Nuclear Magnetic Resonance Spectroscopy (NMR)
The nuclear magnetic resonance spectropy is a nondestructive technique, 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 systems into a core plug. The core plug images provided visual inspections for the core initially saturated with a 3% NH4Cl brine, then contaminated by mud invasion, and finally back flushed with brine for mud removal, respectively.
Contents
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. 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 574+, Mg2+ for A/3+, as well as other ons 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 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 Ca2+ 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(H2O) 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).
Similarly, 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 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 Ba2+ over Ca2+, as indicated by the normalized cation-exchange isotherms.
The normalized anionexchange isoterms indicate that the kaolinite clay prefers 5O|~ over Cl~. It is apparent that the affinity of divalent cations (such as Ca2+) over monovalent cations (such as Na+) is much higher for kaolinite (nonswelling clay) than montmorillonite (swelling clay).
Petroleum-bearing formations contain various metal oxides, including Fe2O3, Fe3O4, MnO2, and SiO2. 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 Hence, the ion-exchange capacity of the metal oxides can be measured by determining the hydroxyl site densities on metal oxides by various methods, including reactions with Grignard reagents, acid-base ionexchange reactions, dehydration by heating, infra-red (IR) spectroscopy, tritium exchange by hydroxyl, and crystallographic calculations (Tamura et al., 1999).
(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.
Hydroxyl-hematite ion-exchange isotherm indicating the amount of hydroxyl ion consumed per unit surface area of hematite vs. the hydroxyl ion concentration in solution (after Tamura et al., 1999; reprinted by permission of the authors and Academic Press).
In Eq. 6-6, £ denotes the zeta-potential of the capillary surface, |i is the viscosity, (££0) is the permittivity, (dU I dp) is the streaming potential pressure gradient, U is the streaming potential, p is pressure, A and L are the cross-sectional area and length of porous media, respectively,(ɸ) is the porosity, and R is the electrical resistance. Johnson (1999) show the dependency of the zeta-potential on the ionic strength and pH of the aqueous solution, obtained by the electrophoresis and streaming potential methods.
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