Characterization of Reservoir Rock 107 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 exami- nation 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 satura- tions (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 recon- structed 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 108 Reservoir Formation Damage 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 Characterization of Reservoir Rock 109 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 deter- mine 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., 1996). Polarized Light Microscopy (PLM) The polarized light microscopy (PLM) technique can be utilized for effectively detecting amorphous substances in porous media because, 110 Reservoir Formation Damage 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. Nuclear Magnetic Resonance Spectroscopy (NMR) The nuclear magnetic resonance spectropy is a nondestructive tech- nique, which measures the spin-lattice and spin-spin relaxation times by means of the radio-frequency resonance of protons in a magnetic field to infer for the petrophysical parameters, including porosity, permeability, and free and bound fluids using specially derived correlations (Unalmiser and Funk, 1998; Rueslatten et al., 1998). Because fines mobilization, migration, and retention in porous media causes porosity variation, the NMR can also be used for examination of core plugs during fines invasion. For example, Fordham et al. (1993) examined the invasion of clay particles within natural sedimentary rocks by injection of suspension of clay particles using the NMR imaging technique. Fordham et al. (1993) show that the proton spin-lattice relaxation time profiles measured at different times indeed indicate the effect of clay fines invasion into core plugs. This information can be used to determine the penetration depth of the clay fines and the effect of fines invasion to permeability. Xiao et al. (1999) state that: The NMR (nuclear magnetic resonance) techniques, namely NMRI (nuclear magnetic resonance imaging) and NMRR (nuclear magnetic resonance relaxation), can support the observations obtained with the return permeability tests, helping in the identification and comprehension of the formation damage mechanisms caused by solids and filtrate invasion in the pores of a reservoir rock. However, the NMR techniques are expensive and time consuming, and better suited for in depth studies (Xiao et al., 1999). Xiao et al. (1999) show typical NMR images and relaxation time curves on invasion of a typical bentonite/mixed metal hydroxide (MMH)/sized carbonate mud system into a core plug. The core plug images provided visual inspections for the core initially saturated with a 3% NH 4 Cl brine, then contaminated by mud invasion, and finally back flushed with brine for mud removal, respectively. Characterization of Reservoir Rock 111 Acoustic Techniques (AT) The acoustic techniques facilitate acoustic-velocity signatures and correlations of the acoustic properties of rocks to construct acoustic velocity tomograms to image the rock damage by deformation, such as elastic and dilatant deformations, pore collapse, and normal consolidation processes (Scott et al., 1998). Scott et al. (1998) describe the acoustic velocity behaviors during compaction of reservoir rock samples. Scott et al. (1998) show a schematic of a confined-indentation experiment used and the acoustic velocity tomograms obtained by the indentation tests. Cation Exchange Capacity (CEC) The total amount of ions (anions and cations) that are present at the clay surface and exchangeable with the ions in an aqueous solution in contact with the clay surface, is referred to as the ion-exchange capacity (IEC) of the clay minerals and it is measured in meq/100 g (Kleven and Alstad, 1996). The total ion-exchange capacity is therefore equal to the sum of the cation-exchange capacity (CEC} and the anion-exchange capacity (AEC): IEC = CEC + AEC (6-1) During reservoir exploitation, when brines of different composition than the reservoir brines enter the reservoir formation, an ion-exchange process may occur, activating various processes leading to formation damage (see Chapter 13). In the literature, more emphasis has been given to the measurement of the cation-exchange capacity, because it is the primary culprit, responsible for water sensitivity of clayey formations (Hill and Milburn, 1956; Thomas, 1976; Huff, 1987; Muecke, 1979; Khilar and Fogler, 1983, 1987). The mechanisms, by which aqueous ions interact with the clay minerals present in petroleum-bearing rock, have been the subject of many studies. Kleven and Alstad (1996) identified two different mechanisms: (1) lattice substitutions and (2) surface edge reactions. The first mechanism involves the ion-exchange within the lattice structure itself, by substitution of A/ 3+ for 57 4+ , Mg 2+ for A/ 3+ , as well as other ions to a lesser degree, and does not depend on the ionic strength and pH of the aqueous solution (Kleven and Alstad, 1996). The second mechanism involves the reactions of the functional groups present along the edges of the silica-alumina units and it is affected by the ionic strength and pH of the aqueous solution (Kleven and Alstad, 1996). The relative contributions of these mechanisms vary by the clay mineral types. It appears that montmorillonite and illite primarily undergo 112 Reservoir Formation Damage lattice substitutions, and surface edge reactions are dominant for kaolinite and chlorite (Kleven and Alstad, 1996). Expansion of swelling clays, such as montmorillonite, increases their surface area of exposure and, therefore, their cation-exchange capacity (Kleven and Alstad, 1996). Theoretical description of the ion-exchange reactions between the aqueous phase and the sedimentary formation minerals is very complicated because of various effects, including ion composition, pH, and temperature (Kleven and Alstad, 1996). The methods used for measurement of the ion-exchange capacity vary by the reported studies. For example, Kleven and Alstad (1996) measured the CEC of clays using Ca 2+ brines without the presence of NaCl and measured the AEC using SO%~ brines. Rhodes and Brown (1994) point out the CEC measurement of clays by commonly used methods, such as the ammonium ion and methylene blue dye adsorption methods, have inherent shortcomings, leading to inaccurate results. Therefore, Rhodes and Brown (1994) have used the adsorption of the colored Co(H 2 O) ion, which yields a very stable hydrated Co(If) complex. Rhodes and Brown (1994) have determined that the CECs of four different Na + - montmorillonites measured by three different adsorption methods differ appreciably. The methylene blue adsorption method generates significantly different results from the cobalt and ammonium ion adsorption methods, which agree with each other within acceptable tolerance. Because the ion-exchange reactions in petroleum-bearing rock are usually treated as equilibrium reactions for practical purposes, ion-exchange isotherms relating the absorbed and the aqueous phase ion contents in equilibrium conditions are desirable. For example, Kleven and Alstad (1996) deter- mined the cation-exchange isotherms shown in Figures 6—4, 6-5, and 6-6, respectively, for single cation-exchange reactions involving Ca 2+ -> Na + (6-2) and Ba l+ -> Na + and binary cation exchange reactions involving Ca 2+ + Ba 2+ -> Na + (6-3) (6-4) Similarly, Figure 6-7 by Kleven and Alstad (1996) shows the typical anion-exchange isotherms for a single anion-exchange reaction involving SOl ~^ d • When more than one ions are present in the system, some are preferentially more strongly adsorbed than the others depending on Characterization of Reservoir Rock 113 Calcium ions in solution, meq/L Figure 6-4. Calcium-sodium ion-exchange isotherms (circles = kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures = 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol. 15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth and Sulphate Ions with Clay Minerals and Sedimentary Rocks," pp. 181-200, ©1996, with permission from Elsevier Science). 14 12 S 10 10 20 30 40 Barium Ions In solution, meq/L Figure 6-5. Barium-sodium ion-exchange isotherms (circles = kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures = 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol. 15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth and Sulphate Ions with Clay Minerals and Sedimentary Rocks," pp. 181-200, ©1996, with permission from Elsevier Science). 114 Reservoir Formation Damage 14 12 10 0 10 20 30 40 50 Calcium and barium ions in solution, meq/L Figure 6-6. Calcium (open figures) and barium (closed figures) ion-exchange isotherms at 70°C (circles = kaolinite and squares = montmorillonite) (Reprinted from Journal of Petroleum Science and Engineering, Vol. 15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth and Sulphate Ions with Clay Minerals and Sedimentary Rocks," pp. 181-200, ©1996, with permission from Elsevier Science). 0,5 0,3 0,2 0,1 0,2 0,4 0.6 Sulphate Ions In solution, meq/L 0,8 Figure 6-7. Sulfate-chloride ion-exchange isotherms at low sulfate concentrations (circles = kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures = 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol. 15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth and Sulphate Ions with Clay Minerals and Sedimentary Rocks," pp. 181-200, ©1996, with permission from Elsevier Science). Characterization of Reservoir Rock 115 the affinities of the clay minerals for different ions. This phenomenon is referred to as the selectivity. Kleven and Alstad (1996) have determined that the kaolinite and montmorillonite clays prefer Ba 2+ over Ca 2+ , as indicated by the normalized cation-exchange isotherms shown in their Figure 6-8. Similarly, their Figure 6-9 showing the normalized anion- exchange isoterms indicate that the kaolinite clay prefers 5O|~ over Cl~. Figure 6-8 also shows that the selectivity is also influenced by the swelling properties of clays. It is apparent that the affinity of divalent cations (such as Ca 2+ ) over monovalent cations (such as Na + ) is much higher for kaolinite (nonswelling clay) than montmorillonite (swelling clay). Petroleum-bearing formations contain various metal oxides, includ- ing Fe 2 O 3 , Fe 3 O 4 , MnO 2 , and SiO 2 . Tamura et al. (1999) propose a hydroxylation mechanism that the exposure of metal oxides to aqueous solutions causes water to neutralize the strongly base lattice oxide ions to transform them to hydroxide ions, according to (6-5) Hence, the ion-exchange capacity of the metal oxides can be measured by determining the hydroxyl site densities on metal oxides by various '0 0.2 0,4 0,6 0,8 1 Extractions of calcium ions In solution at equilibrium Figure 6-8. Normalized calcium-sodium ion-exchange isotherms (circles = kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures = 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol. 15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth and Sulphate Ions with Clay Minerals and Sedimentary Rocks," pp. 181-200, ©1996, with permission from Elsevier Science). 116 Reservoir Formation Damage 0,2 0,4 0.6 0,8 1 Eq. fractions of sulphate Ions In solution Figure 6-9. Normalized sulfate-chloride ion-exchange isotherms (circles = kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures = 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol. 15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth and Sulphate Ions with Clay Minerals and Sedimentary Rocks," pp. 181-200, ©1996, with permission from Elsevier Science). methods, including reactions with Grignard reagents, acid-base ion- exchange reactions, dehydration by heating, infra-red (IR) spectroscopy, tritium exchange by hydroxyl, and crystallographic calculations (Tamura et al., 1999). Figure 6-10 by Tamura et al. (1999) shows a typical isotherm for OH~ ion for hematite. Figure 6-11 by Arcia and Civan (1992) show that the cation-exchange capacity of the cores extracted from reservoirs may vary significantly by the clay content. 5 (Zeta)-Potential When an electrolytic solution flows through the capillary paths in porous media, an electrostatic potential difference is generated along the flow path because of the relative difference of the anion and cation fluxes. Because the mobility of the ions is affected by the surface charge, this potential difference, called the zeta-potential, can be used as a measure of the surface charge (Sharma, 1985). The zeta-potential can be measured by various methods, including potentiometric titration, electrophoresis, and streaming potential. [...]... ^ ^ _ -50 -60 - C 2 4 6 8 10 1: PH Figure 6 -13 Comparison of electrokinetic measurement methods at various pH values, with the initial solution of 1( T3M KCI (Johnson, 19 99; reprinted by permission of the author and Academic Press) 12 0 Reservoir Formation Damage CP3 - NIPER BAG - 1 CP2-CONTROL LOG - — = 0739 20 30 40 50 70 20 30 40 50 60 70 20 30 40 60 60 Swi, % WATER SATURATION, 9! Figure 6 -14 Effect... February 16 -19 , 19 99, pp 273-285 Hicks Jr., P J., "X-Ray Computer-Assisted Tomography for Laboratory Core Studies," Journal of Petroleum Technology, December 19 96, pp 11 20 -11 22 loannidis, M A., Kwiecien, M J., & Chatzis, I., "Statistical Analysis of the Porous Microstructure as a Method of Estimating Reservoir Permeability," Journal of Petroleum Science and Engineering, Vol 16 , 19 96, pp 2 51- 2 61 Johnson,... Chakrabarty and Longo (19 97) expressed the variances of the mineral fractions by: v(f) = [x -X+C - c (6 -12 ) which is the same as the first part of Eq 6 -11 Using Eq 6- 8 without the error term and Eq 6 -11 , the rock properties are estimated by: y = (6 -13 ) Characterization of Reservoir Rock 12 3 and therefore the deviations of the measured and estimated rock properties are given by: e = f-f* (6 -14 ) Then, Chakrabarty... Petroleum Technology, August 19 87, pp 885-898 Xiao, L., Piatti, C., Giacca, D., Nicula, S., & Gallino, G., "Studies on the Damage Induced by Drilling Fluids in Limestone Cores," SPE 50 711 paper, Proceedings of the 19 99 SPE International Symposium on Oilfield Chemistry, Houston, Texas, February 16 -19 , 19 99, pp 10 5 -11 7 Part II Characterization of the Porous Media Processes for Formation Damage Accountability... Oklahoma, September 19 89, pp 205- 218 Muecke, T W., "Formation Fines and Factors Controlling their Movement in Porous Media," JPT, pp 14 7 -15 0, Feb 19 79 Oyno, L., Tjetland, B C., Esbensen, K H., Solberg, R., Scheie, A., & Larsen, T., "Prediction of Petrophysical Parameters Based on Digital 12 6 Reservoir Formation Damage Video Core Images," SPE Reservoir Evaluation and Engineering, February 19 98, pp 82-87... Characterization of Reservoir Rock 11 9 Electrophoresis Streaming Potential C 0) -40- O 0 -6 010 " 10 V Ionic Strength [M] Figure 6 -12 Comparison of electrokinetic measurement methods at various KCI ionic strengths in the 4.4-5.8 pH range (Johnson, 19 99; reprinted by permission of the author and Academic Press) 2 010 - 0- > — • — Electrophoresis —*— Streaming Potential - I I\ -10 - I—J Is 1 o I I -20-30- 1 I V \\ \\... two sandstone reservoirs," Journal of Petroleum Science and Engineering, Vol 10 , 19 93, pp 1- 16, Vol 15 , 19 96, pp 237-250 Cuiec, L., & Robin, M., "Two SEM Techniques to Investigate ReservoirRock Wettability," Journal of Petroleum Technology, November 19 98, pp 77-79 Doublet, L E., Pande, P K., Clark, M B., Nevans, J W., Vessell, R., & Blasingame, T A., SPE 29594 paper, Proceedings of the 19 95 SPE Joint... of various fluid phases and therefore the extent of formation damage in petroleum-bearing formations Because 11 8 Reservoir Formation Damage 6, 0 - O 2.0- o.o 0.0 2.0 4,0 6, 0 Clay Content (%) Figure 6 -11 Cation exchange capacity of the various Ceuta field core samples by Maraven S A., Venezuela (Arcia and Civan, 19 92; reprinted by permission of the Canadian Institute of Mining, Metallurgy and Petroleum)... fluid) in phase, n is the total j'h phase in porous is the saturation or e5=l-(J> (7 -14 ) ei=$Si:j = w,o,g (7 -15 ) SX =1. 0, IX =1. 0, IX =1. 0, 20, =1. 0, 2** =1- 0 (7 - 16 ) 2*cij-Pj i (7 -17 ) The density and velocity of a mixture is given by the volume fraction weighted averages, respectively, as: (7 -18 ) and Pv = 2, . therefore the extent of formation damage in petroleum-bearing formations. Because 11 8 Reservoir Formation Damage 6, 0 - O 2.0- o.o 0.0 2.0 4,0 Clay Content. (%) 6, 0 Figure 6 -11 . Cation exchange. with permission from Elsevier Science). 11 4 Reservoir Formation Damage 14 12 10 0 10 20 30 40 50 Calcium and barium ions in solution, meq/L Figure 6- 6. Calcium (open figures) and barium. Limestone Cores," SPE 50 711 paper, Proceedings of the 19 99 SPE International Symposium on Oilfield Chemistry, Houston, Texas, February 16 -19 , 19 99, pp. 10 5 -11 7.