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 1993 Annual Report

Objective: To report and discuss the progress made during the FFCF project's 1993 calendar year.

Technical Perspective: Hydraulic fracture treatment design requires a myriad of system parameters. Many of these parameters are inherent properties of the reservoir to be hydraulically fractured. The fracturing fluid's flow and particle transport characteristics are important to this design process. The observations made on this research facility will closely relate to the flow behavior expected in an actual hydraulic fracture. These same observations can also be related to the more routine, practical fluid characterization techniques used by the industry.

Results: A high-pressure, high temperature (i.e., 1200 psig, 250 degrees F, parallel plate flow cell has been designed and constructed. An instrumentation and data acquisition system has been assembled that records temperature, pressure and velocity profiles of the fluid in flow cell. A fiber optic based vision system had been incorporated. This novel system provides a low-resolution optical image of the flow field that can be digitally recorded, analyzed, and maps generated to depict proppant concentration as a function of space and time.

Experimental work was completed on impermeable facings that serve as a replaceable lining for the walls of the flow cell. These facing are currently being fabricated using sand and as epoxy resin as a bonding agent. The resin system is also used with an aggregate of silica particles to fabricate permeable facings. These permeable facings will be used in the study of fluid leaking off normal to the main direction of flow.

Experimental and numerical studies of fluid flow are reported. These studies aided in determining the minimum size requirements for fluid characterization experiments. In addition, the flow experiments coupled with analytical studies permitted the determination of the physical size of a much larger low-pressure flow apparatus necessary to adequately characterize the proppant transport properties of complex fracturing fluids.

Technical Approach: The reaction frame, or mechanical body, of the flow cell was designed and constructed by MTS, one of the project's subcontractors. The functional requirements for this design were provided by the entire project group with input from the industry advisors.

The Electrical Engineering group at OU conceived and constructed the instrumentation and data acquisition system. The high-pressure simulator (HPS) was designed to operate at a maximum pressure of 1200 psig (i.e., 12 MM lb.-force load). This required a steel superstructure that inhibits any of the desired observations. Novel techniques of burying equi-spaced optical fibers in the facings to obtain a low resolution optical image of the flow field, imbedded optical glass windows that facilitate the use of the Laser Doppler Velocimetry (LDV), and 3-D graphical representation of measured parameters such as pressure, velocity, and temperature make it possible to observe and analyze what would be impossible with conventional instrumentation.

Laboratory experiments and material property measurements all played a big part in selecting the epoxy resin as a facing material. Cement was initially thought to be the material of choice. Although it did exhibit some desirable physical properties, its mechanical integrity became uncontrollable as facings were actually fabricated.

Experimental and numerical research into turbulent flow and fluid leak-off have provided important information for understanding the application of the prototype. Limitations based on geometry are identified with these same techniques so future equipment and experiments for the FFCF can be designed and planned with clear objectives.

Project Implications: The research and development that has gone into the FFCF prototype 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 94/0065
1.Introduction	1
Goals of the FFCF Project	1
Brief Description of the FFCF	2
The Flow Cell	2
Auxiliary Equipment	4
Instrumentation	4
Research and Development Plan for FFCF Prototype	6
2. Low Pressure, Large Scale FFCF Functional Requirements and Design Concepts	8
MTS Research Activity Summary	8
Objectives of the Low Pressure Fracture Simulator (LPS)	8
Functional Requirements	9
Proposed LPS Structural Specification	17
Discussion of Specification	18
3. Instrumentation and Data Acquisition for Fracturing Fluid Flow Measurement	24
Introduction	24
Data Acquisition and Visualization	24
Vision System	26
Laser Doppler Velociemetry Measurements	28
Particle Tracking	35
4. Operational Procedures for the FFCF High Pressure Simulator	37
Introduction	37
Fabrication and Installation of Facings	37
Facings Fabrication	37
Facing Installation	38
Operation of the MTS Hardware	39
Opening and Closing Procedures for the HPS	39
Operation of the MTS Control System	40
Operation of the Data Acquisition System	40
Honeywell Differential Pressure Transmitters	40
LDV System	42
Temperature System	42
Vision System	42
Other Data Acquisition Computers	42
5. Tests Results from the High Pressure Simulator	43
Introduction	43
Data Analysis	48
Verification Tests	51
Research Tests	62
Conclusions	74
6. Proppant Transport Modeling	77
Basics of Multiphase Mixture Theory	77
Continuum Theory, Corpuscular Theory, and Averaging Theory	77
Notation	78
Mathematical Structure	79
Balance Principles for Constituents	80
Balance Principles for the Mixture	80
Constitutive Equations	81
Restrictions on Constitutive Equations	82
Determinism of the System Equations	82
Boundary Value Problems	83
Conclusion	83
Relation of Microstructure to Constitutive Equations	83
Introduction	83
General Equations for Two-Phase Flow	85
Some Assumptions in Modeling Proppant Flows	89
Hyperbolicity Analysis of the Equations	96
An Application to Wedge Shape Fracture	98
Numerical Stability Consideration	99
Dilute Proppant Transfer	102
Introduction	102
Motion Equations	103
Constitutive Equations	106
Channel Flow	110
Approximate Analytical Solutions	111
Numerical Solutions	114
Dense Proppant Transfer	118
Kinematics and Equations of Balance	118
Constitutive Equations	121
Field Equations	123
Proppant Segregation in Poiseuille Flow	127
Numerical Results	128
Conclusion	131
7. Other Accomplishments Achieved During 1993	133
8. Proposed Work Plan for 1994 and Beyond	134
Introduction	134
Design, Fabrication, and Installation of a Low Pressure Simulator (LPS)	134
Design, Fabrication and Installation of the Computational Hardware, Instrumentation, and Data Acquisition Hardware and Software for the Low Pressure Simulator	137
Rheological and Proppant Transport Research Using the HPS	138
Convective Settling and Encapsulation Studies	139
Dynamic Fluid Loss Studies and Correlation with Laboratory Scale Experiments	139
Characterization of Crosslinked Fluid and Crosslinked Slurry Rheology	139
Investigation of Flow through Perforations	140
Preparation of a Verification Test Plan for the LPS	140
Development of a Strategy/Plan for Soliciting Future Support for the FFCF Project	140
FFCF Building Construction	141
Development of a Systematic Plan for Relocating the HPS to the New FFCF Building	141
Verification and Testing LPS	141
Summary	142