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Introduction

The purpose of this study was to determine how the response of sandstone formations is related to the individual components of acid treatments employed on them. Our goal was to identify the conditions that on the average lead to the greatest response. For this purpose, we collected detailed engineering data on stimulation treatments conducted within Exxon Company, U.S.A. over a three-year period, 1982–1984. During this period, more than 1,400 acid treatments were performed, of which 785 were on sandstone formations. Of the latter, 529 employed a mixture of hydrofluoric and hydrochloric acids. The response of wells treated with this formulation, commonly called mud acid, is the focus of the current study.

Mud acid, unlike hydrochloric acid alone, possesses the ability to dissolve clay materials. Its principal use in sandstone formations is to dissolve clays that have been affected by or are the result of the drilling or completion process. Such clays reduce formation permeability near process. Such clays reduce formation permeability near the wellbore—a condition commonly called formation damage. The mechanism by which mud acid dissolves clays has been well described in the literature and is not the subject of the present work. Because of its high reactivity with clays, mud acid is spent relatively close to the wellbore (from a few inches to at most a foot or so from the sandface). Its sole value, therefore, is to remove damage near the wellbore.

Method of Study

The type of information collected on each stimulation treatment is illustrated in Figure 1 showing the input data forms. In addition to requiring well identification data, date of treatment, outcome (i.e., success or failure), production rate both before and after treatment, reservoir production rate both before and after treatment, reservoir and wellbore data (permeability, porosity, depth, gross and net thickness, pressure and temperature of reservoir), and related factors, the specifics of the treatment were also gathered. For mud acid treatments, these included volumes and compositions of each phase of the treatment as well as the type and amounts of the additives employed in each phase. Where multiple stages were employed, a separate data card was used for each stage. Input data forms were completed in each District soon after the treatment had been conducted and evaluated. The forms were subsequently batch processed in Headquarters with a computer program devised for this purpose.

Definition of Success

The crucial determination on each treatment was whether it was a success or failure. All subsequent evaluation of treatment components related to this assessment. Two methods were employed to determine treatment success. The first, conducted at the District, was essentially an economic assessment. Did incremental production pay out treatment cost within a certain time production pay out treatment cost within a certain time period; or more generally, did treatment results cover job period; or more generally, did treatment results cover job costs to satisfy certain economic yardsticks such as a predetermined profitability index? If the economic predetermined profitability index? If the economic criteria were met at the District, the treatment was ruled a "success" and so indicated on the input data forms. Treatments falling short of the mark were correspondingly ruled "unsuccessful." These assessments were taken at face value in analyzing the data.

A different success determination was also found to be useful. It involved assessing the treatment a success if it resulted in a net positive production buildup. That is, the production rate after the treatment was greater than production rate after the treatment was greater than before. This criterion, while less demanding than the economic success, had the virtue of distinguishing clearly between factors causing a production increase and those leading to a production loss.

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