Modeling Study of Temperature and Fracture-Propagation Effects on the Fracture-Surface Dissolution Patterns and Fractured-Well Productivity in Acid Fracturing
Academic Article
Overview
Research
Identity
Additional Document Info
Other
View All
Overview
abstract
SummaryThe acidfracturing operations in carbonate formations are modeled to evaluate possible improvements in well productivity. Models are developed mainly to estimate the acidpenetration length and fracturesurface etchedwidth profiles. Variable combinations of these two parameters produce significant differences in fracture productivity. To estimate these parameters better, a reliable fracturepropagation model should be coupled with an acid reaction/transport model. Simulating weak acids or dolomiteformation reactivity requires the inclusion of a heattransfer model. The model provided in this study couples these factors as fractures propagate to obtain the fractureconductivity distribution along the length.The fracturepropagation model was created to update the domain of the acid model continuously. In the process, a transient acid convection and diffusion equation is solved and the fracture's etchedwidth profile is calculated. An iterative procedure is then implemented in a temperaturedependent kinetic model that stops when both the temperature and the acid solutions converge. When the injection stops, the acid etching and fluid temperatures are updated as the fracture closes. As the final etching profile is drawn, conductivity is calculated using a correlation that considers formation heterogeneity. The conductivity distribution along the fracture surface can be used to predict the fracturedwell productivity for given reservoir properties.Coupling of the fracture propagation shows a significant difference in the acidmodel solutions, as compared to those assuming a constantfracture geometry. For an extremely high Pclet number that represents a very retarded acid system, a constant drop in the etchedwidth value until reaching zero at the fracture tip is theoretically obtainable. For lower Pclet numbers, the etching profile sharply declines toward the fracture end. This is in contrast to the noncoupled approach, from which a uniform etching profile is obtained at moderatetohigh Pclet numbers. In this research, it was observed that the simulation of acid injection in a noncoupled, constantfracture geometry always overestimates the acidpenetration distance. The etchedwidth distribution and acidpenetration length are temperature sensitive, especially in dolomite formations. Temperature coupling shows that the maximum etching in dolomite formations occurs away from the fracture entrance, as acid reactivity increases. It also shows that the cooling effects of the firststage pad fluid on improving the acidpenetration distance are limited. This work illustrates that the impact of fracture propagation, heat transfer, and simulation of fracture after shutin on productivity prediction is substantial.Simulating acidfracturing operations, assuming a constant final fracture geometry and average single temperature, is time efficient but results in inaccurate solutions. This research quantifies the effects of integrating fracturepropagation and heattransfer models on the acidetching pattern. It also emphasizes the significance of simulating acid fracture during the closure, especially for dolomite formations or weakacid cases, from which a better estimation of the fracture's productivity can be expected. A simplified analytical solution for multiplefluid simulation is also introduced in this work.
