New insights about acid fracture conductivity at laboratory scale
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Copyright 2015, Society of Petroleum Engineers. Recent studies suggest that the fracture face texture affects the acid etching patterns and fracture conductivity of acid fracturing at laboratory scale. This paper presents a systematic study to clarify the effect of fracture topographic features on acid fracture conductivity. The importance is that most of the industry correlations to predict acid fracture conductivity are based on experimental measurements of acid conductivity at laboratory scale. We conducted acid injection tests with fractured cores from the Austin Chalk and San Andres dolomite formations to investigate the effect of injection temperature, pumping time and acid type on fracture conductivity. We also studied the influence of the core fracture surface texture before the injection of acid. Two types of fracture surface textures were used-rough surface and smooth surface- to quantify the effect of surface texture. After the acid injection, we analyzed how conductivity was created by examining the computer-generated profiles of the fracture surfaces. For the smooth fractures, the uneven etching of the rock was responsible for creating roughness on the surfaces and the resulting in conductivity. For rough surfaces, acid conductivity was more related to the deepening of initial features on the sample surface than to creating more roughness. These results suggest that acid conductivity may be created by a combination of both mechanisms, from which deepening may be dominant in homogenous formations. Therefore, it is not correct to disregard the contribution of uneven etching to acid conductivity as suggested by previous publications. Additionally, we found that it is acceptable to perform conductivity tests with either rough or smooth surfaces for soft formations such as the Austin Chalk. The props that kept the fracture open deformed the same for both textures; therefore the conductivity results did not differ significantly. For San Andres dolomite, the conductivity results were different due to the high rock embedment strength of this formation. These results serve as guidelines for performing acid conductivity tests.
