Effects Of Non-uniform Etching On Fracture Conductivity

Acid fracturing is one of the most important treatments to carbonate reservoir.After acid etching the carbonate fracture surfaces non-uniformly, there is some space to let fluid flow under closure stress in the process of acid fracturing. Researches show that acid etched fracture conductivity depends on the morphology of etched fracture surface, so it is significant to study the effect of fracture surface morphology after non-uniform etching by acid on conductivity.Based on the analysis of the acid-rock reaction and acid corrosion mechanism of carbonate rock, a method is put up with to quantify and analyze etched surface morphology before and after acid-rock reaction, used fractal method and digital technique to obtain the parameters related to non-uniform etched fracture surface, and set up a new conductivity model. This model can be used to provide the theoretical basis to choose the available well layer for acid fracturing technology and to evaluate the adaptability of acid fracturing for different carbonate reservoirs. This model could also quantitatively evaluate acid fracturing stimulation effect.(1)Non-uniform etched fracture surface morphology is affected by rock non-homogeneity, flow mal-distribution of acid fluid because of the acid property and uneven fracture surfaces. Conventional experimental facilities cannot simulate acid flow features in formation, like flow regime and streamline distribution. Combining the measurement apparatus of acid-rock reaction kinetics under condition of different acid viscosities with 3D digital analysis method makes the acid flow regime and streamline distribution closed to those in formation, so the measurement apparatus can be used to conduct acid-rock experiments, and the 3D digital analysis method can be used to quantify the change features before and after reactions.(2)Experiments of acid etching fracture surface under conditions of different temperatures, concentrations and flow rates were designed and conducted, and data of etched fracture surfaces before and after reactions were obtained, including width contour maps of fracture space and height reductions of single fracture surface.(3)According to the former study of the research team of reservoir stimulation,fractal method is taken advantage of to calculate the fractal dimensions of grayimages converted from the width contour maps, and with the increasing of temperature, concentration and flow rate, the dimension rises too, so it means that the degree of non-uniform etching is bigger.(4)Height reductions of single fracture surface can represent the width of the fracture after acid–rock reaction, so the kurtosis of height reductions is combined with fractal dimensions to study the effect of non-uniform etching surface on fracture conductivity at the angle of 3D. With the increasing of temperature, concentration and flow rate, the kurtosis reduces, so it means that the degree of non-uniform etching is smaller.(5)Combing parameters(fractal dimensions and kurtosis) of acid etched fracture surface morphology and proven acid etched fracture conductivity model, this paper set up a new conductivity model related to surface morphology.(6)At the closure condition of barometric pressure(0.1MPa), this paper also conducted fracture conductivity measurements. It is found that with the increasing of temperature, concentration and flow rate, fracture conductivity increases. After comparing of measurement and model results shows that the errors are almost under15%, therefore, it is feasible to use parameters related to non-uniform acid etched surfaces to calculate conductivity.