Abstract

This paper details the development of a laboratory testing procedure that simulates the shear and temperature environment that a fracturing fluid is subjected to throughout a fracturing treatment, including the low shear rates (30–100 sec(-1)) experienced in the fracture. Building on previous parametric studies of fluid responses to previous parametric studies of fluid responses to differing test procedures, this new procedure appears to give results that are representative of the viscous properties of the fluid rather than the procedure used to run the test. procedure used to run the test. The validity of our proposed procedure has been verified by testing gels from four suppliers with gel loadings of 40 and 60 lb/1000 gal at temperatures of 200 and 250 degrees F. Acceptance of a procedure such as the one presented in this paper would allow for more accurate control in the design and execution of fracture treatments, and serve as a basis for meaningful, quantitative comparisons between available gel systems.

Introduction

It has long been recognized that the apparent viscosity of a crosslinked fracture fluid is an important factor in the success, or failure, of a hydraulic fracturing treatment. The apparent viscosity of the fracturing fluid directly affects the rate of fluid leakoff, the friction pressure losses in the fracture, the created fracture width, and the ability of the fluid to transport proppant. Due to the opposing nature of several of these parameters, the engineer must optimize the fluid parameters, the engineer must optimize the fluid viscosity for each treatment to maximize the values of propped fracture length and fracture conductivity.

The API issued Recommended Practice 39 (API RP 39) in an effort to standardize the evaluation of fracturing fluids. The techniques described in API RP 39 can only be successfully used to test non-crosslinked, water-base or oil-base gel systems. The most commonly used fracturing fluid is crosslinked hydroxypropyl guar (HPG). Most crosslinked fluids exhibit energy- and time-dependent behavior that cannot be accurately evaluated using the procedures found in API RP 39. Therefore, the testing procedure used to evaluate the viscous properties of the most commonly used fracturing fluids has been left to the discretion of the investigator.

Currently, most fracturing service companies describe the viscous behavior of their fluids in terms of the power law parameters, n' and K'. The shear stress data used to determine these power law parameters are normally collected at shear rates parameters are normally collected at shear rates ranging from 170 to 511 sec(-1) on a Fann Model 50C Rotational Viscometer. Values of n' and K' for the various fluid systems are published for a range of time-temperature conditions. The problem, however, has been that the values of n' and K' for any given fluid are functions of the mixing and testing procedure. Therefore, it has always been difficult procedure. Therefore, it has always been difficult for independent laboratories to either reproduce the data from a given service company, or to meaningfully compare the published data between any two service companies.

In fairness to the service companies, the measurement of the viscous properties of a crosslinked HPG fluid is an extremely difficult problem. The viscoelastic nature of the fluids problem. The viscoelastic nature of the fluids cause them to be sensitive to the shear rate sequence used, the temperature of the gel, the mixing procedures, and the duration of the test. Also, the analysis of the data can be complicated by the elastic nature of the fluids.

The fluid testing procedure described in this paper is the conclusion of five years of research paper is the conclusion of five years of research which concentrated on measuring the viscous properties of fracturing fluids under realistic properties of fracturing fluids under realistic conditions.

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