A new small drillhole minipermeameter probe for in situ permeability measurement: Design, theoretical analysis, operation, and performance characteristics

Extensive collection of outcrop permeability data has taken place during the last decade through utilization of the conventional, surface-sealing minipermeameter technique. Due to its design, this probe type suffers from several limitations as a field instrument, including the inherent restriction to measurement collection near the weathered outcrop surface, and uncertainties associated with seal quality. As a response to these and other issues, a new small drillhole minipermeameter probe was designed. Drilling is followed by drillhole vacuuming, probe insertion, seal expansion, gas flow initiation and in situ calculation of permeability. Advantages of this approach include (1) elimination of the influence of weathering on permeability; (2) provision of a superior sealing mechanism around the air injection zone; and (3) the potential for measurement collection at multiple depths below the outcrop surface. The data analysis methodology associated with performing in situ permeability measurements inside small diameter holes is described. Analysis of field data, which consists of gas injection pressure and mass flow rate, is based on a numerical solution of the ideal gas flow equation in cylindrical coordinates, assuming homogeneous and isotropic conditions over the averaging volume of the measurements. Geometrical factors are determined for the small drillhole minipermeameter probe as a function of varying system dimensions (i.e., drillhole depth, seal thickness, and injection area or headspace), and are expressed in the form of geometrical factor curves. Finally, the theoretical basis for a spatial weighting function in a steady, homogenous, and isotropic flow system is developed utilizing streamline coordinates for both compressible and incompressible flow. A physical iii interpretation of spatial weighting functions in terms of the ratio of steady-state energy dissipation rate per unit volume of porous medium to total energy dissipation rate over the entire flow domain is formulated. Finally, applications of the spatial weighting function are presented based on calculated gas minipermeameter mass flux fields for the conventional surface probe and the new small drillhole probe configuration. The results indicate that for diverging flow field instruments, such as the gas minipermeameter, porous medium volumes in the inlet vicinity are heavily weighted, with volumes near the seal boundaries shown to be extremely important. Knowledge of the spatial weighting function allows one to quantify the size and shape of the sub-volume of a flow system that contributes in a significant manner to a permeability measurement.