Temperature and Geometry Effects on the Fracture Surfaces DissolutionPatterns in Acid Fracturing

摘要

Modeling acid fracturing operations in carbonate formations is performed to evaluate the possibleimprovement in well productivity.Models are developed to mainly estimate the acid penetration lengthand the fracture surfaces etched-width profile.Variable combinations of these two parameters produce asignificant difference in the fracture productivity.To better estimate these parameters,a reliable fracturepropagation model should be coupled with the acid reaction/transport model.Simulating weak acids ordolomite formations reactivity requires the inclusion of a heat transfer model.The model provided in thisstudy couples these factors as fractures propagate to finally obtain the fracture conductivity distributionalong its length.The fracture propagation model continuously updates the domain for the acid model.A transient acidconvection and diffusion equation is solved and the fracture etched-width profile is calculated.An iterativeprocedure is implemented in a temperature dependent kinetic model which is stopped when both thetemperature and acid solutions converge.When injection stops,acid etching and the fluids temperatureare updated as the fracture closes.As the final etching profile is drawn,conductivity is calculated using acorrelation that considers formation heterogeneity.Coupling fracture propagation shows a significant difference on the acid model solutions compared tothat assuming constant fracture geometry.For extremely high Peclet number that represents a very retardedacid system,a constant drop in the etched-width value until reaching zero at the fracture tip is theoreticallyobtainable.For lower Peclet numbers,the etching profile is shown to be sharply declining towards thefracture end.This is in contrast with the non-coupled approach from which a uniform etching profile isobtained at moderate to high Peclet numbers.It is also observed that the simulation of acid injection in noncoupled,constant fracture geometry always overestimates the acid penetration distance.The etched-widthdistribution and the acid penetration length are temperature sensitive,especially in dolomite formations.Temperature coupling shows that the maximum etching in dolomite formations occurs away from thefracture entrance as acid reactivity increases.It also shows that the cooling effects of the first stage pad fluidon improving the acid penetration distance is limited.Simulating acid fracturing operations assuming constant final fracture geometry and an average singletemperature is time efficient but results in inaccurate solution.This paper quantifies the effects of integrating fracture propagating and heat transfer models on the acid etching pattern from which,a better estimate ofthe fracture productivity is expected.