Fluid-Loss Control Differences of Crosslinked and Linear Fracturing Fluids
Authors: Zigrye J.L. , Whitfill D.L. , Sievert J.A. 
Published: December 2007

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 Unpublished conventional fluid-loss test results showed that the fluid loss from the two classes of guar fracturing fluids was controlled best by different types of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