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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 1997 Annual Report


Objective: To report and discuss the progress made on the FFCF Project during the 1997 calendar year.

Technical Perspective: The hydraulic fracturing technique has emerged as an effective method for stimulating oil and gas wells over the past few decades. The proper design of an optimum fracturing treatment involves knowledge of formation stresses, fracturing fluid characteristics and the interaction between the formation and the fluid. Recent technological advances have paved the way to improve the design of such treatments since its implementation in the middle of this century. However, the accurate prediction of the behavior of complex fracturing fluids in existence today under downhole conditions has still not been realized. The Fracturing Fluid Characterization Facility (FFCF) is established in order to provide the Petroleum Industry with a comprehensive, research based organization, whose primary objective is to provide valuable insight into the various mechanisms that govern the flow of fracturing fluids and slurries through a hydraulically created fracture. In this regard, a unique above-the-ground fracture simulator is designed and constructed at the FFCF, labeled "The High Pressure Simulator" (HPS). The HPS is a vertical, variable-width, parallel plate flow apparatus capable of operating at high temperatures (up to 250°F) and pressures (up to 1200 psi). The HPS, in conjunction with auxiliary field equipment, has enabled the replication, to the maximum degree practical, the various processes a fracturing fluid undergoes starting from its preparation on the surface and pumping down the well to its subsequent flowback from the created fracture. The HPS is the most advanced fracture simulator currently available to the industry. Some of its key experimental research areas include fracturing fluid characterization, wall slippage phenomena, dynamic fluid loss, perforation pressure loss, proppant convection and encapsulation and proppant flowback for a variety of fracturing fluids (both linear and crosslinked), slurries and foams. The FFCF is also equipped with a state-of-the-art laboratory, where fracturing fluid characterization is performed on a laboratory scale and can be used to compare with the HPS and industry generated results.

Results: In accordance with the research areas identified by the industry, during 1997 the FFCF concentrated its research efforts on investigation of proppant transport, foam fluid studies, fracturing fluid rheology, perforation pressure loss, proppant flowback feasibility studies, and coiled tubing friction losses. In addition, a host of specialized third party testing for clients was also conducted. Efforts were also made to effectively transfer the research findings to industry via technical publications and presentations. Extensive efforts were also devoted to meet the objectives set forth for the joint industry "Coiled Tubing Consortium." Following are the key results obtained in various research areas.

On the Proppant Transport: The primary objective of the tests devoted to the investigation of proppant transport involved the study of various phenomena governing the settling of proppants, in a fracturing fluid medium inside the HPS, such as convection and encapsulation under dynamic settling conditions. A vision system capable of monitoring the proppant concentration as well as visualization of the proppant movement and bed formation inside the HPS as a function of time is utilized. It is found that the convection is a density-driven mechanism and in low viscous medium, it dominates proppant settling. Increasing viscosity of the lower medium inhibits encapsulation.

On the Foam Fluid Studies: The objective of foam fluid testing is to characterize the rheology of various foam fluids as well as to evaluate their proppant carrying abilities. Foams are increasingly used in stimulation treatments primarily due to their increased proppant carrying capacity, low fluid loss characteristics, and the ability to achieve high flow rates. A limited number of feasibility tests were conducted on aqueous foam and linear gel foams on the HPS. The results were compared with that of those published earlier. In addition, the design and construction of a foam flow loop is currently underway for rheological characterization of aqueous and nitrogen foams so as to offer a comparison with HPS generated data.

On the Borate-Crosslinked Gel Rheology: Studies were conducted on borate-crosslinked fracturing fluids to determine an optimum crosslinker concentration that can be utilized to maximize the economics of a fracturing treatment. Moreover, wall slip studies were also conducted on these gels to further the understanding on borate-crosslinked gels. The crosslinking mechanism is a time-dependent process that involves the breaking and re-healing of bonds at what is termed crosslinking sites (points). This poses a problem in proper characterization of such crosslinked fluid systems. Efforts are being made to accurately predict the behavior of these fluid systems under representative downhole conditions.

On the Perforation Pressure Loss: Accurate prediction of perforation pressure loss is very important in limited-entry hydraulic fracturing treatments. Unfortunately the same governing equation used in the orifice equation has been utilized in the Petroleum Industry to calculate perforation pressure losses. This often leads to erroneous quantification of pressure measurements. In this regard, simple easy-to-use correlations were developed for linear and crosslinked fluids and slurries for calculating perforation pressure losses as a function of various fluid and proppant properties.

On the Proppant Flowback Studies: Proppant flowback following hydraulic fracture stimulation has been a major source of production impairment for oil and gas wells. Wellbore plugging, equipment erosion, and lost fracture conductivity (resulting in production decrease) are some of the other problems associated with proppant flowback. Most sand control methods are effective only for specific time periods and require a large amount of manpower and equipment, dramatically increasing operating costs.

The Fracturing Fluid Characterization Facility has the unique capability of testing fluids and slurries at downhole temperatures and pressures in a large-scale environment using a High Pressure Simulator. The width of the fracture provided by the HPS can be varied during the test just like fractures in reservoirs during treatment. The preliminary studies on proppant flowback performed at FFCF have established the feasibility of conducting proppant flowback studies at the facility. The potential economic impact of being able to eliminate or diminish proppant flowback is significant and has immediate application.

On the Consortium Funded Research: As a part of ongoing efforts to attract potential clients and funding, the Coiled Tubing Consortium was established in order to determine the frictional pressure losses through coiled tubing for a variety of fracturing and drilling fluids through various diameter pipes. A test matrix was designed in collaboration with the consortium members and numerous tests were successfully conducted. The results of Phase I were presented at the Coiled Tubing Consortium Meeting. In addition, as mentioned previously, the proppant flowback consortium is currently in the process of being established.

On the Third Party Testing: The FFCF has successfully attracted several clients in performing specific tests that address their needs and concerns. A comprehensive list of all the companies and organizations that have used the services of the FFCF is presented in the report.

On the Technology Transfer: The purpose of technology transfer is to convey the various research results generated at the FFCF to a user-friendly form that can be utilized readily by the industry. This involves conducting surveys to identify major research areas, transferring information and data that can be applied directly by the industry, and to attract potential clients to generate funding as well as to provide tailor made solutions to their specific problems. A detailed account of the technology transfer activities at the FFCF is provided in the report.

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 was developed to make the facility available to the industry in the form of Third Party Testing. Extensive analyses were performed on data obtained from a set of sand slurry tests. The improved image definition was characterized and a calibration curve of sand concentration as a function of light transmission level was obtained. As a result, the vision system was able to assist in presenting sand concentration levels in graphical form during proppant transport tests.

Project Implications: An analysis of proppant transport data collected with polymer solutions in the HPS has revealed new information concerning the mechanisms of sedimentation in these non-Newtonian fluids. This information will form the basis of new proppant transport models that can be incorporated into the fracture design simulators.

Preliminary foam rheology experiments performed in the HPS are the industry's first attempt to measure foam properties in a large-scale apparatus which simulates the geometry of a hydraulic fracture. Since foam fracturing comprises approximately 30% of the hydraulic fracturing market, it is important that the industry understand as much as possible about the properties of these unique fluids.

New perforation pressure correlations allow one to predict the perforation frictional loss accurately as well as characterize near-wellbore tortuosity. The design of future well completions and stimulation treatments will benefit from this knowledge.

The economic consequences of proppant flowback are enormous. Costly downtime can ensue with a loss in revenue from sales while expensive repairs are being made to the production equipment. The results obtained from the feasibility study at the FFCF and subsequent test results will aid in understanding the various mechanisms for proppant flowback after the hydraulic fracturing treatment.

GRI Project Manager
Brian C. Gahan

Report Contents: