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Title: Fracturing Fluid Characterization Facility

Author: The University of Oklahoma

Sponsor: Gas Research Institute (GRI) and US Department of Energy (DOE)

Report Period: January -- December 1996 Annual Report


Objective: To report and discuss the progress made on the FFCF Project during the 1996 calendar year.

Technical Perspective: The petroleum industry has used hydraulic fracturing techniques to stimulate low and high permeability oil and gas reservoirs to enhance their potential recoveries. Nevertheless, the design and implementation of a scientifically and economically sound fracturing job, due to the lack of knowledge of rheological behavior of hydraulic fracturing fluids under field conditions, remains a challenge. Furthermore, the current level of technical knowledge with research institutes, service companies, and operators does not translate to field applications. One of the principle reasons for this technology gap, is the lack of understanding of the rheological behavior of hydraulic fracturing fluids under field conditions, which primarily relates to the limitations in scaling down the field conditions to the laboratory. This necessitates a need for a comprehensive investigation of these fluids under downhole conditions. The Fracturing Fluid Characterization Facility (FFCF), a field scale simulator was, therefore proposed with the intent of providing the industry with a better understanding of the behavior of these fracturing fluids and their proppant transport characteristics under downhole fracture conditions. The High Pressure Simulator (HPS) simulates, to the maximum degree practical, all conditions experienced by a fracturing fluid from its formation on the surface, down the wellbore, through perforations, into the fracture, and its leakage into the rock formation. Together with the onsite auxiliary equipment, such as mixing and pumping system, pre-conditioning system, data acquisition system, and rheology measuring system, the HPS is the most advance fracture simulator available to conduct research, mimicking field conditions, in the areas such as: rheological characterization of fracturing fluids, proppant transport simulations, proppant transport measurements, perforation pressure loss, coiled tubing friction loss, dynamic fluid loss, and heat transfer characterization of polymer solutions and gels.

Results: During 1996, research efforts were focused on the rheological characterization of fracturing fluids, perforation pressure loss study, dynamic fluid loss study, tubular friction loss study, heat transfer characterization of polymer solutions and gels, and proppant transport simulation. The HPS along with the onsite auxiliary equipment, was utilized to perform various tests. Efforts were also made to effectively transfer the research findings to industry via technical publications, presentations, newsletters, short courses, and seminars. A Coiled Tubing Consortium was initiated to conduct research in the areas related to the application of coiled tubing. In addition, a marketing strategy was prepared to conduct third party testing. Following are the key results obtained in various research areas during 1996.

On the Rheology: Rheological characterization studies of the selected fracturing fluids show that, an optimum crosslinked gel formation characterized by a shear history independent, maximum viscosity is found to exist at a particular pH value for each temperature investigated between shear rates ranging from 20 to 200 sec --1. Also, the maximum viscosities obtained for the various temperatures at the optimum fluid formulation are found to be approximately the same for shear rates considered. In addition, for those pH values at which the gels were shear history dependent, the rheological changes were more predominant in the first minute of test.

On the Perforation Pressure Loss: Comprehensive investigation of pressure loss across perforations led to development of new correlations for perforation pressure loss to provide the coefficient of discharge for linear polymer solutions, titanium/borate-crosslinked gels, and linear slurries prepared with 20/40 mesh sand. Also, the change in the coefficient of discharge due to perforation erosion is found to be a complex function of sand size, sand concentration, perforation diameter, carrier fluid viscosity, flow rate, and pumping time.

On the Dynamic Fluid Loss: The study shows that surface roughness is a major factor in filter cake development. It also indicates that filter cake development is not a requirement for fluid loss control. Further study of fluid loss shows that for linear fluids, fluid loss follows a linear relationship with square-root-of-time, while the fluid loss for crosslinked fluids, shows a departure from this behavior with the time of departure depending on the permeability. Also, increasing the gel concentration enhances fluid loss control and addition of silica flour enhances fluid loss control, however, increasing the concentration provides no further enhancement.

On the Tubular Friction Loss: Research results show that for water, the tubing seam decreases the frictional pressure loss whereas coiled tubing curvature increases the friction pressure drop with magnitude of the effect of curvature being much greater than that of the seam. Moreover, while the tubing seam does not seem to have any effects on frictional pressure loss for the linear solutions, the curvature of the coiled tubing strongly affects the pressure drop. When crosslinked fluid is used, the study for borate-crosslinked gels shows that the frictional pressure loss is a strong function of pH and shear history.

On the Heat Transfer: The overall heat transfer coefficient for borate-crosslinked 35lb/Mgal Guar gel, ranges from 70 and 85 Btu/((F hr ft Ó) shear histories up to 3000 ft. Heat transfer coefficient for linear and crosslinked gels are found to be lower than those of water. The overall heat transfer coefficients for the borate crosslinked Guar are shown to be independent of crosslinker flow rate between 300 and 450 ml/min. The overall heat transfer coefficients, due to a higher degree of crosslinking, is shown to depend on crosslinker flow rates between 450 and 500 ml/min.

Technical Approach: The major goals of the project were to develop a fracture simulator that provides an accurate rheological characterization of hydraulic fracturing fluids under field conditions, and to provide a permanent facility that could be used for testing purposes. To accomplish these tasks, an effective technical program was developed to utilize the HPS to its maximum capabilities. This program was based on industry research needs in areas such as proppant transport, foam fluid characterization, and coiled tubing applications. Accordingly, a Coiled Tubing Consortium was initiated and a technical program developed to make the facility available to the industry in the form of Third Party Testing. In addition, the operational status of the vision system, where proppant transportation can be visualized, was the subject of concern during 1996. Accordingly, extensive analyses were performed on data obtained from a set of sand slurry tests. The improved image definition has been characterized and a calibration curve of sand concentration levels in graphical form.


Project Implications: The major research areas during 1996, to constitute important deliverables are in the areas of borate-crosslinked gel rheology, perforation pressure losses, dynamic fluid loss, coiled tubing friction losses, heat transfer characteristics of polymer solutions, the use of laser doppler velocimetry as a tool for accurate rheological characterization, and proppant transport studies with the aid of a suitable vision system. Through an effective approach, the 1996 research findings were transferred to the industry. In addition, the developed perforation correlation, has been implemented into computer fracturing simulators and the coiled tubing correlation, once revised, will be implemented into computer simulators software. Additional plans in terms of verification and technology transfer activities in these areas are under investigations.

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