Summary

This study was undertaken to investigate the transient behavior of particulate plugging of porous media. The objectives of the experiments were to study in detail a well-characterized system and to check the validity of theoretical predictions. Experiments included injection of clay suspensions under different conditions into sandpacks, measurement of pore-size distribution, and monitoring of permeability and effluent particle concentrations.

Introduction

A study investigating the transient behavior of particulate plugging of porous media was performed on a well-characterized system to check the validity of theoretical predictions.³ Kaolin and bentonite suspensions were injected into 40/170-mesh Ottawa sandpacks. The pore-size distribution of the pack was measured. The permeability and effluent particle concentrations were monitored continuously. Experiments were conducted at different flow rates, pH values, ionic strengths, and particle concentrations with each clay mineral. The effect of each of these parameters on the permeability reduction and effluent particle concentration profile was evaluated. The particle-size distribution of the influent stream was also measured at different pH and ionic strengths.

High ionic strengths, low pH, low flow rates, and high particle concentrations caused more rapid permeability reduction when clays were injected into unconsolidated sandpacks. The particle- and pore-size distributions, together with the surface charges on the particles and pores,^1-3 can be used to estimate rates of permeability decline theoretically. A comparison of theoretical predictions with experimental observations shows a semiquantitative agreement. All trends in the permeability-reduction profile obtained by varying parameters (such as flow rate, pH, etc.) are consistently predicted.

The usefulness of studying well-characterized systems is demonstrated. The results provide a better insight into the mechanisms responsible for particulate plugging. The findings will be useful for estimating and minimizing damage from drilling mud infiltration, waterflooding, and fines migration in unconsolidated sands.

Electrochemical conditions under which particle deposition occurs can be qualitatively predicted. Coagulation effects caused by changes in pH and ionic strength may affect interpretation of the results.

The two variables most commonly monitored in filtration experiments are the pressure drop across the filter bed and the effluent particle concentration profiles. These were also the variables selected for measurement in this study. Experiments were conducted on an unconsolidated sandpack of Ottawa sand. Clay suspensions were injected. This particular system was chosen so that any complications that may arise in more complex systems, such as consolidated sandstones, may be avoided. No dissolution, precipitation, or fines migration occurs in the present system. This ideal system isolates the mechanism of deposition on pore walls from size exclusion and other particle-retention mechanisms. All suspended particles were introduced externally, and their retention should be governed by the principles outlined in the earlier-developed theory.⁴ The study, therefore, permits the investigation of basic particle-retention mechanisms and the domain of parameters in which they are operative. For example, the conditions under which smooth deposition on pore walls is a significant particle-retention mechanism should be outlined and should agree with theoretical predictions.

One of the major problems encountered in the use of experimental results that were reported earlier is the lack of essential information about the systems. Most studies do not give either the particle- or pore-size distributions of the particles and the porous medium used. Indeed, even the mean sizes. are often not reported. The surface chemical conditions, such as the surface potentials, are almost never mentioned but can be approximately determined from the materials used and the operating pH and ionic strength. When even these are not stated, it is impossible to make any comparison between theory and experiment.

A few studies, however, do provide some details of the operating conditions. The work of Donaldson et al.⁵ is one such exception. Suspensions of crushed silica were injected into three different sandstones. The particle- and pore-size distributions were measured and, during injection, the pressure drop and the effluent concentrations were continuously monitored. The porosity of the cores does not change during injection (<0.3%) even though the permeability declines by more than two orders of magnitude. This clearly indicates that permeability decline and particle retention are taking place by the size-exclusion mechanism as opposed to smooth deposition. Unfortunately, no distinction was made between the pressure drop across the sandstone and that across any filter cake that may have formed at the face of the core.

The experiments by Hunter and Alexander⁶ were performed on a system very similar to the one used here. Kaolin clay was injected into a sand column. Gravity was used as the driving force. Under the experimental conditions used, straining was found to be an unimportant retention mechanism because the pores were much larger than the particles. The zeta potential had a significant effect on the amount of particles retained. Larger ionic strengths or smaller zeta potentials caused higher retention. This was attributed to a change in the floccule strength and a change in the particle/porewall interaction.

Experiments in Berea cores and sandpacks were reported by Mungan.⁷ The effect of NaCl and NaOH was investigated in ultrasonically cleaned and conventionally washed sands. In conventionally washed sands, the presence of in-situ clays in brine or NaOH results in damage, primarily owing to blocking of pore constrictions.

Experiments reported by Jones⁸ show the effect of the presence of divalent cations in consolidated cores. It was found that if about 1/10 of the salts dissolved in water are calcium and magnesium salts, pore blockage by clays may be restrained.

Khilar and Fogler⁹ developed a physical model for water sensitivity of Berea sandstone based on the results of experiments. These results may be applicable only to clay-rich cores because of the presence of clay particles before the onset of injection. Also, permeability reduction is primarily a result of particle entrapment at pore throats after release and entrainment.

It was even shown that at high flow rates, particles do not tend to be mobilized unless the salt concentration falls below a critical salt concentration for the system. In all the above cases where clay particles are present in situ, permeability damage results from a reduction in the salinity of the injected fluid. This is a result of the release of clay particles and their subsequent entrapment at pore throats. The system used for the experiments reported here isolates permeability reduction caused by deposition on pore walls from other particle-retention mechanisms.

Theory

The deposition and release rates of particles within a porous medium are determined by forces of interaction among the porous medium, the carrier fluid, and the suspended particles. Such interaction occurs because of electrostatic, Hamaker, or dispersion forces; Born repulsion; hydrodynamic and gravitational forces; and mechanical stresses. An order-of-magnitude analysis of the various forces demonstrates that under typical conditions some forces predominate.