Title: Fracturing Fluid Characterization Facility

Author: The University of Oklahoma

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

Period: January -- December 1994 Annual Report

Objective: To report and discuss the progress made during the FFCF Project's 1994 calendar year.

Technical Perspective: Numerical simulators used today for simulating hydraulic fracturing treatments incorporate assumptions and approximations. Evaluation of the consequences of these assumptions and approximations has not yet been conclusive because controlled experiments on a scale necessary for the evaluation have not been possible. In order to accurately design and control hydraulic fracturing treatments, gas producers and service companies must be able to characterize the performance of the fluids used in creating a fracture and placing the proppant. The FFCF research facility is dedicated to the investigation of fracturing fluid flow behavior and proppant transport phenomena under conditions more representative of the actual fractures.

Results: During 1994 the proof-of-concept testing with the High Pressure Simulator (HPS) was successfully completed and operations of the mechanical and instrumentation systems were verified. Extensive fluid testing was conducted using the HPS to investigate various aspects of fluid rheology, proppant transport, dynamic fluid leak-off, and perforation pressure loss. The testing program has produced results that support some industry assumptions and contradict others. Based on the tests conducted, the following major research results have been identified.
While industry standard Couette viscometers are shown capable of characterizing the rheological behavior of linear polymer solutions, they cannot be used to reliably characterize the behavior of crosslinked fluids.

An investigation of flow through perforations showed that perforation size and fluid viscosity must be considered when selecting a coefficient for use in the standard pressure loss equation. Further, it was found that elevated system pressure has no significant effect on the perforation pressure loss.

Unusual and previously undocumented flow behavior has been observed in the HPS with crosslinked fluids. Flow data collected from various gap widths failed to converge to a single line which was speculated to be a wall slippage. This effect was later eliminated when a new testing procedure using a constant mixing rate with flow diversion was implemented to provide the same shear rate in various gap widths.

Dynamic fluid loss data collected in the HPS using large surface area synthetic and natural rock facings were compared to similar data collected in the laboratory using small cylindrical samples. Fluid loss coefficient calculated from the HPS and laboratory data were found to compare favorably.

An investigation of proppant-laden slurry rheology has shown that the power-law flow behavior index, n, decreases the consistency index, k, increases as the proppant concentration increases. These results confirm previously published information from small laboratory devices.

Technical Approach: For investigating the behavior of fracturing fluids and slurries during and after a hydraulic fracturing treatment, the High Pressure Simulator (HPS) was designed and constructed. It is vertical, variable-width, parallel-plate flow cell capable of operating at elevated temperatures (250 degrees F) and pressures (1200psig). It is currently being used at the FFCF to the maximum extent possible to perform fracturing fluid research.

An instrumentation and data acquisition system has been assembled that records temperature, pressure, and velocity profiles of the fluid in the slot. A fiber optic based vision system facilitate the visualization and accurate measurement of flow behavior of fracturing fluids with and without proppant. The fracture surface can be covered with replaceable 1 inch thick either simulated or natural rock facing. The fluid pre-conditioning equipment has now been installed which includes high-shear coil tubing and low-shear heat exchanger. This allows the simulation of wellbore shear and fracture temperature histories.

Experimental and numerical studies on proppant transport modeling have provided important information for understanding the mechanism of convective proppant transport.

Project Implications: The research and development that has gone into the FFCF HPS will not only serve as the foundation for the future of this project but also provides the industry with valuable insight into some of the more fundamental aspects of fluid flow in a hydraulic fracture. These reports form the basis for advancing the state-of-the-art in rheology research for hydraulic fracturing.

Report Contents:

GRI Report: GRI
95/0091
1. Introduction	1
Summary	1
Rescoping of the FFCF Project	1
The High Pressure Simulator	2
Auxiliary Equipment and Instrumentation	4
Major Research Results and Accomplishments	4
Implications of Major Research Results	6
References	8
2. Equipment Enhancements	9
Summary	9
HPS Upgrade Modifications	9
Preparation and Results of the Natural Rock Facings	11
Fluid Preconditioning Systems	17
Coil Tubing	17
Low-Shear Heat Exchanger	19
References	25
3. Data Acquisition and Instrumentation	28
Summary	28
Tasks Completed in the Instrumentation and Data Acquisition	29
The Determination of Proppant Concentration in the HPS	30
LDV System	37
Data Acquisition and Instrumentation Changes	42
4. R & D Test Results	44
Summary	44
Theory	45
Fracture Friction Loss	45
Perforation Friction Loss	45
Dynamic Fluid Loss	47
Compliance Control	47
Fluid Rheology	48
Introduction	48
Wall Slippage Test	48
Uniform Flow Verification	50
Fracture Friction Loss	50
Slurry Rheology	51
Introduction	51
Procedure	52
Results and Discussion	52
Perforation Pressure Losses	52
Introduction	52
Procedure	53
Results and Discussion	54
Fluid Loss Study	55
Fluid Loss through Synthetic Rock	55
Fluid Loss through Natural Rock	57
Fluid Loss Control in Natural Fractures	59
Fluid Flow/Fluid Loss Interaction	60
HPS Compliance Control Test	63
Procedure	63
Results and Discussion	64
High Pressure Valve Operations	63
Introduction	63
Procedure	63
Results and Discussion	63
Nomenclature	117
References	118
5. Proppant Transport Modeling	119
Summary	119
Drag Force on Particles in Non-Newtonian Fluids	119
Introduction	119
Governing Equations	120
Flow Drag Estimation	125
Discussion	126
Conclusion	129
Convective Proppant Transport Modeling	129
Introduction	129
Discussion	129
Modeling Concepts	130
Expected Results and Their Validation	130
Nomenclature	130
References	132
6. Other Accomplishments Achieved During 1994	133
7. Proposed Work Plan for 1995 and Beyond	135
Introduction	135
Phase I (October 1, 1994 - March 31, 1996)	135