Abstract
The mechanical behaviour of sand aggregates is often studied as a proxy for poorly consolidated sands and highly porous sandstones. Only recently research aimed at understanding sand deformation has started to use techniques that allow for direct observation of the in-situ grain-scale processes. Using state-of-the-art, time-lapse micro X-ray computed tomography (micro-XCT) imaging, the influence of mineralogy on the compaction of sand aggregates has been investigated by performing uniaxial compaction experiments on four different mineral assemblies (quartz, K-feldspar, quartz + K-feldspar and quartz + K-feldspar + clay) at room temperature and dry conditions. For the experiments, a bespoke uniaxial compaction device (sample diameter 2 mm) was constructed and coupled with micro-XCT imaging. This enabled in-situ observation of the strain-accommodating processes during deformation. To verify that the microstructural evolution observed in the small-scale experiments is representative for larger aggregate behaviour, conventional, centimetre-sized, control experiments were performed. The observed inelastic deformation was mainly accommodated by processes such as intragranular cracking and intergranular sliding. At low axial stresses (10 MPa), grain fracturing mainly occurred in K-feldspar grains, if present, along cleavage planes. Only at higher axial stresses, fracturing of quartz grains, if present, was also observed. Presence of clays, in pores and grain contacts, delayed the onset of quartz grain breakage and enhanced porosity reduction as clay in grain contacts facilitated grain sliding and rearrangement.
Highlights
Many hydrocarbon reservoirs consist of highly porous, unconsolidated sands [1] or poorly consolidated sandstones [2]
Studies have shown that compaction at the reservoir level is not fully recoverable, but can involve significant permanent deformation [2,7,8]. The latter is caused by grain-scale processes, such as intergranular and intragranular fracturing, mass transfer processes, and/or frictional sliding along grain contacts, potentially aided by the deformation of thin intergranular clay films [2,3,7,8]
It should be noted that over half of the total axial strain is accumulated during the initial 20 MPa axial stress. This is caused in part by settling of the Teflon liner at low applied stresses (Fig. 4), as liner-free experiments did not show this slow initial ‘onramp’, though they suffered from grain embedment into the PEEK vessel
Summary
Many hydrocarbon reservoirs consist of highly porous, unconsolidated sands [1] or poorly consolidated sandstones [2]. Differential compaction across faults can result in induced seismicity [5,6]. Studies have shown that compaction at the reservoir level is not fully recoverable (i.e. poro-elastic), but can involve significant permanent (i.e. inelastic) deformation [2,7,8]. The latter is caused by grain-scale processes, such as intergranular and intragranular fracturing, mass transfer processes, and/or frictional sliding along grain contacts, potentially aided by the deformation of thin intergranular clay films [2,3,7,8]. It is important to understand and quantify the grain-scale mechanisms leading to reservoir compaction
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
