Pore Systems in Carbonate Rocks and their Influence on Hydrocarbon Recovery Efficiency
Norman C. Wardlaw, 1979. "Pore Systems in Carbonate Rocks and their Influence on Hydrocarbon Recovery Efficiency", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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Part 1. — Porosity in Dolomites
Dolomitization is a selective process and the finer carbonate constituents (mud) tend to be preferentially replaced in sediments containing a mixture of grains and mud.
In the initial stages of dolomitization, that is up to somewhere between 50 and 70 percent dolomite content, the porosity of the host rock either remains constant or tends to decrease slightly. However, at higher dolomite contents, both porosity and permeability commonly increase abrupty. At this stage, the dolomite has a “sucrosic” texture being largely composed of rhombohedra of uniform size with intercrystal porosity. The porosity originates by dissolution of associated calcite which may occur either during the later stages of dolomitization or at some subsequent time.
Dolomites have higher average porosities and permeabilities than limestones because of differences in the size, shape and arrangement of crystals. The tendency for dolomite crystals to assume idio-morphic forms in an anhedral matrix of cal cite can be explained in terms of; a) surface free energy being minimal for low index crystal faces and b) dolomite having a higher surface free energy than calcite.
It is the idiomorphic shape of the dolomite crystals which, combined with the uniformity of crystal size, result in dolomites with “sucrosic” texture. These are quantitatively the most important North American carbonate rock types in terms of oil and gas production.
Part 2. — The Influence of Pore Structure in Carbonates on Hydrocarbon Recovery Efficiency
Oil-recovery efficiency can be evaluated from the results of relative permeability tests conducted on core samples but, because of the difficulty and expense of making these tests, typically fewer than fifteen are available for an entire reservoir. There is a need to devise simpler techniques to evaluate the probable recovery efficiency of reservoir rocks in order that a very much larger sample group may be treated.
The efficiency with which oil can be recovered depends on the fluid properties and on the characteristics of the pore system. The most important characteristics of the pore system are thought to be: pore-to-throat size ratio; throat-to-pore coordination number and; type and degree of nonrandom heterogeneity. On the basis of these characteristics, combined with a knowledge of total porosity, an empirical scheme is proposed which enables an approximate estimation of recovery efficiency from visual observations made from resin pore casts of carbonate reservoir rocks. A large number of samples can be examined rapidly by using this scheme, and the chances of properly evaluating the preformance of a reservoir are improved. Following such an evaluation, a more representative group of samples could be selected for relative-permeability measurements.
Figures & Tables
Geology of Carbonate Porosity
In clastic situations, primary porositv is a direct function of texture and fabric, including size, sorting and shape (Fig. 1). Grain size, sorting, fabric, as well as sedimentary structures are related directly to sedimentary processes acting at the time of deposition (Fig. 1). Each depositional environment is characterized by a distinct suite of processes distributed across the active sediment water interface in a pattern unique for that environment (Fig.2). This suite of processes gives rise to a group of products, including sediment texture, fabric, and structures distributed across the active sediment water interface in a pattern unique for each depositional environment (Figs. 1 and 2). In a prograding or regressive situation, when sedimentation is taking place at the active sediment-water interface, a vertical sequence of sediments is formed which reflects, in an orderly fashion, from deepest at the base, to shallowest at the top, the progressive changes in texture, fabric and sedimentary structures resulting from the progressive changes in processes found along this interface from shallow to deep water (Fig. 3). Each sedimentary environment then, can be characterized by a unique vertical sequence of sediment textures, fabrics and sedimentary structures. It is this unique suite of characteristics that is commonly used for the identification of depositional environments in ancient rock sequences, and most importantly, is used to predict the presence and detailed distribution of the most porous (best sorted, coarsest) potential reservoir facies (Fig. 3).
In a regional setting, the recognition of distinct sedimentary environments and knowledge of logical lateral relationships is the keystone for prediction of the lateral extension or even presence of potential reservoir facies.