An Improved Method for Measuring Fluid Loss at Simulated Fracture Conditions

Abstract

A test apparatus is designed to carry out dynamic and static fluid-loss tests of fracturing fluids. This test apparatus simulates the pressure difference, temperature, rate of shear, duration of shear, and fluid-flow pattern expected under fracture conditions. For a typical crosslinked fracturing fluid, experimental results indicate that fluid loss values can be a function of temperature, pressure differential, rate of shear, and degree of non-Newtonian behavior of the fracturing fluid. A mathematical development demonstrates that the fracturing-fluid coefficient and filter-cake coefficient can be obtained only if the individual pressure drops can be measured during a typical fluid-loss test. Introduction In a hydraulic fracturing treatment, the development of fracture length and width is strongly dependent on a number of key fluid and formation parameters. One of the most important of these parameters is the rate at which the fracturing fluid leaks, off into the created fracture faces. This parameter, identified as fluid loss, also influences the time required for the fracture to heal after the stimulation treatment has been terminated. This in turn will influence the final distribution of proppant in the fracture and will dictate when the well can be reopened and the cleanup process started. Historically, tests to measure fluid loss have been carried out primarily under what is characterized as static conditions. In such tests, the fracturing fluid is forced through filter paper or through a thin core wafer under a pressure gradient, and the flow rate at the effluent side is determined. Of course, the use of filter paper cannot account for reservoir formation permeability and porosity; therefore, the fluid-loss characteristics derived from such tests should be viewed as only gross approximations. The static core-wafer test on the other hand, reflects to some extent the interaction of the formation and fracturing-fluid properties. However, one important fluid property is altogether ignored in such static core-wafer tests. This is the effect of shear rate in the fracture on the rheology (viscosity) of fracturing fluid and subsequent effects of viscosity on the fluid loss through the formation rock. In the past, several attempts were made to overcome the drawbacks of static core-wafer tests by adopting dynamic fluid-loss tests. Although these dynamic tests were a definite improvement over the static versions, each had drawbacks or limitations that could influence test results. In some of the studies, the shearing area was annular rather than planar as encountered in the fracture. In other cases, the fluid being tested did not experience a representative shear rate for a sufficiently long period of time. An additional problem arose because most studies were performed at moderate differential pressures and temperatures. The final drawback in several of the studies was that the fluid flow and leakoff patterns did not realistically simulate those occurring in the field. In the first part of this paper, we emphasize the design of a dynamic fluid-loss test apparatus that possesses none of these drawbacks. In the second part of the paper, test results with this apparatus are presented for three different fluid systems. These systems areglycerol, a non-wall-building Newtonian fluid,a polymer gel solution that is slightly wall-building and non-Newtonian, anda crosslinked fracturing system that is highly non-Newtonian in nature and possesses the ability to build a wall (filter cake) on the fracture face (see Table 1). The fluids were subjected to both static and dynamic test procedures. In the third part of the paper, results of experiments carried out with crosslinked fracturing fluid for different core lengths, pressure differences, temperatures, and shear rates are compared and the significance of the difference of fluid loss is emphasized. Experimental Equipment and Procedure The major components of the experimental apparatus shown in Fig. 1 are a fluid-loss cell, circulation pump, heat exchanger, system pressurization accumulators, and a fluid-loss recording device. The construction material throughout most of the system is 316 stainless steel. The fluid loss is measured through a cylindrical core sample, 1.5 in. [3.81 cm] in diameter, mounted in the fluid-loss cell. Heat-shrink tubing is fitted around the circumference of the core and a confining pressure is maintained to prevent channeling. Fracturing fluid is circulated through a rectangular channel across one end of the core. SPEJ P. 482^

A New General Model of Fluid Loss in Hydraulic Fracturing

“…Comparison with the experimental data of McDaniel et al 8 shows that the model is capable of exhibiting nonlinear behavior matching the laboratory data, which cannot be explained in tenns of the previous simple theory. An important feature of the model is incorporation of the length of the core, which produces nonlinear behavior and can cause large errors in calculating the true value of the leakoff coefficient when the simple fonnulas are used.…”

“…The value of Us obviously depends on the shear rate of the flow along the fractllre face, but systematic data are lacking in the literature. The data of McDaniel et al 8 can be interpreted as supporting this effect, as will be discussed later. Equilibrium is reached when the volume of filtrate reaches Vs =2C w lu s and the transmissibility for V w > Vs is constant, which is obtained by using Vs instead of V w in Eq.…”

“…Similarly, the values of the leakoff coefficient quoted in Table 2 of Ref. 8, which have been obtained by fitting the ends of the curves, will give considerably higher fluid loss compared with the data if applied for the entire time period (for example, 100 mL vs. 65 mL for the highest shear rate). Use of the model allows us to deduce the basic values of C wand IL filt, which can then be used to calculate filtration at conditions different from the test, provided that the variation resulting from other parameters is calibrated by laboratory data.…”

“…Eq. lOa is an improvement over the previous harmonic mean, McDaniel et al 8 give the corresponding formula for C wv when C e is not important (i.e., laboratory experiments) and the equation for conversion from C wv to…”

“…data of McDaniel el al 8. They measured the filtration of linear fluid as well as crosslinked polymer under various shear rates and used cores of different lengths.…”