K.U. Weyer and James C. Ellis
Text version of poster presented at the Hydrogeology of Arid Environments 2012 conference, Hannover, Germany, March 14-17, 2012

© 2012, WDA Consultants Inc.


Introduction

The injection of CO2 at ‘In Salah’ (Krechba gas field) in the Algerian Sahara tests the behaviour of the sequestered CO2 in the subsurface. Figures 1 and 2 depict the occurrence of the Krechba gas reservoir in approximately 1850 m depth in 20 m of Carboniferous sandstone. It is overlain by 900 m of Carboniferous mudstone, 700 m of Lower Cretaceous sandstone, an aquifer, and 200 m of Middle and Upper Cretaceous mudstone. In a newly drilled observation well the water level from the aquifer rose to about the middle of the overlying Cretaceous mudstone. When drilling through the Carboniferous mudstone, loss of circulation was frequently encountered in the upper 400 m and lower 200 m of the mudstone, a caprock in oil field terminology, an aquitard in hydrogeological terminology. It is under debate whether the circulation losses were caused by pre-existing fractures or by hydraulic fracturing (Iding & Ringrose, 2009).


Figure 1: InSAR data of average distance change (close to vertical displacement) evaluated by TRE from August 2004 to March 2007 (from Rutqvist et al., 2010, Fig. 2).


Figure 2: General geology and technical installations at the Krechba gas reservoir (from Rutqvist et al., 2010, Fig. 1).

The Cretaceous aquifers of the Tademait Plateau belong to the ‘Aquifère du Continental Intercalaire’ system (Castany, 1982). Traditionally much of the groundwater flow in the Sahara Basin was seen as originating in the Atlas Mountains and shown to underflow the Tademait Plateau partially from NE to south and partially from NE to SW (Ben Dhia, 1991, his Fig.4 and 5). The guiding concept was the conceptual model that groundwater flow would be limited to aquifers themselves and, in this case, to an aquifer system with an outcrop and thereby recharge area in the Atlas Mountains. In Groundwater Flow Systems theory aquitards at the surface (Figure 3) were shown to be natural recharge areas for deeper aquifers by Freeze & Witherspoon (1967). Tóth (1962) had introduced the concept of Groundwater Flow Systems with recharge and discharge areas whereby the penetration depth can exceed 5 km (Tóth, 2009). In a recharge area the flux of groundwater crosses the groundwater table into the saturated domain; in a discharge area the flux of groundwater is directed from the groundwater body into surface waters or to the surface for evaporation.

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