Capillary suction time (CST) tests with drilling and treatment fluids have been used to characterize formations for swelling tendencies since 1983. However, in most cases, it is not possible to establish comparisons between data sets acquired in different laboratories or at different times. This paper evaluates how certain factors impact CST and offers useable constraints to the method to establish relevant comparisons across different samples. The basic operation of CST tests is simple: slurries of formation materials are prepared in treatment fluids, and the time required for free liquid to travel a calibrated distance in standard porous paper is measured. Swelling formation materials consume free water from the slurry, decreasing the available free water and reducing the permeability of the filter cake, which increases the CST (Underdown and Conway 1992). Several factors can artificially alter the measured time, such as materials blocking the flow of fluid. Extensive experimentation to determine the most prevalent factors and minimize their significance led to a robust, sample-to-sample comparison methodology. The work presented in this paper considers changes in CST response based on the source of formation materials (outcrop samples, drilled cores, and cleaned drill cuttings), the composition [standard spiked samples and natural formation materials analytically characterized by X-ray diffraction (XRD) and cationic exchange capacity (CEC)], and sample preparation (grinding method, particle size, shear rate, shear time, slurry volume, and fluid effects) to determine how each factor impacts the measured CST. Ultimately, a method for obtaining consistent sample preparations and data interpretation for CST is presented. This technique is especially useful in low-permeability formations where other conventional methods, such as core flood tests, cannot be used. In particular, this technique has proven to be very useful when no core formation materials are available and only drilling cuttings can be obtained. To test the robustness of the methodology, shale formation materials from 30 unique wells with clay contents from 6 to 64% were characterized. Measured CST values for each material were used to assess the damage potential in relevant fluids and determine treatment recommendations. By implementing the proper testing constraints, the CST can be used as a reliable, quick, low-cost, easy-to-run, and quantifiable analytical technique for sample-to-sample comparisons. By controlling the parameters involved with sample preparation, CST can be used as a reliable means to accurately determine the formation material's sensitivity to a given fluid on a well-by-well basis.
Hydraulic fracturing is a robust stimulation technique that has been employed for more than 60 years to help increase the recovery rate of hydrocarbons from reservoirs. Hydraulic fracturing fluids are key components of the process. Significant efforts have been made to refine the fluids and advance new technology to help improve the economics, efficiency, and safety of the systems. Specifically, the pursuit of fluids that provide reliable and consistent performance while using lower concentrations of polymer, usually guar or a guar derivative, has been a recurrent point of emphasis in fracturing fluid advancement. There are many advantages of using lower polymer concentrations, including lower costs, improved logistics, and introducing less polymer with its associated residues into the fracture, among others. This paper presents a new fracturing fluid that combines a next-generation boron crosslinker with a new hydroxypropyl guar (HPG) to be crosslinked with 40 to 60% less polymer than used in conventional borate-crosslinked fluids. For this fluid, HPG loadings in the range of 8 to 12 lbm/1,000 gal were used to produce boron-crosslinked stimulation fluids stable up to 200°F. Fluid viscosity testing showed stable fracturing fluids with controlled breaking profiles at temperatures up to 180°F. Dynamic proppant suspension testing indicated that the new fracturing fluid exhibited proppant transport equal to or better than conventional borate fluids. Regained permeability testing using Berea sandstone cores exceeded 75% at 160°F, 85% at 180°F, and 95% at 200°F. Additionally, excellent fluid cleanup was measured by retained proppant pack conductivity with 2 lbm/ft2 20/40-mesh lightweight ceramic proppant. This new boron-crosslinked fluid retains the “rehealable” property and flexibility of conventional borate-crosslinked fluids; however, the polymer is crosslinked at or near the minimum concentration at which the polymer chains can entangle (and are capable of crosslinking), which is an improvement compared to conventional borate-fluids. This concentration is known as the critical overlap concentration, c*, of the polymer. The use of the new crosslinking technology coupled with the new HPG allows for a two-fold advantage in terms of residue reduction. The derivatized polymer requires additional processing, yielding a cleaner polymer with less residue, and the lower polymer dosage results in a further reduction of residue compared to conventional fluids.
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