Investigations of Compositions and Fluid-Fluid Association Mechanisms for Petroleum Fluids

Abstract

Crude oil has become the core resource, irreplaceable for many industries and areas of life. For the longest foreseeable future, most areas will not see a completely fulfilling sustainable replacement for this finite natural resource, particularly as a source of various precursor molecules for medicinal chemistry and advanced functional materials. In the events of raising climate concerns and ever increasing challenges associated with recovering oil from the subsurface, it is necessary to develop novel advanced methods to achieve the best possible oil recovery at minimal cost and environmental impact. Oil is deposited at elevated temperatures and pressures, often deep in geologically challenging conditions. Crude oil of the North Sea is located in tight chalk reservoirs, characterized by high porosity and low permeability and sweep efficiency associated with strong surface adhesion of the oil. Extraction of this organic material requires very accurate understanding and maintaining tight control of the complex phase behavior of the gas/oil mixture, as it passes through the porous material. Oil recovery and refining is a large and complex set of processes, where the most advanced knowledge from multiple fields of engineering and natural sciences has to be utilized. Challenges with recovery, reservoir souring, scale and corrosion, formation of clathrates etc., are all of critical importance due to sheer scale of production, making even minor effects clearly impactful. Crude oil found in different places around the world is not the same. It exhibits unique combinations of physical properties and complex chemical composition giving each crude oil a specific chemical fingerprint. These differences result in vastly different phase behavior in the subsurface, warranting case specific approaches to be designed for the extraction. The goal of this PhD dissertation is to investigate mechanisms governing phase behavior and surface interactions of crude oil in the tight chalk reservoir of the Danish North Sea from the perspective of intermolecular forces. Geographical variations of chemical composition of the crude oil in the North Sea are investigated, as this work is part of the effort to develop custom tailored solutions to be applied for the enhanced oil recovery in the North Sea. Infrared spectroscopy is a powerful method targeting functional groups of molecules, and is therefore used to study polar constituents of the oil. Polar chemical groups have an impact on the physical properties of the fluid. The chemical compositions of several crude oils from the North Sea have been studied alongside chalk samples containing reservoir oil, to compare composition of material at the surface against that in bulk fluids. Residue extracted from the chalk surface is much more rich in polar compounds than bulk oil and even it’s respective heavy fraction from vacuum distillation separations. A method was developed to analyze the crude oil contained in the original drill core samples, sourced in the various geographic locations across the oil fields in the North Sea. The solid phase extraction (SPE) procedure yields separation of a sample of the solvent-extracted crude oil from the core into four fraction, based on the presence of polar functional groups in the molecules. Infrared spectroscopy allows direct access to the characteristic vibrational features of these functional groups, and allows quantify their abundance. Statistical analysis was used to uncover the most important differences in chemistry and find correlations among samples from various geographical locations. The distributions revealed from the principal component analysis (PCA) plots demonstrate that there are distinctive variations in the composition of polar functional groups attributed to specific classes of chemical compounds and that wells can be grouped based on these chemical properties to optimize the conditions of oil production for the specific case. In order to aid the development of enhanced oil recovery processes, various models simulating fluid behavior in the reservoir are used. The most common approach is to use the pressure/ volume/temperature (PVT) phase diagrams obtained in a laboratory for a model fluid composition. The diversity of fluid chemistry in a real reservoir and the complex interaction with the surfaces shows how very limited this approach is in it’s applicability. Therefore one of the objectives of this thesis is to investigate the molecular association mechanisms of microsolvation relevant to molecules found in the oil and associated industrial processes. Again, as crude oil chemistry is infinitely diverse, instead of taking a case approach, it was decided to develop a broadly applicable model based that would take into account the immediate chemical surrounding of the functional groups to exactly describe their local effect on the molecular association in the condensed phase. Infrared (IR) and terahertz (THz) spectroscopy in cryogenic matrices allow access to largeamplitude intermolecular vibrations, that characterize well the non-covalent interactions that cause individual molecules to form clusters. This experimental approach was used to estimate the strengths of hydrogen bonds formed with water of a systematic set of compounds to establish a widely applicable model for the prediction of hydrogen bond acceptor properties of oxygen- and sulfur-based polar groups. We have determined the extent to which the inductive effect influences acceptor properties of oxygen and sulfur atoms, and used the local pair coupled cluster method to analyze the interaction energy of the non-covalent aggregates, providing deeper insight into the role of London dispersion forces on intermolecular interactions in liquid phase. The resulting data array provides a useful experimental reference for the development of advanced thermodynamic models for reservoir scale simulations of fluid-fluid behavior. The molecular associations of dimethyl ether with various classes of oxygen-containing polar compounds were investigated also. Dimethyl ether has received attention as a potentially feasible agent for solvent based enhanced oil recovery, that could be applied in the North Sea. It has the potential to efficiently elute heavier compounds, that tend to strongly adhere to surfaces, and affect physical properties of the flooding fluid. However, a successful application requires thermodynamic models to find the most economy-efficient use of the agent. The investigation also includes light non-polar compounds, such as CO2, OCS and CS2, that are of importance in gas injection production techniques and desulfurization processes. These systems exhibit very weak interactions where dispersion forces are very important. Such systems require particularly sensitive techniques with the spectral range extended into the THz region. We have used para-hydrogen as an alternative to noble gases for cryogenic matrix formation, as the former is transparent in the terahertz range. For this purpose, a cryogenic conversion setup was designed to produce para-hydrogen in a fully automatic way from normal hydrogen, without the need for valuable liquid helium.