On-Site Testing To Define Injection-Water Quality Requirements

Abstract

Equipment and procedures are described for conducting on-site tests to establish filtration and chemical quality, requirements for preventing injector wellbore plugging. Results from sandstone and carbonate formations are presented, and the effects of dissolved iron and oxygen, particulate solids, hydrogen sulfide, and scale formation are described. particulate solids, hydrogen sulfide, and scale formation are described. Introduction An important factor affecting the success of a waterflood project for secondary recovery is the quality of the water project for secondary recovery is the quality of the water being injected. Large costs are involved in the construction and operation of water-treating equipment. However, too often the quality standards required for injecting a particular water into a particular formation are not known in the early stages of the project. The results are (1) damaged and plugged injection wells requiring workover or stimulation, and (2) further expense to modify the treating facilities or even install additional equipment. This paper describes a practical method for determining the specific water-quality standards required for a waterflood. Examples are presented of various water-injection problems that were identified by use of on-site tests, problems that were identified by use of on-site tests, indicating where improvements in water quality were needed to prevent such problems. Discussion Water-Quality Test Methods Generally, three approaches are usually used to determine the quality of injection water:(1)geochemical analyses of the waters involved and description of suspended solids;(2)membrane filter tests to describe the plugging effects of the suspended solids; and(3)flow plugging effects of the suspended solids; and(3)flow tests to study the effects of the injection water on formation core permeability. Important information can be obtained from ail three approaches but, in most cases, only the first and/or second are used. Consequently, no test data are available relating the injection water to the formation in question. Such data can only be obtained by the third approach, or by waiting to see what happens after the waterflood project is well under way. This "wait to see" method can lead to very costly consequences. A combination of the three methods can more adequately define water quality for a given project. Geochemical analyses of the waters involved and description of suspended solids, together with bacteria counts and a corrosion testing program, can identify potential and existing problems. The amount of data potential and existing problems. The amount of data provided by the geochemical analyses varies from project provided by the geochemical analyses varies from project to project depending on the detail of the analytical procedure used. Table 1 illustrates several typical analyses. procedure used. Table 1 illustrates several typical analyses. Corrosion problems are indicated by dissolved and suspended iron compounds, hydrogen sulfide content, and pH. Scaling tendencies and incompatibilities are pH. Scaling tendencies and incompatibilities are indicated by pH and concentration of ions such as barium, carbonates, and sulfate. Analysis of suspended solids can be used to identify the source of the solids, whether they are corrosion products, asphaltenes, or other organic materials, bacteria, fines from water source wells, etc. Membrane filter tests are amply described in the literature. Some generalized water-quality standards based on membrane filter tests, corrosion rates, bacteria counts, and water analyses are available. These standards should be accepted only as general guidelines, with the recognition that the membrane filters have characteristics greatly different from those of the formation being flooded. Membrane filter tests are best applied as an indication of changes in water quality rather than as a means for establishing absolute standards. JPT P. 17