Shallow Gas Development: Stimulation Fluids
Authors: Powell R.J. , Terracina J.M. , McCabe M.A. , Parker M.A. , Becker D.L. 
Published: March 2010

The stimulation treatments for a 40-well Medicine Hat Shallow Gas Project developed by Husky Oil Operations Ltd. are discussed. Field application. of the stimulation designs is presented. Laboratory test results of newly developed breaker technology are also presented.

The new breaker technology is based on encapsulation of an acid source and an enzyme source. These products are specifically designed for use in low-temperature, borate-crosslinked fracturing fluids. Encapsulation of these breakers allows controlled release of the substrate compared to the conventional breaker package of soluble, catalyzed sodium persulphate.

Laboratory data, including rheology and conductivity for fluids containing the new breakers, are compared to fluids containing traditional, low-temperature breakers. The release-rate profiles for the encapsulated breakers are discussed in relation to the pH requirements of enzyme breakers. In laboratory tests, the new breakers result in improved conductivity compared to a catalyzed, sodium persulphate breaker system.

Introduction

The design of breaker systems for hydraulic fracturing fluids in low-temperature (below 50 ° C) applications offers unique challenges. One of these challenges is that, in general, common oxidizing breaker systems must be activated at temperatures below 50 ° C. Typical activators for persulphate-types (S2O82-) of breakers include tertiary amines (NR3)(l), secondary amines (NHR2), and transition-metal complexes. Enzyme breaker systems appear to provide a ready solution for low-temperature situations. A common hemi-cellulase-type enzyme that will degrade guar polymer is active at room temperature (25 ° C). Hemi-cellulase enzyme breakers do not have the same temperature requirements that persulphate breakers do, but enzyme breakers do have specific temperature and pH requirements. Through the process of evolution, enzymes have an optimum temperature value and an optimum pH value at which they will function best. Many enzymes, including hemi-cellulase type enzymes, will function over a fairly broad range of temperature and pH values(2, 3). New enzymes are being discovered on a routine basis. By testing newly available enzymes for application in oilfield related processes, new products are developed that perform better than existing products. Indeed, this is the reason for a field test of this newly developed breaker system. A new enzyme with better pH and temperature tolerances is replacing the previous one. The new enzyme has been encapsulated for many benefits. The two principle reasons are (1) to control the release of breaker allowing for a higher total breaker loading in the fracturing fluid compared to unencapsulated breakers, and (2) to slow the process of denaturization in extreme applications of the breaker. Even though new enzymes allow for wider application due to better pH and temperature stability, enzymes still have their limits. One such hemi-cellulase enzyme product has optimum activity at 30 ° C in a pH 4 fluid. However, exposure of this hemi-cellulase product to a 49 ° C fluid at a pH of 10 quickly denatures the enzyme. The combination of fluid pH and fluid temperature on enzyme activity is significant to the performance of the enzyme breaker.

Another challenge in hydraulic fracturing fluid design is controlling the timing of the breaker action.