On the Efficiency of Hydrogeochemical Sampling of Wells

P. A. Udodov, A. D. Fateev, B. I. Shestakov

(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology and the Problem Geological Laboratory)

Over the last decade, the hydrogeochemical method of prospecting for ore deposits has been introduced into the practice of geological prospecting work. A special place in the complex of hydrogeochemical work is occupied by the sampling of underground waters in wells. It makes it possible to solve a wide range of problems, for example, the presence of various aquifers, the balneological and industrial evaluation of waters, elements of mapping, and many others. The currently accepted procedure for documenting wells does not take into account the study of all these questions. In addition, under the existing procedure, data on paleohydrogeology and on the geochemistry of epigenetic processes, to which much attention is now being given, also remain unilluminated. Here we shall dwell on the significance of hydrogeochemical sampling of wells in prospecting and exploration work for ore minerals and in the drilling of mapping wells in the course of geological surveying. A well drilled in any rocks can provide geochemical information only about the area that it has exposed, i.e., at best, about an area of several square meters. If it passes even within a few meters of an ore body, then often nothing can indicate this. Therefore, sometimes a large amount of drilling is expended to expose ore bodies. There are many such examples in the practice of prospecting and exploration work in Rudny Altai, Gornaya Shoriya, Eastern Transbaikalia, and elsewhere.

Hydrogeochemical sampling increases geochemical information tens of times. If an ore body is located within limits not exceeding the migration distance of the principal ore elements, then its aqueous dispersion stream will be exposed by the well. In the very worst case, the migration distance of the elements mentioned is no less than 200 m in the direction of flow and no less than 100 m across it. This means that hydrogeochemical sampling makes it possible to obtain geochemical information from an area of no less than \(200 \times 100\) m. Moreover, sometimes it is possible to refine the location of an ore body with the aid of hydrogeochemical sampling.

All that has been said above indicates that hydrogeochemical sampling sharply increases the prospecting geochemical efficiency of wells. However, hydrogeochemical sampling undoubtedly requires additional expenditures of time and funds. These expenditures consist of the cost of the sampling itself and the cost of work to prepare

wells for sampling. The magnitude of the expenditures is determined mainly by the geological and hydrogeological conditions of the area and by the stages (scale) of geological prospecting, since these factors determine the methodology of hydrogeochemical sampling of wells.

In practice one has to deal with various conditions of hydrogeochemical sampling. The simplest is the sampling of wells drilled in massive rocks with a thin cover of unconsolidated deposits (5–10 m). Such wells are drilled with flushing by clean water, and for their sampling a short pumping is sufficient to remove the flushing water (Table 1, well 5). It is more difficult to sample wells drilled in fractured rocks overlain by 10–20 m of unconsolidated deposits (Table 1, well 18). Such wells are drilled with a clay solution of various consistency, and for their sampling declaying of the well walls is necessary. Experience shows that in order to declay a well sufficiently for groundwater to enter the well, pumping must be carried out for 4 hours. Still more complicated is the sampling of wells drilled in rocks overlain by a thickness of unconsolidated deposits of 30–60 m. These wells can sometimes be drilled without casing the unconsolidated sequence, using clay solution to stabilize the walls. When such wells are flushed for sampling, as field experience shows, the well walls are declayed and collapse (Table 1, well 24). Therefore, if the aim is hydrogeochemical sampling of such a well, it must be secured through the entire thickness of the unconsolidated sequence with casing pipes.

For the last case we present approximate calculations of the cost of drilling a well and the additional expenditures for preparing it for hydrogeochemical sampling and for the sampling itself. The basis for this calculation may be the data from more than ten years of experience of the staff of the Department of Hydrogeology and Engineering Geology and the problem-oriented geological laboratory for well sampling.

Approximate calculation of the increase in the cost of a well due to conducting hydrogeochemical sampling.

1. Calculation of expenditures for drilling a well
Table 2

2. Calculation of the costs of hydrogeochemical sampling (including preparation for it)

It is evident from Table 2 that hydrogeochemical sampling increases the cost of a well by 5.6%. In calculating the drilling of the well, only direct costs were taken into account; overhead costs were not included. In calculating pumping and sampling, it was assumed that all pipes would remain in the well, whereas in fact in most cases they can be recovered. From what has been said, it is clear that the actual cost increase due to sampling will be considerably smaller.

Proper methodology for hydrogeochemical sampling of wells is of great importance for increasing efficiency. First, hydrogeochemical sampling without preliminary pumping has little effect, since the samples mainly contain drilling fluid; in addition, the walls of the wells are clogged with clay, and their communication with the surrounding aquifers is very difficult, and in a number of cases is completely absent. The maximum duration of pumping required for sampling, under the most difficult conditions, does not exceed 1 brigade-shift, which leads to a cost increase not exceeding 2% of the cost of drilling the well. In choosing sampling depths, one must be guided both by general considerations regarding the geology and hydrogeology of the area and by study of the core from the given well. The greatest interest for sampling is presented by intervals of increased fracturing and mineralized zones.

In addition to interval-by-interval sampling of water, sampling during pumping at certain time intervals is important, since in the course of pumping the depression cone that forms may capture aqueous dispersion flows from the mineralization zone, located both on the flanks of the well and farther down the groundwater flow. The area characterized geochemically will be the larger, the more intensive the pumping. If, after some time, an increase in the content of microcomponents has occurred, then it is sometimes possible to judge the distance to the ore body.

Sampling of drilling fluid during drilling is also of interest. If drilling is carried out with clean water, data can be obtained through possible mixing of the drilling fluid with groundwater. When drilling with a clay mud, this is possible to a much lesser extent, and here spectral analysis of the clay fraction of the mud, which has strong sorption properties, is of interest. A small portion of water separated from the mud can be used for determining heavy metals by the dithizone method. In both cases, hydrogeochemical sampling can provide information on the presence of mineralization here, since

as enrichment of the solution at the expense of the rocks proceeds continuously. For example, wells Nos. 2 and 3, drilled in 1963 near the village of Baranovo, did not expose the ore body, since they were very shallow and the upper part of the bedrock was strongly destroyed, as a result of which no core was recovered. Sampling of the clayey solution and determination with dithizone of the sum of heavy metals (lead, copper, zinc) showed that this sum was 1500–2000 µg/l against its background content of 35 µg/l. Further investigations established that wells Nos. 2 and 3 had come close to the oxidation zone of disseminated sulfide ores.

All the foregoing shows the great effectiveness of using hydrogeochemical sampling of wells (Rudny Altai, Kazakhstan, Kolyvan-Tomsk folded zone). Taking into account the considerable prospecting effect and the slight increase in the cost of drilling operations, it is necessary to introduce hydrogeochemical sampling of wells as a mandatory type in the complex of geological-prospecting investigations.