The Effects of Initial Condition of Fracture Surfaces, Acid Spending and Acid Type on Conductivity of Acid Fracture

Abstract

Predicating the conductivity of acid fracture is not possible using analytical or numerical models due to stochastic nature of acid reaction with rock and complex leak-off behavior of acid. Other alternatives are either empirical correlations or conducting experiments mimicking field treatments. We adopted experimental approach considering scaling down of field conditions to laboratory scale by matching Reynold's number to represents flow along the facture and acid leak-off through fracture faces, and Pecelt number to represent acid transport to fracture walls. Experiments conducted comprise three stages: dynamic etching, surface characterization of etched cores and conductivity measurement. One objective of this work is to understand mechanism through which conductivity is generated in acid fracturing treatment. Moreover, the effect of initial condition of fracture surfaces on conductivity was investigated to quantify the difference between rough-walled and smooth-walled fractures. Another area of interest is the variation of conductivity along the fracture due to acid spending. In previous works, experiments only represented fracture entrance of actual treatment neglecting that acid is spent along the fracture which affects conductivity. Therefore, to better predicate the performance of an acid fracture, experiments at different acid concentrations representing different positions along the fracture were conducted. We also investigate effect of contact time, acid system type and treatment temperature on conductivity using San Andres Dolomite cores. Preliminary results showed that initial condition of fracture surface has a major influence on fracture conductivity where rough-walled fracture predicted higher conductivity by almost an order of magnitude compared to smooth-walled. And mechanism through which conductivity created is by smoothing peaks and valleys creating surfaces mismatch rather than creating asperities. Furthermore, results showed that acid spending does not necessarily cause a decrease in conductivity from entrance toward the tip of a fracture. Also, cross-linked acid is more effective in terms of dissolving rock and controlling fluid leak-off compared to linear gelled acid. Furthermore, more etching does not always translate to higher conductivity suggesting the existence of optimum contact time. Results obtained provide new insights to better understand and predicate conductivity of acid fracture and suggest guidelines to optimize treatment design.