Waste Surveillance in Subsurface Disposal Projectsa

Probably the most time-consuming procedure associated with a deep-well disposal project is found in the follow-up, when the operation is monitored to make sure that the injected material has not escaped from the target aquifer. The usual procedure is to monitor the pressure in the annular space between the injection tubing and the exterior well casing, to monitor the injection pressure and, in extraordinary circumstances, to monitor water levels in an observation well in the nearest aquifer above the one used for disposal. This observation well is normally drilled at a distance of 300 feet or more from the waste-injection well and the theory is that if any of the injected waste is escaping from the target aquifer, this transfer of fluid into the upper aquifer will show up as a pressure increase, as compared to the base line data obtained before injection started. Now there may, indeed, be a slow rise in pressure in the observation well. However, it may be so slow and so small that many years may elapse before it can be conclusively demonstrated that the pressure rise has actually occurred. Then the problem is: Where is the leak and what can be done? The reason for the tardiness in detection is that the observation well will most likely be affected by barometric changes, and these obscure the rise in pressure (water level) caused by the leak. Moreover, if the leak occurs at some distance from both the observation well and the injection well (and this is likely), the rate of fluid escape may be very small, possibly in the order of one percent or less of the injection rate. So if the injection rate is on the order of 700 gpm, we are talking about detecting the escape of 7 gpm of fluid at a distance of many hundreds of feet from the point of observation—an unlikely prospect. The experimental and theoretical work now underway at Louisiana State University may point the way to effective waste monitoring and provide criteria which will lead to the safe containment of wastes for periods comparable to those used by geologists: from 10,000 to 100,000 years. Our experimental work has shown that if the waste is denser than the native fluid in the aquifer (chemical compatibility, non-plugging, safe injection pressures, etc., being assumed), then, in a horizontal aquifer, where the waste may move radially from the injection well, due to the density difference (even though the injection well is completely penetrating) the waste tends to move to the base of the receiving formation. In fact, if the injection rate and the aquifer characteristics are known (or can be estimated to within, say, a factor of 2 or 3) then the maximum distance of travel along the roof of the aquifer can be computed. This implies that if a leak should occur in the confining bed at a greater distance than the computed maximum, the fluid leaving the target aquifer might not be waste, but might well be the brine contained in the target aquifer, presumably a much less dangerous substance. If our objective is to determine the position of the waste front at any time after injection has begun, then it will be necessary to drill monitor wells in the target aquifer and to make multiple completions in the target aquifer from which samples of fluid may be obtained. The question then becomes: Where, and at what depths, shall the well be located and the multiple completions made? Our computational program is designed to answer these questions. The precision of the results depends on the validity of the data that is read into the program. We have verified the computational procedure on a miniature well field that is installed on a miniature aquifer (miniaquifer) in the laboratory, where all of the variables have either been determined or are under our control. Thus the simplest case of injection into a horizontal aquifer at a constant rate has been studied experimentally and we have made 3-dimensional models of the configuration of the waste after stated periods of time. However, aquifers are seldom horizontal and they usually have fluid moving within them (there is a slope to the potentiometric surface). We are working on the computational programs that will take the initial movement of ground water in the aquifer into account if the aquifer is horizontal. We are also working on other programs which take dip into account both in the presence of ground-water movement or in its absence. All of these computational results have been, and will be, tested experimentally by injection projects in the miniaquifer. However, even before the mathematical models have been completed, some very useful qualitative results have been obtained. Among the most important of these is the conclusion that a gently dipping aquifer, with no existing ground-water movement, is a desirable target aquifer—particularly if the waste is, or can be made, denser (higher specific gravity) than the native water. Under such conditions, the volume of pore space available between the injection well and the nearest down-dip boundary, will govern the minimum safe period of containment. The recommendations for long-term waste disposal, including monitoring, involve adequate hydrogeologic evaluation of the containment aquifer prior to building the waste disposal well. Although the initial cost of such a project may be increased over costs now prevailing, a single adequate study will make it possible to design more than one disposal project in an area and make it possible to protect the biosphere from the effects of noxious wastes for a period further into the future than the time interval between the construction of the first of the pyramids in Egypt and the present day.