Laboratory Studies for the Design and Analysis of Hydraulic Fractured Stimulations in Lenticular, Tight Gas Reservoirs
Abstract: This paper was selected for presentauon by an SPE Program Commmee following rewew of mformahon conlamed in an abstract submmed byIhe author(s) Contents of the paper. as presented. have not been rewewed by Ibe Somety of Petroleum Englfieers and are subject 10 correchon by the author(s). The material, as presented, does not necessarily reflect any !JOstlton of the Society of Petroleum Engineers, Its officers. or members Papers presen!ed at SPE meetings are subject 10 pubhcauon rewew by Ed!torlal Committees of [h… Show more
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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.
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.
Formation damage studies using artificially fractured, low-permeability sandstone cores indicate that viscosified fracturing fluids can severely restrict gas flow through these types of narrow fractures. These studies were performed in support of the Department of Energy's Multiwell Experiment (MWX). The MWX program was a coordinated research effort to study methods to evaluate and enhance gas production from low-permeability lenticular reservoirs of the Western United States. Extensive geological and production evaluations at the MWX site indicate that the presence of a natural fracture system is largely responsible for unstimulated gas production. The laboratory formation damage studies were designed to examine changes in cracked core permeability to gas caused by fracturing fluid residues introduced into such narrow fractures during fluid leakoff. Polysaccharide polymers caused significant reduction (up to 95%) to gas flow through cracked cores. Polymer fracturing fluid gels used in this study included hydroxypropyl guar, hydroxyethyl cellulose, and xanthan gum. In contrast, polyacrylamide gels caused little or no reduction in gas flow through cracked cores after liquid cleanup. Other components of fracturing fluids (surfactants, breakers, etc.) caused less damage to gas flows. The results of fluid leakoff tests indicated that polysaccharide polymers caused a filter cake buildup at or near the crack entrance while polyacrylamide polymers did not cause a filtercake buildup within the time period of the tests. For xanthan gum gels filtercake buildup was reduced for gels containing polymer breakers. For gels containing polymer breakers, 100 mesh sand was an effective fluid-loss control agent for narrow fractures. Other factors affecting gas flow through cracked cores were investigated, including the effects of net confining stress and non-Darcy flow parameters. Results are related to some of the problems observed during the stimulation program conducted for the MWX. Introduction The MWX has been an extensive program to characterize and stimulate gas production from low-permeability lenticular gas reservoirs of the Western United States. Three closely spaced wells were drilled into the Cretaceous Mesaverde group in Garfield County near Rifle, Colorado. After extensive geological and geophysical characterization, a series of stimulation treatments were performed in sandstones of the paludal, coastal, and fluvial intervals of the Williams Fork formation at the MWX site. A number of reports have been published which describe the work that has been performed at the MWX site. Core studies indicated that dry core matrix permeabilities to gas at reservoir stress conditions were less than 10 40d and frequently less than 3 40d. At typical levels of water saturation for the reservoir, these values may be reduced by an order of magnitude. Porosities ranged from 3 to 12%. Clays generally averaged less than 10% and were predominantly illite and mixed-layer clays. P. 551^
Summary Extensive production testing and core analyses show that production from thelow-permeability sandstones in the Mesaverde formation at the U.S. DOEMultiwell Experiment (MWX) field laboratory is dominated by natural fractures. Stimulation data strongly suggest that damage to the narrow natural fracturessignificantly affects post-stimulation production. This paper summarizes thefield data that show evidence for production from natural fractures and fordamage to those narrow natural fractures by stimulation fluids, resulting indecreased gas production. The nature of these interactions was clarifiedthrough laboratory studies on the degradation of stimulation fluids and thepermeability damage to artificially fractured core exposed to the fluids. Evidence confirms that stimulation-fluid damage to the narrow natural fracturesrestricted the production of gas in early MWX stimulations, consistent with thepartial reversibility of fluid damage to natural fractures following along-term shut-in. A controlled breaker system was designed for use withtemperature-stable biopolymer foam. Use of this fluid system in a later MWXstimulation substantially increased gas production and reduced the evidence of damage. Introduction Three closely spaced, vertical wells were drilled through the Mesaverdeformation in the Piceance basin near Rifle, CO. The Mesaverde formation at the MWX site contains diverse depositional environments (Fig. 1): shoreline/marine, delta plain (lower, paludal environment; upper, coastal environment), fluvial, and paralic. Core was taken from all zones of production operations. Coreanalyses included a study of the reservoir properties of the matrix rock and athorough characterization of the natural fractures in the core. Results fromproduction operations showed that natural fractures dominate prestimulationproduction in all cases. Moreover, experience suggests that damage to theseapparently narrow natural fractures is a very important factor affectingpost-stimulation production. Field data indicate interactions between thestimulation fluids used and the natural fracture systems. These interactionsprobably entailed the penetration of stimulation fluids into the naturalfracture systems with a resultant decrease in permeability. The evidenceincludes the following.Observations of high treating pressures duringstimulations caused primarily by large stress contrasts between sandstones andthe abutting materials.Evidence of increased leakoff at high pressuresduring treatments (most likely into natural fractures). This appears to occurabove a threshold pressure and may result in an increase in leakoff by a factor of 50. It may be the mechanism responsible for early screenouts observed in twostimulations.Post-stimulation well-test data that clearly show the creation of a conductive fracture. We must, however, include damage to the naturalfractures that intersect the created fracture to match the low productionrates. A separate laboratory program was initiated to support the stimulationdesign. This program was expanded as a result of production problems causedlargely by the interactions of stimulation fluids with the natural fractures. Four laboratory studies were performed: analysis of stimulation fluids forpolymers and decomposition products to estimate the state of thestimulation-fluid gel in the formation; use of artificial fractures in the coreto simulate the effects of stimulation fluids on natural fractures(permeability damage and leakoff); exposure of these artificial fractures to fine-mesh sand; and design of a breaker system for use with a biogel polymer tominimize damage to the natural fracture systems. The three objectives of thispaper are to establish through MWX field and laboratory data the importance of interactions between natural fractures and stimulation-fluid systems, todetermine the interaction between the natural fracture and thestimulation-fluid systems by describing laboratory studies on the degradation of the fluid systems and permeability damage of artificial fractures ex posedto stimulation fluids, and to describe a stimulation-fluid system developed foruse in tight sandstones containing narrow natural fractures. MWX Stimulations and Fluid Systems Productions and stimulations were conducted during 1983–87 in each majorlenticular depositional environment: in paludal Zones 3 and 4 together; in thecoastal yellow sand, and in each of the fluvial Sands B, C, and E (Table 1). Minifractures were conducted to determine stimulation parameters of fractureclosure, fracture-height growth, and leakoff for use in the final design of theassociated propped stimulations. The minifractures were designed to beapproximately pad volumes. Propped stimulations were to evaluate fracturingbehavior in lenticular reservoirs and to increase production. Water-basedgelled hydroxypropyl guar (HPG) system with a methanol prepad and a 75% qualitynitrogen foam with gelled biopolymer (xanthan gum) (20 lbm/1,000 gal [2.4kg/m3] in the liquid phase) were used. Table 2 summarizes the various treatmentcompositions. All fluid systems contained bactericide and surfactant (see Refs.1 through 10 for further details). HPG Stimulation-Fluid Systems. HPG gel with methanol prepads was used in thepaludal-zone stimulation operations. Two minifractures and a proppedstimulation were conducted in the paludal interval (Table 1). The minifracturefluids contained an HPG gel with an oxidizer breaker concentration of 1 to 2lbm/1,000 gal [0.12 to 0.24 kg/m3] (Table 2). The fluid system contained a claystabilizer. The stimulation fluid contained a Ti crosslinked HPG gel (Table 2). Breaker concentration was low -- 0.25 to 0.50 lbm/1,000 gal [0.03 to 0.06kg/m3]. The breaker was used in only the last two stages (in about 30% of thefluid pumped) because of the possibility that the prevailing formationtemperatures (210F [99C]) would cause the stimulation fluid to lose viscosityand drop the proppant. Post-operation production was less than the pretreatmentproduction, suggesting significant formation damage. Because of productionproblems after the propped paludal zone stimulation, a small-volume, highlyoxidizing breaker treatment was attempted (Table 2). This treatment, whichremoved a possible gel block in the sandpack, was conducted below the fractureclosure pressure to prevent any natural fractures from opening. There was noobvious production increase as a result of the treatment; in fact, this highlyoxidizing treatment may have caused some formation damage. 3 No production wasrealized until a modified packer assembly was installed to facilitateproduction of returned fluids.
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