Abstract
Abstract. Cementation of potential reservoir rocks is a geological risk, which may strongly reduce the productivity and injectivity of a reservoir, and hence prevent utilisation of the geologic subsurface, as it was the case for the geothermal well of Allermöhe, Germany. Several field, laboratory and numerical studies examined the observed anhydrite cementation to understand the underlying processes and permeability evolution of the sandstone. In the present study, a digital rock physics approach is used to calculate the permeability variation of a highly resolved three-dimensional model of a Bentheim sandstone. Porosity-permeability relations are determined for reaction- and transport-controlled precipitation regimes, whereby the experimentally observed strong decrease in permeability can be approximated by the transport-limited precipitation assuming mineral growth in regions of high flow velocities. It is characterised by a predominant clogging of pore throats, resulting in a drastic reduction in connectivity of the pore network and can be quantified by a power law with an exponent above ten. Since the location of precipitation within the pore space is crucial for the hydraulic rock properties at the macro scale, the determined porosity-permeability relations should be accounted for in large-scale numerical simulation models to improve their predictive capabilities.
Highlights
Mineral dissolution and precipitation are micro-scale processes, which may significantly alter the internal rock structure and affect the effective hydraulic behaviour of the system at the macro-scale
The geochemical reaction regime strongly impacts the evolution in permeability of the digital rock sample as illustrated by the large variations between transport-controlled and reaction-controlled porosity-permeability relations
The observed strong decrease in permeability can be approximated by the simulated porositypermeability relation for a transport-controlled precipitation, assuming mineral growth in regions comprising flow velocities > 75th percentile (Fig. 2b)
Summary
Mineral dissolution and precipitation are micro-scale processes, which may significantly alter the internal rock structure and affect the effective hydraulic behaviour of the system at the macro-scale. Predicting changes in rock permeability is of paramount importance for most applications related to geologic subsurface utilisation (Jacquey et al, 2015; Kleinitz et al, 2001; Regenspurg et al, 2015), especially regarding the productivity and injectivity of a reservoir. Pape et al (2005) analysed the observed local clogging phenomena due to anhydrite precipitation in the porespace of related drill hole samples and studied the pore size geometry. They distinguished between two facies types with different diagenetic history: (1) a fine-grained and uncemented sand with small pores, mechanically compacted during diagenesis and (2) a coarser sand with larger pores, almost completely cemented by anhydrite after its compaction
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