Effect Of Natural Fracture On Wormhole Propagation Behavior

There are developed natural fractures in carbonate reservoirs that have a significant influence on the process of acid seepage and the pattern of wormhole.Because of the pattern of wormhole determining the result of matrix acidizing,it is of great significance to study the effect of natural fracture on the wormhole propagation behavior.This paper focuses on the issues of wormhole propagation behavior in fractured carbonate reservoir.First of all,the natural fracture model with statistically significance is established by the method of Monte Carlo.By randomly simulating the natural fracture parameters fitted by different probability distribution functions,the natural fracture distribution and porosity distribution data are obtained.Secondly,the mathematical model and the numerical model of two-scale continuum wormhole propagation are established.The coupling of the natural fracture model and the proposed model is used for simulating the physicochemical process of acid flow and reaction in fractured carbonate reservoir.Finally through extensive numerical simulation,wormhole propagation behavior under fracture porosity distribution is studied in comparison with matrix porosity distribution;the effect of the natural fracture parameters(fracture density,fracture length,fracture orientation)on wormhole propagation behavior is investigated;the effect of model size on propagation wormhole behavior is conducted;wormhole penetration distance is predicted under matrix and fracture porosity distribution.Research shows that the wormhole propagation velocity with natural fracture,other things be equal,is higher than velocity without natural fracture.Acid solution interconnects the dispersed natural fractures and extends forward to form the main wormhole.Due to the distribution of natural fracture,acid wormholing could be severely tortuous.At small fracture density,the wormhole propagation velocity is apparently increased with the increase of fracture density.At lower fracture density,wormhole propagation velocity increased significantly with increasing fracture density.At fracture density higher than 2.0/cm2,it doesn't show obvious increase.When fracture length is more than 2.5cm,wormhole propagation velocity is not influenced obviously by the increase of fracture length.Besides fracture orientation has a significant impact on the pattern of wormhole,wormhole propagation velocity decreases with the increase of angle between the orientation of natural fracture and acid injection,and wormhole tends to became more tortuous and more branched.Pore volume to breakthrough may be lower with natural fracture than without natural fracture,when the angle is over than 45 o.Under typical acid properties and injection conditions,the length of wormhole propagation is around 6.5m without natural fracture,and around 11.5m with natural fracture.This study provides a theoretical basis for the matrix acidizing design in fractured carbonate reservoirs.