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The models for colloidal mobilization in porous media are often based on Derjaguin‐Landau‐Verwey‐Overbeek interaction energy profiles. Despite abundant evidence on mobilization of particle clusters, the models to date have been limited to single colloid‐surface Derjaguin‐Landau‐Verwey‐Overbeek calculations. We show through visualization tests, qualitative and quantitative modeling, and data evaluation, that the widely accepted single colloid‐single surface isolation does not always adequately describe colloid behavior. The experiments were performed with kaolinite and latex particles mobilized from sand grains and glass beads in a transparent visualization cell. The particles were mobilized under increasing velocity and pH and decreasing salinity. The detachment conditions were determined by a torque balance of electrostatic, drag, lifting, and gravity forces, combined with a theory of a contact area between a single soft particle and a plane surface. The tests show that the calculations based on the mechanical equilibrium of a single particle on a plane surface significantly underestimate the critical detachment velocity. A better prediction is obtained using a cluster approximation. High‐resolution images taken during mobilization tests show 2‐12 particle clusters on media surface and in its proximity. The mechanical‐equilibrium calculations show that the attaching torque for clusters with assumed size and cylindrical shape is significantly higher than that for single particles. The critical detachment velocity of clusters, derived from the mechanical‐equilibrium force and torque balances, is higher than for single particles and provides a better fit to the experimental data. Our study suggests that a cluster approximation can accurately predict the mobilization of colloids in porous media.

Coal permeability declines due to fracture closure during the dewatering stage. A new technique for stimulation of natural coal cleats through ultra-fine and ultra-light high-strength particle injection into a coal fracture system is proposed. Coupling this technique with hydraulic fracturing treatment resulted in particles entering cleats under leak-off conditions. The following optimal water-based coreflood experimental conditions were determined by applying Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to the interaction between glass particles and the coal matrix: stability of a particle-based suspension (no agglomeration); repulsion between particles and the coal matrix; and, immobilisation of coal natural fines. At these conditions, these particles were placed inside cleats and were not attached to the cleat entrance, leading to less external cake formation; no formation damage due to fines migration was observed. The experimental study was carried out on some bituminous coal samples. Micro-sized glass particles were injected into a coal core at minimum effective stress until core permeability decreased to a value predetermined by a mathematical model. An increase of the effective stress to its maximum value by injection of particle-free water resulted in an approximate three-times increase in coal permeability, when compared to the original value. The proposed technique can be used for stimulation of a natural fracture network in conventional and unconventional reservoirs, as well as for the enhancement of conductivity of micro-fractures around the hydraulically induced fractures. These particles can be used as a non-damaging leak-off additive during hydraulic fracturing stimulation treatments leading to long-term fracture conductivity.

The coal permeability declines due to fracture closure during the production and pressure depletion. The recently proposed technique for stimulation of natural coal cleats consists of the injection of microsized high-strength particles into a coal natural fractured system below the fracturing pressure. Coupling this technique with hydraulic fracturing treatment resulted in particles entering cleats under leal-off condition. In the current paper it is shown that the particles must be deposited at specific conditions of the particle-coal repulsion, ensuring the absence of external cake formation. The new method was successfully validated through laboratory injection of microsized glass particles into fractured coal cores. Application of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory resulted in determination of experimental conditions favourable for particle-particle and particle-coal repulsion; these conditions also immobilize the natural fines. At these conditions, no particle attachment to coal surface and no particle agglomeration were observed, thus the conditions exclude formation damage due to external cake formation, particle attraction to coal rock and fines migration. The previously developed mathematical model was used for determination of the duration of particle injection into a coal core at minimum effective stress. Particle placement resulted in almost three-time increase in coal permeability, thus confirming the mathematical model used. The curve for well productivity index-vs-stimulation zone radius reaches maximum at some critical value of stimulation radius; the maximum is determined by the mathematical model. Placing particles beyond this critical radius results in reduction of well productivity index, due to significant hydraulic losses experienced by suspension flowing through narrowing cleat apertures during production stage. Applying the proposed novel technology during hydraulic fracturing treatment leads to improvement in productivity of coal seam gas wells and other unconventional resources (shales, tight gas and geothermal reservoirs) through enhancement of interconnectivity among microfractures around the hydraulically induced fractures.

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