Application Of Helical Screw Rheometer For Rheological Measurements
Authors: Lord David L. , Shackelford Don 
Published: March 2010

Increasing complexity of hydraulic fracturing fluids has intensified the need for improved on-site quality control methods. Currently the helical screw rheometer (HSR) is considered by most to be a laboratory curiosity. However, the HSR has unique features which make it an ideal on-line fluid sampling and testing device for use during hydraulic fracturing treatments. These features are exploited in an HSR unit which has been successfully adapted to function in the field environment. Because its operation is based on microprocessor technology implementation of instrument control, data acquisition, and data analysis functions have all required special consideration. This paper describes how these various instrument functions were made operational. Data acquisition and analysis junctions are further illustrated with viscometric data collected with a number of different fracturing fluid base gels.

Introduction

The increasing complexity of hydraulic fracturing fluids has intensified the need for improved quality control procedures. For example, crosslinked fluids are now being used extensively because they have enhanced viscosity and proppant transport capabilities over linear gels at elevated temperature, The satisfactory performance of these crosslinked fluids in the wellbore and fracture depends on base gel pH, temperature, and rheological properties(l). When base gels are batch mixed, representative samples can be taken from individual frac tanks and their properties evaluated. Any frac tank found to contain an unsatisfactory fluid can have its contents adjusted to specification prior to pumping the job. When base gels are continuously mixed, the opportunity to sample preblended base gels is no longer available. Operators will often accept this limitation to take advantage of the many benefits derived from continuously mixing base gels using liquid gel concentrate technology(2).

The lost opportunity to sample preblended base gels during continuous mix operations has been overcome through the introduction of on-line sampling and monitoring devices. On-line monitoring of base gel pH and temperature are relatively straightforward procedures using standard instrumentation and therefore will not be discussed in any detail in this paper. Online monitoring of base gel rheological properties, however, is an entirely different matter. In the absence of standard instrumentation for this purpose, the industry is applying many different techniques.

Pipe viscometers, operating with constant flow through two or more pipes in series, are the most prevalent among the various devices currently being used(3-S). The schematic of a typical pipe viscometer is shown in Figure 1. Each pipe size in the viscometer, typically 10 mm to 35 mm ID, provides individual shear stress and shear rate values which are deduced from differential pressure and flow rate measurements, respectively. Shear stress vs shear rate points obtained simultaneously from two or more pipe sizes are analyzed to characterize the fluid rheology. Pipe viscometers tend to be bulky in size for a number of reasons. Each pipe must be 2 m to 3 m long to provide a measurable level of differential pressure while providing adequate entrance and exit lengths to ensure that accurate values are being obtained.