Equilibrium Minifracturing Test and Analysis
Authors: Poulsen D.K. 
Published: December 2007

This work presents a new type of in-situ test for determining growth parameters for a hydraulic fracture. The equilibrium minifrac test discussed in this work (1) more directly measures fluid-loss rate than does the commonly used pump-in/shut- in test and (2) maintains a constant or near-constant pressure drop between the fracture and the reservoir boundary, eliminating the need to consider pressure dependencies of fluid, formation, and filtercake properties in the analyses.

This work also presents techniques that are developed in this study for analyzing the in-situ test. These techniques (1) do not require knowledge of the relationship between fluid-loss rate and pressure, (2) are independent of fracture growth behavior, (3) allow the use of most any fracture growth model or simulator, (4) are independent of rock behavior, (5) do not require knowledge of in-situ fluid-flow parameters, and (6) do not require knowledge of fracture height.

Introduction

With very few exceptions, hydraulic fractures are created in porous, permeable formations. Because the pressure required to fracture a formation exceeds the pressure of the fluid originally contained in the formation, the fracturing fluid has a tendency to leak off at rates dependent on properties of the fracturing fluid, the formation, and the formation fluids.

Proper design of hydraulic fracturing treatments requires accurate prediction of the rate at which the fracturing fluid leaks off. The fluid-loss rate directly affects the amount of fluid available for further fracture extension, in turn affecting the size of the created fracture or conversely, the fluid volume needed to create a fracture of given dimensions. In addition, in proppant fracturing the fluid-loss rate determines the volume of pad fluid required to prevent proppant bridging at the fracture tip. The fluid-loss rate also determines how the proppant concentrates as the fluid moves down the fracture.

This, in turn, affects the friction loss in the fracture and the pumpability of the fluid, as well as the conductivity of the induced fracture. In acid fracturing, the fluid-loss rate affects the rate of acid transfer to the fracture face.

Until 1979, fluid-loss rates were exclusively determined using coefficients obtained by combining laboratory-determined empirical coefficients with theoretically calculated coefficients. The laboratory-determined coefficients were used to characterize the filtercake-building properties of the fracturing fluid and fluid-loss control additive system. The calculated coefficients, used to characterize the resistances due to flow of the filtrate into the formation and to compression of the reservoir fluid, were determined from properties such as assumed filtrate viscosity; reservoir fluid viscosity; reservoir permeability, porosity, and compressibility; and the differential pressure between the fracture and the reservoir boundary; all of which are assumed to remain constant. The only verification of the calculated fluid-loss behavior was whether the fracturing treatment could be performed as planned; however, this assumed that all other factors in the design process, such as model applicability, fracture height, etc., were correct.

In 1979, pump-in/shut-in analyses were introduced as means for determining certain fracture-growth parameters, primarily the effective fluid-loss coefficient and fracturing fluid efficiency, from the pressure decline during the shut-in period following a fracturing treatment. The analysis of pressure decline data, particularly that taken following smaller (minifracture) treatments performed before the main fracturing treatment, has become well accepted and is often performed in the petroleum industry. Although many analysis techniques have been developed to overcome shortcomings of the original pump-in/shut-in analysis or extend its applicability, a number of limitations remain. This work introduces a new type of fracture test that overcomes many of these limitations through a more direct measurement of the fluid-loss rate.

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