The Effects of Acid Contact Time and the Resulting Weakening of the Rock Surfaces on Acid Fracture Conductivity

Abstract

Abstract Acid fracturing is a stimulation process in which lasting conductivity after fracture closure is created by uneven etching of the fracture face. The success of an acid fracturing treatment depends on the conductivity created being retained under overburden stress. Fracture conductivity is usually predicted using correlations based on limited tests conducted at low injection rates that may misrepresent hydrodynamic effects that occur in the fracture. We have conducted a new series of acid fracture conductivity experiments at conditions more representative of field conditions to explore the correlation between the experimental conditions and the rock properties with the final conductivity. In order to reproduce field conditions in the lab, we have used a lab set up properly scaled through matching of dimensionless numbers. Tests were conducted with Indiana limestone, San Andres dolomite, and Texas Cream chalk. A gelled acid system was used with varying contact times. The etched fracture surface was characterized and the rock embedment strength was measured before and after acidizing in addition to standard conductivity measurements for each experiment. In this paper, the results from a systematic experimental study are presented. Experimental results illustrate that the fracture surface etching pattern is a dominant factor for fracture conductivity. If channels are created, the fracture will have retaining conductivity after closure. Rock strength is another important parameter in acid fracturing. Formations with low initial Rock Embedment Strength (RES) values or a lager reduction in RES after acid injection have lower conductivity after closure. Measurements and analyses of rock embedment strength provide understanding of the conductivity behavior in acid fracturing. The results of preliminary tests for a North Sea chalk are also presented and discussed in the paper. Introduction Acid fracturing is a stimulation technique for which success depends on the created conductivity being retained under closure stress. Conductivity is mainly a competition between two phenomena: the etching of the rock surface and the weakening of the rock strength by the acid. Under closure stress, in addition to uneven etching, fracture conductivity depends on the ability of the rock to retain mechanical integrity. A few previous studies of acid fracture conductivity have considered the role of rock strength in the conductivity retained with elevated closure strength. The original correlation of acid fracture conductivity was presented by Nierode-Kruk 1. The correlation is based on the experimental results, and the conductivity is directly related to the amount of the rock dissolved during acid injection, with the hardness of the rock in terms of embedment strength correlated with the conductivity.