Cap Rock Entry Pressure as Function of Rock Type and the Relation with NMR
The main objective of this integrated petrophysical laboratory and geology study of a mixed carbonate and clastic caprock was to determine the relation between rock-type and threshold pressure. The data was used to assess if reservoir pressure of the UGS site could be raised to desired levels. A secondary objective was to determine the relation of entry pressure with NMR logs.
- Core description, X-ray diffraction and thin section petrography to determine cap rock facies types and to interpret the depositional setting.
- CT-scanning of whole core and selected core plugs covering each lithofacies
- Pore characterization using mercury injection (MICP), Scanning Electron Microscopy (SEM) and thin section
- Gas entry pressure measurement, Pulse Decay Nanopermeability and Laboratory Nuclear Magnetic Resonance (NMR) to measure threshold pressure, permeability, porosity and T2 relaxation time.
- Interpretation and integration of laboratory and geological data and correlation with wireline and NMR logs
- Threshold pressure is controlled by cap rock type. Each rock type having a distinct pore size distribution and pore characteristics linked to grain size and clay content. Highly variable threshold pressure was found in diagenetically overprinted facies.
- T2 log mean values are poorly correlated to threshold pressure because these values represent only an average pore throat diameter, whereas the largest pore throat determines the entry pressure. The larger pores are represented by the longer NMR T2 curves and the T2 cut-off at 95% showed a strong correlation with entry pressure and can be used to correlate with NMR logs.
- The study has led to a thorough understanding of the sedimentology and textural characteristics of the caprock; especially the lithotypes that ultimately define the maximum pressure the rock can withstand. The spatial geometry of this lithologically variable, fluvial-lacustrine depositional system need to be modeled as the facies distribution as well as diagenetic overprint has important implications for seal integrity and ultimately the extent to which the pressure can be raised.