Abstract
A flow loop was used to evaluate the stability of fracturing fluids at high temperatures. The design provides enough pressure to prevent vaporization of water-base systems up to 350F [177C]. Crosslinked polymer systems from four service companies were evaluated at 180 and 245F [82 and 118C]. The tests showed that crosslinked fracturing fluids degrade with temperature and shear, losing much of their viscosity and proppant-carrying capacity in a few hours. Thermal stability is a major factor in selecting gels for fracturing deep, high-temperature reservoirs.
Introduction
Job failures in deep, hot wells can be caused by "sand-outs." These sandouts, or "screenouts," can result from inadequate carrying capacity (reduced viscosity) or too high a fluid loss (dehydration) for the polymer loading of the fracturing fluid. This study investigates the reduction in viscosity of crosslinked fracturing fluids with time at temperatures and shear rates approximating downhole conditions. A flow loop was used to investigate the rheological properties of fracturing fluids as a function of time under shear at temperatures as high as 245F [118C]. The pipe loop configuration was chosen because our field experience indicated that rotational viscometers were too "kind" to fracturing systems. We experienced sandouts that should not have happened if the crosslinked fracturing systems used had the flow characteristics that rotational viscometry indicated they had. The flow loop configuration also avoids some of the problems inherent in rotational viscometers, such as fluid climbing the shaft and contamination of the sample. Flow loops have been used to condition and evaluate drilling fluids at high temperatures. However, some of these instruments were not designed to give quantitative results. Our instrument permits measurement of apparent viscosity at known shear rates, flow index, and consistency index, all at high temperatures. This paper describes the flow loop, test procedures used, and results obtained. The flow loop gives reproducible results at high temperatures and allows the evaluation of the rheological properties of fracturing fluids under flow conditions nearer those encountered in actual fracturing jobs than do rotational, high-temperature instruments.
Previous Work Previous Work Very little information has been reported on temperature stability of crosslinked fracturing fluids, especially under shearing conditions. Elbel and Thomas discussed the use of viscosity stabilizers for high-temperature fracturing. Conway et al. subjected crosslinked fracturing fluids to shear and to high temperatures. Hsu and Conway described the development of more stable crosslinked gels for use in deep, hot formations. All these investigators used the Fann 50 viscometer for their work, although Conway et al. used a pump to shear the samples before testing.
Flow Loop Description
The flow loop was built to evaluate the flow properties of drilling fluids, fracturing fluids, heavy crudes, and waxy crudes. The current test system can operate up to 350F [177C] and 250 psi [1,724 kPa]. The flow loop schematic is shown in Fig. 1. The test fluid is poured into the mixing vessel (Pfaudler) and then flows through the pump. The capacity of the system--including the heat exchangers, the test section, and the mixing unit--is about 25 gal [0.095 m3]. A high-accuracy, oval gear flowmeter was used for flow rate measurement. The meter was used only intermittently because it is a high- shear device that was originally intended to handle oil-base systems. A low-shear magnetic flowmeter has been added to the loop. The magnetic flowmeter shears the test fluids much less than the gear meter and can thus be left on continuously when testing shear-sensitive fracturing fluids. A differential pressure transducer measures pressure drop over the 20-ft-long [6.1-m], 0.957-in.-ID [2.43-cm] test section. The system is heated with a hot oil heater. An in-line, variable-shear-rate cup and bob viscometer allows continuous measurement of apparent viscosity at test temperature and pressure. It also-permits the running of rheograms to measure the flow pressure. It also-permits the running of rheograms to measure the flow parameters of the test fluid at any time during a test. parameters of the test fluid at any time during a test. A remote indication panel provides displays of flow rate, pressure drop, temperatures, shear stress, and shear rate. These values also are recorded on paper and magnetic tapes. A detailed description of the equipment, including recent improvements, is given in the Appendix. The improvements include the magnetic flowmeter mentioned previously, an automated data collection and reduction system, and a smaller pump.
SPEJ
p. 249
Measurement techniques for steady shear viscosity of Mozzarella-type cheeses at high shear rates and high temperature