Permeability decline in argillaceous sandstone formations has tendered a Permeability decline in argillaceous sandstone formations has tendered a broad spectrum of explanations and related remedial treatments. This decline has often been attributed to the mobilization, migration, and plugging of fine particles in the formation pore spaces. While previous plugging of fine particles in the formation pore spaces. While previous investigations have generally been directed toward either a chemical or a mechanical characterization of the damage mechanism, this experimental study was undertaken to comprehensively analyze both the chemical and the mechanical interactions.
In linear core tests, the flow of a chemically compatible, wetting fluid resulted in severe permeability loss when the fluid velocity exceeded a critical value. The flow of a chemically incompatible, wetting fluid resulted in a total loss of permeability which exhibited no dependency on fluid velocity or volumetric throughput. Pretreatment of the core with a cationic polymer (clay stabilizer) prevented permeability damage due to chemical incompatibility. However, a cationic polymer pretreatment could not protect the core against damage caused by exceeding the critical velocity.
Introduction
For several years fines migration has been recognized as a source of permeability damage and productivity decline in both consolidated and permeability damage and productivity decline in both consolidated and unconsolidated formations. Investigations of the fines damage mechanism have basically originated from two different perspectives — chemical versus mechanical mobilization, migration, and plugging.
CHEMICAL MECHANISM: Laboratory work in the area of fines migration induced by chemical interactions has historically involved demonstrations of rapid and drastic permeability decline resulting from fresh water contact of clay-containing formations. Several investigators have described the mechanism by which clays expand, disperse, migrate, and plug, and on this basis have devised remedial treatments by which damage can be prevented or inhibited.
As reported by Veley (1969), the factors contributing to the binding of clay particles in compact, oriented aggregations are van der Waals forces, recrystallization and chemical alteration, sorption of organic matter from oil, mutual sorption of ions between adjacent unit layers, electrostatic attractions between positively charged edges (at neutral or lower pH) and negatively charged faces, and the thermodynamic drive to reduce interfacial free energy by reducing surface area. Opposing these considerations are the factors contributing to clay particle expansion and dispersion such as hydration of exchangeable cations, hydration of particle surfaces, repulsion of interacting atmospheres of exchangeable cations (double layer theory), desorption or chemical removal of sorbed binding matter, neutralization of positive charges on particle edges, mechanical shear, and thermal (Brownian) motions.
Of all these effects, Veley maintained that the relative expansion and compression of the afore-mentioned cation atmosphere was critically important in determining whether clay particles would disperse. As noted by Reed (1972), the cation atmosphere develops as a result of the inherent negative charge on almost all clay minerals due to substitution of Al+3 for Si+4 in the tetrahedron sheet and Mg+2 or Fe+2 for Al+3 in the octahedron sheet. The negatively charged lattice is neutralized by cation adsorption on the clay surfaces. However, the cations dissociate in solution establishing an atmosphere of positively charged ions near the surface of the negatively charged particle. This is the aforementioned electric double layer and particles exhibiting this ionic configuration repel one another.
Attraction of cations toward the particle (i.e. compression of the double layer) is a function of the particle's total charge density and the surrounding cations' total effective charge. The diffusion forces are strongly dependent on the concentration of ions in the bulk solution.
