Crosslinked Fracturing Fluids Research Articles

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.

Fluid-Loss Control Differences of Crosslinked and Linear Fracturing Fluids

Summary Three fracturing fluidsa crosslinked guar, a delayed hydrating guar, and a linear guarwere tested for fluidloss control at 1-hour intervals while being conditioned in a heated, pressurized flow loop. Each fluid was tested with three different fluid-loss additive systems: diesel, silica flour (SF), and a diesel/SF combination. Additionally, the crosslinked system was tested with diesel plus an anionic surfactant (AS) and a diesel/SF combination plus the anionic surfactant. These tests show that the fluid loss of crosslinked fracturing fluids is best controlled by using diesel in combination with a surfactant or a properly sized particulate material. The fluid loss of linear guar particulate material. The fluid loss of linear guar fluids is controlled best with particulate additives. Therefore, it is important to consider the type of fracturing fluid that is being used for a particular job when planning which fluid-loss additives to use. Introduction Excessive fluid loss into the formation during a fracturing job poses serious problems. Screenouts, sandouts, and gelouts are some of the terms used to describe the premature termination of the fracturing job. The filtrate viscosity and deposits of guar on the fracture face provide some fluid-loss control. The fluid loss can be controlled even more by adding materials such as inert solids or hydrocarbons to the fracturing fluid. Unpublished conventional fluid-loss test results showed that the fluid loss from the two classes of guar fracturing fluids was controlled best by different types of additives. Particulate additives worked best in linear fluids and diesel worked best in crosslinked fluids. In an attempt to better define fluid-loss control needs as well as develop an improved test technique, a heated, pressurized flow loop was used to condition the fracturing pressurized flow loop was used to condition the fracturing fluid containing fluid-loss additives before the fluid-loss testing. Combinations of additives tested earlier by conventional means were compared with the results obtained during this program. Fluids and Fluid-Loss Additives The tested fracturing fluids were a crosslinked guar (XG60), a standard guar (LG60), and a two-component delayed hydrating guar (DG60). The fluid formulations are given in Table 1. Unfortunately, the different fluids contained different amounts of methanol and KCl. These variations resulted from each set of tests being requested by field personnel for different reasons. It is possible that these slight differences in composition affected the fluid-loss control of the fluids. The following fluid-loss additive systems were tested in all of the fluids:5 vol% diesel,20 lbm SF/1,000 gal [2.4 kg SF/m3], and20 lbm SF/1,000 gal [2.4 kg SF/m3] plus 5% diesel. A diesel/anionic surfactant system and diesel/ plus 5% diesel. A diesel/anionic surfactant system and diesel/ AS/SF system also were tested in the XG60 fluid. These systems contained 2 gal AS/1,000 gal [239 g/m3] fluid.SF was chosen as the properly sized particulate material to be used in this study. The specific SF used had a population particle size mean value of 2 microns [2.0 m] with a standard particle size mean value of 2 microns [2.0 m] with a standard deviation of 1.6 microns [1.6 m]. It should be emphasized that single-component materials, such as SF, and multicomponent systems that contain a variety of materials and particle sizes are available. It is important that these particulate materials contain a particle-size distribution that will plug the pore throats of formation being treated effectively. All SF's or other similar materials may not be effective because of the lack of a good particle-size distribution for a given formation. Flow Loop Test Equipment and Procedure The flow loop was used to subject the fracturing fluids to downhole temperature and shear conditions for fluidloss testing. The viscosity of the fluids was measured using a pipe viscometer and a Brookfield in-line viscometer. Each fracturing fluid was mixed in a holding tank in the afternoon before testing and allowed to stand overnight. The next morning the methanol and fluid-loss additives were mixed into the fluid. If the fluid was to be crosslinked, the crosslinker solution was prepared at this time. The fluid was transferred from the holding tank to the flow loop and circulated for 10 or 11 minutes at a shear rate of 350 seconds, which is the maximum for the pump. The base apparent viscosity was measured during the first 5 minutes. If the fluid was to be crosslinked, the crosslinker was injected after 5 minutes. After pumping at the high rate, the flow rate was decreased to give a shear rate of 20 seconds and the heating cycle was started. JPT p. 315

Pipe Viscometer Study of Fracturing Fluid Rheology

Abstract The large increase in the use of crosslinked fracturing fluids in the past decade has led to the need for accurate evaluation of their flow properties. Traditional rotational viscometers are inadequate for studying the rheology of these crosslinked fracturing fluids, hereafter called "gels." A recently developed coiled-pipe viscometer is described, and gel response to various temperature and shear histories during preparation and testing is presented as determined by the pipe viscometer. If gel flow properties in the fracture are to be studied accurately with any viscometer, then preparation of the gel, thermal energy input, and mechanical energy input to the test sample should be controlled to duplicate the gel of the fracture. The pipe viscometer developed has the following attributes. Gel preparation is an integral part of the viscometer. Each element of the gel is subjected to the same degree of shear while mixing. The gel develops in-line as it moves to the test section of the viscometer. There are no stagnant periods. Heat transfer occurs from the walls of the conduit to the fluid in flow as in the fracture. Parallel flow sections are incorporated for scale-up information. Results are presented to elucidate the mechanism of gel formation and deterioration, gel stability at high temperatures, and the possible occurrence of slip flow in the fracture. The coiled-pipe viscometer shows potential to stimulate fluid preparation and flow in the fracturing process.

Evaluation of Fracturing Fluid Stability by Using a Heated, Pressurized Flow Loop

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 350°F [177°C]. Crosslinked polymer systems from four service companies were evaluated at 180 and 245°F [82 and 118°C]. 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.

Chemical Model for the Rheological Behavior of Crosslinked Fluid Systems

Summary The use of crosslinking agents to improve viscosity in polysaccharide polymer fluids is a widespread practice in hydraulic fracturing. The viscosity obtained from the use of a particular crosslinking agent depends entirely on the parameters of the in-situ chemical reaction to be performed at the wellsite. The major parameters encountered, such as concentration of polymer and crosslinking agent, pH, temperature, and shear regimen, will dictate the apparent viscosity of the product generated by the reaction. A mechanistic model for this crosslinking reaction is presented along with a description of the general effects of concentration, pH, temperature, and shear levels. Macroscopic observation of an ideal "complexed" gel is discussed using the most significant reaction parameters. Data show that the rheological properties of a crosslinked fracturing fluid are time-dependent and vary widely, depending on the reaction parameters to be encountered at the wellsite during a fracture treatment. Introduction Since their introduction as stimulation fluids to the industry in 1968, the use of crosslinked fracturing fluids has grown steadily. Today, they account for approximately 35% of the total volume of aqueous gels used in stimulation treatments. These fluids provide several advantages over non-crosslinked gels:greater viscosity per pound of polymer,friction reduction,wider fractures,better sand transport,more viscosity in high-temperature applications, andversatility and adaptability to a wide variety of treatment conditions. Before a comparison between various crosslinked fluids can be made, it should be recognized that the rheological data are highly dependent on the experimental conditions under which they were obtained. One of our primary objectives is to emphasize the importance of some of the experimental conditions. There are many water-soluble polymers that can be crosslinked with a variety of crosslinking agents to form fracturing fluids. However, only a rather limited number of polysaccharide gelling agents have found extensive commercial application in fracturing fluids. Table 1 shows the many chemical elements that have been used successfully to crosslink polysaccharides materials. Each element has its own unique pH. oxidation state, and concentration range for optimal crosslink formation. Although many metals require specific salt and/or chelated derivatives as the delivery form, the resulting crosslinked gels exhibit many common properties. This paper is restricted to the natural polysaccharides (cellulose and guar gum) and their nonionic derivatives (Fig. 1). We use examples of crosslinking agents from Table 1 to illustrate the effect of shear, pH. temperature, and type of coordination on the general properties exhibited by crosslinked fluids. Experimental Procedure Viscosity measurements were made on a Model 50 or Model 39 Fann viscometer using a variety of bob and sleeve combinations as described in Ref. 10. The crosslinking reactions were performed by first prehydrating a 0.48 to 0.72 wt% solution of the base polymer (40 to 60 lbm/1,000 gal) in a blender for 30 minutes in the presence of an adequate buffer concentration to control pH. The ph-control agents used as buffers include fumaric acid, hydrochloric acid, acetic acid, formic acid, sodium bicarbonate, sodium carbonate, and sodium hydroxide. JPT P. 315^