Fluid Placement Model for Stimulation of Horizontal or Variable Inclination Wells

Abstract Acidizing is a common method of stimulating horizontal wells. The acidizing process is fundamentally different when applied in a horizontal well compared with application in a vertical well. The fluid distribution in a horizontal well is affected by a longer welibore length. a broader variation in the reservoir properties along the wellbore, and possibly different mechanical means to place the fluids in the wellbore. A comprehensive fluid placement model linked with a reservoir acidizing simulator is essential to precisely design acidizing treatments for horizontal wells. This paper presents a model of fluid placement in a horizontal well. The model predicts the placement of injected fluids by tracking the interfaces between different fluids in the wellbore. It is capable of tracking multiple interfaces for multiple injection stages in horizontal wells. For injection with coiled tubing, the model allows tubing tail movement during injection. It also handles simultaneous injection from the annulus and from a tubing string. A unique feature of the model is that it considers gravity under-riding of the denser injected acid when the wellbore is not perfectly horizontal. Both analytical and discretized solutions of the model are presented in the paper. Examples in the paper illustrate the effects of such factors as velocity of tubing movement, annular injection, non-uniform distribution of flow into the reservoir, and gravity segregation under-riding in inclined wellbore sections on fluid placement in an acidizing treatment. The wellbore placement model has been linked with a matrix acidizing model calculating damage dissolution and skin reduction near the wellbore. An example is provided comparing different placement techniques such as bullheading and injection through coiled tubing with and without simultaneous injection through the annulus. The simulator can help to determine optimum tubing tail locations and optimal injection volumes of acids, to select the most appropriate diverting methods, and, hence, to maximize the benefits of an acidizing treatment. P. 473