Asphaltene precipitation and deposition have been a formation damage problem for decades, with the most devastating effects being wettability alteration and permeability impairment. To this effect, a critical look into the laboratory studies and models developed to quantify/predict permeability and wettability alterations are reviewed, stating their assumptions and limitations. For wettability alterations, the mechanism is predominantly surface adsorption, which is controlled by the asphaltene contacting minerals as they control the surface chemistry, charge, and electrochemical interactions. The most promising wettability alteration evaluation techniques are nuclear magnetic resonance, ζ potential, and the use of high-resolution microscopy. The integration of such techniques, which is still missing, would reinforce the understanding of asphaltene interaction with rock minerals (especially clays), which holds the key to developing a strategy for modeling wettability alteration. With regard to permeability impairment, surface deposition, pore plugging, and fine migration have been identified as the dominant mechanisms with several models reporting the simultaneous existence of multiple mechanisms. Existing experimental findings showed that asphaltene deposition is non-uniform due to mineral distribution which further complicates the modeling process. It also remains a challenge to separate changes due to adsorption (wettability changes) from those due to pore size reduction (permeability impairment).

We have demonstrated the effect of micrite content on the acoustic properties of well-controlled microstructures that are created in the laboratory to closely mimic those of carbonate rocks. In particular, we examined the effect of micrite content on the acoustic velocity, sensitivity of velocity to pressure, and changes in velocity resulting from dissolution upon saturation with a reactive fluid. We followed Dunham’s classification and fabricated the samples by mixing coarse (sand-sized) and very fine (micrite-sized) calcite grains in different ratios and then cold compressing the mixture. The acoustic velocities were measured under benchtop conditions and as functions of confining pressure before and after the injection of a CO2 aqueous solution. The benchtop measurements indicated that the addition of micrite made the frame of the carbonate samples stiffer. Because the sensitivity of velocity to pressure decreases as the content of micrite increases, we hypothesized a stiffer pore structure in micrite-richer fabrics. Moreover, the content of micrite seems also to affect the change in elastic moduli upon dissolution. Micrite-rich samples experience a drop in elastic moduli after fluid injection, with respect to the moduli measured under dry conditions. This was interpreted as being likely due to dissolution, which weakens the rock frame. Such an effect seems to overcome the stiffening that results from dispersion mechanisms under high-frequency conditions.

Clays, hydrous aluminous phyllosilicates, have a significant impact on the interpretation of physical measurements and properties of porous media. In particular, the presence of paramagnetic and/or ferromagnetic ions like iron, nickel, and magnesium in clays can complicate the analysis of nuclear magnetic resonance (NMR) data for porous media characterization. This is due to the internal magnetic field gradient induced by the clay minerals. In this study, we aim to investigate the impact of clay content on spin–spin relaxation time (T₂), which is strongly influenced by the pore surface chemistry. Seven rock core plugs, characterized with variable clay content, were used for this purpose. The clay mineralogy and volume were determined by means of quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). The T₂ relaxation time was measured using a Carr–Purcell–Meiboom–Gill (CPMG) sequence with variable echo spacing (T_E). The maximum percentage difference in dominant T₂ values (MRDT₂) between shortest and longest echo spacing was subsequently correlated with clay content obtained from QEMSCAN. Our results show that the reduction in T₂ distribution with increasing echo time T_E is more significant in samples characterized by higher clay contents. The MRDT₂ was found to be strongly correlated with clay content. An analytical equation is presented expressing MRDT₂ as a function of clay content providing a quick and non-destructive approach for clay content estimation. Moreover, the MRDT₂–clay content relationship showed a nonlinear behavior: MRDT₂ increases drastically as the clay content increases up to 15%, beyond which the rate of MRDT₂ change with clay content diminishes. This behavior could be attributed to the clay distribution. At higher clay contents (above 15%), it is more likely for clay to form clusters (structural clays), which will not significantly increase the clay surface in contact with the pore fluid. Further, experimental data suggests that ignoring the impact of clay on internal magnetic gradients and T₂ signal may result in considerable underestimation of the actual pore size distribution.