Surface Characterization and its Effect on Fracture Conductivity in Acid Fracturing

Abstract

Abstract Conductivity in acid fracturing after fracture closure depends on the contact of the non-uniformly etched fracture surfaces created by the acid. The surface topography of the fracture faces after acid injection is rarely investigated and, therefore, poorly described. The concept of average fracture width has been used in the fracture conductivity models to predict fracture conductivity in acid fracturing. Previous study showed that the characterization of fracture faces after acid etching plays an important role in fracture conductivity. If channels and wormholes are created, fracture conductivity cannot be described by the conventional model. For more evenly distributed etching, surface topography characteristic strongly influences the resulting conductivity. With a surface profilometer, a laser device that accurately measures the height of the asperities on the rock surface, we obtain a digital image that can be used to quantitatively characterize the etched surface topography after acidizing. Using this data we are able to define the channel geometry and etching patterns as numerical values. The distribution of these numerical values is later converted and analyzed as surface texture parameters and standard statistical variables for rough surface characterization. When two roughness parameters of each corresponding fracture surface are superposed, the fracture conductivity can be related to the statistical parameters of etching. In this paper we present experimental results for a wide range of acid fracturing conditions that provide the variables and controlling parameters relating roughness of the fractures surface to fracture conductivity measurements. The findings of this study is important to understanding the effect of etcing pattern of fracture face on fracture conductivity behavior, and it also helps to develop lab scale fracture conductivity correlations in acid fracturing. Introduction Conductivity of acid fracturing stimulation is caused by the contribution of uneven etching surfaces that hold fracture open after closure. Obviously, resulting conductivity of an acid fracture strongly depends on the heterogeneity of carbonate formation. In the past, there have been studies that predict conductivity of acid fracturing correlated to the formation rock mechanics properties and geometry of the fracture through experiments (Nierode and Kruk, 1973, Anderson and Fredrickson, 1987, Beg, 1996, Ruffet and Onaisi, 1997, and Gong and et. al., 1998). The studies used an average fracture width from the total amount of rock dissolved by injected acid at zero stress as initial condition, and then let the conductivity decline as the fracture width reduces when closure stress is applied. Fracture surface is simply assumed uneven by acid reaction, but the concept of "uneven" was not included when develop the correlations for conductivity. Our previous study (Pournik and et al., 2007, Malagon and et al., 2006, and Melendez and et al., 2007) suggested that while the average etched fracture width is one of the important parameters in acid fracture conductivity, the characteristics feature of etched surface plays an equal important role in fracture conductivity. A randomly distributed uneven surface because of the random feature of rock properties functions completely differently when the rock properties are correlated in certain dimension, and therefore the etched surface may also have a correlated feature. To investigate the characteristics of etched surface and its role on acid fracture conductivity, we have carried out an experimental study of acid fracturing conductivity. The main targets aimed at studying the parameters that affect fracture conductivity, develop new correlations that considering other factors rather than just embedment strength on conductivity, and generate guidelines for acid fracturing design. We examined the means of statistically characterizing the etched surface, and tried to determine if there is a connection between surface etching pattern and resulting conductivity and explore the possibility of including the surface etching pattern in conductivity correlation for acid fracturing treatment.