Downhole Fluid Analysis and Asphaltene Science for Petroleum Reservoir Evaluation

Abstract

Petroleum reservoirs are enshrouded in mysteries associated with all manner of geologic and fluid complexities that Mother Nature can inspire. Efficient exploitation of petroleum reservoirs mandates elucidation of these complexities; downhole fluid analysis (DFA) has proven to be indispensable for understanding both fluids and reservoir architecture. Crude oil consists of dissolved gases, liquids, and dissolved solids, known as the asphaltenes. These different fluid components exhibit fluid gradients vertically and laterally, which are best revealed by DFA, with its excellent precision and accuracy. Compositional gradient analysis falls within the purview of thermodynamics. Gas-liquid equilibria can be treated with a cubic equation of state (EoS), such as the Peng-Robinson EoS, a modified van der Waals EoS. In contrast, the first EoS for asphaltene gradients, the Flory-Huggins-Zuo (FHZ) EoS, was developed only recently. The resolution of the asphaltene molecular and nanocolloidal species in crude oil, which is codified in the Yen-Mullins model of asphaltenes, enabled the development of this EoS. The combination of DFA characterization of gradients of reservoir crude oil with the cubic EoS and FHZ EoS analyses brings into view wide-ranging reservoir concerns, such as reservoir connectivity, fault-block migration, heavy oil gradients, tar mat formation, huge disequilibrium fluid gradients, and even stochastic variations of reservoir fluids. New petroleum science and DFA technology are helping to offset the increasing costs and technical difficulties of exploiting ever-more-remote petroleum reservoirs.

A New Model for Describing the Adsorption of Asphaltenes on Porous Media at a High Pressure and Temperature under Flow Conditions

“…[11][12][13][14][15] This aggregation behavior of asphaltenes is also highly dependent on the chemical nature of the crude oil, temperature, compositions and the true vertical depth of the well that is directly associated with the reservoir pressure. 16 For instance, the asphaltenes in condensate oils with an average gas-oil-ratio (GOR) of 350 m 3 /m 3 behave as molecules with a size of 1.5 nm based on the Yen-Mullins model. 17,18 In black oils with a GOR of 90-180 m 3 /m 3 , asphaltenes behave as aggregates with an approximate size of 2 nm; however, for low GOR oils, such as heavy and extra heavy crude oils, asphaltenes may behave like clusters with a minimum size of 5 nm.…”

A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

“…This reservoir is known to be connected; the observation of thermodynamic equilibration of the asphaltenes and liquid phase components is consistent with reservoir 13 connectivity. 4,14,[48][49][50] As is often found, the asphaltenes offer complementary means to evaluate connectivity because a large gradient of asphaltenes is expected (cf. Fig.…”

Addressing hysteresis and slow equilibration issues in cavity-based calculation of chemical potentials

“…Not only aqueous solubility is of industrial importance: in the oil and gas industry crude oil fractions may be defined by their relative solubilities in hydrocarbon solvents. For example an asphaltene fraction is defined as soluble in toluene and insoluble in n-heptane 16 . Asphaltene phases typically consist of a mixture of different molecular species with a mean molecular weight of around 700u 17 .…”

“…Asphaltenes are prone to aggregation that is driven by changes in condition, such as pressure, temperature or compositional changes by blending/incorporating with other elements such as other incompatible oils or gases. [16][17][18][19][20][21] .…”