Enhancing Petrophysical Property Prediction from Well Logs Using Digital Rock Physics Technique - Case Study of a North Sea Sandstone Reservoir

Abstract

Abstract This paper presents a field case study, which integrates Digital Rock Physics (DRP) techniques and downhole logging measurements, to enhance the prediction of formation absolute permeability and drainage capillary pressure from well log data. The approach first generates numerically a digital rock model representing the formation at each given depth of interest, and then uses the model as input to simulate fluid flow and capillary-dominated fluid displacement for the determination of capillary-pressure curve and absolute permeability. Rock models are constrained by formation parameters derived from logging data, and created by simulating the way the clastic rock formed over millions of years, with periods of grain sedimentation, compaction and diagenesis. All input parameters for the rock modeling including the density, porosity, grain mineralogy, grain-size distribution (GSD), cement and clay content, are obtained directly or indirectly from downhole logging measurements. The method's feasibility and applicability are demonstrated by applying the DRP modeling technique to one set of field data from a North Sea well contained a complete suite of logs. Cores are extensively taken from the well-consolidated sandstone formations. Laboratory measurements provide the measured permeability and capillary pressure for comparison. At various depths, the predicted permeability of rock models is in good agreement with the measured values of core sampes from the same depth, and mostly falls within the same order of magnitude as the measurements. Compared to Nuclear Magnetic Resonance (NMR)-based Coates model, the DRP-based approach gives a better predicted permeability when both methods use default values in their parameters. In addition, three depths representing different sandstone zones are randomly selected to investigate the effect of surface relaxivity on the predicted GSD, permeability and capillary pressure of rock models. When the surface relaxivity of 3 μm/s is used in this study, the DRP permeability at these depths matches very well with core measurements as well as the mobility calculated from formation tester measurements. Also, their entry capillary pressures are consistent with those from core measurements. However, the simulated pressure curves differ in shape from the measured ones, which may be attributed to limited variation of grain sizes used in the rock modeling. The promising results stemming from this study confirm the feasibility of combining the DRP technique with downhole logging measurements to accurately and reliably predict formation petrophysical properties in a timely and cost-effective manner.