Introduction

Introduction to Mineralogy

Petrophysics is the study of rock properties and their interactions with fluids (gases, liquid hydrocarbons, and aqueous solutions). The geologic material forming a reservoir for the accumulation of hydrocarbons in the subsurface must contain a three-dimensional network of interconnected pores in order to store the fluids and allow for their movement within the reservoir. Thus, the porosity of the reservoir rocks and their permeability are the most fundamental physical properties with respect to the storage and transmission of fluids. Accurate knowledge of these two properties for any hydrocarbon reservoir, together with the fluid properties, is required for efficient development, management and prediction of future performance of the oilfield.

The purpose of this text is to provide a basic understanding of the physical properties of porous geologic materials, and the interactions of various fluids with the interstitial surfaces and the distribution of pores of various sizes within the porous medium. Procedures for the measurement of petrophysical properties are included as a necessary part of this text. Applications of the fundamental properties to subsurface geologic strata must be made by analyses of the variations of petrophysical properties in the subsurface reservoir.

Emphasis is placed on the testing of small samples of rocks to uncover their physical properties and their interactions with various fluids. A considerable body of knowledge of rocks and their fluid flow properties has been obtained from studies of artificial systems such as networks of pores etched on glass plates, packed columns of glass beads, and from outcrop samples of unconsolidated sands, sandstones, and limestones. These studies have been used to develop an understanding of the petrophysical and fluid transport properties of the more complex subsurface samples of rocks associated with petroleum reservoirs. This body of experimental data and production analyses of artificial systems, surface and subsurface rocks make up the accumulated knowledge of petrophysics. Although the emphasis of this text is placed on the analyses of small samples, the data are correlated to the macroscopic performance of the petroleum reservoirs whenever applicable. In considering a reservoir as a whole, one is confronted with the problem of the distribution of these properties within the reservoir and its stratigraphy. The directional distribution of thickness, porosity, permeability, and geologic features that contribute to heterogeneity governs the natural patter of fluid flow. Knowledge of this natural pattern is sought to design the most efficient injection-production system for economy of energy and maximization of hydrocarbon production [1].

Petrophysics is intrinsically bound to mineralogy and geology because the majority of the world's petroleum occurs in porous sedimentary rocks. The sedimentary rocks are composed of fragments of other rocks derived from mechanical and chemical deterioration of igneous, metamorphic and other sedimentary rocks, which is constantly occurring. The particles of erosion are frequently transported to other locations by winds and surface streams and deposited to form new sedimentary rock structures. The petrophysical properties of the rocks depend largely on the depositional environmental conditions that controlled the mineral composition, grain size, orientation or packing, and amount of cementation and compaction.

Mineral Constituents of Rocks: A Review

The physical properties of rocks are the consequence of their mineral composition. Minerals are defined here as naturally occurring chemical elements or compounds formed as a result of inorganic processes. The chemical analysis of six sandstones by emission spectrograph and X-ray dispersive scanning electron microscopy [2] showed that the rocks are composed of just a few chemical elements. Analysis of the rocks by emission spectroscopy yielded the matrix chemical composition since the rocks were fused with lithium to make all of the elements soluble in water and then the total emission spectrograph was analyzed. The scanning electron microscope X-ray, however, could only analyze microscopic spots on the broken surface of the rocks. The difference between the chemical analysis of the total sample and the spot surface analysis is significant for consideration of the rock-fluid interactions. The presence of the transition metals on the surface of the rocks induces preferential wetting of the surface by oil through Lewis acid-base-type reactions between the polar organic compounds in crude oils and the transition metals exposed in the pores [3]. The high surface concentration of aluminum reported in Table 1.1 is probably due to the ubiquitous presence of clay minerals in sandstones.

The list of elements that are the major constituents of sedimentary rocks (Table 1.1) is confirmed by the averages of thousands of samples of the crust reported by Foster [4] (Table 1.2). Just eight elements make up 99% (by weight) of the minerals that form the solid crust of the Earth; these are the elements including oxygen, listed in the first seven rows of Table 1.1 from analysis of six sandstones. Although the crust appears to be very heterogeneous with respect to minerals and types of rocks, most of the rock-forming minerals are composed of silicon and oxygen together with aluminum and one or more of the other elements listed in Table 1.2.

The chemical compositions and quantitative descriptions of some minerals are listed in Tables 1.3 and 1.4. Some of the minerals are very complex and their chemical formulas differ in various publications; in such cases the most common formula reported in the list of references was selected.