Acid Fracturing: The Effect of Formation Strength on Fracture Conductivity

Abstract

Abstract A few studies considered the role of rock strength on fracture conductivity retained at elevated closure strengths. Based on experimental results and field data, many correlations were derived with a consideration of both the surface roughness and rock mechanical properties. These studies found that acid fracture conductivity is sometimes lower with longer acid contact times than with shorter ones. This is apparently the result of the weakening of the rock structure along the face of the fracture with increased acid exposure, such that the conductivity decreases more rapidly with increasing the closure stress. Based on extensive literature survey of three types of carbonate rocks, it was found that chalk has the lowest rock embedment strength and the fractures closed at much lower stresses compared with limestone or dolomite. Dolomite has the highest rock embedment strength and best conductivity results compared with other rocks tested. A sensitivity analysis was performed to determine the effect of rock embedment strength, and dissolved rock equivalent conductivity on the fracture conductivity. The results of this analysis showed that dissolved rock equivalent conductivity determines the maximum conductivity at a closure stress equal to zero, while the value of rock embedment strength determines the decline in conductivity with increasing the closure stress. This analysis shows that the value of dissolved rock embedment conductivity doesn't affect the difference that exists in some of the developed correlations, while the value of rock equivalent strength has a significant impact on this difference. Finally, the rock embedment strength is a key parameter that affects the fracture conductivity. This paper discusses current correlations to predict fracture conductivity, and addresses the influences of several key parameters on the fracture conductivity. Introduction The success of the acid fracturing treatment depends on the created conductivity being retained under closure stress. Conductivity is mainly a competition between two phenomena: the etching of the rock surface and the weakening of the rock strength by the acid. Under closure stress, in addition to uneven etching, fracture conductivity depends on the ability of asperities to maintain mechanical integrity. The final conductivity of the fracture depends on factors that create the conductive path and those that maintain it open under closure pressure. The simple part of this process is creating the conductive path because several parameters can be used to achieve this like: acid concentration, soaking time, using different acid systems, and additives. While, the hard part is how to keep this path open because it depends on the remaining hardness of the asperities. As hard it will be, as the conductivity will be maintained. Therefore there is a need to optimize the parameters in part one to maintain the hardness of the rock. Broaddus et al. (1968) stated that large quantities of acid and long contact times may weaken the asperities and will cause crushing of them under the closure stress. Nierode and Kruk (1973) showed that the fracture conductivity can be predicted from the closure stress, rock embedment stress, and dissolvent rock equivalent conductivity. They mentioned that the fracture flow capacity resulting from acid reaction is very high, except when the rock embedment strength, rock solubility is low, or the closure stress is high. Anderson el al. (1989) stated that formation characteristics will have the dominant effect on the etched conductivities as a result of fracture acidizing. Chalk formation is a typical example for the etched surfaces that can be easily closed due to its natural softens. A serious of dynamic etching tests should be used to determine the optimum acid system for a given formation. The fracture conductivity data obtained from dynamic etching tests can be used as input data for a computerized acid design.