Integration of Geochemical, Mud Gas and Downhole Fluid Analyses for the Assessment of Compositional Grading - Case Studies

Abstract

Abstract Identifying compartmentalization, quantifying connectivity, and assessing the presence of compositional grading are critically important to reservoir management, particularly in deepwater projects where uncertainties are large and mistakes are costly. Compositional grading has been known for over 50 years, but the topic received little attention until the 1980's when sufficiently advanced analytical methods became available to assess the phenomenon. Individually, geochemistry, downhole fluid, and mud gas analyses have provided valuable insights into compositional grading, but each analytical method relies on different fluid traits and has different implications. When these analytic methods are systematically combined and consistently applied, the synergy delivers a much more accurate and robust picture of the reservoir and the fluids therein. In this paper, we review two case studies in which we have combined multiple techniques for the assessment of compositional grading in different settings. We demonstrate that new technologies combined with real-time monitoring and control and a more integrated evaluation approach produce a more robust interpretation of the fluids and yield insights into reservoir architecture. Introduction Sage and Lacey (1938) define compositional grading as "variations in the composition of the liquid phase of natural reservoirs, which are continuous through significant ranges in elevation". Therefore, the requirements for compositional grading are that the reservoir is interconnected and that fluid properties such as gas/oil ratio (GOR), saturation pressure, API, Saturation/Aromatics ratio, gas mole fraction, etc. vary with elevation. The magnitude in grading of these properties can vary greatly, depending on the geological and geochemical history of the reservoirs. One must distinguish compositional grading from the fluid property changes commonly observed in vertically stacked reservoirs. For instance, the fill/spill mechanism at work in many stacked reservoirs results in each reservoir filling up by petroleum spiraling up from deeper reservoirs via faults and other pathways, by hydraulic leakage from the crest of the underlying reservoir, or by capillary leakage. As the source matures with time, later petroleum charges become less dense, and each reservoir fills downward from the top due to buoyancy and displaces the earlier heavier charge. As such, there is a tendency for the average API gravity, GOR and bubble point to increase in successively deeper reservoirs. If, on the other hand, the charge entering the trap is denser than the existing hydrocarbon column, filling will occur at the oil/water contact and may not readily mix with the rest of the column. The range of API gravity in a trap initially reflects the maturity of the source rock kitchen during trap filling, constrained by the capacity of the trap while the range of GOR and the bubble point of oil in a trap reflect the pressure and temperature history of the trap (Stainforth, 2004). Compositional grading can be caused by a variety of factors and often indicates a state of non-equilibrium, but it can also be observed in equilibrated systems when chemical potential gradients are balanced by gravitational potential gradients. Temperature gradients can also contribute to concentration variation. In light oils with gravity greater than 35 degrees API, strong compositional grading will often occur where the reservoir fluid is near its critical point. In heavier oils, compositional grading can be due to a number of causes or a combination thereof. These include water washing, evaporative fractionation, incompetent sealing shales, dynamic charge of differing fluids, and segregation of asphaltenes. We will detail two case studies in which we have employed multiple techniques for the assessment of compositional grading. At the end, it will be apparent how an integrated approach yields a more robust interpretation of the fluid grading and a better understanding of reservoir architecture.