Shallow Gas Development: Stimulation Fluids

Abstract 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 cross linked fracturing fluids. Encapsulation of these breakers allows controlled release of the substrate compared to the conventional breaker package of soluble, catalyzed sodium persulfate. 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 persulfate 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 persulfate-types (S2O82-) of breakers include tertiary amines (NR3)3l secondary amines (NHR2)3 and transition-metal complexes. Enzyme breaker systems appear to provide a ready solution for low temperature situations. A common herni-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 persulfate breakers do, but enzyme breakers do have specific temperature and pH requirements. One such hemi-cellulase enzyme product has optimum activity at 30 °C in a pH 4 fluid. However, exposure of this berni-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 activity of the enzyme breaker. Another challenge in hydraulic fracturing fluid design is controlling the timing of the breaker action. This can be done by altering the physical form of the breaker through encapsulation technology. A hydraulic fracturing fluid should create a fracture, transport proppant throughout the fracture, and then degrade to a near water-thin fluid, leaving a clean proppant pack. The nature of a breaker is to degrade the fracturing fluid. How and when this degradation takes place determines the success of a hydraulic fracturing treatment. Conventional breaker systems (enzyme or oxidizing) begin acting immediately to reduce fluid viscosity. To delay the breaking action, breaker products can be encapsulated. Encapsulation technology has been known since the 1930s but has only been used recently in the oilfield industry. Both scale inhibitors2 and breakers3–5 have been encapsulated to improve the performance of these products. In the case of a breaker system, an ideal performing product is one that remains inactive until the end of pumping and fracture closure and then instantly breaks fluid viscosity to 1 cp. Figure I shows examples of break profiles observed with current breaker systems compared to the ideal case. In the conventional form of the breaker, an immediate loss of viscosity is observed.