AnaLog Services, Inc. has long been involved in research on mechanical Integrity (MI) and monitoring issues concerning the Underground Injection Control (UIC) program. Several years ago, we conducted an extensive testing program on a variety of water flow meters with interesting results (see Water Meter Trivia). What follows is the body of a letter dated July 6, 1999, to EPA Region IV:
"I am directing this letter to you hoping you will see that it reaches the appropriate technical people.
"Some time ago, Region IV supplied a copy of a November, 1985 document titled Guidance Document on Evaluation of Injection Well Manifold Monitoring Systems, EPA 570/9-85-005. Said document was prepared by Woodward-Clyde Consultants for U.S. EPA under Contract Number 4W-2548-NTSX.| Home | Tech & Tips | UIC |
"The document in question contains an unbelievable mistake. It is stated as a fact that for a given flow meter, flow rate changes can only be detected above the "accuracy line" for that meter (see Figure 7, following page 12). This mistake grows from the common misconception that precision and accuracy are the same; they are not.
"In fact, a typical oil field waterflood meter may have a repeatability (precision) one to two orders of magnitude better than its accuracy. That is to say, a 100 barrel per day meter (about three gallons per minute) with an accuracy of say 2% will actually register much less flow than the two barrels per day the report suggests. Woodward-Clyde would have us believe that a leak of less than two barrels per day would not be noticed, but that is hogwash. Our 100 barrel per day meter can see leaks on the order of a few gallons per day! Thus, the ability of a flow meter to see small changes in flow is more related to precision than it is to accuracy.
"How am I so sure about this? Because we routinely monitor the injection into shallow "tight" injection wells where the daily injection rate is below what Woodward-Clyde says we should even be able to detect! And we do it with good precision and reasonable accuracy. Further, the "guts" used in three popular oil field flow meters are identical to the "guts" used in countless millions of domestic water meters, but with different housings and gearing. If these meters behaved as Woodward-Clyde believes they do, all our water bills would be much lower!
"The implications of this error are profound. First, the importance of individual well flow monitoring has apparently been grossly undervalued by EPA from early in the UIC program. Second, EPA has tolerated, even embraced, manifold monitoring apparently in part because the value of individual well flow meter monitoring was so poorly understood. And finally, it seems likely that this horrible misunderstanding has colored EPA's attitude about the 40 CFR §146.8(b)(3) Records of Monitoring MIT (Retest).
"I personally believe that manifold monitoring is one of the worst things ever allowed in the UIC program, but I recognize that the practice is here to stay because of the politics involved. However, individual well flow meter monitoring should be encouraged whenever possible. Individual well flow meter monitoring is the very best day to day protection we have. I challenge you to look at this issue; I believe it is very important. This horrible mistake is the kind of bad science that gives EPA a bad name."
This letter was forwarded from Region IV to EPA headquarters in Washington, DC, then back to Region IV, but so far no answer has been forthcoming. It is troubling that some of the early basic scientific research that forms the underpinning of the UIC program is so badly flawed.
Technical Note: A distinction is made between the terms "precision" and "accuracy" when said terms are applied to flow measurement instrumentation. Precision refers to the repeatability of the flow measurements, with homogenous media, regardless of whether or not the measured values are widely displaced from the true flow value as a result of systematic errors (such as "slip"). Precision can be expressed by the "standard deviation" or various derivations thereof. Accuracy simply refers to the agreement between the flow measured by a given flow meter, and the actual "real world" flow. The precision of typical flow meters may be one or two orders of magnitude better than the accuracy of said meter. Therefore, typical flow meters are able to resolve changes in flow much smaller than the accuracy rating of a given meter. In using flow meters to detect leaks, accuracy is not nearly as important as precision with respect to detection of small leaks; it matters not if a flow meter is accurate to say ± 2 barrels per day if it can resolve differences in flow of one gallon per day!