In 1981, over 1,000 new wells were drilled below 15,000 feet1 . Although bottom hole temperatures in these wells vary widely, all have a number of similar characteristics which make hydraulic fracturing treatments very difficult.
These similarities may include high bottom hole pressures and temperatures, small diameter tubing, high closure pressures, low permeabilities, and tubing contraction problems while treating. The high treating pressures encountered necessitate the use of fluids with minimal pumping friction pressures. The high closure pressures often require the use of high strength proppants which are more difficult to transport than sand. Low formation permeabilities dictate the need for long propped fractures to achieve the desired production increase and thus long term fluid stability is required. To minimize contraction in long tubing strings, treating fluids often must be preheated (100–140°F). The high initial fluid temperatures can alter fluid properties downhole.
The expected effective viscosity of treating fluids as a function of time and temperature are of key importance to the design of stimulation treatments for these wells. In most cases, crosslinked gels are the fluid of choice for stimulation treatments, and most crosslinked fluidsare shear history dependent, therefore possessing no unique rheological characteristics. The variable conditions encountered during the treatment such as tubing size, pump rate, surface fluid temperature and pipe time influence the rheological properties observed.
A laboratory method has been developed to more concisely evaluate the role of the above parameters and data will be presented demonstrating the magnitude of the effects on the time-temperature stability of the crosslinked fluid. Results from both a laboratory and field evaluation of various techniques will be discussed. By achieving conditions where most of the crosslinked fluid viscosity is obtained in the fracture, a much higher viscosity and pipe shear independent fluid is obtained. The rheological data generated on the fluid is also more reliable.