Reservoir compaction: What role does petrographic heterogeneity play in the Groningen Gas field?

Abstract

Fluid extraction from geological formations for purposes of subsurface utilization (e.g. hydrocarbon production, fluid storage, geothermal energy production) leads to pore pressure drop in reservoirs. The weight of the rock layers above the reservoir is partially carried by both the reservoir pore pressure and by the reservoir rock itself. Therefore, if fluids are extracted from the subsurface, the reservoir will experience an increase in compressional stress, which may lead to compaction of the reservoir rock. One type of reservoir rock that are highly susceptible to diagenetic processes and compaction due to pore pressure changes are porous sandstones. As the compressional strength of sandstone reservoirs is directly related to the petrographic composition of the rock, understanding the impact of mineralogical composition and textural relationships on reservoir compaction is key. An example of a sandstone reservoir where production related compaction occurs and is associated with surface subsidence and seismicity is the Groningen gas field, situated in the north-eastern part of the Netherlands. However, a detailed model for the reservoir petrography does not exist for the Groningen gas field. The aim of this study is to identify petrographic controls that have an impact on geomechanical behaviour of the gas field by means of optical microscopy (OM) and scanning electron microscopy (SEM) in order to develop a predictive petrographic model. Grain properties, grain displacement, grain contacts, packing texture and paragenetic sequences are studied on a selection of cored wells in the gas field. Mineralogical composition and diagenetic history is determined by OM and its subsequent impact on sandstone compaction. Different phases of clay have been identified by FESEM and EDS that surround clays and occupy the pore space, which locally inhibits cementation of quartz, feldspar or dolomite. Therefore,  the timing and extent of clay growth likely play an important role for the geomechanical stability of the reservoir sandstones. This project will contribute to our understanding of the reservoir heterogeneity of the Groningen gas field and improves our knowledge of subsurface response to subsurface utilisation.