Premature Decline of Production Temperature – Can Tracer Test Tell Why?

Abstract

For a hydrothermal reservoir operated by means of well-doublet circulation, supposed to deliver at commercial levels for at least four decades and showing, within less than three years since the onset of fluid circulation, a conspicuous decline of production temperature (with no apparent changes in reservoir hydraulics), rapid inter-well drainage or flow-path ‘short-circuiting’ is being presumed, e. g., by a transmissivity ‘window’ of either natural origin (falling below detection limits of prior geophysical exploration), or induced by wellbore treatments at very early stages of reservoir development (unintended ‘stimulation’ resulting in ‘misaligned’ increase of permeability). A tracer test was initiated aiming to elucidate the existence and properties of the presumed flow-path shortcut. Interpreting and evaluating the measured tracer signal in terms of solely advective processes (a piston-flow additive model enjoying some popularity) leads to the somewhat paradoxical ‘finding’ of a (seemingly) unambiguous correlation between the thermal lifetime expectation and the ‘measured’ fluid residence time, while remaining unable to infer, from the measured tracer signal, the actual size of the presumed transmissivity ‘window’. Scoping analytical and numerical simulations of heat and tracer transport by advection-dispersion and matrix diffusion reveal some useful, though limited correlation patterns between temperature decline and tracer breakthrough. Based on such, preliminary findings from the tracer test help constrain the range of plausible properties for the presumed flow-path shortcut.