The Vertical Hydraulic Conductivity of an Aquitard at Two Spatial Scales

Abstract

Aquitards protect underlying aquifers from contaminants and limit recharge to those aquifers. Understanding the mechanisms and quantity of ground water flow across aquitards to underlying aquifers is essential for ground water planning and assessment. We present results of laboratory testing for shale hydraulic conductivities, a methodology for determining the vertical hydraulic conductivity (K(v)) of aquitards at regional scales and demonstrate the importance of discrete flow pathways across aquitards. A regional shale aquitard in southeastern Wisconsin, the Maquoketa Formation, was studied to define the role that an aquitard plays in a regional ground water flow system. Calibration of a regional ground water flow model for southeastern Wisconsin using both predevelopment steady-state and transient targets suggested that the regional K(v) of the Maquoketa Formation is 1.8 x 10(-11) m/s. The core-scale measurements of the K(v) of the Maquoketa Formation range from 1.8 x 10(-14) to 4.1 x 10(-12) m/s. Flow through some additional pathways in the shale, potential fractures or open boreholes, can explain the apparent increase of the regional-scale K(v). Based on well logs, erosional windows or high-conductivity zones seem unlikely pathways. Fractures cutting through the entire thickness of the shale spaced 5 km apart with an aperture of 50 microns could provide enough flow across the aquitard to match that provided by an equivalent bulk K(v) of 1.8 x 10(-11) m/s. In a similar fashion, only 50 wells of 0.1 m radius open to aquifers above and below the shale and evenly spaced 10 km apart across southeastern Wisconsin can match the model K(v).

Large-scale impact of CO2 storage in deep saline aquifers: A sensitivity study on pressure response in stratified systems

“…The (homogeneous) properties of all sealing layers are representative of shale formations suitable for trapping CO 2 , with small permeability and high capillary entry pressure for supercritical CO 2 . With the focus on the multilayer impact of CO 2 injection, which depends strongly on the properties of the sealing layers, we have varied seal permeability over a wide range: k s = 1.0 Â 10 À16 to 1.0 Â 10 À21 m 2 -based on shale permeabilities reported in Neuzil (1994), Domenico and Schwartz (1998), Hovorka et al (2001), and Hart et al (2006) plus one case with an impermeable seal for comparison. Notice that the van Genuchten model was used to calculate the capillary pressure and relative permeability of the two-phase flow in all simulation cases (Van Genuchten, 1980).…”

Research project on CO2 geological storage and groundwaterresources: Large-scale hydrological evaluation and modeling of impact ongroundwater systems

“…to 10 -16 m 2 (Neuzil, 1994;Domenico and Schwartz, 1998;Hovorka et al, 2001;Hart et al, 2006). Hart et al (2006) recently measured the permeability of shale aquitards of the Maquoketa Formation in Wisconsin, and found the permeability to range from 2.5 10 -21…”

“…In this case, the saline aquifer acts like a "semi-closed" system ( Figure 3, bottom), allowing some fraction of the displaced brine to migrate into and through the overlying and underlying sealing units, which in turn would increase the storage capacity for CO 2 . The importance of this vertical inter-layer communication will mostly depend on the permeability of the seals, which can vary widely depending on their hydrogeological characteristics (e.g., Neuzil, 1994;Domenico and Schwartz, 1998;Hovorka et al, 2001;Hart et al, 2006 The quick-assessment method is based on the fact that the injected CO 2 needs to displace native brine of an equivalent volume, and that this equivalent volume is comprised of three volume contributions that can be easily calculated: (1) the additional pore volume within the storage formation provided by pore and brine compressibility in response to pressure buildup; (2) the additional pore volume within the sealing units provided by pore and brine compressibility in response to pressure buildup; and (3) With these goals in mind, this section is structured as follows: Section 3.2 introduces the hypothetical formation-seal systems analyzed and discusses the different sensitivity cases with varying geometric and hydrogeologic properties; Section 3.3 describes the quickassessment method for estimating the pressure buildup and storage capacity in closed and semi-closed systems; Section 3.4 gives simulated results showing the observed pressure buildup and CO 2 plume evolution for a range of conditions; and Section 3.5 demonstrates the applicability and validity of the quick-assessment method through comparison against the simulated results.…”

Inactive supply wells as conduits for flow and contaminant migration: conditions of occurrence and suggestions for management

“…Hanson et al (2004) modeled groundwater flow in the Santa Clara Valley of California and estimated that approximately 19% of flow from shallower to deeper strata occurred through wells. Hart et al (2006) investigated potential flow mechanisms responsible for a discrepancy between core-scale and regional estimates of vertical conductivity for an aquitard in Wisconsin, USA. Using a combination of numerical and analytical modeling, they found that flow through a single well could be approximately 700 l/min and a spacing of one well every 10 km would be sufficient to account for the difference in flow implied by the different estimates of hydraulic conductivity.…”