A. Ted Watson

Adsorption measurements in Devonian shales

Much of the natural gas in the Devonian shales in the Appalachian basin is stored as a condensed phase, which may be in solution in organic material or adsorbed on surfaces. A laboratory apparatus and measurement procedures for accurate determination of the relatively small amounts of condensed phase typical of Devonian shale samples have been designed. A temperature control system was developed so that selected temperatures can be maintained precisely. Adsorption isotherms on various shale samples have been measured, and various factors which affect gas-shale adsorption are discussed.

Quantitative NMR imaging of multiphase flow in porous media

Quantitative description of multiphase flow in porous media and local saturation distributions at steady states are of fundamental importance for petroleum recovery. The use of MRI provides an unprecedented means for obtaining such information. In this paper, profile imaging techniques for quantitative evaluation of fluid saturations during flow experiments in porous media are developed. The procedures for overcoming problems arising from very short, fluid saturation-dependent and spatial variation of T2, which are common in porous media, were addressed. The general methods developed should also be applicable to similar inhomogeneous biological systems. Experimental NMR imaging measurements of two-phase displacement were conducted in several limestones and sandstones representing various different types of pore structures, including a macroscopically homogeneous structure, a laminated structure, and a sample that exhibits porosity at different scales. The advantages of using each different type of profile imaging sequence to investigate flow in different types of porous structures are demonstrated. Images showing many features of multiphase flow, including nonuniform flow through different bedding structures, are obtained during the flow experiments. The use of profile images for obtaining many important petrophysical properties, such as permeability, porosity, saturation, and pore structural information, is discussed.

Measurements and analysis of fluid saturation-dependent NMR relaxation and linebroadening in porous media

NMR T1, T2 and spectral linewidth measurements were conducted in rock samples with different saturations obtained by drainage experiments. Saturation dependent T1 was found to follow a power-law relationship and the inverse of linewidth to follow a linear dependence with saturation. These phenomena can be explained by considering that the fluids are distributed in different sized pores for different saturation states. A method is proposed for calculating wetting phase relative permeabilities using T1 data at various saturations.

Fluid saturation‐dependent nuclear magnetic resonance spin‐lattice relaxation in porous media and pore structure analysis

Nuclear‐magnetic‐resonance spin‐lattice relaxation measurements were conducted in a Bentheimer sandstone sample for drainage experiments involving gas‐liquid fluid phases with ten different saturation levels ranging from complete water saturation (Sw=1) down to Sw=0.14. A monotonic decrease in relaxation times was observed as the water saturation was lowered over this broad saturation range. This phenomenon is explained by considering that for a drainage process the liquid (wetting) phase was drained from different‐sized pores at different saturations with larger pores being drained first. The relaxation decay curves corresponding to each saturation state were analyzed using both stretched exponential and discrete multiexponential functions. In particular, the effect due to bulk fluid relaxation was eliminated so that a more appropriate relationship between pore size distribution and the relaxation rate is obtained. From these analyses, the relative variation of pore size distributions corresponding to di...

CHARACTERIZATION OF FRACTURED PERMEABLE POROUS MEDIA USING RELAXATION-WEIGHTED IMAGING TECHNIQUES

The inversion time (TI) weighted inversion-recovery spin-echo (IRSE) imaging technique was used for laboratory characterization of fractures and porous matrix in permeable media. The method is based on the fact that relaxation rates in fractures and those in surrounding porous matrix are substantially different. Thus, by selectively suppressing imaging signals from either fractures or porous matrix, one can highlight fluid distributions in either of these regions so that fractures can be unambiguously identified and surrounding porous structures can be characterized. The advantages over conventional NMR spin-echo imaging are demonstrated with various fractured porous rock types. A technique to speed acquisitions of images of fluid distributions in porous matrix is also demonstrated.

Estimation of porous media flow functions

Properties important for describing the flow of multiple fluid phases through porous media are represented as functions of state variables (fluid saturations). A generalized procedure is presented to obtain the most accurate estimates of the multiphase flow functions from the available experimental data. The procedure is demonstrated for several different experimental designs, including a novel experiment in which fluid saturations are measured using nuclear magnetic resonance imaging. A method to evaluate the accuracy of the estimates is presented, and its use for assessing experimental design is demonstrated.

An Analytical Model for History Matching Naturally Fractured Reservoir Production Data

This paper presents a method for analyzing production data from naturally fractured gas reservoirs. A normalized time is used to modify analytical solutions to model gas flow in finite, dual-porosity reservoirs. Procedures for validating the dual-porosity reservoir model, determining the suitability of simpler reservoir models, and determining the best parameter estimates are illustrated with field data from naturally fractured reservoirs.

Characterizing the role of desorption in gas production from Devonian shales

Previous investigators suggest that more than one half of the gas stored in the Devonian Shales may exist in an adsorbed state. However, adsorption is considered to be an unconventional mode of gas storage and is not often accounted for in conventional reservoir engineering analysis. This article examines the role that desorption may play in gas production from Devonian Shale reservoirs. The results suggest that accounting for gas desorption can have a significant effect on production forecasts and estimates of gas reserves. A methodology is presented for detecting the presence of gas desorption and for estimating the parameters that describe the desorption process from Devonian Shale production data. The accuracy of these parameter estimates and the effects of stimulating the desorption mechanism are also examined.

Determination of Permeability Distributions Using NMR Velocity Imaging

This study develops a methodology for determining the absolute permeability distribution in a porous media sample using velocity data obtained from NMR imaging experiments. An objective function describing the discrepancy between observed and simulated data is reduced by iteratively updating the permeability. This parameter estimation scheme is based on an iterative method which uses optimal control theory to refine the estimates. Although this theory is developed for both isotropic and anisotropic porous media, the permeability reconstructions examined in this paper are restricted to the isotropic case. Synthetic data are used to investigate the impact of varying the noise in the experimental data, the degree of parameterization, the relative weighting of the regularization term in the objective function, and the amount and type of data required to obtain a satisfactory permeability reconstruction. These synthetic data are extracted from the solution of numerical experiments that have utilized an assumed permeability distribution. The methodology is also applied to data gathered in laboratory experiments for water flow in a sandstone sample.

The use of agarose gels for quantitative determination of fluid saturations in porous media

The use of agarose gel reference standards for quantifying petrophysical properties in porous media is described. The specific interest is to determine the values of fluid saturations and porosity in oil bearing rocks; the MRI methodology for estimating these properties is discussed. It is shown that the relaxation times of the gel reference standard and the relaxation times of the fluid contained in the porous media affect the estimation process. The determination of porosity and fluid saturations can be greatly simplified if the relaxation times of the reference standard and the relaxation times of the fluid are closely matched. Gel concentration of paramagnetic impurities in the form of copper ions is used to control the longitudinal relaxation properties. The relaxation properties of agarose gels, as a function of agarose and paramagnetic impurity concentrations, have been measured at 2.0 T. The data are well fitted by a simple polynomial in agarose concentration and paramagnetic impurity concentration. Finally, a one-dimensional imaging example of use of agarose gels as reference phantoms is discussed.