Leakoff Control and Fracturing-Fluid Cleanup in Appalachian Gas Reservoirs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98145, "Case Histories: Damage Prevention by Leakoff Control and Cleanup of Fracturing Fluids in Appalachian Gas Reservoirs," by J. Paktinat, SPE, J.A. Pinkhouse, SPE, and C. Williams, SPE, Universal Well Services Inc.; G.A. Clark, SPE, ConocoPhillips; and G.S. Penny, SPE, CESI Chemical, prepared for the 2006 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 15–17 February. Laboratory and field data collected in the studies detailed in the full-length paper illustrate that addition of a microemulsion (ME) to a fracturing fluid has significant advantages over conventional surfactant treatments when water recovery, increased effective fracture length, and well productivity are of concern to the operator. Introduction A continuing challenge in Appalachian basin gas wells is post-fracturing fluid recovery in low-pressure, low-permeability reservoirs. Most wells are stimulated with water-based fracturing fluids and produce back less than one-half of the injected fluids even with the use of conventional surfactants that lower the air/water interfacial tension. Large quantities of fluid are trapped in the reservoir surrounding the wellbore, and in the case of hydraulic fracturing, the fluid is trapped in the region surrounding the fracture and within the fracture itself. This trapped fluid has a detrimental effect on the relative permeability, effective flow area, fracture length, and well productivity. More factors than simple air/water surface tension influence cleanup of injected fluids. A factor often over-looked is interfacial tension between rock and injected fluid, which is of prime importance in dictating capillary pressure and capillary end effects in gas wells. Fig. 1 shows a hydraulic fracture with injected fluid surrounding the propped fracture. Gas breaks through at the point of least resistance (e.g., at the fracture tip or near the wellbore), leaving injected fluid trapped. The gas then channels at the top of the fracture, bypassing a large percentage of the injected fluid. Water saturation at the fracture face and within areas of the propped fracture adversely affects the relative permeability to gas, significantly impairing gas flow into and through the propped fracture. In low-pressure environments, swabbing is used to initiate gas flow. Often, even long swabbing times achieve only 50% cleanup. Surfactant-Property Comparison Conventional Surfactants. Surfactants are a group of chemicals consisting of hydrophobic and hydrophilic tails that alter the surface activity of aqueous media. When a surfactant is dissolved in an aqueous medium, its hydrophobic group distorts the hydrogen bonds between the water molecules around the hydrophobic group, resulting in decreased surface tension between the hydrophobic groups and water. Both hydrophobic and hydrophilic groups of surface-active agents play an important role in this phenomenon. The hydrophobic portion normally is made up of hydrocarbons ranging from C8 to C18 and can be aliphatic, aromatic, or a mixture of both. The main sources of hydrophobes are natural fats, oils, petroleum fractions, synthetic alcohols, or polymers. The classification of the surfactant comes from the hydrophilic group of the surfactant. This portion identifies surfactants as being anionic, cationic, zwiterionic, or nonionic. In this work, the nonionic surfactants are used because of their compatibility with fluid systems used in the Appalachian basin. Two classes used are the aliphatic ethoxylates (AEs) and nonyl phenol ethoxylates.