Calculating Wormhole Propagation and Skin Factor in Carbonate Reservoirs during Directional Wells Acidizing

Matrix acidizing is a method for improving well inflow performance. In this operation, acidic solution is injected into the carbonate formation to increase near wellbore permeability by rapid creation of irregularly shaped channels named “wormholes”. Other common carbonate stimulation techniques include: hydraulic fracturing, acid fracturing, hydro-blasting and combination of acidizing and cased-hole perforation. In this research, we aim to calculate wormhole propagation in a horizontal carbonate later (bed dip=0) during acidizing a directionally drilled well which is open-hole or cased-hole and thus modeling skin factor evolution with time. For this purpose, Buijse-Glasbergen semi-empirical field scale wormhole propagation model based on regression with acid efficiency curve, a model for unsteady-state pressure distribution in the formation caused by a directionally drilled well and a model for single phase Newtonian fluid flow in the wellbore considering inflow/outflow from wellbore wall are coupled which by solving them simultaneously, wormhole propagation in each time step can be calculated. Based on the obtained results, wormhole propagation decreases with reservoir layer depth due to the fact that by starting acidizing job, permeability of the upper portions of the layer will increase more than the lower parts as a result of the sooner contact of the acid with them. Thus, these parts will receive larger volumes of acid and greater wormhole penetration depths. Also, wormhole propagation radius will be increased in a cased-hole completion in comparison with the open-hole and thus  carbonate reservoirs acidizing results will be improved. It is required to consider optimum number of sublayers used in simulation based on reasonable precision and runtime to obtain accurate results. Increasing wellbore inclination causes wormhole propagation to be increased to a specific depth, but it does not necessarily improve overall acidizing results and reducing skin factor. Minimum skin factor occurs in a specific inclination which must be considered in planning directional wells which their reservoirs will require acidizing certainly.