A Small-Scale Fracture-Conductivity Study
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 106272, "Small- Scale Fracture Conductivity Created by Modern Acid-Fracture Fluids," by M. Pournik, SPE, C. Zou, SPE, C.M. Nieto, M.G. Melendez, D. Zhu, and A.D. Hill, SPE, Texas A&M University, and X. Weng, Schlumberger, prepared for the 2007 SPE Hydraulic Fracturing Technology Conference, College Station, Texas, 29-31 January.
A series of acid-fracture-conductivity tests was conducted that simulated flow in a hydraulic fracture, both in the main flow direction along the fracture and in the fluid-loss direction. Three commonly used acid-fracturing fluids were tested at 200 and 275°F. The acid-fracture-conductivity apparatus is similar to a standard American Petroleum Institute (API) fracture-conductivity cell, but with the ability to hold core samples that are 3 in. thick in the leakoff direction.

Introduction
Acid fracturing, a well-stimulation process in which acid dissolves reservoir rock along the face of the hydraulically induced fracture, is expected to create lasting conductivity after fracture closure. However, conductivity after fracture closure requires that the fracture face be nonuniformly etched by the acid while the strength of the rock is maintained at high levels to withstand closure stress. At low closure stress, the etched pattern of the fracture face should have a dominant influence on the resulting fracture conductivity as long as the rock strength can with-stand the load. As the closure stress is increased, surface features along the fracture faces may be crushed, which makes fracture conductivity more dependent on the rock strength than on the initial etching pattern.
Success of the acid-fracturing process depends on the resulting fracture conductivity, which is difficult to predict because it depends on a stochastic process and is affected by a wide range of parameters. Most predictions of conductivity are made with the empirical correlation developed by Nierode and Kruk. This correlation was based on experiments with 1-in.-diameter, 2- to 3-in.-long fractured cores with no fluid loss through the rock samples.
To ensure that laboratory experiments represent field conditions, the phenomena that occur in the acid-fracturing process must be scaled properly. In this study, experimental conditions were scaled to give the same magnitude of acid transport along the fracture, acid leakoff, and acid reaction at the fracture face as occurs in field treatments.

Acids
Three acid systems were used in the conductivity tests: a gelled-acid system with an acid-soluble polymer as a gelling agent, an acid-in-oil emulsion, and an acid/viscoelastic-surfactant acid solution. The polymer-gelled-acid system contained 15% HCl, 2.5% gelling agent, and a corrosion inhibitor. The emulsified acid was a mixture of 28% HCl and diesel with an emulsifier and a corrosion inhibitor. The diesel forms the external phase of the emulsion. The viscoelastic-surfactant acid has unique "self-diverting" characteristics. The fluid viscosity increases significantly as the acid spends, which creates effective diversion of the acid in matrix acidizing. The same characteristics have been found to reduce the acid-leakoff rate and increase stimulation effectiveness in acid fracturing. The viscoelastic diverting acid system used in this study consists of 15% HCl, 7.5% viscoelastic surfactant, and a corrosion inhibitor.