Since the advent of hydraulic fracturing as a well stimulation tool, a variety of fluids and treatment methods have been proposed and used to improve well productivity. Fluids used in this process over the years include: water, lease oil, refined oil, water-oil emulsions, acid-oil emulsions, gelled oil, gelled water J and gelled acid.
A recent development has been the introduction of thick fluids with low pipe friction for hydraulic fracturing. The use of thick fluids can increase Fracture conductivity by creating wider fractures, carrying larger and higher concentrations of propping agent and improving prop distribution. In many cases the increased Fracture conductivity will result in greater production increases.
This paper describes the new very-thick fracfluids and low-damage Fluids to improve well productivity. Early attempts to increase fracture conductivity with thick fluid were only moderately successful. Acid-in-oil emulsions were used extensively in the 1950's1. These emulsions had excellent sand carrying ability, but pipe frictional pressures developed during injection Frequently prevented proper application for maximum productivity increase. The cost of the acid-oil-emulsions was also a deterrent to widespread use as the size of treatments increased. Later, Kristianovich2 renewed the interest in use of thick fluids to improve the fracturing process. The first successful application of thick frac fluids was the heavy, refined oil-water dispersions developed by Esso Production Research3. These fluids Fulfilled the requirements of highly-viscous fluids and established new Fracturing technology. Increased friction pressures generated by this type fluid are overcome by special treating techniques in which a water ring is employed between the thick oil and the pipe wall. Although this type of fracturing fluid is still used in treating specific formations, handling difficulties have reduced its overall appeal.
The recently developed thick fluids described herein exhibit low friction pressure down tubular goods in addition to high viscosity in the fracture. These Fluids generally provide relatively low formation damage and Fracture conductivity damage. These are an important factor in providing increased productivity where formation sensitivity or detrimental saturation changes are prevalent. The properties, application, and design of four new aqueous frac fluids and two new oil-based systems are described. These fluids are more expensive than conventional fluids, therefore proper application is required.
Well conditions which may indicate the need for the highly viscous fluids are high overburden pressures, low reservoir pressure, soft formations, high permeability, high temperature, rapid decline in production, rate limitations which prevent optimum design with conventional fluids, or intervals where control of frac heigh is needed.
The key design factor to determine the effect of these factors fracture conductivity under existing fracture closure pressure. High overburden pressures particularly with low reservoir pressure, give high fracture closure pressured which may result in low fracture conductivity. The proper choice of propant and greater fracture width may be necessary at these high fracture closure pressures (above 4000 psi).