Hydraulic and Mechanical Impacts of Pore Space Alterations within a Sandstone Quantified by a Flow Velocity-Dependent Precipitation Approach

Maria Wetzel,Michael Kühn,Thomas Kempka

doi:10.3390/ma13143100

Abstract

Geochemical processes change the microstructure of rocks and thereby affect their physical behaviour at the macro scale. A micro-computer tomography (micro-CT) scan of a typical reservoir sandstone is used to numerically examine the impact of three spatial alteration patterns on pore morphology, permeability and elastic moduli by correlating precipitation with the local flow velocity magnitude. The results demonstrate that the location of mineral growth strongly affects the permeability decrease with variations by up to four orders in magnitude. Precipitation in regions of high flow velocities is characterised by a predominant clogging of pore throats and a drastic permeability reduction, which can be roughly described by the power law relation with an exponent of 20. A continuous alteration of the pore structure by uniform mineral growth reduces the permeability comparable to the power law with an exponent of four or the Kozeny–Carman relation. Preferential precipitation in regions of low flow velocities predominantly affects smaller throats and pores with a minor impact on the flow regime, where the permeability decrease is considerably below that calculated by the power law with an exponent of two. Despite their complete distinctive impact on hydraulics, the spatial precipitation patterns only slightly affect the increase in elastic rock properties with differences by up to 6.3% between the investigated scenarios. Hence, an adequate characterisation of the spatial precipitation pattern is crucial to quantify changes in hydraulic rock properties, whereas the present study shows that its impact on elastic rock parameters is limited. The calculated relations between porosity and permeability, as well as elastic moduli can be applied for upscaling micro-scale findings to reservoir-scale models to improve their predictive capabilities, what is of paramount importance for a sustainable utilisation of the geological subsurface.

Highlights

Geochemical fluid-rock interactions such as precipitation and dissolution of minerals alter the microstructure of rocks, and thereby affect their physical behaviour at the macro scale
The permeability is determined from the flow field by solving the steady-state Stokes equation [42], whereas effective rock stiffness is calculated by a static finite element method [43]
Variations in rock morphology are compared for the initial state (φ = 23.4%) and the altered microstructure at a porosity of 15.3%, since the sample is rendered impermeable at lower porosities for the high flow velocities (HFV) scenario

Summary

Introduction

Geochemical fluid-rock interactions such as precipitation and dissolution of minerals alter the microstructure of rocks, and thereby affect their physical behaviour at the macro scale. The prediction of the resulting changes in effective hydraulic and mechanical rock properties is of paramount importance for numerous natural geochemical systems and commercial applications such as geothermal energy production [1,2], hydrocarbon exploration and exploitation [3,4], nuclear waste disposal [5,6] as well as energy and gas storage [7,8,9]. The spatial distribution of mineral nucleation and growth is complex and controlled by various factors including chemistry [16,17], transport properties [18,19], mineralogy [20,21], temperature [22,23] and pore morphology [24,25], whereby the impact of each factor depends on the particular process. A preferential mineral deposition is observed for small pores [16,26] or narrow throats [27] as well as larger pores, which is explained by lower supersaturation thresholds due to interfacial energy effects [25]

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