Measuring Water Quality and Predicting Well Impairment

Abstract

The economic performance of a waterflood or a water disposal project can be significantly affected by suspended solids in the injection water. Here are methods and a theory that can be used to interpret water quality data obtained with membrane filters or cores and to predict well impairment caused by suspended solids. predict well impairment caused by suspended solids. Introduction In a waterflood or a water disposal project the possibility exists that suspended solids will cause the possibility exists that suspended solids will cause the injection wells to become impaired. Filtration can usually reduce the concentration of suspended solids; however, the cost of water treating should be balanced against the cost of other alternatives, such as periodic stimulation or replacement of injection wells. In some cases extensive water treating can be justified, but under other circumstances it will be more profitable to inject untreated water. Water quality is affected by several types of contaminants, including suspended silts, clays, scale, oil and bacteria. Any of these may be the predominant source of improvement in a particular injection water and environment. Formation cores, artificial cores. and membrane filters have been used in the industry to monitor suspended solids and to evaluate water quality. Some studies have defined water quality in terms of filtration rates or other experimental data. The disadvantage of these empirical definitions is that they cannot be directly related to well impairment. This paper proposes a measure of water quality that is defined as the ratio of the concentration of suspended solids to the permeability of the filter cake formed by those solids. The water quality ratio can be obtained directly from membrane or core filtration data and can be used to calculate the rate of formation impairment. Formation Impairment from Suspended Solids In considering the effects of suspended solids, some measure of the rate of impairment is needed. A convenient way to estimate how long an injector can be used before stimulation is required is to calculate its half-life. The half-life is defined as the time required for the injection rate to decrease to 50 percent of its initial value. The time required to reach some other fractional reduction in rate can also be calculated. Impairment from suspended solids is thought to occur by one of the following mechanisms (see Fig. 1):The solids form a filter cake on the face of the wellbore (wellbore narrowing);The solids invade the formation, bridge, and form an internal filter cake (invasion);The solids become lodged in the perforations (perforation plugging); andThe solids settle to the bottom of the well by gravity and decrease the net zone height (wellbore fillup). Each of the four basic impairment mechanisms is modeled in Appendix A for a constant-pressure-drop process, Equations are derived that express the time process, Equations are derived that express the time required for the injection rate to decline to some fraction a of its initial value. For each mechanism, this time can be expressed as the product of the two function, F and G. P. 865