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

Abstract

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.

An Approximate Solution Of Linear Flow In Naturally Fractured Reservoirs

“…The product T r X r is calculated to be 0.32 x 106 mdm 3 /mPa.s (11.32 x 106'md-ft3/cp) from equation (15). This leads to a hydraulic fracture half-length equal to 76.2 m (250 ft).…”

Impacts of Diverse Fluvial Depositional Environments on Hydraulic Fracture Growth in Tight Gas Reservoirs

“…For practical purposes, the wells in this study are considered to be vertical. Several fluid systems such as slickwater, crosslinked gel, and hybrid systems as well as different types of proppant are typically used when hydraulic fracturing in tight gas reservoirs (Sattler et al 1991;Cipolla et al 2010;Palisch et al 2010). In this study, the cases were modeled using slickwater, using a reference treatment design consisting of approximately 1,500 bbl, with a viscosity of 1 cp, and 73,000 lbs of 20/40 Brady sand, based on common practices in analog fields.…”

Completion Operations in Low Permeability Deep Basin Gas Reservoirs: To Use or Not to Use Aqueous Fluids, That is the Question

“…The clean-up can be a very slow process in tight and/or depleted reservoirs (9,(13)(14)(15)(16) . Furthermore, the process of evaporation is much slower in tighter rocks (9)(10)(11)(12) .…”