2.6 Overlying and adjacent aquifers

Generally, the degree of mineralisation of formation waters increases with increasing depth, although locally other settings may occur, e.g. in deep karst or in arid and coastal environments. Hence, overlying and adjacent Aauifers may comprise saline and/or freshwaters. Measuring the geochemical evolution of subsurface formation waters is one tool to directly detect the potential impact of leaking CO2, brine or other fluids into overlying or adjacent aquifers. These measurements require fluid sampling on a regular basis. monitoring could be undertaken in boreholes that penetrate the reservoir or in monitoring wells that penetrate overlying formations. Measurements could include parameters, such as: pH, alkalinity, HCO3-, dissolved gases, hydrocarbons, major and minor elements, TIC, TOC, stable isotopes, redox potential, specific conductance, TDS, density, natural and introduced tracers. It is important to design the sample retrieval system that will conserve the properties that are required for analysis. Fluid mixtures (CO2, brine plus any other relevant fluid or hydrocarbons) will density-separate in the wellbore, and this fractionation will increase as fluids move upward through tubing and gases expand and become less dense. Temperature and solubility relationships will also change, for example gas in solution will evolve. If needed, several techniques can be used to reduce these complications.

Extraction of fluids is labour-intensive, requiring a gas lift or pumping system except where pressure or gas saturation are high enough to lift fluids to the surface. Commercial downhole sampler systems can be deployed on wireline or slickline to collect samples at reservoir pressure and temperature and then conserve this volume during transport to the surface. Well drilling and completion may cause contamination of the near wellbore environment with allochthonous fluids that must be reduced and corrected for.

A U-tube sampler was designed for the Frio Project that allows samples to be returned to surface at near reservoir pressures (Freifeld and Trautz, 2006). The U-tube is composed of a double length of high pressure stainless steel tubing with a check valve open to the reservoir. Formation fluid is collected in the U-Tube, driven at reservoir pressure into evacuated sample cylinders at the surface by high pressure, ultra-pure nitrogen. Free gas in the sample and gases coming out of solution are pumped from the top of the gas separator through a quadrupole mass spectrometer analyser and a landfill gas analyser to measure changes in gas composition in the field (Fig. 2-18). Geochemical analysis must also be matched to the analysis requirements, which may require measurement of gas and liquid fractions at known pressure and temperature, collection of field parameters, filtration, stabilisation, labelling, storing, and shipping of samples.

Investigating aqueous geochemistry provides detailed information needed to confirm model predictions on CO2 migration and potential leakage pathways. In particular, it is the only technique available that has promise to document dissolution and mineral trapping or, conversely, any geochemical interactions that may lead to increased risk (e.g. damage to formation, confining zone, or engineered system).

Fig 2-18-neu

Fig. 2-18: Schematic drawing of the U-tube sampling technology (Freifeld and Trautz, 2006).