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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)

Period: January -- December 1995 Annual Report


Objective: To report and discuss the progress made on the FFCF Project during the 1995 calendar year.


Technical Perspective: The hydraulic fracturing techniques for well stimulation has been applied successfully for well stimulation of low and high permeability reservoirs for numerous years. Over the years, considerable advances have been made in improving economics have always been the key to the success and it is more so when the reservoirs under consideration are marginal. It requires carefully optimized treatment design and a sound economic program. These are major challenges the oil and gas industry are facing today. Fluids are widely used for the stimulation of wells. In the past, significant progress has been made in the development of new fluids and proppant. However, increased efforts are needed to achieve a through understanding of true behavior and characterization of fluids under field conditions. The FFCF research facility flow behavior and proppant transport characteristics of these fluids under downhole fracture conditions.

Results: During 1995, efforts were focused upon fulfilling the objectives outlined in the Phase I of the Research and Development Plan. Extensive fluid testing and evaluation was undertaken by conducting tests employing the High Pressure Simulator (HPS). New equipment and instrumentation were acquired for this purpose and enhancements to existing instrumentation were also made. Fluid testing and evaluation were concentrated in the four major research areas: Fluid rheology/fluid behavior, proppant transport, dynamic fluid leak-off, and perforation pressure loss. The key research results are outlined below:

Dynamic fluid loss data collected in the HPS, using large surface areas synthetic and natural rock facings, show that crosslinked gels do not follow classical square-root-of-time behavior on moderate to low permeability rock. Plugging of the permeable matrix by crosslinked gels was determined to cause the development of limiting or linear fluid loss behavior as a function of time. The time at which the onset of this behavior begins was found to decrease as the permeability decreases.

At early times during a treatment, the fluid loss control efficiency of crosslinked gels is better than those of polymer solutions because they control spurt loss better. Polymer solutions become more efficient at longer times because the continue to follow square-root-of-time behavior while gels develop limiting or linear fluid loss behavior as a function of time.

Extensive borate-crosslinked gel testing shows that the proper pH must be selected to obtain optimum fluid performance under specific field conditions. Gels formulated under optimum pH conditions can have apparent viscosities which are an order of magnitude larger than those formulated under non-optimum conditions. Gels formulated under non-optimum conditions exhibit significant shear history sensitivity while those formulated under optimum conditions do not.

Perforations pressure loss measurements have shown significant dependence on proppant size, proppant concentration, and perforation diameter. Measurements made in 3/8 inch diameter perforations with small proppant, such as 20/40 mesh sand, display the expected increase with increasing concentration while slurries made with larger sand, such as 12/20 mesh sand, do not. With the smaller proppant slurries, the increase in perforation pressure loss with increasing concentration is much greater than what would be expected based simply on an increase in slurry density. Also, small proppant size slurries have exhibited higher perforation pressure loss.

The overall heat transfer coefficients measured with linear polymer solutions and borate-crosslinked fracturing gels were found to be much smaller than those for water. Surprisingly, heat transfer coefficients for the viscous borate-crosslinked gels were larger than those of the linear polymer solutions, but still smaller than those for water.

Technical Approach: The High Pressure Simulator (HPS) has been designed and fabricated for measuring fluid properties and investigating fluids behavior in conditions representative of downhole environment. It is a vertical, parallel-plate, variable gap width flow cell capable of operating at 1200 psi and 250 degrees F. This state-of-the- art simulator has numerous unique capabilities which are used to perform research and development activities in several research areas. It is currently being used at the FFCF to maximum extent possible to address the near-term results issues. The facility is well equipped with the fluid mixing, storage, and pumping equipment as well as necessary instrumentation to provide reliable and accurate flow data. During 1995, the testing capabilities of the FFCF were improved considerably through various additions and strategic modifications to the individual equipment. The fluid preconditioning system, the heat exchanger, and the tubular friction loss measurement system have provided another dimension to the testing capabilities. The addition of the two laboratory rheometers enables us to provide direct comparison and correlation of viscometric and viscoelastic fluid property data.

An instrumentation and data acquisition system records temperature, pressure, density, flow rates, gap widths, and velocity profiles of the fluid in the slot. A fiber optic vision system allows the fluid and proppant visualization and accurate measurement of slurry flow behavior in the flow cell. In 1995, major improvements were made in the LDV and vision systems. The upgrade in the LDV system have made the near-wall velocity profile measurements possible. The changes made in the vision system have improved the sensitivity of the overall optical system.

Numerical and experimental studies with proppant-laden fluids have increased the understanding of proppant transport phenomena.

Project Implications: Some of the tests done to-date have confirmed and supported previous assumptions used in fracture stimulation. For example, we confirmed that borate viscosity in pH sensitive and in the process provided additional data to show the importance of quality control to keep these systems at their optimum viscosity. Other tests are leading to new surprises/findings. For example, a new perforation friction loss correlation has been developed which will lead to better interpretation of net pressure and more accurate estimate of downhole pressure during stimulation. Additional focused experiments are planned as the project emphasizes the dissemination of results obtained to-date, and show how it can make a difference in field implementation of stimulation treatments. By dissemination of information through various channels, including getting select findings implemented in fracture models, this project will be able to make the fracturing process more efficient by either reducing stimulation costs or producing more gas for the same investment.

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