Reservoir and Cap Rock Monitoring

Abstract

One aim of the CLEAN project was to develop and test monitoring methods for the reservoir cap rock and the reservoir itself. It is shown here that advanced injection and production profile evaluation can be achieved using a combination of pressure, temperature and spinner flow meter data. Using distributed temperature sensing, temperature profiles in gas-filled wells can be acquired, as the sensor cable can be stationary during the measurement allowing for simultaneous thermal equilibration along the entire logged profile. A first field test of the developed hybrid wireline logging system was successfully performed under static conditions and the feasibility of warm-back monitoring was shown based on the results of numerical simulations for a possible CO₂ injection scenario. A combined approach using the developed hybrid system for enhanced production logging during injection followed by warm-back monitoring within a subsequent shut-in period would allow for accurate determination of the spatial extent and injectivity of individual CO₂ injection intervals.

Application of the PNG (pulsed neutron gamma) method for estimation of saturation changes is hampered under the considered conditions because of the low contrast between CO₂ and natural gas. However, considering the effect of drying-out of the bound water with associated salt precipitation, which is expected to occur close to injection wells, could be monitored with PNG logs depending on the volume of bound brine.

In preparation to seismic MSP/VSP experiments numerical models were calculated, which helped to find boundary conditions for the survey design. In this framework it was possible to identify the receiver depths with largest amplitude changes (more than 5 %) depending on the offset, the dimension of density and velocity changes and the expansion radius of the CO₂ front. As expected, the amplitude changes during the replacement of the reservoir gas by CO₂ are very small. For a direct detection of the CO₂ front using the seismic wave field, large pressure and/or temperature changes would be required. The concentration of an active seismic monitoring in the Altmark must be focused on the appearance of possible leakages, which cause larger velocity changes. Tests with synthetic datasets showed that seismicity location with the diffraction summation method is possible. For this purpose four stations or rather four boreholes are necessary.

For CO₂ injection projects, stable isotopes and isotope mass balance calculations are new and promising tools to monitor migration of CO₂ plumes and to accurately quantify the amount of CO₂ dissolved in water respectively. The baseline monitoring of the Altmark site yielded a good overview of baseline variations of the DIC (dissolved inorganic carbon) and isotope values, which are clearly distinguishable from the CO₂ to be injected. Laboratory results revealed theoretical trends. They indicate that with high pCO₂, δ¹³C_DIC and δ¹⁸O$_{{{{\rm{H}}_2}{\rm{O}}}}$ add up as valuable monitoring tools for different respective intervals of DIC concentrations. In these applications of isotope monitoring, fractionation plays a major role as they influence the isotope ratios resulting from dissolution and speciation processes. Thus, the determination of their temperature and pressure dependence as well as the influence of salinity are important to hone isotope techniques to serve as accurate tools for the assessment of CCS reservoirs before and after injection.

The microbial community of the 3.5 km deep nearly depleted gas reservoir was analysed by molecular genetic techniques. Comparative analysis of the fluid samples from three different wells, which differ in their temperature profiles and operative history are presented. DNA fingerprinting indicated the presence of microorganisms similar to previously identified microbes from thermophilic and anaerobic environments in deep hypersaline and hot reservoir environments. Phylogenetic analyses revealed a high microbial diversity including different H₂-oxidising bacteria (hydrogenophaga, acidovorax, ralstonia and pseudomonas), thiosulfate-oxidising bacteria (diaphorobacter), dissimilatory metal reducers (pantoea), aromatic-degrading deep sediment inhabitants, (sphingomonas), extremophiles (bacillus), and biocorrosive thermophilic microorganisms. Additionally several sequences of microorganisms similar to representatives from different saline, hot, anoxic, deep environments were detected, which had not been cultivated previously. Cell numbers of SYBR Green total cell counts were less than 102 cells/ml and the cells were usually attached to particles.