Abstract

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. Method selection can be crucial to treatment success - and an increasing number of options exist - each with its own set of limitations and uncertainties. Preferences and success vary for matrix and fracture acidizing - in vertical and deviated wellbores, in sandstones and carbonates and in cased and perforated, gravel packed, and openhole completions. Method selection and implementation can be daunting but greatly rewarding - calling for creativity and field experimentation.

This paper focuses on the important role of acid placement and diversion, and the types, purposes, benefits and pitfalls of the methods currently in practice. The importance of treatment placement was evident and recognized in the earliest acid treatments conducted in the late 19th century. Although this need has been recognized since the dawn of acidizing, at no point in its history has a diversion method found universal reliability and acceptance. Insufficient interval coverage is perhaps still the most common reason why acid jobs often fail to meet expectations. A well-conceived treatment in all other aspects of design (damage assessment, selection of fluids and additives, and volumes) can count for nothing if the treatment does not enter or cover those portions of the interval with the greatest need of stimulation.

Since the first commercial acid treatments in the 1930s, mechanical placement has evolved from crude rubber "packers" to advanced coiled tubing technologies. Chemical and particulate diverters have evolved from chicken feed to specialized chemical systems, including self-diverting fluids. With chemical diversion, different methods have come into and fallen out of favor - replaced by new ideas, or those forgotten and subsequently revived.

Within its historical perspective, this paper discusses present-day acid placement and diversion methods, their best applications and their limitations - with a view and emphasis on industry needs and direction for the future.

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 utilization 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 prior to the turn of the 20th 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 (Williams et al. 1979). Frasch was issued the first patent on acidizing on March 17, 1896 (Increasing the Flow of Oil Wells, U.S. Patent No. 556,669).

In this brief but brilliant thesis based on use of hydrochloric (HCl) acid to stimulate brine production from a limestone formation, Frasch anticipated a number of the elements of modern-day acidizing, including the need for a "rubber packer" to isolate the zone and force the acid into the formation to be treated.

Although this need was recognized from day one, the absence of proper acid placement is probably still the biggest reason acid jobs fail (besides, that is, failure to correctly diagnose production or injection impairment). A well-conceived, properly designed treatment in all other respects (formation damage assessment, selection of acid types, concentrations, volumes, additives) can go for naught if the treatment is not properly placed. The zone of interest must be sufficiently contacted by stimulation fluids.

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