Anhydrite cementation and compaction in geothermal reservoirs: Interaction of pore-space structure with flow, transport, P-T conditions, and chemical reactions

In order to analyze local clogging phenomena due to the precipitation of anhydrite in the pore-space we performed a petrophysical study on cores and data from three boreholes probing the Rhaetian hydrothermal aquifer in northern Germany (Allermöhe, Neuruppin 1/88, Neuruppin 2/87). The pore size geometry of sandstones was studied using, among other methods, pulsed field gradient-nuclear magnetic resonance (PFG-NMR). We found two facies types with clearly different diagenetic history: (1) a fine-grained sand with small pores, mechanically compacted during diagenesis into an average sandstone of fractal pore geometry; (2) a coarser sand with larger pores, almost completely cemented by anhydrite after compaction had reduced the pore-space to a porosity of 30%; thus no further mechanical compaction occurred and the pores remained smooth. In contrast, the fine-grained facies was not cemented, possibly because anhydrite crystal nuclei are unstable in pores of insufficient size. With regard of these two facies types we developed: (1) an approach for calculating vertical logs of anhydrite and permeability content from gamma density measurements in boreholes or on core; (2) a chemical reaction model to calculate the amount of precipitation and dissolution of each mineral species at high temperature and salinity. These petrophysical and geochemical models are integrated into the numerical simulation tool SHEMAT for simulating coupled flow, heat and species transport, and chemically induced permeability changes. A comparison of the diagenesis of the cemented and un-cemented facies types yields an improved understanding of the geological conditions required for anhydrite cementation. Specifically, with regard to the studied Rhaetian sandstones, numerical reactive flow simulations demonstrate that intense, local anhydrite cementation may occur when saline formation waters mix with hot brines ascending on faults.