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Compositional hindered transport model for primary and enhanced oil recovery from fractured wells in unconventional tight-oil plays, A
Calisgan, Tugce
Calisgan, Tugce
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2025
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Abstract
This research was conducted as a project within the Unconventional Reservoir Engineering Project (UREP) consortium at the Marathon Center of Excellence for Reservoir Studies (MCERS) in the Petroleum Engineering Department of Colorado School of Mines for the partial fulfilment of the PhD degree requirements in petroleum engineering. The objective of the research was to investigate hindered transport in nanoporous media during primary and enhanced oil recovery from unconventional tight-oil plays.
The recovery from tight-oil reservoirs is less than 10%; therefore, enhanced oil recovery (EOR) has been an industry focus not only for improving project economics but also for limiting the environmental footprint of oil and gas production from unconventional reservoirs. Furthermore, the industry primarily relies on conventional ideas and models to assess the effectiveness of EOR technologies and agents used in unconventional plays. However, in unconventional reservoirs, the effectiveness of the EOR methods can be improved by deciphering the key mechanism of mass transfer across phase boundaries and the fracture- matrix interfaces. The main defect of this approach is to emphasize displacement processes, which limits the focus of EOR to improve the mobility of fluids in the fracture network and larger pores. Because most of the stranded oil is in the matrix nanopore systems, transferring EOR agents from highly conductive fractures to tight matrix pores, achieving deeper matrix penetration, and alleviating the conditions immobilizing hydrocarbon components must be the key considerations to achieve quantitatively meaningful increases in cumulative recovery. This perception shifts the EOR focus from conventional, advective, displacement considerations in micro and macropore systems to intermolecular and surface phenomena governing the advective-diffusive transport and retention of fluids in fractured nanoporous matrix, which we will call hindered transport in this work.
The fact that produced oils mainly consist of the light end of the hydrocarbons indicates that heavier hydrocarbons may be filtered, hindered, and retained in the reservoir. Hindered transport has also been observed in core experiments and attributed to various mechanisms, such as molecular sieving, steric hindrance, and adsorption on the pore surfaces of the tight matrix. The impact of hindrance on production depends on the sizes, distributions, and connections of the pores and pore-throats, mineralogy of the rock, and composition of the reservoir fluids and injectants. Previous experimental work with mini cores of Niobrara formation has indicated that CO2 injection may reduce hindrance and mobilize heavier hydrocarbons into the oil stream. This work aims to convert the experimental observations to a mechanistic model, develop a numerical model for hindered transport in naturally fractured nanoporous media, provide deeper understanding of the physical phenomena contributing to hinderance, and assess the importance of hindrance on field scale by numerical simulations.
The key aspect of the model developed in this work is that hindrance of heavier hydrocarbons caused by adsorption and sieving does not only affect the accumulation terms but also alters the advective and diffusive flux components of the mass balance equation. Molecular dynamics simulations have indicated that diffusioosmosis caused by adsorption may change the permeability and relative permeability coefficients. Moreover, molecular interactions in crowded media have been reported to cause deviations from regular diffusivity coefficients or cause anomalous diffusion.
While the effects of compositional changes on component fluxes and the contribution of adsorption to the accumulation of components are regularly accounted for, accumulation due to sieving and flux alterations caused by adsorption, sieving, and crowding are not considered in the conventional models of fluid flow in porous media. Therefore, the central contribution of this work is to delineate major hydrocarbon retention mechanisms and propose a mechanistic model for hindered transport of hydrocarbons and CO2 in naturally fractured nanoporous media encountered in unconventional reservoirs. The main task of the study is to construct a one-dimensional, compositional numerical model, which considers fluid transfer between fracture and matrix media with the effects of molecular interactions, surface forces, and steric and dynamic effects on flow and transport. Obtaining the adsorption and sieving parameters used in the mechanistic model is not in the scope of this work. Although substantial work is needed to build a database of the hindered transport properties under practical conditions of interest, the values and ranges provided in the literature, which were obtained by experiments, theoretical models, and molecular simulations in previous studies, suffice for our objective to demonstrate the utility of our model and highlight the relative significance of hindrance in transport in fractured nanoporous media.
Besides discussing the basis, providing a mechanistic model, and developing small- and large-scale computational models, this work demonstrates the significance of molecular level phenomena and diffusion-driven exchange between matrix and fracture media. Importance of the matrix-block sizes (natural fracture densities) and considering transient matrix diffusion in numerical models are shown. The results emphasize the use of the appropriate multicomponent diffusive flux relations and highlight the inadequacy of the bulk and binary diffusion coefficients to model multicomponent diffusion in crowded media. It is shown that, hinderance causes considerable changes in compositional distributions under the same production conditions. The pressure dependency of the molecular diffusion, adsorption, and sieving parameters is more impactful at lower pressure ranges or over large pressure changes.
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