A Rapid Method for Determining Fracturing-Fluid Leakoff Coefficient and Spurt Loss
Authors: Fan Yong , Chen Zhongming 
Published: December 2007

Minifrac test is a routine practice for determining design parameters and interpreting fracturing behavior. The traditional analysis for fracturing-pressure-decline data employs either type-curve matching or a G-function plot to determine an overall leakoff coefficient. No spurt loss is addressed. This paper introduces a rapid method for simultaneously determining fluid leakoff coefficient and spurt loss from minifrac tests.

This work is based on Carter's leakoff model with considerations of the different fluid-loss mechanisms. The fluid leakoff term is reformulated, then incorporated with mass balance and principles of solid mechanics. Consequently, a plot of fracture net pressure vs. time function forms a straight line, where the slope of the straight line is proportional to the fluid leakoff coefficient, and the intercept determines the spurt loss. The deviation of the early pressure-decline data from the straight line is attributed to the fracture growth after pumping ceases. This technique is easily applicable to the three models commonly used in fracture design and has no need to assume the fracture surface evolution factor, traditionally termed upper/lower bound limits.

Four case studies of actual treatments using published data are presented to demonstrate this method and its interpretation. Besides the fluid leakoff coefficient and spurt loss, the other parameters that can be determined, as shown in the real field data analysis, are the fluid efficiency and fracture dimensions, including the fracture growth after pumping. When compared to the traditional method, this method yields sufficiently accurate results, offers more features, and is much easier to use in practice.

Introduction

Minifrac test, also known as fracture calibration treatment, is small-scale fracturing that is conducted in the target zone with the same fluid to be used in the subsequent main treatment. The test is aimed at interpreting a fracture geometry and determining design parameters by a means of fracturing-pressure analysis. Since its inception, fracture calibration treatment has become an effective tool and routine practice for evaluating and improving treatment design, particularly before massive fracturing treatment is implemented.

The systematic study of fracturing-pressure analysis started in the late 1970s when massive hydraulic fracturing in tight gas formations became common. In 1979, Nolte and Smith introduced a basis for interpreting pressure behavior during a fracture treatment. They presented a means by plotting the fracture net pressures vs. treating time on log-log scales for inferring fracturing behaviors during unrestricted extension, confined height, excessive height growth, and restricted penetration. Nolte also presented analysis of fracturing-pressure decline that permitted the inference of fracture geometry and design parameters. Since then, a series of papers regarding modifications and improvements on minifrac analysis and its applications have been published by Nolte. A detailed review on this subject was printed in two textbooks. The pressure decline analysis, generally called Nolte analysis in the oil industry, has been widely used for obtaining the treatment design parameters.

Recently, Fan presented a new interpretation model for determining formation permeability, fracture filtercake resistance, and pressure-dependent leakoff coefficient from fracturing-pressure decline. In his paper, the leakoff rate is determined by the pressure profile with considerations of variable filtercake, non-Newtonian invasion effects, and superposition of fracture pressure. The model can be used to convincingly interpret the minifrac test data collected in tight-gas reservoirs, low-permeability oil reservoirs, and frac&pack candidates of medium to high permeability formations.

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