Multiwell Experiment Research Articles

Significance of Coring-Induced Fractures in Mesaverde Core, Northwestern Colorado (1)

Several types of mechanically induced fractures occur in Mesaverde core from the U.S. Department of Energy's Multiwell Experiment wells. Two of these types of fractures have significance for the determination of in-situ stress orientations. Scribe-line fractures originate at orientation grooves on the surface of the core and are initiated by scribing knives in the core barrel minutes after the core is cut free from the surrounding rock. In sandstone core, these fractures strike parallel to the maximum in-situ horizontal stress direction that has been determined in these wells by anelastic strain recovery measurements. Coring-induced petal and petal-centerline fractures form just below the advancing core bit. Their strike is also controlled by the in-situ stress field. Loc lly, however, strike may be secondarily influenced by shear stress created by bit rotation, reorienting petal fracture strikes in the direction of rotation. Nevertheless, in many situations the orientations of these two types of fractures may be used to determine the orientation of the horizontal in-situ stress field if the core is oriented. Even in unoriented core, the relative orientation between in-situ stress and natural fractures may be determined.

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.

Altered-Stress Fracturing

Summary Altered-stress fracturing is a concept whereby a hydraulic fracture in one well is reoriented by another hydraulic fracture in a nearby location. A field test was conducted in which stress changes of 250 to 300 psi [1.7 to 2.1 MPa] were measured in an offset well 120 ft [37 m] away during relatively small minifractures in a production well. Results show that altered-stress fracturing is possible at this site and others. Introduction The in-situ stress field that exists at depth in a rock mass is known to control many oil and gas stimulation parameters, such as the orientation and height of a hydraulic fracture. In some reservoirs, the stress distribution is unfavorable for an effective stimulation, and considerable research has been directed toward mitigating undesirable effects. Particular examples important for natural gas production are a hydraulic fracture that is not optimally oriented with respect to well layout or permeability anisotropy, and fracture height growth into a low-stress zone above or below the reservoir. Controlling such undesirable growth has been primarily through modifications of the stimulation design or the drilling program. While such modifications as altering the well layout have been successful in some cases, they may not be practical in others. A second approach exists, however, that usually is overlooked: instead of changing the stimulation or drilling program, it may be possible to alter the stress field to a more favorable orientation or magnitude. While altering the stress field at 5,000- to 10,000-ft [1500- to 3000-m] depths may seem pretentious, we should remember that significant stress perturbations are induced by any process that changes reservoir pressure or fractures the rock. Because of the high pressures needed for fracture dilation and the large surface area created. a hydraulic fracture can effectively alter the stress field. This paper elaborates on the concept of stress alteration through hydraulic fractures and the ways it can be applied. We describe an initial field test, where stress tests were conducted in one well while a second well was hydraulically fractured to measure the stress change in the vicinity of a hydraulic fracture. The discussion includes analytic calculations for homogeneous media and finite-element calculations for layered media for comparison with the field results. We describe reservoir simulations that show the productivity enhancement expected from application of this concept and discuss practical capabilities of this altered-stress fracturing technique. Background Large natural-gas resources are found in tight, lenticular gas sandstones, particularly in the western U.S. Most of these lenticular sands have matrix permeabilities less than 1 or 2 ud and limited lens sizes, which are not encouraging for economical production under any foreseeable conditions. Many of these reservoirs, however, are also naturally fractured, resulting in effective reservoir permeabilities of tens of microdarcies as measured in well tests. These permeabilities are just sufficient to allow economical production in some areas and to warrant additional research in others. Such a situation exists at the U.S. DOE's Multiwell Experiment (MWX) Site, near Rifle, CO, in the Piceance basin. 6 A three-well research site was developed to characterize tight gas sands and to evaluate stimulation and production methods. The non-marine, lenticular sands of the Mesaverde group are the target reservoirs, at depths from 4,000 to 7,500 ft [1219 to 2286 m]. Most of these lenticular sands have 0.1- to 2- ud matrix permeabilities, but limited natural fracture systems increase the effective reservoir system permeabilities to 10 to 50 ud. Studies of outcrops and more than 4,100 ft [1250 m] of core-more than 1,100 ft [335 m] of it oriented-have shown that the natural fractures are primarily unidirectional, resulting in an anisotropic permeability distribution. This has been substantiated with three-well interference test results that show permeability anisotropy ratios of 30:1 to 100:1. In the few lenses where cross-fracture sets are observed, permeability anisotropies are still large. Reservoir thicknesses range from a few to 40 ft [12 m], with estimated lens widths of 100 to 2,500 ft [30 to 762 m], depending on the depositional environment. Because these reservoirs were deposited as sand channels or meander belts, lens lengths are expected to be much greater than the widths. Finally, the in-situ stress field is aligned with the natural fractures so that hydraulic fractures parallel the natural fractures. Stimulation and production of these reservoirs is not easy. Hydraulic fractures will intersect only a few of the natural fractures, and productivity enhancements of only about two to four times can be expected. In addition, these natural fractures are extremely narrow and appear to be easily damaged by stimulation gels. JPT P. 990^

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.

Tight-Sand Development Potential in the Southern Rulison Area, Garfield County, Colorado

Summary Reservoir characterization and simulation of natural gas production were used to assess the production potential for an extension area within the Piceance Creek basin (Garfield County, CO). Many of the basin's reservoirs are lenticular sands characterized as having low porosities, low permeabilities, and relatively high water saturations. One such horizon is the Mesaverde group, which consists of discontinuous sandstone lenses distributed within a shale matrix and is present throughout the Piceance Creek basin. Prior studies of the commercial Rulison field in the east-central portion of the basin indicated that compressional forces created an anticlinal nose aligned northwest/southeast through part of the field. These forces created a conductive natural fracture system that enhances the permeability of the Mesaverde. This paper shows a relationship between Rulison field and an area south of Rulison that encompasses about 36 sq miles [93.2 km]. The area has been sparsely drilled (averaging one well per two sections and nonuniformly spaced); thus, the possibility exists for further commercial development through infill drilling. The southern Rulison area is characterized in structure, percent sand, and critical reservoir parameters including permeability. Production rates, cumulative gas production, and drilling and completion data are used to infer permeabilities by history matching with a dual-porosity reservoir simulator. Results include a reservoir description of the area and economic evaluations of several investment scenarios that will help to assess the potential resource. Introduction The Piceance Creek basin of northwest Colorado contains an estimated 33 Tcf [934 × 10–9 m3] of maximum recoverable gas and is the setting for the U.S. DOE's Multiwell Experiment (MWX). The Natl. Petroleum Council estimated the volume of recoverable gas at 12.9 Tcf [365 × 10–9 m3), assuming a gas price of $2.50/Mcf [$8.83/100 M], a 15% discounted cash flow rate of return, and the current level of technology. The basin comprises Garfield, Mesa, and Rio Blanco counties with minor extension into Delta, Gunnison, and Moffat counties. The Tertiary Age Wasatch formation and Cretaceous are Mesaverde group are typically the gas-productive horizons throughout the basin. Situated near the center of the basin in Garfield County is the Rulison field. Discovery of the field and its early development occurred in the late 1950's and early 1960's, with sparse drilling in the southern sections of Township 6 South, Range 94 West (T6S, R94W) and scattered drilling throughout T7S, R94W and T7S, R95W. Wasatch completions have generally been restricted to T6S, R94W with development on 160 acres/well [65 ha/well]. The Mesaverde has been drilled on 320 acres/well [130 ha/well] and is the focus of this study. Most of these wells penetrated the entire lenticular section of the Mesaverde (Williams Fork), with a few deeper wells reaching the marginal marine deposits in the lowermost Mesaverde. Northwest Exploration (now part of the Williams Co.) further developed the area during the early 1980's. Thirteen Mesaverde wells were completed in T6S, R94W. For this investigation, T6S, R94W will be referred to as the Rulison field and T7S, R94W and T7S, R95W as the southern area. The geology and natural gas production of the study area were evaluated to determine why the southern area had not been developed to the extent of the Rulison field. The Rulison field has been well characterized; therefore, the present effort concentrates on the southern area. In appraising the southern area as a developmental prospect, similarities to the Rulison field are identified and quan-tified. Fig. 1 is a generalized map of the study area, the two townships of interest are southwest of Rifle, just south of the Colorado River.

In-Situ Stresses in Low-Permeability, Nonmarine Rocks

Summary In-situ stress measurements have been performed over a 2,500-ft [760-m] interval in the nonmarine section of the Cretaceous Mesaverde in the Piceance basin in Colorado. These measurements included 52 hydraulic fracture measurements of the minimum in-situ stress and 22 anelastic strain recovery (ASR) measurements. Stress data obtained in sandstones, shales, mudstones, siltstones, and coals show the effect of lithology on the magnitudes of the stresses. Introduction Many oil and gas production and completion activities depend on the in-situ stresses in and around the reservoir rock, and the degree of success of these activities can often be influenced by determination of the stresses and their inclusion in the design process. Some important examples include hydraulic fracture growth, proppant crushing in hydraulic fractures, fracturing during waterflooding, wellbore stability, and the behavior of natural fractures during any process in which the pore pressure and/or stresses are altered. Knowledge of the stress field at depth does not come easily, however. Calculations of the stresses are relatively crude because the three-dimensional stress field depends on a complex history of properties and loading conditions and may include anelastic, plastic, and fracturing episodes. While much of the effect of this past history may be negligible in many cases, stresses may also be locked in sufficiently so that the total history must be known. Additionally, we would also need to know other conditions, such as drained vs. undrained behavior of the pore fluids, whether the tectonic loads should be considered stress or strain boundary conditions (important for layered media), the history of diagenesis, and details of the constitutive model(s) of the bulk rock. In all but the simplest cases, we will have difficulty performing credible stress calculations. The previous discussion suggests that stress measurement is the preferred option, and much research has been performed to develop usable stress measurement techniques that can be applied at depth in a wellbore. These includethe well-known hydraulic fracturing technique;various core analyses, such as ASR, differential strain curve analysis (DSCA), and differential wave velocity analysis;wellbore condition logs (televiewers and calipers) to examine eccentricity and breakouts;hydraulic fracture diagnostics to estimate the stress field orientation; andother ideas. The subject of this paper is two-fold:a compendium of all in-situ stress measurements conducted in nonmarine rocks at the Multiwell Experiment (MWX) in the Piceance basin of Colorado anda critical analysis of the techniques, results, and implicat ions. This study builds on many previous stress measurements and analyses at this site, many of which have been in marine rocks. MWX Site The MWX25 is an experiment funded by the U.S. DOE to characterize low-permeability reservoirs in the western U.S. and to test technology for the production of natural gas from these marginal resources. The site is located in the Piceance basin in west-central Colorado near the town of Rifle. The facility consists of three closely spaced wells (100 to 220 ft [30 to 67 m] apart) painstakingly char-acterized by the recovery and analysis of more than 4,100 ft [ 125m] of core, advanced logging programs, detailed well testing, frequent stress testing, and highly instrumented stimulation experiments. The Mesaverde group, the tight sand target, is between the depths of 4,100 and 8,200 ft [ 1250 and 2500 m] at this location.

Differences in Fracture Characteristics and Related Production: Mesaverde Formation, Northwestern Colorado

Summary Reservoirs in the Mesaverde formation in the Piceance Creek basin, northwestern Colorado, consist of naturally fractured, low-permeability sandstones. Permeability along the natural fractures is the primary mechanism of gas production. However, the characteristics of the natural fractures change within the formation at boundaries roughly coincident with changes in depositional environment, as seen in 4,200 ft [1280 m] of core from the U.S. DOE's Multiwell Experiment (MWX) wells. Depositional environment controlled the characteristics of the matrix rock and reservoir heterogeneity as well as the fracture permeability system. Therefore, natural gas production rates from the different zones in the formation can be correlated with the natural fracture characteristics from that zone. The fracture systems consist of unidirectional regional fractures that provide highly anisotropic reservoir permeability trends. Locally, secondary fractures were superimposed on the regional system, and higher production rates resulted. Production rates are low elsewhere, and little well-to-well communication across the fracture trend occurs. Moreover, stimulations of these reservoirs have often been unsuccessful because the hydraulic fractures propagate parallel to the natural fractures.

Characterization and Implications of Dickite-Mineralized Fractures in Mesaverde Core from Piceance Creek Basin: ABSTRACT

The presence of authigenic dickite, a polymorph of kaolinite, in fractures in the gas-productive Mesaverde Formation of the Piceance Creek basin may indicate a locally interconnected fracture system. Core used in this study is from the US Department of Energy's Multiwell Experiment located in the Rulison field in northwestern Colorado. The Mesaverde is fractured throughout the 4000-ft section cored at this location. Although spray calcite is the most common fracture mineralization throughout the section, dickite mineralization commonly occurs as a secondary phase in fractures within a roughly 400 ft-thick interval. The petrography of the host rock where these fractures are concentrated is similar to the petrography of other core intervals where dickite mineralization and/or irregular fractures were not detected. No apparent difference is seen in the mineral constituents or the diagenetic alteration of the minerals present, suggesting that the source of the dickite may have been extraformational. If the dickite is extraformational, the variation in the orientations of these anomalous dickite-mineralized fractures, coupled with the presence of dickite on some other types of fractures, implies that at some time in the depositional and burial history of the rock the dickite fractures represented an interconnected fracture system. Microporosity is associated withmore » the dickite mineralization, and this fracture system may still be locally interconnected and permeable making this a potentially gas-productive interval. Special consideration of drilling and/or stimulation fluid interaction with dickite may be necessary.« less

Advances in Borehole Seismic Fracture Diagnostics

Summary Recent redesign of the hardware, software, and data-reduction techniques associated with a triaxial borehole seismic system (BSS) have made possible better estimates of hydraulic fracture geometry. The redesigned triaxial system now incorporates three geophones per axis and provides 30 times the downhole gain of the replaced system, resulting in improved signal-to-noise ratios. This stronger signal, with an increased digitization rate for each of the six simultaneously digitized channels from two borehole seismic tools, has made possible the acquisition and processing of data that previously were inaccessible. The new electronics makes use of a null system and an improved calibration system that includes a synthetic event generator to produce sinusoidal signals of specified amplitude, frequency, and phase. These make readjustments to maintain system balance while the tool is in place downhole. A maximum likelihood event location scheme, incorporating an algorithm based on the use of directional statistics, is used to compute the location of microseisms and error estimates for these locations. Based on the ability to locate perforation shots, a 25-ft [7.6-m] uncertainty in the location of individual microseisms is the estimated accuracy of the redesigned system. This resulted in a fairly high level of confidence in the determination of the azimuth of the Nov. 1, 1986, hydraulic fracture in the fluvial zone at DOE's Multiwell Experiment (MWX) site.

Pressure Core Measurements in Tight Sandstone Lenses During the Multiwell Experiment

SummaryA pressure-coring operation was performed in two tight sandstone lenses of the Mesaverde group during the drilling of one well of DOE's Multiwell Experiment (MWX). The goals were (1) to obtain accurate water saturation data; (2) to measure the amount and composition of gas on a foot-by-foot basis; and (3) to recover fluids for water-chemistry analysis. This application of pressure coring had not been tried before; it was felt to be a novel approach for accurate water-saturation measurements in tight sandstone core.Several changes to usual pressure-coring procedures were adopted, including the development of an organic, noninvasive coring fluid compatible with the oil-based drilling fluid used to help preserve the original water saturation. The pressure-core assembly was modified so that gas samples could be taken directly from the inner barrel and would allow the capture of any gas not trapped during the process; however, none was found. Three 15-ft [4.6-m] -long pressure-core barrels were taken from two sandstone lenses 20 and 30 ft [6.1 and 9.2 m] thick.Water-saturation data were obtained on the pressure core by both Dean-Stark distillation and vacuum-oven drying. The analyses of all recovered core suggest that water-extraction procedures should be carried on for a long time and that the Dean-Stark procedure may not be the water-extraction procedure of choice for tight sandstone core. The failure of one core barrel to hold pressure turned out to provide a bonus, because identical analyses were made on pressurized and unpressurized core from the same sandstone. These analyses showed no appreciable water-saturation difference on rock with the same porosities. No fluid was recovered during thawing alone; the rock was too tight. Gas was obtained in the pressure-coring process and measured on a foot-by-foot basis.

Results of the multiwell experiment in situ stresses, natural fractures, and other geological controls on reservoirs

Hundreds of millions of cubic meters of natural gas are locked up in low‐permeability, natural gas reservoirs. The Multiwell Experiment (MWX) was designed to characterize such reservoirs, typical of much of the western United States, and to assess and develop a technology for the production of this unconventional resource. Flow‐rate tests of the MWX reservoirs indicate a system permeability that is several orders of magnitude higher than laboratory permeability measurements made on matrix‐rock sandstones. This enhanced permeability is caused by natural fractures. The single set of fractures present in the reservoirs provides a significant permeability anisotropy that is aligned with the maximum in situ horizontal stress. Hydraulic fractures therefore form parallel to the natural fractures and are consequently an inefficient mechanism for stimulation. Successful stimulation may be possible by perturbing the local stress field with a large hydraulic fracture in one well so that a second hydraulic fracture in an offset well propagates transverse to the natural fracture permeability trend.

Geologic and Engineering Implications of Production History From Five Mesaverde Wells in Central Piceance Creek Basin, Northwest Colorado

SummaryFive gas wells completed in low-permeability rocks of the Upper Cretaceous Williams Fork and Iles formations in the central part of the Piceance Creek basin, northwest Colorado, have been on continuous production for about 2 years. A general comparison is made between these wells and the Dept, of Energy 's Multiwell Experiment (DOE MWX) wells about 30 miles [48 km] to the southeast. The five wells were completed with gelled KC1 water/sand fractures in various intervals over a 3,000-ft [910m] section of largely fluvial and paludal rocks of the Williams Fork and Iles formations of the Mesaverde group. Flow has apparently stabilized at low flowing tubing pressures at a rate of 30 to 125 Mcf/D [0.84 to 3.5 × 103 m3/d] per well, with minor water and variable condensate volumes. On the basis of production information to date, attempts to select intervals with better-than-average effective permeabilities by state-of-the-art techniques were not successful. The study points out several important differences in the Mesaverde group between the area of the five wells and the MWX site. Formation temperatures are higher and levels of thermal maturity are lower in the area of the five wells than at the MWX site. High formation pressures were encountered in the lower part of the Williams Fork and Iles formations at the MWX site, while thus far, only near normal to slightly below normal pressures have been encountered in the Mesaverde group in the area of the five wells. Gas shows and gas production characteristics in the area of the five wells appear to be related in part to the availability of gas-generating source rocks. Gas shows are sparse in the upper 1,000 to 1,700 ft [305 to 520 m] of the Williams Fork formation, where good source rocks are uncommon. Gas shows generally occur in only the upper parts of sandstones in this interval, suggesting that discrete gas/water contacts (GWC 's) are present. Completion attempts in this interval are commonly hindered by higher-than-average volumes of formation water. In contrast, almost continuous gas shows occur throughout the remainder of the Williams Fork formation and underlying Iles formation, where good source rocks are abundant. Discrete GWC 's are rare in this interval. Completions in this part of the section are capable of sustaining continuous gas production. The differences in the Mesaverde group between the MWX site and the area of the five wells point out the need to expand the MWX data base to include other gasproducing areas in the Piceance Creek basin.

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

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

Fracturing and Testing Case Study of Paludal, Tight, Lenticular Gas Sands

Summary 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.

Case Study of Gas Migration in the Wasatch and Mesaverde Formations of the Piceance Basin, Colorado

Summary The U.S. DOE Morgantown Energy Technology Center (METC) conducted a reservoir simulation study and an engineering analysis on gas migration for two tight lenticular sandstone units, the Wasatch formation and the Mesaverde group, in Garfield County, CO. The study quantified potential gas loss through migration from the Naval Oil Shale Reserve (NOSR) No. 3 (Fig. 1) to nearby producing wells located in the Rulison field and forecasted production for proposed NOSR No. 3 offset wells located at previously selected sites, offsetting commercial wells outside the reserve. After the reservoirs were characterized, production was forecasted by use of a reservoir simulator to history match actual production. A dual-porosity reservoir simulator, developed specifically for naturally fractured gas-producing reservoirs, and reservoir engineering calculations were used in the study. The study followed two extensive DOE field testing programs, a nearby multiwell experiment (MWX), and a special Mesaverde test well (1XM9) drilled in NOSR No. 3 (see Figs. 1 and 2). These DOE field tests provided geologic information and values for reservoir parameters that control production from this unconventional gas resource. In addition to the results from the two field experiments, the study used a detailed geologic analysis and production data from nearby industry wells to characterize the producing formations.

Fracture Diagnostics Results for the First Multiwell Experiment's Paludal Zone Stimulation

Summary A dual seismic system designed to monitor the growth of hydrofractures was fielded during the first stimulation at the U.S. DOE-sponsored Multiwell Experiment (MWX) during Dec. 1983. The MWX consists of three 8,000-ft [2438-m] wells drilled in low-permeability gas reservoirs. The wellbores are spaced less than 200 ft [61 m] apart over most of their length. With this configuration, microseismic activity created by the fracture treatment in one well could be observed with downhole geophone packages in the two adjacent wells. Responses of the two triaxial borehole seismic tools (BST's) were monitored in real time. Upon detection of a seismic event of known location, orientations of the BST's were determined from the polarization of the compressional wave arrivals. From knowledge of the horizontal and vertical angles of incidence and an empirical estimate of seismic velocities, the locations of the fracture-induced seismic sources were estimated. Thus the location of the microseisms could be estimated and, in turn, the fracture height and extent inferred. This paper presents the results of fracture diagnostics by use of this borehole seismic system for the first MWX stimulation. This information is compared and combined with the results obtained from other diagnostic techniques to provide an integrated estimate of the fracture geometry resulting from the stimulation.

Porosity, Permeability, and Pore Structure of the Tight Mesaverde Sandstone, Piceance Basin, Colorado

SummarySpecial core analyses on 44 tight Mesaverde sandstone samples from the U.S. DOE Multiwell Experiment (MWX) were combined with petrographic investigations to relate the porosity and permeability of the cores to the pore structure of the rocks. Core analysis was performed on 1-in. [2.54-cm] -diameter horizontal plug samples with a computerized steady-state-flow measuring device that routinely measures gas flow rates with a resolution of better than 10−6 std cm3/s.All samples were selected from intervals expected to be gas-productive on the basis of wireline well logs and were taken from the portion of the interval that showed the lowest gamma ray log response. The core plugs were measured for dry permeability to gas, relative permeability at various water saturations, porosity to gas, and PV compressibility. Petrographic samples were taken directly off the plug ends and were analyzed with both an optical microscope and a scanning electron microscope (SEM). The petrographic study was explicitly directed toward observing the flow paths and pore structure deduced from the core analysis data.Petrographic observations revealed that the pore geometry of tight sandstone falls into three general categories: grain-supported primary pores, secondary solution pores connected by narrow intergranular slots (the most common pore structure in the Mesaverde), and matrix-supported grains. Reservoir properties measured from core analysis correlated fairly well with the observed pore geometry and showed trends associated with various depositional environments in the formation.

Determination of Widths of Meander-Belt Sandstone Reservoirs from Vertical Downhole Data, Mesaverde Group, Piceance Creek Basin, Colorado

Paleohydrologic reconstructions have been applied to downhole measurements made in the U.S. Department of Energy's Multi-Well Experiment (MWX) wells in order to derive sand-body widths for the nonmarine part of the Mesaverde Group (Cretaceous) in the east-central part of the Piceance Creek basin of northwestern Colorado. These sediments were deposited in a predominantly meandering fluvial system. Paleochannel depths, recognized in the wells from the heights of fining-upward trends in core, can be converted to channel widths,using a published formula. The resulting channel widths are used to calculate meander-belt amplitudes (sand-body widths) from relationships derived from published empirical data. The estimate of width derived from this technique can be compared with sa d-body widths derived by three other methods. These various techniques give compatible results, indicating the validity of the downhole technique.