Role of Internal Motions and Molecular Geometry on the NMR Relaxation of Hydrocarbons

The role of internal motions and molecular geometry on $^1$H NMR relaxation times $T_{1,2}$ in hydrocarbons is investigated using MD (molecular dynamics) simulations of the autocorrelation functions for in{\it tra}molecular $G_R(t)$ and in{\it ter}molecular $G_T(t)$ $^1$H-$^1$H dipole-dipole interactions arising from rotational ($R$) and translational ($T$) diffusion, respectively. We show that molecules with increased molecular symmetry such as neopentane, benzene, and isooctane show better agreement with traditional hard-sphere models than their corresponding straight-chain $n$-alkane, and furthermore that spherically-symmetric neopentane agrees well with the Stokes-Einstein theory. The influence of internal motions on the dynamics and $T_{1,2}$ relaxation of $n$-alkanes are investigated by simulating rigid $n$-alkanes and comparing with flexible (i.e. non-rigid) $n$-alkanes. Internal motions cause the rotational and translational correlation-times $\tau_{R,T}$ to get significantly shorter and the relaxation times $T_{1,2}$ to get significantly longer, especially for longer-chain $n$-alkanes. Site-by-site simulations of $^1$H's along the chains indicate significant variations in $\tau_{R,T}$ and $T_{1,2}$ across the chain, especially for longer-chain $n$-alkanes. The extent of the stretched (i.e. multi-exponential) decay in the autocorrelation functions $G_{R,T}(t)$ are quantified using inverse Laplace transforms, for both rigid and flexible molecules, and on a site-by-site bases. Comparison of $T_{1,2}$ measurements with the site-by-site simulations indicate that cross-relaxation (partially) averages-out the variations in $\tau_{R,T}$ and $T_{1,2}$ across the chain of long-chain $n$-alkanes. This work also has implications on the role of nano-pore confinement on the NMR relaxation of fluids in the organic-matter pores of kerogen and bitumen

Fluid-Rock Characterization for NMR Well Logging and Special Core Analysis

The overall objective of this effort is to develop, build and test a high-speed drilling motor that can meet the performance guidelines of the announcement, namely: 'The motors are expected to rotate at a minimum of 10,000 rpm, have an OD no larger than 7 inches and work downhole continuously for at least 100 hours. The motor must have common oilfield thread connections capable of making up to a drill bit and bottomhole assembly. The motor must be capable of transmitting drilling fluid through the motor'. To these goals, APS would add that the motor must be economically viable, in terms of both its manufacturing and maintenance costs, and be applicable to as broad a range of markets as possible. APS has taken the approach of using a system using planetary gears to increase the speed of a conventional mud motor to 10,000 rpm. The mud flow is directed around the outside of the gear train, and a unique flow diversion system has been employed. A prototype of the motor was built and tested in APS's high-pressure flow loop. The motor operated per the model up to {approx}4200 rpm. At that point a bearing seized and the performance was severely degraded. The motor is being rebuilt and will be retested outside of this program

NMR Spin-Rotation Relaxation and Diffusion of Methane

The translational-diffusion coefficient $D_T$ and the spin-rotation contribution to the $^1$H NMR relaxation time $T_{1J}$ for methane (CH$_4$) are investigated using MD (molecular dynamics) simulations, over a wide range of densities $\rho$ and temperatures $T$, spanning the liquid, supercritical, and gas phases. The simulated $D_T$ agree well with measurements, without any adjustable parameters in the interpretation of the simulations. A minimization technique is developed to compute the angular-velocity for non-rigid spherical molecules, which is used to simulate the autocorrelation function $G_{\!J}(t)$ for spin-rotation interactions. With increasing $D_T$ (i.e. decreasing $\rho$), $G_{\!J}(t)$ shows increasing deviations from the single-exponential decay predicted by the Langevin theory for hard spheres, and the deviations are quantified using inverse Laplace transforms of $G_{\!J}(t)$. $T_{1J}$ is derived from $G_{\!J}(t)$ using the kinetic model "km" for gases ($T_{1J}^{km}$), and the diffusion model "dm" for liquids ($T_{1J}^{dm}$). $T_{1J}^{km}$ shows better agreement with $T_1$ measurements at higher $D_T$, while $T_{1J}^{dm}$ shows better agreement with $T_1$ measurements at lower $D_T$. $T_{1J}^{km}$ is shown to dominate over the MD simulated $^1$H-$^1$H dipole-dipole relaxation $T_{1RT}$ at high $D_T$, while the opposite is found at low $D_T$. At high $D_T$, the simulated spin-rotation correlation-time $\tau_J$ agrees with the kinetic collision time $\tau_K$ for gases, from which a new relation $1/T_{1J}^{km} \propto D_T$ is inferred, without any adjustable parameters

Surfactant Based Enhanced Oil Recovery and Foam Mobility Control

Surfactant flooding has the potential to significantly increase recovery over that of conventional waterflooding. The availability of a large number of surfactant structures makes it possible to conduct a systematic study of the relation between surfactant structure and its efficacy for oil recovery. A mixture of two surfactants was found to be particularly effective for application in carbonate formations at low temperature. The mixture is single phase for higher salinity or calcium concentrations than that for either surfactant used alone. This makes it possible to inject the surfactant slug with polymer close to optimal conditions and yet be single phase. A formulation has been designed for a particular field application. It uses partially hydrolyzed polyacrylamide for mobility control. The addition of an alkali such as sodium carbonate makes possible in situ generation of naphthenic soap and significant reduction of synthetic surfactant adsorption. The design of the process to maximize the region of ultra-low IFT takes advantage of the observation that the ratio of soap to synthetic surfactant is a parameter in the conditions for optimal salinity. Even for a fixed ratio of soap to surfactant, the range of salinity for low IFT was wider than that reported for surfactant systems in the literature. Low temperature, forced displacement experiments in dolomite and silica sandpacks demonstrate that greater than 95% recovery of the waterflood remaining oil is possible with 0.2% surfactant concentration, 0.5 PV surfactant slug, with no alcohol. Compositional simulation of the displacement process demonstrates the role of soap/surfactant ratio on passage of the profile through the ultralow IFT region, the importance of a wide salinity range of low IFT, and the importance of the viscosity of the surfactant slug. Mobility control is essential for surfactant EOR. Foam is evaluated to improve the sweep efficiency of surfactant injected into fractured reservoirs as well as a drive fluid for ASP flooding. UTCHEM is a reservoir simulator specially designed for surfactant EOR. It has been modified to represent the effects of a change in wettability produced by surfactant injection

Modeling micelle formation and interfacial properties with iSAFT classical density functional theory

Surfactants reduce the interfacial tension between phases, making them an important additive in a number of industrial and commercial applications from enhanced oil recovery to personal care products (e.g., shampoo and detergents). To help obtain a better understanding of the dependence of surfactant properties on molecular structure, a classical density functional theory, also known as interfacial statistical associating fluid theory, has been applied to study the effects of surfactant architecture on micelle formation and interfacial properties for model nonionic surfactant/water/oil systems. In this approach, hydrogen bonding is explicitly included. To minimize the free energy, the system minimizes interactions between hydrophobic components and hydrophilic components with water molecules hydrating the surfactant head group. The theory predicts micellar structure, effects of surfactant architecture on critical micelle concentration, aggregation number, and interfacial tension isotherm of surfactant/water systems in qualitative agreement with experimental data. Furthermore, this model is applied to study swollen micelles and reverse swollen micelles that are necessary to understand the formation of a middle-phase microemulsion

Integrated, Multi-Scale Characterization of Imbibition and Wettability Phenomena Using Magnetic Resonance and Wide-Band Dielectric Measurements

The petrophysical properties of rocks, particularly their relative permeability and wettability, strongly influence the efficiency and the time-scale of all hydrocarbon recovery processes. However, the quantitative relationships needed to account for the influence of wettability and pore structure on multi-phase flow are not yet available, largely due to the complexity of the phenomena controlling wettability and the difficulty of characterizing rock properties at the relevant length scales. This project brings together several advanced technologies to characterize pore structure and wettability. Grain-scale models are developed that help to better interpret the electric and dielectric response of rocks. These studies allow the computation of realistic configurations of two immiscible fluids as a function of wettability and geologic characteristics. These fluid configurations form a basis for predicting and explaining macroscopic behavior, including the relationship between relative permeability, wettability and laboratory and wireline log measurements of NMR and dielectric response. Dielectric and NMR measurements have been made show that the response of the rocks depends on the wetting and flow properties of the rock. The theoretical models can be used for a better interpretation and inversion of standard well logs to obtain accurate and reliable estimates of fluid saturation and of their producibility. The ultimate benefit of this combined theoretical/empirical approach for reservoir characterization is that rather than reproducing the behavior of any particular sample or set of samples, it can explain and predict trends in behavior that can be applied at a range of length scales, including correlation with wireline logs, seismic, and geologic units and strata. This approach can substantially enhance wireline log interpretation for reservoir characterization and provide better descriptions, at several scales, of crucial reservoir flow properties that govern oil recovery

Detection and Production of Methane Hydrate

This project seeks to understand regional differences in gas hydrate systems from the perspective of as an energy resource, geohazard, and long-term climate influence. Specifically, the effort will: (1) collect data and conceptual models that targets causes of gas hydrate variance, (2) construct numerical models that explain and predict regional-scale gas hydrate differences in 2-dimensions with minimal 'free parameters', (3) simulate hydrocarbon production from various gas hydrate systems to establish promising resource characteristics, (4) perturb different gas hydrate systems to assess potential impacts of hot fluids on seafloor stability and well stability, and (5) develop geophysical approaches that enable remote quantification of gas hydrate heterogeneities so that they can be characterized with minimal costly drilling. Our integrated program takes advantage of the fact that we have a close working team comprised of experts in distinct disciplines. The expected outcomes of this project are improved exploration and production technology for production of natural gas from methane hydrates and improved safety through understanding of seafloor and well bore stability in the presence of hydrates. The scope of this project was to more fully characterize, understand, and appreciate fundamental differences in the amount and distribution of gas hydrate and how this would affect the production potential of a hydrate accumulation in the marine environment. The effort combines existing information from locations in the ocean that are dominated by low permeability sediments with small amounts of high permeability sediments, one permafrost location where extensive hydrates exist in reservoir quality rocks and other locations deemed by mutual agreement of DOE and Rice to be appropriate. The initial ocean locations were Blake Ridge, Hydrate Ridge, Peru Margin and GOM. The permafrost location was Mallik. Although the ultimate goal of the project was to understand processes that control production potential of hydrates in marine settings, Mallik was included because of the extensive data collected in a producible hydrate accumulation. To date, such a location had not been studied in the oceanic environment. The project worked closely with ongoing projects (e.g. GOM JIP and offshore India) that are actively investigating potentially economic hydrate accumulations in marine settings. The overall approach was fivefold: (1) collect key data concerning hydrocarbon fluxes which is currently missing at all locations to be included in the study, (2) use this and existing data to build numerical models that can explain gas hydrate variance at all four locations, (3) simulate how natural gas could be produced from each location with different production strategies, (4) collect new sediment property data at these locations that are required for constraining fluxes, production simulations and assessing sediment stability, and (5) develop a method for remotely quantifying heterogeneities in gas hydrate and free gas distributions. While we generally restricted our efforts to the locations where key parameters can be measured or constrained, our ultimate aim was to make our efforts universally applicable to any hydrate accumulation