The relationship between rheology and fluid loss and the effects of shear history, temperature, crosslinker and fluid-loss additive on fluid loss of hydroxypropyl guar gels are discussed. Tests were performed under dynamic conditions using an apparatus performed under dynamic conditions using an apparatus which incorporates in-line mixing of crosslinked fluids, so that fluids whose fluid-loss properties are measured have rheological characteristics similar to those generated under field mixing conditions.
The fluid-loss properties of the fluid used in a hydraulic fracturing job are very important to the success of the treatment. Leakoff affects the fracture volume to fluid volume ratio achieved and so is a consideration in both the engineering and the economics of the treatment. Realistic measurements of fluid-loss properties are necessary so that proper fluid and properties are necessary so that proper fluid and fluid-loss additive(s) selection can be made from among the many available and so that proper job design for a given fluid can be achieved.
Efforts have been made in the past to get more realistic measurements by studying fluid loss under dynamic conditions. Tests were performed so that the fracturing fluid was kept moving under pressure past the test surface area. This type of testing began with Hall and Dollarhide, who observed that fluid loss was proportional to time rather than to (time). It has continued through the years, with experimental conditions changing to match the fluid types and treating conditions used at the time.
A number of fluid systems are in use today. They include noncrosslinked polysaccharides, borate-crosslinked gels, transition metal-crosslinked gels and delayed-crosslinking systems. On the basis of composition alone, these fluids might be expected to exhibit different fluid-loss characteristics. But a recent study 7 has shown that not only fluid chemistry but also the method of preparation affects fluid rheology. The rheology of a fluid prepared in a lab blender is quite different from the rheoloyy of a fluid generated by injecting crosslinker into a polymer solution flowing in a tube. Under flowing conditions, fluid rheology may also affect fluid loss. Accurate measurement of fluid-loss properties requires that the fluids be generated in a manner which simulates field mixing as closely as possible.
This paper describes an apparatus for studying fluid loss under dynamic conditions. The apparatus includes equipment for crosslinking the fluids as they are flowing through tubing. The effects of fluid composition, shear history, temperature and fluid-loss additives are presented.
A schematic diagram of the apparatus used is shown in Figure 1. Base fluid containing hydroxypropyl guar, water and various additives was prepared in large mix tanks and then pumped into the test system with a positive-displacement triplex pump. Crosslinker or crosslinker activator was injected into the polymer stream as it entered a static mixer so that crosslinking polymer stream as it entered a static mixer so that crosslinking was accomplished on the fly. The crosslinker-polymer mixture traveled through tubing prior to heating to provide the desired shear history for the fluid. The shear history could be varied by changing the pump rate or tubing arrangement. In general, the fluid was sheared at least 11 sec at a shear rate greater than 1,600 sec(-1) and at least 90 sec at a shear rate greater than 123 sec(-1). Specifics are presented in Table 1.
The fluid was then heated to test temperature in a heat exchanger 6.1 m in length. Heating occurred in one to three minutes at 15 to 45 sec shear rate. The hot fluid passed through 18.3 m of 1.09-cm ID tubing (123 to 370 sec(-1)) before reaching the fluidloss cell. Once through the fluid-loss cell, the fluid passed through a dome-loaded, back-pressure regulator used to pressurize the system and into a large waste tank. Thus, the fluid makes a single pass across the core, better simulating fluid loss near the wellbore than fluid loss far out in the fracture.