Hydrologic Behavior of Unsaturated, Fractured Tuff: Interpretation and Modeling of a Wellbore Injection Test

Abstract

A previously reported injection zone test in the Tshirege Member of the Bandelier tuff has been interpreted and modeled using a variety of conceptual models ranging from a single continuum model (SCM) to a dual permeability model (DKM). The agreement of the numerical models and the data was quite acceptable, both qualitatively and quantitatively. The migration of water injected in an open interval of the wellbore exhibited flow that was controlled by gravity and, to a lesser extent, capillary forces. A sensitivity analysis showed that the behavior is well captured with laboratory determined hydrologic parameters and less so if rocks with higher capillary suction are simulated. More complex model formulations that explicitly account for fractures were also tested, but it was found that these models reproduce the field behavior only when parameters are selected that minimize the role of fractures. Therefore, the SCM is favored due to its simpler formulation and ease with which it is implemented in large‐scale flow and transport simulations. Discrete fracture simulations are used to illustrate that fractures are predicted to play a minor role in most subunits of the Bandelier tuff: high matrix permeability allows water entering fractures to quickly imbibe into the matrix, thereby reducing the tendency of these rocks to exhibit preferential fracture flow. In contrast, water flow through Basalts present on the Pajarito Plateau is predicted to be dominated by fractures.