Acid-Fracture Conductivity Correlations for a Specific Limestone Based on Surface Characterization

Abstract

Acid-fracture conductivity prediction is a key aspect in acid fracturing design and consequently in the oil and gas production forecast and economic evaluation. Acid-fracture conductivity correlations have been developed for some groups of calcareous rocks with a wide range of properties distribution. The experimental part of these studies has been based on samples from quarries and outcrops. This paper presents the development of conductivity correlations, which differs from previous studies by focusing on a specific oil producer limestone (Quissamã-ESP), by use of samples from well whole cores, and by performing comprehensive 3D surface characterization of the acid-etched fractures. Fractures were acid-etched in the lab with four types of acid systems at proper similarity conditions. The experimental study included core samples preparation, surface scanning with a laser profilometer, and rock strength and conductivity measurements. The 3D surface characterization not only supported the development of a conductivity correlation based on usual parameters, fracture width and rock embedment strength, but also enabled the development of correlations based on surface characterization parameters. It was also useful to manage experimental artifacts, to define the etching pattern, and to obtain more accurate fracture width calculation. The results showed that acid-etched fracture surfaces are rougher than usual engineering surfaces and present negative skewness, high kurtosis, and an intermediate isotropy -mostly random but with some spatial orientation. Four types of exponential acid-fracture conductivity correlations were developed for the Quissamã-ESP limestone, all them more accurate than the Nierode and Kruk correlation, adopted as a reference. The initial conductivity in the most accurate correlation is a function of a surface characterization parameter, while the rate of conductivity change with closure stress is a function of the rock embedment strength. The use of surface characterization parameters can improve the prediction of the initial conductivity, but not the rate of conductivity change with closure stress. Further studies will be necessary to improve the rate of conductivity change with closure stress prediction and to evaluate the possibility of structure creation, such as channels, in the acid-fractured surfaces.

Limits of Acid Fracture Conductivity Correlation Improvement Through Acid Fracture Surface Characterization

“…The application of the Nierode and Kruk (1973) correlation to the Quissamã-ESP limestone showed mean deviation of 112.4%, SD of 92.4%, maximum deviation of 456.1%, maximum deviation in the range of closure stress of normal production of 127.0% (Rodrigues 2011). Thus, the developed correlation represents a significant improvement in conductivity prediction for the Quissamã-ESP.…”

“…The full results of the experimental study can be found in Rodrigues (2011). This item presents only the data directly related to the main topic of this paper.…”

“…This item presents only the data directly related to the main topic of this paper. 1) generating a list of C 1 and C 2 values for each test (Table 1), which were the basis for conductivity correlations development (Rodrigues 2011). Then a refinement process was applied on the exponential model (Eq.…”

“…The rock embedment strength and the fracture conductivity measurements are presented in Rodrigues (2011). …”

“…To calculate the parameters Rodrigues (2011) implemented the equations presented by Dong et al (1993), Thomas (1999), Mainsah et al (2001), Whitehouse (2002), and Blunt and Jiang (2003) in MATLAB codes. The filter used is the one recommended in the ISO-13565-1 (1998) standard, proper for surfaces with deep valleys, such as the acid wormholes.…”