Abstract
Acid fracturing is a stimulation process in which the dissolution of the fracture faces creates conductivity after fracture closure. The resulting fracture conductivity depends highly on the amount and pattern of etching in addition to the strength of asperities. While there has been significant amount of studies on enhancing fluid systems to generate more optimum etching pattern, etched width, and etched length, there has been minimal work on understanding how the acid systems influence rock strength and affect closure of fracture under stress.
A systematic review of all available data on rock strength used in acid fracture modeling was conducted with emphasis on determining errors involved in each measurement. Furthermore, a detailed review of documented effect of acid on rock strength was also undertaken to understand how acid changes fracture face strength. Finally, the impact of errors in rock strength values used in conductivity correlations was studied in order to determine the importance of using correct values for rock strength.
Based on analysis of measured rock strength measurements, an average of 20% has been observed in data with greater errors for dolomite formations. Furthermore, there are great variations among rock strength values among different studies, with variations up to six times. In terms of effect of acid on rock strength, the measured weakening of rock of an average of 15% is within the measurement error limits and hence no conclusion can be made on acid weakening rock strength. Effect of errors in rock strength values used in conductivity correlations suggests significant errors in conductivity estimation, especially for softer rocks and higher closure stresses.
There have been many developments in enhancing the performance of acid fractured wells. However one major reason for the lack of success is limited understanding of the effect of acid on rock strength and incorrect estimation of rock strength. In order to understand the effect of acid fracturing on rock strength and resulting fracture conductivity under closure stress, a better method of measuring rock strength is needed in addition to detailed study on the effect of treatment parameters including acid type and contact time on rock strength variation.
