General minifrac analysis type curves for analysis of heterogeneous formations have been developed and are presented in this paper. These type curves apply to a wide variety of formations including coal beds and naturally fractured formations. The new type curves have been developed for the Perkins and Kern model. The paper presents the development of equations and type curves, and discussion of the technique and criteria for their application.
Conduct and results of a minifrac test in a coal seam are presented in this paper. The test is analyzed using the developed technique. The data in this case fits the new type curve extremely well. The match with existing type curves was poor.
A minifracture test is essentially the creation of a small fracture without using any propping agent. It is performed a few hours to several days prior to the main fracturing treatment. The objectives of a minifrac are to gain knowledge of fluid-loss and fracture geometry. For design purposes, the most important parameter calculated from a minifrac test is the leakoff coefficient. Fracture length and width fluid Efficiency, and closure time may be also calculated.
Techniques for analyzing a minifrac test have been developed.1–4 In the initial paper introducing minifrac analysis, Nolte1 presented the technique employing the Perkins and Kern model. In developing the technique, it was assumed that fracture length relates to time as a power law function.
Equation (1) (Available in full paper)
In SPE 8341, Nolte l,2 assumed that the exponent, "a, may be 0.50 or 1.0. He showed that error with the use of either exponent is less than 16%. Lee3 rederived the equations for minifrac analysis for K-Z and radial models using probably the more realistic exponent of 2/3. Modifications of these techniques to account for effect of fluid compressibility and temperature change during theminifrac test were also presented in the literature.5
Because in these techniques the formation is presumed homogeneous, the derived equations do not accurately apply to a naturally-fractured formation in general or to coal bed seams in particular.
This paper presents a modification to the existing minifrac analysis technique that makes it applicable to naturally fractured formation and coal seams. The physical justification and significance of the developed model are presented. The developed model was applied to a minifrac test, conducted in a coal bed. The data and results of analysis are presented.
Current minifrac models assume that leak-off rate is inversely proportional to square root of time, which indicates that the formation is assumed to be homogenous and back pressure in the formation builds up with time, resisting fluid flow. Experimental work on filter cake growth6 has shown that an exponent different from 0.5 may be encountered. In this paper this exponent is called leakoff exponent. In development of models given later in this paper, it is assumed that this leakoff exponent may be different from 0.5. Field examples will show the validity of this assumption.