Reservoir Fluid Geodynamics: The Chemistry and Physics of Oilfield Reservoir Fluids after Trap Filling

Abstract

Oilfield reservoirs exhibit a wide array of complexities that have great impact on the efficiency of oil production. Major challenges include delineating overall reservoir architecture and the distributions of the contained fluids. Reservoir crude oils consist of dissolved gases, liquids, and dissolved solids (the asphaltenes); the corresponding compositional variations and phase transitions within reservoirs greatly impact production strategies and economic value. Standard workflows for understanding reservoir (rock) architecture are subsumed in the discipline “geodynamics”, which incorporates the initial rock depositional setting and subsequent alterations through geologic time to yield the present-day reservoir. However, reservoir fluids are not generally treated in such a systematic manner. Petroleum system modeling provides the timing, type, and volume of hydrocarbon fluids that charge into reservoirs. However, there is little treatment regarding how these fluids change after filling the reservoir. A significant limitation had been the lack of thermodynamic treatment of asphaltenes in reservoir crude oils. Consequently, projecting reservoir fluid properties away from the wellbore has been problematic. “Reservoir fluid geodynamics” (RFG) is the newly formalized discipline that incorporates changes in the distributions of reservoir fluids and phase transitions over geologic time. A key enabling advance is the recently developed ability to treat asphaltene gradients in oilfield reservoirs using the Flory–Huggins–Zuo equation of state (FHZ EoS) with its reliance on the Yen–Mullins model of asphaltenes. In addition, in situ downhole fluid analysis in oil wells provides accurate vertical and lateral fluid gradients in reservoirs in a cost-effective manner. Thermodynamic equilibrium can now be recognized; equilibrated fluids imply connected reservoirs, meaning a single flow unit. Disequilibrium fluid gradients imply ongoing or recent fluid processes in geologic time. The analysis of 35 oilfields (with more than 100 oil reservoirs) has allowed the identification of various reservoir fluid geodynamic processes. Some processes, such as biodegradation, have long been studied; nevertheless, even in these cases, inclusion of the thermodynamic modeling yields accurate predictions of distributions of key fluid attributes. Many other RFG processes are elucidated herein and are shown to impact major reservoir concerns for production. The resulting fundamental understanding of the physics and chemistry of these RFG processes enables measurements made at the wellbore to be used as a basis for accurate prediction of fluid properties throughout the reservoir.

Overview of Asphaltene Nanostructures and Thermodynamic Applications

“…Key issues that have been addressed include reservoir connectivity , and its inverse compartmentalization, , baffling, , viscous oil and tar mat formation, , upstructure mobile tar, aquifer support, asphaltene onset pressure, , wax precipitation, solution gas gradients, timing of charge, fault block migration, , biogenic gas charge into oil, and spill–fill trap filling, , along with traditional geochemical concerns of biodegradation, ,, gas washing, and water washing. ,, These wide-ranging concerns treated in various RFG oilfield studies establish that the development and application of asphaltene thermodynamics extends far beyond issues that are directly impacted by asphaltenes, such as tar mat formation. Instead, RFG exploits thermodynamics, which itself is unlimited in its applications. ,,,− …”

Yen–Mullins Model Applies to Oilfield Reservoirs

“…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.…”

“…Asphaltene gradients are the most recent reservoir fluid property to be addressed by a proper thermodynamic treatment, − and they provide the most stringent test of thermodynamic equilibrium. ,− Asphaltene gradients can be measured accurately, especially through the use of in situ fluid measurements in oil wells, referred to as “downhole fluid analysis” (DFA) in oilfield well logging . The latest generation of well logging tools that performs DFA can measure fluid gradients accurately and efficiently .…”

“…As noted, compositionally equilibrated reservoir fluids imply reservoir connectivity. If reservoir fluids are not equilibrated in a connected reservoir, then frequently a fluid process can be identified in recent geologic time that precludes equilibrium. ,,,− The collection of the fluid processes in geologic time constitute the newly codified technical discipline “reservoir fluid geodynamics” (RFG) (Figure ). …”

Clarifying of the Geological Structure of the Operated Oil Deposit on the Composition of Oil Removed From Various Wells

“…Несмотря на широкое использование высокотехнологичных геофизических методов разведки и современных вычислительных технологий для обработки полученной информации, геологическое строение введенных в эксплуатацию нефтяных месторождений уточняют на основе полученных данных о материальном составе извлекаемых пластовых флюидов [1,2]. Особый интерес проблема детализации геологического строения представляет для эксплуа-тируемых многопластовых месторождений, когда в результате применения интенсивных технологий повышения нефтеизвлечения возможно появление неконтролируемых межпластовых перетоков.…”