A Study of Connectivity and Baffles in a Deepwater Gulf of Mexico Reservoir Linking Downhole Fluid Analysis and Geophysics

Abstract

Reservoir architecture and the size and reservoir quality of producing bodies remain a central concern particularly in deepwater. In this case study, high-quality seismic imaging delineated the sand bodies and an intervening shale break between two stacked sands. Wireline evaluation in each well consisted of advanced DFA (Downhole Fluid Analysis), formation sampling and pressure measurements, borehole imaging and petrophysics. Reservoir fluid geodynamic analysis of Wireline asphaltene gradient measurements indicate that each sand body is laterally connected and that the shale break could be a baffle. Geodynamic analysis of reservoir architecture employing seismic analysis and wellbore imaging and petrophysical logging concludes the same. All other PVT and geochemical data are compatible with this assessment; nevertheless, the DFA-measured asphaltene gradients are shown to be superior to all other fluid measurements to determine reservoir architecture. The concurrence of high-resolution seismic imaging with advanced wireline for both formation and reservoir fluid geodynamics enables building robust geologic models populated with the accurate fluid structures of the reservoir. History matching months of production match most probable reservoir realizations which are now the basis of reservoir simulation. Future exploration with step-out wells are being optimized with this powerful workflow.

Yen–Mullins Model Applies to Oilfield Reservoirs

“…Figure 11 shows two wells in an oilfield that consist of both an upper sand and a lower sand. 6,15,139 The DFA gradient data match expectations for a dispersion of asphaltene nanoaggregates, which is consistent with the asphaltene concentration in these oils of several percent (and varying in accordance with the measured DFA gradient). In addition, the offset in asphaltene gradients between the crude oils in the upper and lower sands means that they are not connected, at least not well, especially considering the small physical distance between the sands of tens of feet.…”

“…The oils in the two sands were at different pressures as a result of the lack of connectivity coupled with oil production preferentially from the lower sand. 6,15,139 This proves that the DFA interpretation of poor connectivity at best between the upper and lower sands is correct. Asphaltene equilibration analysis for black oils with a nanoaggregate dispersion of asphaltenes is a powerful method of reservoir evaluation.…”

“…All other fluid properties [GOR, density, viscosity, and Ts/(Ts + Tm)] are consistent with the asphaltene gradient analysis, but none has the sensitivity of the asphaltenes. 6,15,139 Energy & Fuels plot confirms that the molecular and nanoaggregate parts of the oil column appear connected and that the nanoaggregates are just above the water. Asphaltene Nanoaggregates.…”

“…An industry-wide study of 28 deepwater Gulf of Mexico reservoirs showed that 75% of the reservoirs underperformed in production rate or total recovery primarily as a result of unrecognized compartmentalization . Many reservoir studies have confirmed that if the reservoir crude oils are in thermodynamic equilibrium, then the reservoir is connected; it takes massive fluid compositional redistribution to equilibrate reservoir fluids, thus requiring connectivity. ,− In contrast, pressure equilibration among different compartments requires only small mass transport and does not require excellent connectivity in reservoirs. − Pressure communication is a necessary but insufficient condition to establish flow communication in reservoirs.…”

“…Figure 11. Reservoir with a black oil of moderate asphaltene concentration of a few percent 6,15,139. Data from two wells are shown; each well has an upper sand and a lower sand.…”