Characterization of Asphaltene Transport over Geologic Time Aids in Explaining the Distribution of Heavy Oils and Solid Hydrocarbons in Reservoirs

Abstract

Asphaltenes are a very important component of reservoir fluids. They have a huge impact on crude oil viscosity and are a Flow Assurance concern. They can undergo a phase transition, giving rise to tar mats that seal aquifers precluding aquifer sweep. Local tar deposits can act as a drilling hazard. Upstructure tar (or bitumen) deposition can occur which can flow with produced light hydrocarbons greatly reducing the productivity index. In EOR, miscible gas injection can also give rise to asphaltene deposition. Characterizing these disparate observations is now performed within a simple overarching framework. Here, we combine asphaltene nanoscience, thermodynamics, and fluid mechanics to model asphaltene-rich fluid flow and asphaltene deposition that occur in reservoirs in geologic (or even production) time. This analysis successfully accounts for extensive measurements in several reservoirs in different stages of similar processes. Reservoir black oils with a late, light hydrocarbon charge experience asphaltene instability. This instability does not necessarily cause precipitation; instead, weak instability can cause a change in the nanocolloidal character of asphaltenes without precipitation. Consequently, this less stable asphaltene remains in the crude oil and is thus mobile. This process can result in fluid density inversions and gravity currents that pump asphaltene 'clusters' in oil over reservoir length scales relatively quickly in geologic time. These asphaltene clusters then establish very large asphaltene and viscosity gradients at the base of the reservoir. If the light hydrocarbon instability event continues, a regional tar mat can form. In contrast, if the light hydrocarbon charge is sufficiently rapid, the displacement of the contact between the original and new reservoir fluids overtakes and precipitates asphaltenes locally producing deposition upstructure often near the crest of the field. In this paper, several reservoirs are examined. Two reservoirs have massive, current gas charge and have bitumen deposition upstructure. Another reservoir is shown to be midway through a slower gas charge, with the asphaltene instability causing migration of asphaltenes from the top to the base of the oil column in the form of clusters creating large asphaltene gravity gradients. Another reservoir is shown to have this process completed yielding a 50 meter column of heavy oil at the base of the oil column underlain by a 10 meter regional tar mat. This integrated analysis enables a much simpler understanding of many production issues associated with asphaltenes and provides a way forward for treating disparate asphaltene problems within a single framework.

The dynamic Flory-Huggins-Zuo equation of state

“…The asphaltenes in the heavy oil column above are equilibrated and excellently match the Yen-Mullins model. [52] The numerical schematics in Fig. 5 is consistent with the compositional distribution of this reservoir.…”

“…Clusters observed in oilfield studies have generally been close to 5.0 nm [3,44]. The FHZ EoS is increasingly utilized in oilfield studies [52]. Other formalisms, such as SAFT (statistical associating fluid theory) modeling, have also been used to describe asphaltene gradients in oilfields [53].…”

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

“…3 The adsorption and/or deposition of asphaltenes onto formation grains is one of the primary causes of formation damage in oil reservoirs, which has been one of the major unresolved flow assurance problems in the oil industry. [4][5][6][7] Asphaltenes, which are the heaviest and most polar compounds in crude oil, are typically defined as the solubility class obtained from crude oil by fractionation using solvents.…”

“…21 studied the adsorption of n-C 7 asphaltenes from mineral oil on to several hydrophilic clays minerals. 3 The authors found that the adsorption is mainly due to the interactions of the polar parts of the asphaltenes with the polar parts of the surface resulting in a hydrophobic shell of asphaltenes and allowing the adsorption of a second layer of asphaltenes that is more hydrophilic for the buildup of outward-facing polar functional groups. 19,21 Several studies 21,[35][36][37] have shown that kaolinite and calcite are very active minerals for asphaltene adsorption and that kaolinite have the highest adsorption capacity.…”

Dynamics of Tar Mat Formation Due to Asphaltenes Accumulation Under Gas Charge in Reservoirs

“…Some Pliocene reservoirs are still actively charging and have not had sufficient time to equilibrate or even to complete their charge. [5] In these cases, thermodynamic equilibration of the fluids is simply precluded and fluid properties variation, such as gas-oil ratio (GOR) and saturation pressure in low GOR fluids, and the asphaltene onset pressure [6] can occur. Consequently, the corresponding fluid gradient is often not equilibrated.…”