Fracturing Fluid Leakoff Under Dynamic Conditions Part 2: Effect of Shear Rate, Permeability, and Pressure
Authors: Vitthal S. , McGowen J.M. 
Published: December 2007

This paper discusses the results of an industry consortium established to understand and allow modeling of the fluid leakoff process in hydraulic fracturing applications. Over 1,000 laboratory experiments structured to study the effects of shear rate, permeability, differential pressure, temperature, gel concentration, fluid-loss additives, and fluid type have been conducted. Fluid-loss data was measured with a state-of-the-art experimental setup that included fluid preconditioning loops for high shear and low shear regimes and a novel cell design that minimized flow irregularities. Part 2 presents the results of experiments involving linear hydroxypropyl guar (HPG) gel, HPG/borate systems, and HPG/titanate systems at shear rates between 0 and 200 sec-1. pressures between 500 to 10,000 psi, a temperature of 180 F, and core permeabilities between 0.1 md and 1000 md.

This study found that fluid loss under dynamic conditions can be significantly higher than under static conditions. Significant differences in fluid-loss behavior were observed between linear gels, transition-metal crosslinked gels, and borate-crosslinked systems. The behavior of linear gels was sensitive to permeability and pressure, but insensitive to shear rate. The behavior of crosslinked gels was more sensitive to shear rate, but less sensitive to permeability and pressure. The fluid loss of all fluids tested could be modeled though the mechanisms of non-Newtonian viscous invasion, classical filter-cake deposition, and filter-cake resuspension. This paper presents guidelines for fluid-loss prediction for these systems with appropriate fluid-loss mechanisms.

The findings presented in this paper are vital for the prediction of fracture fluid leakoff within fracture simulators. Understanding which fluid loss mechanism is predominant is essential in analyzing pressure decline data from minifracturing treatments and in using this information in predicting treatment placement.

Introduction

Traditionally, hydraulic fracturing has been limited to relatively low-permeability (&lt; 10 md) reservoirs. In recent years, the use of hydraulic fracturing has expanded significantly because of the success of frac-and-pack treatments. As a result, fracturing treatments are now performed in reservoirs with permeabilities up to 1 darcy with beneficial results. The objectives of fracturing low-permeability and high-permeability reservoirs are different and defined by reservoir parameters. In low-permeability reservoirs, the desired objective is typically to obtain a long narrow fracture without obtaining a tip screenout. In high-permeability fracturing, a tip screenout to maximize conductivity is desired. Clearly, understanding the fluid loss is critical in both situations to either prevent or encourage the development of a tip-screenout. Field experience shows that fluid-loss behavior is highly dependent on the reservoir characteristics and is often different from that reported when the standard API tests are used. This discrepancy can be partly attributed to limitations in the standard API procedure and use of static rather than dynamic data. A good summary of the importance of laboratory-measured filtration data and past experimental investigations is provided in the companion paper. A short review of the principal concepts and literature is provided here, followed by an overview of the paper contents.

Factors Affecting Fluid Loss of Fracturing Fluids. Experimental data presented in this paper shows that two distinct phases of fracturing fluid loss appear to exist:an early high leakoff phase before a competent filter cake is established across the face of the formation, typically referred to as spurt loss, anda phase where all fluid loss is controlled by the leakoff through the filter cake.

The major factors influencing spurt loss and filter-cake behavior are listed as follows:

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