The placement of acid over the entire wellbore interval can be the key to successful stimulation treatments in wells with long (often horizontal) completion intervals. This paper discusses computer simulations performed to determine how fluids are distributed in the wellbore during bullheaded and circulated acidizing treatments. One of the novel aspects of these simulations is the inclusion of wellbore effects arising from the transient flow of acid and diverter along the wellbore. The results give an insight into a possible cause of some of the poor stimulation performances observed in the field when typical volumes of low-viscosity acids were injected without diversion or selective-placement techniques. Such situations can result in the acid coming into contact with only a small fraction of the treatment interval. The simulations also show how viscosifying the acid could have improved the stimulation performance.
This paper outlines the basic concepts of the simulations and some of the general trends observed from the results. In particular, the inclusion of transient wellbore flow calls for the modification of recently published guidelines for various diversion techniques.