Permeability structure estimation around a borehole using numerical models of thermal re-equilibration

Abstract:

Permeability structure plays an important role in controlling fluid flow and pressure distribution within the subsurface. Although in-situ measurements are preferable, traditional aquifer tests generally provide only a single bulk estimate for the open interval within a borehole. Here, we describe a new method for estimating permeability structure and depth variations around boreholes by analyzing variations in thermal re-equilibration after circulation of cold fluids. Previous studies have observed depth variations in thermal re-equilibration after drilling in time-series measurements of temperature at various depths within a borehole and interpreted as resulting from variations in permeability and extent of density-driven infiltration of cold borehole fluids into the formation. We investigate the relationship further through numerical model simulations of double-coupled fluid flow and heat transport within permeable zones around a borehole. Because cold water is denser than warm water, hydrostatic conditions with a cold borehole can drive fluid flow into formations in equilibrium with the geotherm. Compared with low-permeability conduction-dominated scenarios, we find that increasing the permeability within an isolated zone promotes infiltration of cold fluids and results in longer thermal re-equilibrium times at those depths. The results are consistent with previous interpretations and suggest that a combination of distributed temperature sensing time-series and numerical modeling can be used to efficiently estimate permeability at multiple depths around boreholes.