Simulating borehole hydraulic cross-connection in a multi-layered fractured sedimentary rock aquifer informed by high-resolution datasets.

Abstract:

Using high-resolution datasets is imperative to develop hydraulically-informed, process-based conceptual site models (CSMs) that can be used to build numerical models to predict groundwater flow system behaviour for various scenarios. Fractured sedimentary bedrock aquifers contain features that enhance flow such as fractures (e.g. joints and bedding partings) and vuggy porosity within the porous rock matrix, creating a complex network of preferential flow features. However, using a suite of discrete fracture network and matrix (DFN-M) methods has enhanced the hydrogeological characterization of these aquifers. Some of the DFN-M techniques are conducted in an open borehole that creates a hydraulic cross-connection between aquifer units and disturbs the natural local flow regime. The high-resolution data are used to delineate hydrogeologic units (HGUs) providing an appropriate framework for numerical modelling for simulation of system responses. Groundwater flow is often modelled using the ‘equivalent porous medium’ (EPM) approach, assuming that the fractures are numerous and well-connected such that the hydraulics of the fractured aquifer system can be reasonably represented using porous media parameters. Despite the simplifications, EPM generally provides satisfactory results representative of the local/regional groundwater flow system; however, it is much more challenging to represent contaminant transport.

This study investigates the conditions around a single bedrock borehole cored through a Silurian-aged dolostone aquifer. High-resolution datasets, providing information about lithology and fracture network variability, are used to distinguish several distinct HGUs. This aquifer is an important water supply source regionally, serving approximately 1M people, in southern Ontario, Canada. A suite of novel hydrophysical, geophysical and geological techniques are being deployed in the fractured dolostone aquifer to inform groundwater flow under ambient and open hole conditions. Based on the robust CSM framework informed by the high-resolution data, a 3D numerical model has been developed using FEFLOW (13 layers and 100 m total thickness; areal extent of 100 km²; triangular mesh with131,274 elements and 71,876 nodes). The model is run for several open interval length scenarios (i.e. representing various open well depths) to compare changes in hydraulic head and flow through the HGUs. Thus, the cross-connection effects on HGU response cones and flow re-distributions are demonstrated. Particle tracking is used to emphasize the important influence of hydraulic cross-connection on contaminant distributions and migration pathways. The EPM flow model demonstrates how: 1) the flow conditions are changed at multiple depths around the open borehole, potentially leading to erroneous estimation of natural aquifer conditions if not properly accounted for; and 2) these high-resolution data can inform both the natural and altered conditions.