Integrated Core Analysis for Fractured Reservoirs: Quantification of the Storage and Flow Capacity of Matrix, Vugs, and Fractures
- N. Bona (Eni-E&P) | F. Radaelli (Eni-E&P) | A. Ortenzi (Eni-E&P) | A. De Poli (Eni-E&P) | C. Peduzzi (Eni-E&P) | M. Giorgioni (Shell Italia E&P SpA)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2003
- Document Type
- Journal Paper
- 226 - 233
- 2003. Society of Petroleum Engineers
- 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.6.9 Coring, Fishing, 5.5.2 Core Analysis, 4.3.4 Scale, 5.8.7 Carbonate Reservoir, 5.4.2 Gas Injection Methods, 5.1.5 Geologic Modeling, 5.6.2 Core Analysis, 4.1.2 Separation and Treating, 5.6.4 Drillstem/Well Testing, 4.1.5 Processing Equipment, 5.1 Reservoir Characterisation
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Answering general questions such as "Where is the oil?," "How much oil is there?," and "Can we extract it?" is a challenging task for a large fractured field in southern Italy. Various studies were conducted to gain more insight into the way oil is distributed in the rock and the producibility of the different structures observable on the cores (matrix, vugs, and fissures).
These included the cryogenic scanning electron microscope (CryoSEM) and gas chromatography (GC)-pyrolysis, pore-size-distribution measurements, SEM analysis on thin sections, and a number of nonconventional techniques that were designed specifically for that type of rock. Nuclear magnetic resonance (NMR) imaging was conducted on several whole core samples and the different porosity contributions (microporosity, vugs, and fissures) defined on a 3D basis. An analytical approach based on the percolation theory was used to separate the permeability contributions and define the conditions under which vugs and fissures may form a conducting system. The inputs were distributions of pore throats, throat length, coordination number, fissure orientation, and porosities. Wettability is a key parameter for production estimates, and we used a technique for measuring it in both microporosity and fissures, which makes use of dielectric constant measurements. All the data contributed to our current understanding of the reservoir.
The field being analyzed is located in the southern Apennines in Italy. Oil has been discovered in laterally and vertically extensive fractured carbonate reservoirs of the Apulian Platform, characterized by stratigraphically wide-ranging sediments dominantly deposited in shallow-water environments. The oil column may reach in excess of 1 km but have an average porosity of only 1%, which challenges all types of petrophysical measurements. While most of the production is achieved through "discrete events" (i.e., large fractures and dissolution), it is believed that a substantial amount of moveable hydrocarbons is contained in the tighter parts of the reservoir. Only a few examples of production at the wellbore have been observed from these systems, all with low productivity indices (the average permeability from well testing is 2 md). However, recharging of oil to discrete events is believed to be an important aspect in the life of the field. Cores represent the main source of petrophysical information for this part of the reservoir.
A sketch of the rock model at the core scale is illustrated in Fig. 1. It consists of five components: (1) matrix (either intergranular or intercrystalline microporosity with entry ports < 0.1-1 micron); (2) isolated vugs (void spaces of variable size, from 10 microns to some millimetres that are not intersected by fissures or fractures); (3) connected vugs (void spaces of variable size that are intersected by fissures or fractures); (4) fissures (small fractures with effective apertures ranging from 1 to 10 microns); and (5) fractures (fractures having effective apertures above 10 microns). By "effective aperture" or simply "aperture" of a fracture, we mean the distance between two parallel plates that show the same capillary behavior as the fracture.
While matrix, vugs, and fissures form a homogeneous background at the core scale, fractures do not. At bigger scales, other "objects" exist (namely, a hierarchy of larger fractures) that are not measurable on cores. The objective of the study was to characterize the net volume of the reservoir at the core scale. The following stages were made:
Identification of the oil-bearing objects.
Evaluation of their contribution to the overall porosity.
Definition of the permeability contributions.
Estimation of their wettability.
Where Is the Oil?
Both the petrophysical characteristics of the rock and the properties of the oil are markedly heterogeneous. Examination of the cores shows decimeter-sized areas with oil traces that contain centimeter- sized scattering. In some cases, these scattered zones are hydrocarbon-bearing in a nonuniform way, indicating a sort of fractal behavior, which clearly reflects system fracturing on different scales. All this made it extremely difficult to evaluate the net/gross pay.
Mercury intrusion tests performed on small chips show that there are different structures at the core scale. The resulting poresize distribution functions exhibit a peak at about 50 microns, another peak at approximately 5 microns, and a plateau for capillary sizes < 0.1 micron (Fig. 2). These correspond to the fracture, fissure, and matrix systems that have been introduced in the previous section. To understand if the oil is present in all three systems, we performed GC-pyrolysis (GHM) tests on matrix samples and CryoSEM analyses on fissured chips. The GHM tests did not reveal hydrocarbons in the matrix, and the CryoSEM analyses consistently showed oil traces only in the fissure system. The surface of a fissure is illustrated in Fig. 3a (the dark areas represent the oil), while Fig. 3b shows a detail of the matrix, with pores and isolated vugs that are completely filled by the water. The nature of the fluids was ascertained through chemical microanalysis, with the water containing only oxygen and the oil carbon. Our conclusion was that the rock is totally water-saturated for capillary sizes < 1 micron. In practice, of the five rock constituents identified at the core scale, the matrix should not contribute to the reserves, while the fissures and the fractures are probably oil-bearing. The isolated vugs, therefore, contain water, but the connected vugs should have oil in them.
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