General Guidelines for Batch Treatments of Polylactic Acid for Diversion in Multistage Matrix Acidizing Treatments

Abstract In the last decade, there have been several publications describing the use of Polylactic Acid (PLA) as a diverting agent in multistage matrix acidizing treatments. The solid polyester is particularly useful as a diverting agent because it hydrolyzes in the presence of heat and water leaving no residue in the formation, negating the need for any clean-up fluids. Most of the publications thus far focus on characterizing the physical and chemical attributes of the diverter and demonstrating its effectiveness in field trials. This paper develops some general guidelines for the application of PLA diversion in multistage matrix acidizing treatments by conducting sensitivity analysis on the design parameters of three field cases. To carry out these sensitivity analyses, an empirically derived model that describes how PLA creates a skin factor during multistage matrix acidizing treatments is incorporated in a near wellbore simulator. This simulator also tracks the fluid interfaces, models the transient reservoir and wellbore flow, and calculates the wormhole propagation rate and a number of completion skin factors. The design parameters that will be investigated include whether PLA should be sized to enter the wormhole or to bridge at the sand face, diverter concentration, specific cake resistance, the number of diverter stages, and the total volume of acid pumped. The efficiency of the diversion treatments will be compared based on the overall final skin factor and uniformity of the skin profile along the well. This paper reveals that PLA is significantly more effective in creating a uniform skin profile if it is designed to bridge the wormhole openings rather than to enter them. It is also shown that increasing the diverter concentration or using a diverter with a higher specific cake resistance can create a more uniform skin profile but there is an inverse relationship with the overall skin factor. It is shown that using the same volume of the diverter in a few diverter stages is better than pumping all of the diverter at once, but that there is a limit where using a larger number of stages does not add any benefit. The study highlights the need to fully stimulate the high permeability zones before pumping the PLA, and finally, it suggests that the total volume of acid used should be higher when PLA diversion is used to offset the adverse effects on the overall skin factor.