Permeability Damage to Natural Fractures Caused by Fracturing Fluid Polymers

Abstract

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^

Fracturing fluids

“…Studies [257] on formation damage using artificially fractured, low permeability sandstone cores indicated that viscosified fracturing fluids can severely restrict the gas flow through narrow fractures. Poly(saccharide) polymers such as hydroxypropyl guar, HEC, and xanthan gum caused a significant reduction of the gas flow through the cracked cores, up to 95%.…”

Fracturing Fluids

“…Studies (Gall et al, 1988) on formation damage using artificially fractured, low permeability sandstone cores indicated that viscosified fracturing fluids can severely restrict the gas flow through narrow fractures. Polysaccharide polymers, such as hydroxypropyl guar, HEC, and xanthan gum caused a significant reduction in gas flow through the cracked cores, by up to 95%.…”

Managing Uncertainty Related to Hydraulic Fracturing Modeling in Complex Stress Environments with Pressure-Dependent Leakoff

“…In some cases, increased viscosity, fluid volumes, or inadequately size fluid loss material can reduce productivity by damaging the fracture systems or failing to effectively counteract PDL. [22][23][24][25][26] In laboratory tests designed to model PDL behavior, it has been found that proper sizing and concentration of fluid loss material is essential for PDL control. 26 Further, increasing the fluid viscosity or gelling agent can sometimes dilate fissures or enhance the PDL effects, which can be potentially damaging.…”