The Effect of Chemical Environment on the Time-Dependent Compaction Behaviour of Quartz Sands

Hydrocarbons play an important role in the energy system and will remain doing so for some time in the future. However, in some locations, decades of hydrocarbon production have resulted in reservoir compaction and surface subsidence. Such effects can have significant unwanted impact on the environment and surface infrastructure. Aside from poro-elastic effects, which are relatively easily predictable, compaction of reservoir rocks can potentially also be time-dependent due to the time-dependent deformation (creep) of the rock matrix. Previous studies on compaction of both clastic and carbonate reservoir rocks and model aggregates have shown that the chemical environment plays an important role in determining the extent of creep. The aim of this research is to further assess the effect of chemical environment on creep in quartz aggregates.
To do this, we are conducting uniaxial compaction experiments on aggregates of Ottawa quartz sand (d = 212 – 250 μm), at room temperature conditions or at elevated temperature of 75 °C to mimic reservoir conditions. The experiments consist of two stages: 1) load cycling up to 35 MPa effective stress under lab dry conditions, and 2) creep at a constant effective stress of 35 MPa. During the creep stage, the sample is either dry (lab air, dry CO2), or vacuum-flooded (silica-saturated solution, wet CO2, 0.1 M aluminium chloride solution). The pore fluids and gases (except for lab air) are introduced and maintained during the experiment at 10 MPa total pressure.
The experiments show that wet experiments produce more creep (lab air vs silica-saturated solution), while less creep occurs in the CO2 and aluminium chloride experiments compared to the silica-saturated solution experiment. The observations suggest that the time-dependent deformation of the quartz aggregate is mainly related to subcritical crack growth or stress corrosion cracking, resulting in grain failure and subsequent rearrangement of grain fragments. The inhibiting effect of CO2 is inferred to be related to the acidic environment CO2 creates, which reduces the crack growth rate. A similar effect is inferred to occur in the presence of aluminium chloride. Apart from creating an acidic environment, it has been suggested that aluminium ions reduce the solubility of quartz, which further reduces the rate at which subcritical cracking can occur. The results suggest that perhaps acidic additives or CO2 injection may have applications in reducing creep in clastic reservoirs.