Compaction of granular carbonates under conditions relevant to diagenesis and fault sealing. Geologica Ultraiectina (332)

Abstract

Carbonate reservoir rocks contain more than 60% of the world’s oil reserves and 40% of its gas reserves. The evolution of the reservoir quality, i.e. their porosity and permeability, is for a large part controlled by compaction due to pressure solution (chemical compaction). Pressure solution also forms an efficient mechanism of fault sealing in carbonate rocks. Moreover, during hydrocarbons production, and after injection of CO2 into carbonate reservoirs, pressure solution may lead to vertical compaction creep and surface subsidence. Quantifying pressure solution compaction in carbonate rocks is therefore a subject of substantial practical and economic importance for hydrocarbons exploration and production, and for assessing the safety of geological storage of CO2 in depleted carbonate reservoirs. This thesis reports the results of an experimental study of pressure solution compaction of granular calcite aggregates under hydrocarbon reservoir conditions. The aims were to verify the operation of pressure solution by reproducing pressure solution microstructures, to determine the effects of variables such as stress, temperature, grain size, pore fluid chemistry and fluid flow on the rate of the process, and to reveal and quantify the rate limiting mechanism of pressure solution. An experimental procedure was developed to isolate pressure solution from mechanical compaction, by applying pre-compaction at lab dry samples. No or minor creep could be measured in the pre-compacted samples when reloaded with pore fluid of various hydrocarbons. The pressure solution creep tests were performed on dry pre-compacted samples with pore fluids of CaCO3 saturated solution. Fine-grained calcite or natural carbonate materials were used and the experimental conditions either at room temperature and 1-4 MPa applied stress or at 150 ºC and effective stresses of 20-47 MPa. Typical microstructures for intergranular pressure solution were reproduced. These consisted of indented and sutured grain contacts, over growths, and dissolution pits and precipitated layers developed on reference crystal surfaces inserted into the granular samples. Compaction creep rates were found to be always inversely proportional to grain size and increase with increasing of effective stresses. Temperature had only a minor positive effect on compaction rates in the temperature range up to 150 ºC. Pore fluid salinity ranging from 0.1-0.5 M NaCl increased the pressure solution compaction rate. Addition of common impurity ions of Mg2+ and phosphate seen in formation water and seawater to the pore fluids drastically slowed down the compaction creep. Purer calcite samples achieved much larger strains than impure limestone samples. Flow-through promoted creep rates at larger strains (> 5%) while no effects on strain rates occurred at lower strains. Measurements of the Ca2+ concentration from compacting samples in intermittent flow-through runs revealed a build up to high super-saturations of CaCO3 during compaction, notably at high strains. Evidence showed that pressure solution is the dominant mechanism for compaction creep and diffusion along grain boundary is the rate limiting step at low strains becoming precipitation controlled at higher strains (>5-7%). The experimentally derived values of the diffusion product DS (diffusion coefficient times effective thickness of water film) for IPS in calcite at low strain is between 2.9810-18 to 3.7310-19 m3/s at 150 C. As impurity ions like Mg2+ and HPO42- can drastically slow down IPS due to the effect on the precipitation reaction, it is likely that IPS in calcite in nature is much slower than seen in Lab experiments on pure systems.