Multiwell Experiment Research Articles

Diagnostic assessment of reservoir response to fracturing: a case study from Hydraulic Fracturing Test Site (HFTS) in Midland Basin

This paper outlines a data collection and diagnostics case study involving multiple horizontal shale wells. We look at well production profiles using rate transient analysis, differences in near wellbore complexity, geologic variations within the area of interest, as well as compositional differences in the rocks based on cores obtained from within the stimulated reservoir. The Hydraulic Fracturing Test Site is a multi-well experiment involving stimulation of unconventional shale wells in the southeastern Midland portion of the Permian Basin. The targeted formations include both the upper as well as the middle Wolfcamp formations, also referred alternatively as Wolfcamp A and Wolfcamp B. Data integration and analysis shared in this paper help us understand the various geologic controls impacting well productivity, particularly the wide variance observed between the Wolfcamp A and Wolfcamp B formations. Rate transient analysis indicates similar system permeabilities for stimulated wells. However, we observe higher effective fracture half-lengths for upper Wolfcamp wells. Using observations from 3D seismic interpretations (such as pad scale faults) as well as petrophysical and image log data, we highlight the substantial differences in stimulation as we move along the well laterals from the heel toward the toe sections. These differences are further reconciled with observations from zones with high data density at the core locations through stimulated rock, as well as independent data such as microseismic emissions. At the test site, Wolfcamp A was found to be relatively quartz rich with significant heterogeneity whereas Wolfcamp B is richer in clay and organic content. This impacts the geomechanical characteristics of the rock mass with much higher natural fracture density in the shallower interval. Thus, the fracture growth is more uniform in the deeper interval and more heterogeneous with branching likely in upper interval. Increased complexity also leads to consistently better productivity from the wells in the shallower interval as demonstrated from RTA results. This case study is unique because it provides valuable insights from actual sampling of the stimulated zones in hydraulically fractured wells and helps understand impact of various factors that contribute toward variability in well production. The findings from this study provides insights into need for optimization of completion designs in the various Wolfcamp landing zones, such as optimization of cluster or fracture spacing in various Wolfcamp intervals. In addition, it provides a useful template for data collection and research direction in future field test sites of similar nature in unconventional reservoirs.

Sensitive Mie scattering immunoagglutination assay of porcine reproductive and respiratory syndrome virus (PRRSV) from lung tissue samples in a microfluidic chip

A microfluidic immunosensor utilizing Mie scattering immunoaggultination assay was developed for rapid and sensitive detection of porcine reproductive and respiratory syndrome virus (PRRSV) from lung tissue samples of domesticated pigs. Antibodies against PRRSV were conjugated to the surface of highly carboxylated polystyrene microparticles (diameter=920nm) and mixed with the diluted PRRSV tissue samples in a Y-shaped microchannel. Antibody–antigen binding induced microparticle immunoagglutination, which was detected by measuring the forward 45° light scattering of 380nm incident beam using microcallipered, proximity fiber optics. For comparison, multi-well experiments were also performed using the same optical detection setup. The detection limit was determined to be 10−3TCID50ml−1 for PRRSV dissolved in PBS, while those of previous RT-PCR studies for PRRSV were 101TCID50ml−1 (conventional assays) or <1TCID50ml−1 (quantitative real-time assays). Mie scattering simulations were able to predict the shape of the PRRSV standard curve, indicating that any non-linearity of the standard curve can be interpreted purely as an optical phenomenon. Each assay took less than 5min. A strong correlation could be found between RT-PCR and this method for the lung tissue samples, even though their respective detection mechanisms are different fundamentally (nucleic acids for RT-PCR and virus antigens for light scattering immunoagglutination assay). Several different dilution factors were also tested for tissue samples, and 1/10 and 1/100 were found to be usable. If the microfluidic chips are used only once (i.e. without re-using them), both superior sensitivity and satisfactory specificity can be demonstrated. Specificity studies revealed the presence of Type II PRRSV and non-presence of Type I PRRSV and that the microfluidic chip assay could detect Type II North American strain of PRRSV for the animals tested. This work demonstrates the potential of the Mie scattering immunoassay on a microfluidic chip towards real-time detection system for viral pathogens in domesticated animals.

Sensitivity of velocities to overpressure within heterogeneous tight gas sand reservoirs

Tight gas sands account for approximately 19% of the total U.S. gas production, and may contain more than 35% of the U.S. recoverable gas resources (Oil and Gas Investor, 2005). In order to test technology for the production of gas from tight gas sands, the U.S. Department of Energy sponsored a Multiwell Experiment (MWX) in the Rulison gas field in the Piceance Basin, Colorado (Northrop et al., 1983). Drilling commenced in 1981, the field program was completed in 1988, and reporting was completed in 1990. In this article, we examine the relation between acoustic-wave velocities and pore pressures using data acquired during this experiment, and show how acoustic velocities can be used to estimate pore pressure. This is important, because pore pressure is an important factor determining the amount of gas in place, but is difficult to measure directly in low-permeability formations.

Combination of Raman Microscopy, Multiwell Plate Experimental Designs, and BTEM Analysis for High-Throughput Experimentation

Both nonreactive and reactive multiwell plate experiments were combined with Raman microscopy and band-target entropy minimization (BTEM) analysis. The multicomponent nonreactive experiments showed that accurate pure component spectral estimation is possible without recourse to any spectral libraries. The multicomponent reactive experiments showed that, in addition to accurate pure component spectral estimation, concentration profiles can be obtained for quantitative purposes. In the present case, the solvent and time dependence of a cycloaddition reaction was addressed as the high-throughput experimentation issue. A total of 1152 experimental spectra were collected and analyzed. Two methods were used, namely, (A) each solvent set was individually analyzed and (B) the entire set of spectra, from 4 different solvents, were analyzed all together. Method B provided very satisfactory results. The present study with combined Raman-multiwell plate-BTEM analysis establishes proof of concept. The new approach appears to be applicable to other frequently conducted combinatorial/high-throughput experimentations. These include, but are not restricted to, chemo- and regioselective studies, solid-phase syntheses, etc.

Development of a FEM Reservoir Model Equipped with an Effective Permeability Tensor and its Application to Naturally Fractured Reservoirs

The numerical simulation of naturally fractured reservoirs is commonly based on the continuum dual-porosity concept. However, the model needs to assume that the fractures form very regular patterns, different from field observations. In order to overcome this problem, we proposed a method that can consider the characteristics of the real fracture system and developed a two-phase transient finite element method (FEM) (TENFEM) model able to implement an effective permeability tensor. The permeability tensors were estimated by using a single-phase, steady-state FEM (EPC) model also coded in this work, where Darcy's law in matrix and cubic law in fracture were adopted for considering the flow characteristics in the discrete fracture network (DFN) approach. The developed models were applied to the paludal sand reservoir of a multiwell experiment (MWX) site to demonstrate validity and applicability of the models. The estimated average permeability in the MWX is almost identical compared to the results of the well test analysis. Then the numerical simulation with the TENFEM model was performed by using the transient pressure recorded from a gas well test on the MWX #1. From the results of bottom hole pressure and pressure distribution within the system, it was noted that the model with effective permeabilities generated almost identical results against the DFN model in the aspects of the behavior and the direction of fluid propagation with pressure decline, and therefore the proposed model is considered to be more efficient in terms of the computation time as well as the required storage capacity. Also from the upscaling study, it was shown that even with fewer numbers of grid blocks the developed model could effectively describe the directional flow caused by the fracture characteristics such as orientation and density of the fracture.

A model for lignin alteration—part II: numerical model of natural gas generation and application to the Piceance Basin, Western Colorado

The model presented here simulates a network of parallel and sequential reactions that describe the structural and chemical transformation of lignin-derived sedimentary organic matter (SOM) and the resulting generation of mobile species from shallow burial to approximately low-volatile bituminous rank. The model is calibrated to the Upper Cretaceous Williams Fork Formation coal of the Piceance Basin at the Multi-Well Experiment (MWX) Site, assuming this coal is largely derived from lignin. The model calculates the content of functional groups on the residual molecular species, C, H, and O elemental weight percents of the residual species, and moles of residual molecular species and mobile species (including components of natural gas) through time. The model is generally more sensitive to initial molecular structure of the lignin-derived molecule and the H 2O content of the system than to initial temperature, as the former affect the fundamental reaction paths. The model is used to estimate that a total of 314 trillion cubic feet (tcf) of methane is generated by the Williams Fork coal over the basin history.

A model for lignin alteration—part I: a kinetic reaction-network model

A new quantitative model is presented which simulates the maturation of lignin-derived sedimentary organic matter under geologic conditions. In this model, compositionally specific reactants evolve to specific intermediate and mobile products through balanced, nth order processes, by way of a network of sequential and parallel reactions. The chemical kinetic approach is based primarily on published observed structural transformations of naturally matured, lignin-derived, sedimentary organic matter. Assuming that Upper Cretaceous Williams Fork coal in the Piceance Basin is primarily lignin-derived, the model is calibrated for the Multi-Well Experiment(MWX) Site in this basin. This kind of approach may be applied to other selectively preserved chemical components of sedimentary organic matter.

A Reaction-Transport-Mechanical Approach to Modeling the Interrelationships Among Gas Generation, Overpressuring, and Fracturing: Implications for the Upper Cretaceous Natural Gas Reservoirs of the Piceance Basin, Colorado

Predicting reservoir characteristics in tight-gas sandstone reservoirs, such as those of the Upper Cretaceous units of the Piceance basin, is difficult due to the interactions of multiple processes acting on sediments during basin development. To better understand the dynamics of these systems, a forward numerical model, which accounts for compaction, fracturing, hydrocarbon generation, and multiphase flow (BasinRTM) is used in a one-dimensional simulation of the U.S. Department of Energy's Multiwell Experiment (MWX) site in the Piceance basin. Of particular interest is the effect of gas generation on the dynamics of the system.

Comparison and Verification of Electric-Log-Derived Rock Stresses and Rock Stresses Determined From the Mohr Failure Envelope

Summary This paper evaluates the correlation between values of minimum principal in-situ stress derived from sonic logs and those obtained from triaxial core tests and coefficient for earth at rest correlations. The minimum principal in-situ stress is then compared to actual field mini-frac test data, which accurately determine the minimum principal in-situ stress and are used to verify the accuracy of the correlations. The cores and the mini-frac stress test were obtained from two wells, the Gas Research Inst.'s (GRI's) Staged Field Experiment (SFE) #1 well through the Travis Peak Formation in the East Texas Basin, and the U.S. Dept. of Energy's Multiwell Experiment (MWX) wells located west-southwest of the town of Rifle, Colorado, near the Rulison gas field. Results from this study indicate that of the two approaches used in obtaining the minimum principal in-situ stress, the normalized Mohr failure envelope approach is in better agreement with the measured stress values (from mini-frac tests) in sandstone, while the sonic log approach gives closer match in shale.

Density-dependent growth regulation of pig retinal pigment epithelial cells in vitro.

Transplantation of the retinal pigment epithelium (RPE) involves delivering donor RPE onto a bare area of host Bruch's membrane that is surrounded by a confluent monolayer of RPE. We investigated the effects of different plating densities and the presence of an adjacent confluent monolayer on the growth characteristics and final morphology of pig RPE in vitro. In single-well experiments, porcine RPE were plated in 24-well plates at densities varying from 1 to 75 cells/mm2. Triplicate plates were counted on the 3rd and 10th days after plating and at confluence. A multiwell chamber was built to allow cells plated at different densities to be bathed with conditioned media from adjoining wells. In single-well experiments, plating at low densities increased the time to reach confluence and resulted in fewer, larger and more fusiform RPE at confluence. In multiwell experiments, the growth rate of cells plated at low density decreased as the amount of high-density medium increased in communicating wells and led to smaller, rounder cells at confluence. The presence of low-density RPE in adjoining wells increased the growth rate of RPE plated at high density and produced fewer, larger and more fusiform cells at confluence. Newly plated RPE grew more slowly when confluent monolayers of RPE were present in adjoining wells. Plating density is a critical factor in determining the growth rate and the final morphology of RPE in tissue culture. The presence of a neighboring confluent monolayer of RPE inhibits the growth rate of newly plated RPE in vitro.

Application of isotope tracer techniques for assessing the seepage of the hydropower dam at Tri An, South Vietnam

Some symptoms of excessive seepage of water were observed at the earthfill dam of the 400 MW Tri An hydropower plant in South Viet Nam. To provide experimental information for assessing the situation, isotope tracer experiments were performed using 24 existing piezometric boreholes encompassing a 1100 m long middle section of the dam. Filtration velocities measured were in the ranges (0.2–30) cm d−1 in the dam body and (0.5–75) cm d−1 in subsurface strata. No abnormally permeable horizons were detected. Permeability coefficients were calculated and compared with that obtained in pre-project hydrogeological surveys and by field tests during dam construction. Tracer experiments yielded permeability of the same range for the subsurface aquifer but from one to two orders of magnitude higher for the dam body. Such a dramatic change of permeability of the dam body me dia is interpreted as a result of interaction with seeping water after reservoir impoundment. A transient erosion process took place during several years. The losses of water by seepage through the dam body and infiltration to underground water were estimated as 1100 m3 d−1 and 15,000 m3 d−1 respectively, less than 1/2000 of the quantity of water consumed for power generation. A multi-well interconnection experiment was performed to determine the velocity and direction of subsurface flows.

Stimulation-Fluid Systems for Naturally Fractured Tight Gas Sandstones: A General Case Study

Summary Extensive production testing and core analyses show that production from the low-permeability sandstones in the Mesaverde formation at the U.S. DOE Multiwell Experiment (MWX) field laboratory is dominated by natural fractures. Stimulation data strongly suggest that damage to the narrow natural fractures significantly affects poststimulation production. This paper summarizes the field data that show evidence for production from natural fractures and for damage to those narrow natural fractures by stimulation fluids, resulting in decreased gas production. The nature of these interactions was clarified through laboratory studies on the degradation of stimulation fluids and the permeability damage to artificially fractured core exposed to the fluids. Evidence confirms that stimulation-fluid damage to the narrow natural fractures restricted the production of gas in early MWX stimulations, consistent with the partial reversibility of fluid damage to natural fractures following a long-term shut-in. A controlled breaker system was designed for use with temperature-stable biopolymer foam. Use of this fluid system in a later MWX stimulation substantially increased gas production and reduced the evidence of damage.

Hydraulic Fracturing in Tight, Fissured Media

Summary Large volumes of natural gas are found in tight, fissured reservoirs. Hydraulic fracturing can enhance recovery, but many complications, such aspressure-sensitive or accelerated leakoff, damage and complex fracturing, ariseduring treatment of such reservoirs. Special procedures generally should beconsidered during breakdown and procedures generally should be consideredduring breakdown and fracturing of these reservoirs. In addition, the use ofalternative stimulation strategies may be beneficial. Introduction Large quantities of natural gas exist in low-permeability reservoirs ofseveral western U.S. basins. The Natl. Petroleum Council identified greaterthan 600 Tcf [greater than 17 × 10–12 m3] of recoverable gas in both lenticularand blanket reservoirs. The U.S. Geologic Survey recently estimated that thereis 420 Tcf [12 × 10–12 m3] of gas in place in the Piceance basin alone and4,970 Tcf [141 × 10–12 m3] in the Greater Green Piceance basin alone and 4,970Tcf [141 × 10–12 m3] in the Greater Green River basin. If only a smallpercentage of this gas is recovered, these basins can provide significantreserves. Characteristics of these reservoirs make gas production difficult andgenerally uneconomic. Matrix rock permeabilities are typically less than 1 udunder in-situ conditions. Fissures, or natural fractures, exist in most ofthese reservoirs, but the fissures are often quite marginal, increasing theeffective permeability of the reservoir by only one to three orders ofmagnitude. Nevertheless, the presence of fissures is critical to the recoveryof gas from these rocks. These fracture systems also have often been found to be highly anisotropic, with most fissures aligned along a single axis. Permeability anisotropy iscaused by the preferred orientation of the fissure system and probably bystress anisotropy as well. Hydraulic fractures often are probably by stressanisotropy as well. Hydraulic fractures often are aligned with the fissures, soproductivity improvement ratios are low. Complex fracturing is likely in fissured media, owing to close alignment ofthe fissures and the hydraulic fracture and to the likelihood of developingoffsets when cross fissures are encountered. Such fracturing may result in hightreatment pressures, early screenouts, and significantly reduced fracturelengths. Enhanced leakoff is common in fissured reservoirs. Leakoff into the fissuresmay be quite complex because of a variety of complicating factors. The fissuresmay have a relatively constant conductivity during hydraulic fracturing, or theconductivity may be a function of the net stress on the fissure so that leakoffincreases during a treatment. In extreme cases, the fissures may begin todilate and leakoff accelerates rapidly. Dealing with these complications is particularly difficult in a tightreservoir because the fissures must be treated carefully in leakoff control orcomplex fracturing. The fissures are the production mechanism and must not bepermanently damaged or the reservoir will not produce. This paper discusses the complications associated with hydraulic fracturingin tight, fissured media and presents strategies for controlling or alleviatingsome of the deleterious effects. Some alternative technologies are alsosuggested. Much of this paper is based on results of the U.S. DOE Multi-well Experiment (MWX) and mineback experiments at the DOE's Nevada Test Site. Leakoff The most obvious effect that permeable fissures have on a hydraulic fracturetreatment is enhanced leakoff. If the leakoff is relatively insensitive topressure, as expected in fissures with a vuggy porosity, then it can bedetected with a minifracture and controlled with a suitable additive. However, the additive must not totally plug the fissures, or it must be easily cleanedout during flowback. In fissures with relatively planar porosity, such as nonmineralized cracks, the fissure aperture may be highly pressure-sensitive. During a hydraulicfracturing treatment, the pressure within the fissure increases as fluid leaksinto it, so the normal stress on the fissure decreases and its conductivityincreases. This change is somewhat compensated for by an increased normalstress caused by the opening of the hydraulic fracture, but the overall effectis a decrease in the net stress on the fissure and a pressure-dependentleakoff. Castillo developed analyses for pressure-dependent leakoff. Castillodeveloped analyses for pressure-sensitive leakoff where the leakoff coefficientis proportional to pressure-sensitive leakoff where the leakoff coefficient isproportional to pressure to the 0.5 or 1.0 power. Pressure behavior of astress-sensitive pressure to the 0.5 or 1.0 power. Pressure behavior of astress-sensitive fissure, however, is considerably more complex.

Paleomagnetic Core-Orientation for Characterizing Reservoir Anisotropy: Case Histories from Fractured Reservoirs in Abu Dhabi and Venezuela

Over the past decade, the authors have tested and refined the paleomagnetic core-orientation technique, they first applied on the U.S. Department of Energy's Multiwell Experiment in the early 1980s. Paleomagnetic orientation accuracy within 5{degree} is routinely achievable, as demonstrated in over 100 tests against dipmeter, seismic, borehole televiewer, and mechanical core-orientation (conventional multishot and MWD) techniques. These tests reveal that mechanical core-orientations commonly suffer from torques acting on downhole equipment and on the core. In contrast, the paleomagnetic core-orientation technique is not prone to downhole mechanical problems, uses no rigtime, is cost-effective, and can selectively orient individual core pieces, including ASR and DSCA samples. Accurate paleomagnetic orientations can be obtained even from cores drilled decades ago. In Abu Dhabi, paleomagnetically-oriented 10-year-old slabbed cores from Habshan and Dukhan carbonates yield fracture patterns consistent with the geologic structure in the Asab and Sahil fields. In Venezuela, paleomagnetic orientation of fresh and decade-old archived cores of Cogollo limestones and La Luna black shales yield fracture and bedding directions consistent with dipmeter data and outcrop observations around Lake Maracaibo. Paleomagnetic core-orientation permits access to the vast resource of reservoir-anisotropy data in previously unoriented cores in storage throughout the world. Paleomagnetic core-orientation is a versatilemore » technique with proven accuracy, not only in vertical wells, but also in slant holes and horizontal wells.« less

Regional Fractures II: Fracturing of Mesaverde Reservoirs in the Piceance Basin, Colorado (1)

Cretaceous strata in the east-central Piceance basin of northwestern Colorado were subjected to several phases of stress due to horizontal tectonic compression, burial, and uplift. These rocks contain a regional set of west-northwest extension fractures, observed in abundant vertical and deviated core from the U.S. Department of Energy's Multiwell Experiment and Slant Hole Completion Test wells. Time-depth relationships, fracture orientation, and fluid inclusion analyses indicate that the fractures formed at about 36-40 Ma, during a phase of increased Laramide west-northwest compression, and in a pressure-temperature regime compatible with the geologically reconstructed maximum burial depths. This regional fracturing is an example of load-parallel extension fracturing and basinwide dilatancy at depth, under conditions of high pore pressure and anisotropic, tectonically created, horizontal stress.

A Reservoir Simulation Study of Naturally Fractured Lenticular Tight Gas Sand Reservoirs

The results of parametric studies of two naturally fractured lenticular tight gas reservoirs, Fluvial E-1 and Puludal Zones 3 and 4, of the U.S. Department of Energy Multi-Well Experiment (MWX) site of Northwestern Colorado are presented and discussed. The three-dimensional, two-phase, black oil reservoir simulator that was developed in a previous phase of this research program is also discussed and the capabilities further explored by applying it to several example problems. The simulation studies lead to the conclusion that 1) at early times the reservoir performance does not depend on lenticularity; 2) the initial reservoir performance does not depend on natural fracture concentration, although at later times the performance predictions of systems with lower natural fracture concentrations begin to fall below the ones with higher concentrations; 3) porosity change with time and pressure leads to double performance prediction reversals when comparing gas flow rates and cumulative gas production from naturally fractured and non-naturally fractured tight gas reservoirs; 4) the assumption of zero capillary pressure in the fractures can lead to erroneous predictions in the simulation of naturally fractured tight gas reservoir performance; and 5) the simulator developed in a prior phase of this project is capable of handling a reservoir block that is blanket sand, lenticular, completely fractured, partially fractured or completely unfractured and is amenable to an anisotropic heterogeneous reservoir whether the reservoir is fractured or not.

Case Study of a Stimulation Experiment in a Fluvial, Tight-Sandstone Gas Reservoir

Summary A successful stimulation experiment was conducted in a fluvial sandstone of the Mesaverde formation at the U.S. DOE's Multiwell Experiment (MWX) Site in the Piceance basin of Colorado. The stimulation experiment consisted of stress tests, a three-well prefracture interference test, step-rate/flowback tests, a minifracture, a full stimulation treatment, borehole geophone diagnostics during fracturing, and a postfracture interference test.

Fluid Flow in Healed Tectonic Fractures

Summary Core samples recovered during the course of the Multi-well Experiment (MWX)contained numerous mineralized vertical tectonic fractures a few millimeterswide. Permeabilities of these calcite-filled fractures were higher but of thesame order as the surrounding matrix. Increases in permeability of up toseveral-thousand-fold were obtained by treatment with permeability of up toseveral-thousand-fold were obtained by treatment with acetic acid. Acidfracturing provides a possible approach to formation stimulation. Introduction Core recovered during the course of the MWX in the Mesaverde formationexhibited numerous tectonic fractures. The majority of fractures in thepenetrated sandstones are vertical. Their abundance in the formation, particularly in the fluvial zone, is indicated by their presence in 4-in.-diameter cores. The fractures are readily identifiable because ofmineralization, which, in most cases, appears to fill the fracture space. Thepermeability of the rock matrix, however, is generally only a few microdarcies, and it is not obvious whether the mineralized fractures serve as a conduit oras a barrier to gas flow. The flow properties of mineralized fractures weretherefore included as part of a broader investigation of the petro-physicalproperties of low-permeability petro-physical properties of low-permeabilitysandstones from the MWX. Gas production from the Mesaverde formation measuredduring the course of the MWX was often one to three orders of magnitude greaterthan predicted from measurements of matrix permeabilities. It was concludedthat natural fractures contribute significantly to observed production. A modelof fracture distribution, based on MWX results and other observations, wasproposed for horizontal fluvial reservoirs. proposed for horizontal fluvialreservoirs. It was postulated that damage to natural fracture systems duringthe course of hydraulic fracturing could explain the reduction of productivityto below prefracture values. productivity to below prefracture values. Finleyand Lorenz recently reported on a detailed study of fracture distribution andtype in MWX cores. A compilation of these results shows the relative abundanceof vertical calcite-healed fractures. In some of the mineralized fractures, crystal growth from the fracture walls is only partial. Thus, in the reservoir, some fracture areas may have very high conductivity and as-yet-unknowncontinuity, which may dominate production behavior. The presence of numerousvertical mineralized fractures (VMF's), however, poses a number of questionswith respect to their contribution to reservoir behavior and response tostimulation. In this paper, we summarize an investigation of (1) flowproperties of core plugs containing VMF's relative to the neighboring matrix, (2) relative permeability behavior of VMF cores, and (3) permeabilityenhancement through refracturing and acidization of mineralized fractures. Experimental Methods Porosities, Surface Areas, and Porosities, Surface Areas, andPermeabilities. Porosities were determined Permeabilities. Porosities weredetermined gravimetrically by liquid saturation. Reported surface areas weremeasured by the single-point BET method with nitrogen as adsorbent and heliumas a carrier. Numerous variables and uncertainties arise in attempts to measurethe flow properties of low-permeability gas sands. properties oflow-permeability gas sands. Determination of in-situ stresses and satisfactorymethods of representing formation conditions in the laboratory arelong-standing problems. Permanent changes in core properties problems. Permanent changes in core properties can arise from drilling, strain-relaxationeffects, cutting of test plugs, contact with drilling filtrate, drying, andother aspects of core storage. Other considerations that enter into laboratorytest procedures are core orientation, choice of test fluid, pore pressures, transient effects, variation in pressures, transient effects, variation in meanconfining pressure in measurements for Klinkenberg plots, procedures forchanging water saturations in relative permeability measurements, and possibleleakage of fluid between the core surface and sleeve material. These factorscan contribute to obtaining results of limited significance. Thus, testing oflow-permeability gas sands involves compromises. Permeability, desorptionisotherms, and relative permeability measurements confirm that it is obviouslypreferable to use preserved cores (cores that have not been dried preservedcores (cores that have not been dried since recovery from the field) ratherthan dried cores, so that irreversible changes in clay structure and hydrationstate can be avoided. However, only a small number of the available MWX coreshad been protected from evaporation after recovery, and in many instances, itwas found that the adopted sealing procedure did not prevent drying. This, coupled with the problems of having enough preserved core samples containingfractures and then readily identifying them in their sealed state, made the useof preserved core infeasible. preserved core infeasible. JPT P. 1310

Aquifer thermal energy storage: A numerical simulation of field experiments in China

This paper describes the results of computer simulations for three seasonal aquifer thermal energy storage experiments which have been carried out recently in China. The governing equation of the mathematical model used to simulate the experiments contains convection, conduction and heat dispersion terms. A general description of the experiments, three‐dimensional mathematical model and numerical method used is presented. The simulated temperatures agree very well with the observed field data. Total mean values of the absolute errors are 0.5°C for the single‐well experiment, 0.7°C for the double‐well experiment and 0.4°C for the multiwell experiment; relative error ranges from 2.8% to 4.5%. The simulated production temperatures and energy recovery ratios, ϵ, (ϵsimu = 0.638 versus ϵobs = 0.640 and ϵsimu = 0.729 versus ϵobs = 0.753 for different experiments) also agree very well with the field data. Discussions are also given on the determination and choice of various parameters used in the simulations. The results strongly indicate the validity of the model and simulation procedures used. It is necessary to consider the heat transfer through the fluid phase by heat dispersion in the governing equation.

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.