New Mexico Geological Society Annual Spring Meeting — Abstracts

Eleven screened intervals representing two geologic units have been tested in five new wells at Los Alamos National Laboratory. All of the wells are completed with multiple screens. This paper addresses preliminary results of slug-testing these wells by a straddle-packer/injection method.

Resulting hydraulic-conductivity (K) data range from <1 ft/d to 10's of ft/d. Results vary not only between but also within each of the two major geologic units covered by testing: Cerros del Rio lavas and Puye Formation sediments. In order to better explain the variation in K, data were divided into classes (<1, 1-10, 10-20 and >20 ft/d) and the lithologic character of material behind the screen tested, based on geologic and geophysical logging, was tabulated. Two of the six materials yielding K values in the 0-1 ft/d class are associated with clay-rich basalt flow bases or interflows. Curiously, the remaining four low K values are associated with gravel intervals, all of which made water during drilling. Both values in the 1-10 ft/d class are associated with fractured basalt. Both values in the 10-20 ftld class are associated with unaltered pumiceous gravel. The single value in the >20 ftJd class was obtained from fractured basalt.

These observations fall within published ranges of K for such materials with one notable exception: values obtained for some of the Puye Formation sediments are much lower than expected. Why would gravels that produced much water during drilling yield the lowest K values when tested? Two possible explanations are that the screen is plugged or head was insufficient to permit injection of water into these materials. The first can be ruled out, as all screened intervals seem to have been adequately developed prior to testing. The second explanation is tempting, as the screened intervals involved are the deepest in their respective wells, lying at depths of 827-1645 ft. Although limited head data suggest vertical gradient is upward in one of the wells, it is not clearly up or down in the other. A third possibility is suggested by neutron logging data that show extensive vertical continuity of free pore water in higher-K sediments, but a lack of such continuity in low-K sediments. This difference may indicate that both vertical and lateral continuity are required for a higher-K response to testing in the Puye Formation, even if the borehole penetrated multiple isolated higher-K zones that produced significant water when connected during drilling. Analysis of the controls of hydraulic conductivity in these materials, as determined by testing, continues.