Reservoir Connectivity, Water Washing and Oil to Oil Correlation: An Integrated Geochemical & Petroleum Engineering Approach

Abstract

Oil Fingerprinting by gas chromatography techniques are among the most sensitive and accurate tools utilized to study reservoir compartmentalization and oil to oil correlations. The greatest challenge of the technique, however, lies in recognizing and identifying if any oils have undergone post-generative alteration processes such as water washing or biodegradation that may prohibit accurate correlation. These effects, even when subtle, may alter or transform oils resulting in misleading interpretations and invalid outcomes. Understanding the controls on the oil composition is critical for all fingerprinting studies and can promote new characterization methods to ensure any negative post-generative alteration effects are mitigated. The study aims to illustrate the use of various geochemical fingerprinting methods to assess the reservoir connectivity between two oil accumulations. Furthermore, the effect of water washing on the oil compositions, and its relationship with gas to oil ratio (GOR) and salinity were investigated. A total of 11 oil samples from an Ordovician siliciclastic formation across 10 locations in a field were analyzed for API gravity light hydrocarbons (C5-C7) and whole-oil fingerprinting (C8 to C20) gas chromatography (GC) characterization methods. Light hydrocarbons (LHC) analysis used to correlate different oils to their sources was interpreted using five specially selected source dependent ratios plotted in a C7 star diagram. If the oils have a similar pattern, this indicates a similar source and vice versa. The results suggest that the oils can be correlated to two different source rocks that charged the study area from independent northeast and southwest directions. A second set of light hydrocarbon ratios sensitive to water washing and biodegradation effects suggested a noticeable water washing trend increasing to the north. The whole-oil fingerprinting analysis employs a multivariate statistical model across all the samples to determine the most variant 12 ratios from the chromatograms to construct a specialized star diagram. From this analysis, five separate reservoir compartments were identified. It was further observed that a set of samples from a specific compartment differed in one of the 12 ratios. This ratio was plotted against the water washing transformation ratio from LHC and revealed a strong positive correlation. The difference in this ratio is attributed to water washing. Both parameters suggest that water washing possesses strong negative correlations with total dissolved salts (TDS) of the formation water and gas to oil ratio (GOR). These relationships accentuate the potential of utilizing the geochemical ratios to predict the GOR and consequently improve the production planning. The study shed the lights on the potential utilization of new rapid and cost-effective geochemical methods to predict some production engineering parameters.