Abstract A UK repository concept currently under consideration for the disposal of intermediate-level radioactive waste and some low-level waste not suitable for surface disposal involves using large quantities of cementitious materials for construction, grouting, waste containers, waste isolation matrix and buffer/backfill. CO 2 generated from the degradation of organic material in the waste will result in cement carbonation and associated mineralogical changes. Hydraulic and gas permeability tests were performed on Nirex Reference Vault Backfill (NRVB) cement at 40 °C and either 4 or 8 MPa. Carbonation reactions using CO 2 gas halved the permeability of the NRVB under simulated repository conditions. A greater decrease in permeability (by three orders of magnitude) was found during carbonation using dissolved CO 2 . Mineralogical changes were found to occur throughout the cement as a result of the reaction with CO 2 . However, a narrow zone along the leading edge of a migrating reaction front was associated with the greatest decrease in porosity. Fluid pressures increased slightly due to permeability reductions but fluid flow still continued (albeit at a lower rate) preventing the build-up of overly high pressures. Overall, the observed reductions in permeability could be beneficial in that they may help reduce the potential for fluid flow and radionuclide migration. However, continued carbonation could lead to potential issues with regards to gas pressure build-up.

Abstract Our objective—to summarize the effects of carbonate diagenesis on seismic records—is directly linked to an industry goal of continually improving our capability of seismically imaging reservoir, seal, and hydrocarbon distributions. Carbonate diagenesis affects reservoir and seal distributions mainly through processes of differential leaching, compaction, and mineralogic replacement. The results of previous studies (both in-house and published) indicate that all processes of carbonate diagenesis are detectable by conventional seismic techniques if the combination of spatial scale and net rock property change is sufficient. The best results are seen when large changes in rock properties occur over large volumes of rock, such as in basinwide, depth-related mineralogic phase changes or in spatially extensive subsurface leaching. In many actual exploration cases, however, the seismic signatures of carbonate diagenesis are too subtle for easy detection, either because the rock-altering process acted over too small a volume or because the resultant changes in rock properties were too small. This seismic detection problem is modest at burial depths of 1000 m but is serious at burial depths ≥4000 m. Improved resolution of diagenetically altered carbonates must come through (1) increased use of higher resolution seismic techniques, such as crosswell tomography, and (2) more rigorous integration of rock, log, and seismic data in a data management environment that allows for iterative model testing, such as 3-D visualization software.

Abstract This paper describes how meteoric cementation enhanced the hydrocarbon trapping and/or producing potential of three limestones. Petrophysical effects of meteoric diagenesis on carbonates vary between two perfect end members of pure seal formation and pure reservoir enhancement . Net porosity and permeability changes are inferred to be a simplistic function of water availability and the exposed terrane’s chemical reactivity Meteoric tight zones form under conditions of low water availability and high terrane reactivity (e.g., a semi-dry climate exposure of Mg-calcite sediment). Solution-enhanced reservoirs form under conditions of high water availability and low terrane reactivity (e.g., a rain forest exposure of stoichiometric dolomite). Examples of meteoric tight zones are shown in cores from west Texas, offshore China, and central Oman. Petrographic and geochemical data were used to define the causes of reservoir degradation. From an exploration/ exploitation standpoint, these intervals form potential top-seals for hydrocarbon trapping and/or intraformational permeability barriers that compartmentalize hydrocarbon production. More generally, meteoric tight zones may be a critical trapping factor in many similar hydrocarbon accumulations—both producing (but not recognized as such) and prospective. A more thorough investigation through the current inventory of fields might show meteoric seal formation is as economically important in trap formation as its much better studied “karsting” counterpart. Either end member should be easily recognized by its unusual petrographic and geochemical signature and overwhelming petrophysical effect on the rock. Methodical searches for meteoric diagenetic traps should be most productive in areas with moderate drilling density (for rock control), relatively simple facies distributions (to minimize facies prediction problems), and accentuated paleotopography (where cross-formational meteoric tight zones can form appropriate trapping geometries). Drowning successions should also be more prospective because of a greater incidence of ephemeral exposure of unstabilized sediments during brief sea level drops.