SPE Production & Operations
Volume 26, Number 2, May 2011, pp. 195-202

Summary

Because of irregular fracture surfaces caused by heterogeneities such as variations in local mineralogy and variations in leakoff behavior, we use a correlation to calculate fracture conductivity in acid fracturing. An acid-fracture-conductivity correlation consists of two parts: conductivity at zero closure stress and the rate of conductivity change with closure stress. Existing correlations do not consider the effect of variations in formation properties and were developed on the basis of laboratory experiments that use core samples with dimensions of only a few inches. On the other hand, acid-fracture simulators have grid sizes of several feet to tens of feet. Therefore, correlations should be scaled up properly when used in a fracture simulator. This paper presents new correlations to obtain the conductivity at zero closure stress. This conductivity can then be incorporated in acid-fracture-conductivity models that consider the effect of closure stress.

In this study, an extensive numerical study was performed using an intermediate-scale acid-fracture model with total dimensions comparable to a gridblock size in an acid-fracturing simulator and grid sizes comparable to core-sample sizes used in laboratory acid-fracture-conductivity tests. In this model, the distributions of permeability and mineralogy along the fracture faces are geostatistically generated. The model generates fracture-surface-etching profiles as a function of acid contact time, from which we can obtain fracture-width distributions when the fracture surfaces have come in contact at low closure stress. We then calculate the fracture conductivity by solving for the flow rate through this irregular domain for a fixed dp across the domain. By analyzing the relationship between the fracture conductivity created and statistical properties of the permeability and mineralogy distributions, we developed new acid-fracture-conductivity correlations for low closure stress. These correlations can be used directly to better predict the primary coefficient in the widely used Nierode-Kruk (Nierode and Kruk 1973) correlation of acid-fracture conductivity. This work allows the Nierode-Kruk (N-K) correlation to be scaled up to the dimensions appropriate for use in an acid-fracturing simulator.

© 2011. Society of Petroleum Engineers

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History

• Original manuscript received: 2 March 2010
• Meeting paper published: 9 June 2010
• Revised manuscript received: 6 July 2010
• Manuscript approved: 11 November 2010
• Published online: 7 April 2011
• Version of record: 16 May 2011