flow was not realized until mid-July when a revised packer assembly was installed. Nevertheless, rates in excess of 170 MCFD (4810 m /d) could not be sustained during the ensuing post-frac testing period. period. Extensive laboratory investigations were initiated to examine the production problems. These studies involved many aspects of the core program and the analyses which had been performed prior to both stimulation operations. The purpose of this report is to examine (a) early core analysis data, (b) laboratory core studies supporting stimulation design, and (c) post-frac laboratory investigations and analyses, post-frac laboratory investigations and analyses, and then to integrate these results with field data to clarify and interpret the results of both the Phase I and II stimulation operations. Phase I and II stimulation operations. BACKGROUND CORE DATA The objectives of the MWX core program are to provide a physical description of the reservoir and provide a physical description of the reservoir and to support well testing and-stimulation. Prior to MWX, there was very little specialized core data (e.g., restored state permeabilities as a function of water saturation, capillary pressures, etc.) available from tight sandstone formations such as the Mesaverde. In fact, many of the first such measurements were made on WWX core. P. 397
The Department of Energy has initiated a Multi-well Experiment with the objectives: (1) characterization of low permeability, lenticular gas sands and (2) evaluation of state-of-the-art and developing technology for their production. The location is in the Rulison Field in the Piceance Basin, Garfield County, Colorado where the lenticular Mesaverde sequence and the underlying blanket Corcoran-Cozzette are the formations of interest and lie at depths between 4,000 and 8,600 ft. Features of this research-oriented field experiment include: (1) three close-spaced wells (100 to 500 ft) for reservoir characterization, conventional well tests, interference testing, well-to-well geophysical profiling, and placement of diagnostic instrumentation adjacent to fracture treatments; (2) complete core taken through the formations of interest; (3) a comprehensive core analysis program; (4) an extensive logging program featuring conventional and experimental logs; (5) determination of in situ stress in sand lenses and bounding shales; (6) application of geophysical techniques to determine sand lens orientation; (7) use of seismic, electrical potential and tilt diagnostic methods for hydraulic fracture characterization; and (8) a series of stimulation experiments. Analysis of data from these activities will yield the following kinds of information: (1) geologic characterization emphasizing the morphology, properites and variability of lenticular sands; (2) a reservoir model for lenticular sand production; (3) a fracture model including correlations between stress, geometry and lens orientation; (4) core-log-well test correlations for improved formation evaluation, and (5) economic assessments. This paper is a survey of the findings to date. It includes a discussion of a study of the most relative outcrops; the drilling program; core analysis; and well-to-well correlation of the geology. Two wells have been drilled, the first was continously cored through the Mesaverde section from 4,310 to 6,820 ft. In the second well, 950 ft of core was taken, which include all the zones of interest for stimulation experiments. Significant overpressuring was encountered and a large number of fractures were observed in the core, some open up to 3/4-inch width with partial mineralization. Preliminary indications are that natural fractures may play a much larger role in gas production from the low permeability lenticular sand than had previously been thought.
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