Modelling the hypothetical methane-leakage in a shale-gas project and the impact on groundwater quality

Abstract

The hypothetical leakage of methane gas caused by fracking a 1,000-m deep Cretaceous claystone horizon at Damme, Germany, is simulated in a TOUGH-REACT reactive-transport model with 5,728 elements. A hypothetical leakage zone connects the Cretaceous horizon with a Quaternary potable-water aquifer (q1). Methane gas rises up to the q1 horizon in less than 2 days in all calculated scenarios. The simulations include the major constituents of groundwater as well as the seven most hazardous trace components that are natural constituents of groundwater (As, Cd, Cr, Ni, Pb, Se and U). The general trend is characterised by depletion of the natural hazardous components with decreasing acidity and oxygen fugacity in the relevant pH range (7-9). Nevertheless, the concentrations of elements whose dominant aqueous species are negatively charged in this pH range (Cr and Se) rise against the general trend due to desorption reactions. Slight enhancement effects are produced by the dissolution of contaminant-bearing oxides such as Cr-bearing goethite. In summary, the geological risks of a fracking operation are minor. The technical risks are more important. This is especially the case when rising methane gas gets into contact with fracking fluid that accidentally escapes through faulty well seals.

Trace Element Behavior in Methane‐Rich and Methane‐Free Groundwater in North and East Texas

“…van Breukelen and Griffioen (2004) and van Breukelen et al (2004) observed and modeled an increase in pH and calcite precipitation during anaerobic degradation of methane at the fringe of a landfill plume. Although only a modeling exercise, Schwartz (2015) reported similar results during anaerobic methane oxidation with an increase in pH as well as a drop in most trace element concentrations, consistent with the assumed conceptual model that sorption controls their concentrations.…”

Numerical modeling of fracking fluid migration through fault zones and fractures in the North German Basin

“…Model calculations on methane migration often result in a release to the atmosphere (e.g. Sauter et al 2012;Schwartz 2015). As long as it can migrate as a free phase, methane is driven upward due to its significantly lower density and therefore is more likely to reach surficial aquifers (if continuously permeable structures exist).…”

“…In contrast to most studies published so far, fluid flow is simulated not only in highpermeability zones (e.g. Myers 2012;Sauter et al 2012;Chesnaux et al 2013;Schwartz 2015) but also in open fractures.…”

“…In a reactive transport model, Schwartz (2015) detected a high risk for methane migration caused from fracking in a 1, 000-m deep Cretaceous horizon (Wealden) in the North German Basin. He included a hypothetic leakage zone of very high permeability (k = 10 −8 -10 −10 m 2 ) which cross-cuts the thick clay formation; however, no explanation on the geological background of such a structure was given.…”

“…The model follows a generic approach that is applicable for the North German Basin and comparable geological settings. The study considers flow conditions at basin scale, taking into account (near-natural) regional flow, whereas Myers (2012) and Schwartz (2015) modeled fracking fluid migration at a very small scale, with lateral extensions of 450 × 450 m and 88 × 90 m, respectively. The flow direction in their models is therefore strongly controlled by the proximity to the boundary conditions.…”