Effects of fluid evolution dynamics on reservoir fluid distributions

Abstract

Understanding the evolution of hydrocarbon reservoirs is crucial for fluid mapping and optimizing well placement and reservoir performance. Not fully understanding controls on reservoir fluids leads to inaccurate fluid mapping, identification of false compartments, poor well placement, reservoir underperformance, and wrong reserves. During the evaluation and development planning of a shallow 120-ft-thick black-oil reservoir, nonlinear depth profiles of fluid properties were observed across a 1,200-ft anticlinal closure, forming a gentle trend of increasing viscosity within the top quarter of the oil column, followed by a steeper deterioration with depth. The nonlinear trends presented uncertainties in evaluating vertical fluid connectivity, with direct impact on field development and optimal production strategies. In order to ascertain whether the observed trends represent vertical compartmentalization or disequilibrium due to competing fluid evolution geodynamics within a single fluid system, a systematic workflow was developed, primarily utilizing PVT data to deduce the fill-biodegradation model, integrated with geochemistry, basin modeling, and petrophysical analysis. The reservoir remained below 66 °C throughout its history, exposing it to biodegradation during active charging. Biodegradation in deeper sections consumes the lighter hydrocarbons and increases the heavy-ends, leading to decreased gas-oil ratio, bubble-point pressure, and oil formation volume factor toward the oil-water contact. Conversely, ongoing recharge at the crest introduces lighter components, hence increasing the gas-oil ratio, bubble-point pressure, and oil formation volume factor. The isomer ratio of butane decreases with depth as expected in multiply charged reservoirs but then inflects systematically to higher values with increasing biodegradation toward the oil-water contact due to preferential biodegradation of n-butane over isobutane. The ratio provides a practical means for the assessment of reservoir dynamics independent of gravitational segregation, hence aiding in fluid predictions in biodegraded systems, especially where samples are either contaminated or unavailable for geochemical analysis. These geodynamic processes, coupled with the downward decline in reservoir quality and the concomitant increase in pore-water saturation, resulted in a quasi-separation of a single fluid system into different PVT regions and nonlinear depth profiles of fluid properties. This model negates the current vertical compartmentalization theory and improves fluid mapping and pre-drill prediction away from well control. Compositional simulation will be attempted next to capture inferred reservoir fluid geodynamics.