Instrumental and Laboratory Techniques for Characterization of Reservoir Rock
Evaluation of reservoir formation sensitivity to changing conditions during petroleum reservoir exploitation requires a multi-disciplinary team effort and the integration of various instrumental and analytical approaches (Kersey, 1986; Amaefule et al., 1988; Unalmiser and Funk, 1998). Some methods, such as well test interpretation, may be used to infer for limited information on a few critical parameters of reservoir formation. However, direct easurements of core properties at reservoir conditions are preferred, because they provide the most realistic information about the petroleum-bearing formations. The fundamental analytical techniques available for laboratory evaluation of core samples for sensitivity and damage potential are briefly described in this chapter. For operational principles and detailed descriptions, the readers are referred to manufacturers' manuals and other pertinent sources.
Contents
Formation Evaluation
Knowledge of reservoir formation characteristics is essential information required for studies of reservoir formation damage and interpretation of laboratory and field tests. As stated by Doublet et al. (1995), "Reservoir properties and heterogeneities can be effectively defined using four scale levels (Doublet et al., 1995 credit; Kelkar, 1991, 1993 for this information):
- Microscopic:
micro scale data
— pore and grain size distributions
— pore throat radius
— rock lithology
- Macroscopic:
core scale data
— permeability
— porosity
— saturation
- Megascopic:
simulator grid block scale data
— wireline logs
— seismic data
- Gigascopic:
reservoir scale data
— pressure transient tests
— geologic model
The extent of the information required depends on the scale of the studies planned. Various techniques are being developed for measurement of reservoir formation properties.Weber (1986) describes the various types of information that can be acquired by different logging techniques. However, Weber (1986) cautions that: "Combination of modern logs can give much information in non-cored wells, but only after proper calibration via core studies.
Characterization and quantification of the properties of the rocks, pore, and fluid systems require an integration of disciplines (Gunter et al., 1997). Skopec (1992) defines that "Reservoir characterization is a process for quantitatively predicting reservoir properties to reduce geological uncertainties and define reservoir spatial variability." Skopec (1992) adds that "Rock characterization is one component in the reservoir haracterization scheme. Skopec (1992) describes the multidisciplinary effort necessary for rock characterization.
The petrophysical integration tasks can be grouped into three stages:
1 characterization of the rock and pore types, and the fluids and the flow functions,
2 construction of a petrophysical model, and
3 testing of the reservoir description by various approaches.
Typical instrumental techniques used for characterization of core samples are described in the following.
X-Ray Diffraction (XRD)
The X-ray powder diffraction analysis (XRD) is a nondestructive technique that can provide a rapid and accurate mineralogical analysis of less than 4 micron size, bulk and clay contents of sedimentary rock samples (Amaefule et al., 1988). This is accomplished by separately analyzing the clays and the sand/silt constituents of the rock samples (Kersey, 1986). The X-ray diffraction technique is not particularly sensitive for noncrystalline materials, such as amorphous silicates and, therefore, an integrated application of various techniques, such as polarized light microscopy, X-ray diffraction, and SEM-EDS analyses, are required (Braun and Boles, 1992). Hayatdavoudi (1999) shows the typical X-ray diffraction patterns of the bulk and the smaller than 4 micron size clay fractions present in a core sample.
X-Ray CT Scanning (XRCT)
X-Ray CT (computer-assisted tomography) scanning is a nondestructive technique, which provides a detailed, two- and three-dimensional examination of unconsolidated and consolidated core samples during the flow of fluids, such as drilling muds, through core plugs and determines such data like the atomic number, porosity, bulk density, and fluid saturations (Amaefule et al., 1988; Unalmiser and Funk, 1998). This technique has been adapted from the field of medical radiology (Wellington and Vinegar, 1987). As depicted by Hicks Jr. (1996), either an X-ray source is rotated around a stationary core sample or the core sample is rotated while the X-ray source is kept stationary. The intensity of the X-rays passing through the sample is measured at various angles across different cross sections of the core and used to reconstruct the special features of the porous material. The operating principle is Beer's law, which relates the intensity of the X-ray, through the linear attenuation coefficient, to the physical properties of materials and different fluid phases in the sample (Wellington and Vinegar, 1987; Hicks Jr., 1996). A schematic of a typical X-ray scanning apparatus is shown by Coles et al. (1998). The image patterns can be constructed using the linear attenuation coefficient measured for sequential cross-sectional slides along the core sample as shown by Wellington and Vinegar (1987). These allow for reconstruction of vertical and horizontal, cross-sectional images, such as shown by Wellington and Vinegar (1987). Three-dimensional images can be reconstructed from the slice images as illustrated by Coles et al. (1998). Tremblay et al. (1998) show the cross-sectional and longitudinal images of a typical wormhole, perceived as a high permeability channel, growing in a sand-pack. Such images provide valuable insight and understanding of the alteration of porous rock by various processes.
X-Ray Fluoroscopy (XRF)
The X-Ray fluoroscopy technique is used for determining the drilling mud invasion profiles in unconsolidated and consolidated core samples and it is especially convenient for testing unconsolidated, sleeved core samples (Amaefule et al., 1988). Amaefule et al. (1988) show a typical X-ray fluoroscopic image.
Scanning Electron Microscope (SEM)
The rock and fluid interactions causing formation damage is a result of direct contact of the pore filling and pore lining minerals present in the pore space of petroleum-bearing formations. The mineralogical analysis, abundance, size, and topology and morphology of these minerals can be observed by means of the scanning electron microscopy (SEM) (Kersey, 1986; Amaefule et al., 1988). Braun and Boles (1992) caution that, although the SEM can provide qualitative and quantitative chemical analyses, it should be combined with other techniques, such as the polarized light microscopy (PLM) and the X-ray diffraction (XRD) to characterize the crystalline and noncrystalline phases, because amorphous materials do not have distinct morphological properties. An energy dispersive spectroscopy (EDS) attachment can be used during SEM analysis to determine the iron-bearing minerals (Amaefule et al., 1988). Various specific implementations of the SEM are evolving.
For example, the environmental SEM has been used to visualize the modification of the pore structure by the retention of deposits in porous media (Ali and Barrufet, 1995). The cryo-scanning electron microscopy has been used to visualize the distribution of fluids in regard to the shape and spatial distribution of the grains and clays in the pore space (Durand and Rosenberg, 1998). The SEM has also been used for investigation of the reservoir-rock wettability and its alteration (Robin and Cuiec, 1998; Durand and Rosenberg, 1998). The SEM operates based on the detection and analysis of the radiations emitted by a sample when a beam of high energy electrons is focused on the sample (Ali and Barrufet, 1995). It allows for determination of various properties of the sample, including its composition and topography (Ali and Barrufet, 1995). Typical SEM photomicrographs are shown by Amaefule et al. (1988). The environmental SEM images shown by Ali and Barrufet (1995) illustrate the modification of the pore structure by polymer retention in porous media. As can be seen by these examples, the SEM can provide very illuminating insight into the alteration of the characteristics of the porous structure and its pore filling and pore lining substances.
Thin Section Petrography (TSP)
The thin section petrography technique can be used to examine the thin sections of core samples to determine the texture, sorting, fabric, and porosity of the primary, secondary, and fracture types, as well as the location and relative abundance of the detrital and authigenic clay minerals and the disposition of matrix minerals, cementing materials, and the porous structure (Kersey, 1986; Amaefule et al., 1988). Amaefule et al. (1988) show the examples of typical thin section photomicrographs.
Petrographic Image Analysis (PIA)
As stated by Rink and Schopper (1977), "The physical properties of sedimentary rocks strongly depend on the geometrical structure of their pore space. Thus, a geometrical analysis of the pore structure can provide valuable information in formation evaluation." The petrographic image analysis (PIA) technique analyzes the photographs of the cuttings, thin sections, or slabs of reservoir core samples using high-speed image analysis systems to infer for important petrophysical properties, including textural parameters, grain size and distribution, topography, directional dependency of textural features, pore body and pore throat sizes, porosity, permeability, capillary pressure, and formation factor (Amaefule et al., 1988; Rink and Schopper, 1997; Oyno et al., 1998). The images of the rock surfaces can be obtained by photographing on paper using standard cameras or digital video cameras attached to a microscope, but computer-aided digital storage and analysis of images provide many advantages (Oyno et al., 1998). Saner et al. (1996) show typical thin section photomicrographs of typical carbonate lithofacies. The photographs shown by Ehrlich et al. (1997) indicate the packing flaws in typical sandstone samples. Coskun and Wardlaw (1996) show the porel size spectra and binary images of five pore types of some North Sea sandstones. Such images can be analyzed by various techniques to determine the textural attributes and to derive the petrophysical characteristics of the petroleum-bearing formation (Rink and Schopper, 1977; Ehrlich et al., 1997; Coskun and Wardlaw, 1993, 1996; loannidis et al., 199
Polarized Light Microscopy (PLM)
The polarized light microscopy (PLM) technique can be utilized for effectively detecting amorphous substances in porous media because, being optically isotropic, amorphous substances can be distinguished from the majority of the crystalline matter, except for the optically isotropic halides (Braun and Boles, 1982). The polarized light microscopy is based on distinguishing between various substances by the difference in their refractive indices. Braun and Boles (1982) recommend supporting the PLM method by at least another method, such as the scanning electron microscopy combined with the energy dispersive X-ray spectrometry (SEM-EDS) and the X-ray diffraction (XRD) method.
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