Understanding the anomalous temperature data from the Large Block Test at Yucca Mountain, Nevada

Abstract

The Large Block Test (LBT) at Yucca Mountain, Nevada, is unique because of its size, which is large enough to include a number of large fractures, while still small enough so that boundary conditions and heterogeneities can be adequately controlled or characterized. Extensive mapping of the test block has established the presence of many small and large fractures. Preheat air injection testing has also revealed that the block is highly heterogeneous, with fracture permeability varying more than four orders of magnitude. We hypothesize that these large fractures and the resulting heterogeneity play a significant role in the development of coupled thermal‐hydrological (TH) processes in the LBT. A large volume of TH data, including temperature and saturation measurements, has been collected from the LBT. Some of these temperature data from the LBT, particularly those recorded by sensors TT1‐14 and TT2‐14, appear anomalous. We show that these anomalous temperature data can be understood only if heterogeneity is invoked. As an example, we show that the heat pipe signature, an indication of the extent of TH coupling, in TT2‐14 is significantly longer in duration than that in TT1‐14. We show that such a difference can be explained only if one includes a high‐permeability layer, the existence of which has been indicated by air injection testing, through the location of sensor TT2‐14. Similarly, the oscillating temperature pattern in all sensors of borehole TT2 during 4470–4500 hours of heating is shown to be the result of rainwater flowing down a high‐permeability fracture. The irregular temperature pattern in borehole TT1 happens because of rainwater reaching the source of heat quickly through a highly permeable, inclined fracture and vapor rising upwards through a less permeable vertical fracture. We also demonstrate that the anomalous temperature pattern recorded by TT1‐14 during 2500–3500 hours of heating can be explained in terms of continuous upward movement of vapor and the downward flow of condensate through the fractures. We conclude that the temperature data from the LBT are the results of coupled TH processes occurring in a heterogeneous environment.