Utilizing Core Fluorescence to Reduce Uncertainty in the Petrophysical Interpretation of Thin and Low Relief Carbonate Oil Reservoirs.

Abstract

The water saturation (Sw) interpretation from resistivity logs has been observed to be highly uncertain in Oilfield A. By integrating core fluorescence observations from underutilized ultraviolet (UV) light core photography images, it is possible to QC these interpretations in challenging thin reservoirs, thinly bedded reservoirs, compacted reservoirs and low relief structures with significant oil volumes at intermediate oil saturation. High-quality core is routinely taken in appraisal wells in Oilfield A. A standard core analysis work programme has been applied to these cores including core gamma, Conventional Core Analysis (CCA), Whole Core Analysis (WCA), Mercury Injection Capillary Pressure (MICP) and core photography. Core photography includes both white light and UV imaging. A detailed core description of the cores has subsequently been made. This includes the description of core fluorescence in UV core photography images which show varying intensities of hydrocarbon saturation. The core fluorescence description is integrated with all other available well data including the log Sw interpretation. The results highlight the uncertainty in the conventional resistivity-based determination of Sw. As part of the modeling workflow in Oilfield A, log Sw interpretation and the proposed saturation height model (SHM) were QC’ed by integrating all available data from appraisal wells. By comparing hydrocarbon core fluorescence from UV light core photography images to the log Sw interpretation, the following results were possible: High resistivity "shoulder bed" effects, resulting from the low-resolution resistivity log averaging the juxtaposition of low porosity non-reservoir and reservoir intervals, were removed from petrophysical interpretation. Core fluorescence from UV light core images was observed to be intermittent on a foot-to-decifoot scale in Reservoir 5 due to cementation associated with chemical compaction features. The high-resolution core fluorescence description was used to challenge the continuous hydrocarbon saturation interpreted from logs in Reservoir 5. The integration of these observations allowed reconciliation with results from repeat formation tester pump-outs and well tests, enabling a more realistic Sw distribution in the reservoir model. Hydrocarbon saturation from log Sw was interpreted to continue deep into several reservoirs in Oilfield A. The core fluorescence description from UV light core photography showed hydrocarbons were only present at the top of these reservoirs. In reservoirs with homogeneous porosity profiles but vertically changing rock-types, the resistivity log is not able to distinguish high resistivity caused by hydrocarbon saturation from high resistivity caused by cementation and/or reduced permeability. The finding challenges using the same values of ‘m’ and ‘n’ in Archie log Sw interpretation throughout a reservoir. Integration of hydrocarbon core fluorescence from UV light core images added significant value to the understanding of Sw distribution in Oilfield A. Apparently contradictory data from repeat formation tester pump-outs and well tests were reconciled, and previous hypotheses were challenged by integrating this underutilized data type. Simple steps are used to integrate these data to reduce Sw uncertainty in heterogeneous carbonate reservoirs, particularly those with a large proportion of STOOIP at intermediate oil saturation due to their low-relief structure.