Acid Placement and Diversion

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 124141, ’The Art and Practice of Acid Placement and Diversion: History, Present State and Future,’ by L.J. Kalfayan, SPE, Hess Corporation, and A.N. Martin, SPE, BJ Services, originally prepared for the 2009 SPE Annual Technical Conference and Exhibition, New Orleans, 4-7 October. With the increasing need for highly cost-effective well production-enhancement applications, acid stimulation is becoming increasingly popular. To be successful, acidizing procedures require distribution of stimulation fluids across and within the desired treatment interval. Historically, this has been approached with mechanical placement or chemical diversion of treatment fluids. The full-length paper focuses on the important role of acid placement and diversion, and on the types, purposes, benefits, and pitfalls of methods currently in use. Introduction Determination of the proper fluid placement is perhaps the most crucial factor in acid-treatment design in both carbonates and sandstones. Treatment success can hinge on it. More often than not, some method or combination of placement or diversion methods is required to distribute acid across (and within) the zone(s) of interest, but no one method or combination of methods is going to be effective in all cases. Of increasing importance in acid-treatment design, in both matrix and fracture acidizing, is use of prior experience and analogous-field examples, fluid-entry diagnostics, and modeling wherever possible. The importance of treatment placement was evident and recognized in the earliest acid treatments in the late 19th century. Herman Frasch, chief chemist for the Standard Oil Solar Refinery in Lima, Ohio, at the time, is credited with the invention of the acidizing technique. Frasch was issued the first patent on acidizing on 17 March 1896. The absence of proper acid placement is probably still the biggest reason that acid jobs fail (that is, besides failure to correctly diagnose production or injection impairment correctly). A well-conceived, properly designed treatment in all other respects (e.g., formation-damage assessment and selection of acid types, concentrations, volumes, and additives) can go for naught if the treatment is not placed properly. The zone of interest must be contacted sufficiently by stimulation fluids. Outside of short, homogeneous formation intervals (which are rare), perfect “coverage” of the treatment zone often is not possible, unless diversion techniques are used. Acid Placement Matrix acid treatments, whether for sandstones or for carbonates, require similar methods to ensure optimum placement. Unless steps are taken to promote efficient acid placement, the stimulation fluids will tend to follow the path of least resistance, meaning the acid preferentially will pass into the interval with the highest conductivity. Often, this section of the well requires the least stimulation. An acid stimulation that is placed solely into this already-productive interval (or already swept interval, for an injection well) will have less effect than if the acid were placed into a less-conductive formation (assuming, of course, that this second interval has potential for improvement).