Development and application

In this paper, theoretical and experimental approaches to flow, hydrodynamic dispersion, and miscible and immiscible displacement processes in reservoir rocks are reviewed and discussed. Both macroscopically homogeneous and heterogeneous rocks are considered. The latter are characterized by large-scale spatial variations and correlations in their effective properties and include rocks that may be characterized by several distinct degrees of porosity, a well-known example of which is a fractured rock with two degrees of porosity---those of the pores and of the fractures. First, the diagenetic processes that give rise to the present reservoir rocks are discussed and a few geometrical models of such processes are described. Then, measurement and characterization of important properties, such as pore-size distribution, pore-space topology, and pore surface roughness, and morphological properties of fracture networks are discussed. It is shown that fractal and percolation concepts play important roles in the characterization of rocks, from the smallest length scale at the pore level to the largest length scales at the fracture and fault scales. Next, various structural models of homogeneous and heterogeneous rock are discussed, and theoretical and computer simulation approaches to flow, dispersion, and displacement in such systems are reviewed. Two different modeling approaches to these phenomena are compared. The first approach is based on the classical equations of transport supplemented with constitutive equations describing the transport and other important coefficients and parameters. These are called the continuum models. The second approach is based on network models of pore space and fractured rocks; it models the phenomena at the smallest scale, a pore or fracture, and then employs large-scale simulation and modern concepts of the statistical physics of disordered systems, such as scaling and universality, to obtain the macroscopic properties of the system. The fundamental roles of the interconnectivity of the rock and its wetting properties in dispersion and two-phase flows, and those of microscopic and macroscopic heterogeneities in miscible displacements are emphasized. Two important conceptual advances for modeling fractured rocks and studying flow phenomena in porous media are also discussed. The first, based on cellular automata, can in principle be used for computing macroscopic properties of flow phenomena in any porous medium, regardless of the complexity of its structure. The second, simulated annealing, borrowed from optimization processes and the statistical mechanics of spin glasses, is used for finding the optimum structure of a fractured reservoir that honors a limited amount of experimental data.

Flow phenomena in rocks : from continuum models to fractals, percolation, cellular automata, and simulated annealing

wide acceptance of the conclusion that most reservoirs are at wettability conditions other than VSWW. This conclusion has led to a resurgence of interest in satisfactory proce­ dures for measuring reservoir wettability and determining its effect on oil recovery, especially with respect to waterflooding. De­ termination of reservoir wettability and its effect on oil recovery by methods that in­ volve core samples will be referred to as ad­ vanced core analysis for wettability (ACAW). Reservoir wettability is not a simply de­ fined property. Classification of reservoirs as water-wet or oil-wet is a gross oversim­ plification. Various procedures for meas­ uring wettability have been proposed. Two methods of quantifying wettability based on rocklbrine/oil displacement behavior, the modified Amott test 2 and the USBM test, 3 are in common use. Each method depends on water saturation measurements and re­ lated capillary pressures or flow conditions to define a wettability scale. The tests show that reservoir wettability can cover a broad spectrum of wetting conditions that range from VSWW to very strongly oil-wet. With­ in this range, complex mixed-wettability conditions given by combinations of preferentially water-wet and oil-wet surfaces have been identified. In this paper, the 'adopted scales of reservoir wettability and their relationships to interface boundary con­ ditions are considered together with the dra­ matic effects that wettability can have on oil recovery.

Wettability and Its Effect on Oil Recovery

Permeability of crystalline and argillaceous rocks

Hydrocarbon production, gas recovery from shale, CO2 storage and water management have a common scientific underpinning: multiphase flow in porous media. This book provides a fundamental description of multiphase flow through porous rock, with emphasis on the understanding of displacement processes at the pore, or micron, scale. Fundamental equations and principal concepts using energy, momentum, and mass balance are developed, and the latest developments in high-resolution three-dimensional imaging and associated modelling are explored. The treatment is pedagogical, developing sound physical principles to predict flow and recovery through complex rock structures, while providing a review of the recent literature. This systematic approach makes it an excellent reference for those who are new to the field. Inspired by recent research, and based on courses taught to thousands of students and professionals from around the world, it provides the scientific background necessary for a quantitative assessment of multiphase subsurface flow processes, and is ideal for hydrology and environmental engineering students, as well as professionals in the hydrocarbon, water and carbon storage industries.

Multiphase Flow in Permeable Media: A Pore-Scale Perspective

A quantitative method for measuring the wettability of porous media containing brine and crude oil has been developed by the USBM using capillary pressure curves determined with a centrifuge. These measurements were reproducible within a standard deviation of 8.2 percent. The determination of fractional wettability has been compared with a method involving a combination of imbibition and displacement. Results are in agreement qualitatively as to whether a sample was water-wet or oil-wet. When tested with outcrop cores of Torpedo Sandstone, wetting tendencies of oils from widely different fields range from highly water-wet to almost neutral. Silicone-treated cores vary in extent of wettability in a predictable manner, becoming more oil-wet as the concentration of silicone used in core preparation is increased. Wettability, used as a parameter in designing linear mathematical models for predicting recovery efficiency, is equal in significance to viscosity and permeability.

Wettability Determination and Its Effect on Recovery Efficiency

Research conducted to determine the potential of nuclear explosions to stimulate gas production verifies that the gas permeability of tight sandstone cores is markedly decreased with increasing overburden pressure. Water saturation also reduces the gas permeability by a large amount. The relative permeability, however, does not change significantly with overburden pressure.

Rex D. Thomas

Papers

Wettability Determination and Its Effect on Recovery Efficiency

Effect of Overburden Pressure and Water Saturation on Gas Permeability of Tight Sandstone Cores