Philip M. Singer

NMR T2 Distributions in the Eagle Ford Shale: Reflections on Pore Size

Production of oil from organic shale reservoirs is a function of porosity, hydrocarbon saturation, pore pressure, matrix permeability, and hydraulic fracture surface area plus fracture conductivity. Hydraulic fracture surface area, porosity, saturations and pore pressure dominate initial production rates. Matrix permeability becomes increasingly important in sustaining production later in time. Permeability measurements to oil from organic shale core samples are not commercially available today. However, permeability to oil is believed to be a function of pore throat size, wettability, and water saturation, the same as a conventional reservoir. This work investigates pore size, wettability, and expelled hydrocarbon volumes using log and core-based nuclear magnetic resonance data from the Eagle Ford Shale focused on the comprehensive evaluation of one well. Comparisons with core porosity measurements, scanning electron microscope images (SEM) and mercury injection capillary pressure tests (MICP) are compared with the nuclear magnetic resonance (NMR) interpretation for calibration and validation. The NMR T2 distribution is partitioned into regions of bound and producible free fluid. Two types of pore systems are present in the Eagle Ford Shale; kerogen-hosted (OM) and inter/intra particle (IP). Bore hole logs indicate the upper Eagle Ford Shale is dominated by IP porosity, and the lower Eagle Ford Shale is dominated by OM porosity. Core NMR indicates OM pores are hydrocarbon wet while IP pores have mixed wettability. Core pore fluids are not representative of in-situ conditions as the lighter portion of the hydrocarbons have been expelled during core recovery. Comparison between log and core measured NMR allows the quantification of the expelled hydrocarbon those zones with the “best” producibility. Understanding which portion of a shale reservoir contains producible fluids impacts target zone selection.

Molecular dynamics simulations of NMR relaxation and diffusion of bulk hydrocarbons and water

Molecular dynamics (MD) simulations are used to investigate 1H nuclear magnetic resonance (NMR) relaxation and diffusion of bulk n-C5H12 to n-C17H36 hydrocarbons and bulk water. The MD simulations of the 1H NMR relaxation times T1,2 in the fast motion regime where T1=T2 agree with measured (de-oxygenated) T2 data at ambient conditions, without any adjustable parameters in the interpretation of the simulation data. Likewise, the translational diffusion DT coefficients calculated using simulation configurations agree with measured diffusion data at ambient conditions. The agreement between the predicted and experimentally measured NMR relaxation times and diffusion coefficient also validate the forcefields used in the simulation. The molecular simulations naturally separate intramolecular from intermolecular dipole-dipole interactions helping bring new insight into the two NMR relaxation mechanisms as a function of molecular chain-length (i.e. carbon number). Comparison of the MD simulation results of the two relaxation mechanisms with traditional hard-sphere models used in interpreting NMR data reveals important limitations in the latter. With increasing chain length, there is substantial deviation in the molecular size inferred on the basis of the radius of gyration from simulation and the fitted hard-sphere radii required to rationalize the relaxation times. This deviation is characteristic of the local nature of the NMR measurement, one that is well-captured by molecular simulations.

A more accurate estimate of T2 distribution from direct analysis of NMR measurements.

In the past decade, low-field NMR relaxation and diffusion measurements in grossly inhomogeneous fields have been used to characterize pore size distribution of porous media. Estimation of these distributions from the measured magnetization data plays a central role in the inference of insitu petro-physical and fluid properties such as porosity, permeability, and hydrocarbon viscosity. In general, inversion of the relaxation and/or diffusion distribution from NMR data is a non-unique and ill-conditioned problem. It is often solved in the literature by finding the smoothest relaxation distribution that fits the measured data by use of regularization. In this paper, estimation of these distributions is further constrained by linear functionals of the measurement that can be directly estimated from the measured data. These linear functionals include Mellin, Fourier-Mellin, and exponential Haar transforms that provide moments, porosity, and tapered areas of the distribution, respectively. The addition of these linear constraints provides more accurate estimates of the distribution in terms of a reduction in bias and variance in the estimates. The resulting distribution is also more stable in that it is less sensitive to regularization. Benchmarking of this algorithm on simulated data sets shows a reduction of artefacts often seen in the distributions and, in some cases, there is an increase of resolution in the features of the T(2) distribution. This algorithm can be applied to data obtained from a variety of pulse sequences including CPMG, inversion and saturation recovery and diffusion editing, as well as pulse sequences often deployed down-hole.

Role of internal motions and molecular geometry on the NMR relaxation of hydrocarbons

The role of internal motions and molecular geometry on 1H NMR relaxation rates in liquid-state hydrocarbons is investigated using MD (molecular dynamics) simulations of the autocorrelation functions for intramolecular and intermolecular 1H-1H dipole-dipole interactions. The effects of molecular geometry and internal motions on the functional form of the autocorrelation functions are studied by comparing symmetric molecules such as neopentane and benzene to corresponding straight-chain alkanes n-pentane and n-hexane, respectively. Comparison of rigid versus flexible molecules shows that internal motions cause the intramolecular and intermolecular correlation-times to get significantly shorter, and the corresponding relaxation rates to get significantly smaller, especially for longer-chain n-alkanes. Site-by-site simulations of 1Hs across the chains indicate significant variations in correlation times and relaxation rates across the molecule, and comparison with measurements reveals insights into cross-relaxation effects. Furthermore, the simulations reveal new insights into the relative strength of intramolecular versus intermolecular relaxation as a function of internal motions, as a function of molecular geometry, and on a site-by-site basis across the chain.