The Mechanism Of Drag Reduction With Hydrophobic Nanoparticles In Reservoir Microchannels

Drag reduction by hydrophobic nanoparticles (HNPs) is a new technology to enhance water injection or decrease injection pressure in reservoir micro-porous tube. It is of great importance to enhance water injection and then to improve oil recovery for low permeability reservoir. Although results from laboratory and field tests have showed that significant drag reduction could be achieved through this technology, the development and application of the technology is severely constrained because of its unclear mechanism. A drag reduction mechanism based on slip effect on nano-structural interface is proposed according to the characters of porous structure in reservoir and validated by a series of experiments. Specific tasks and their accomplishments are given below.1) Based on slip effect on nano-structural interface, a drag reduction mechanism is proposed. After HNPs are poured into formation, emulative adsorption appears between HNPs and water molecules, then HNPs are adsorbed on porous walls and hydrophobic nanoparticles layers (HNPLs) are formed to replace hydrated film. These layers have both micro- and nano-structures' and superhydrophobic properties, and when a driving pressure is applied to the water stream, the slip effect takes place in reservoir microchannels, which leads to larger effective diameter. As a result, the flow rate increases and the flow resistance is greatly reduced.2) According to the definition of slip length and basic fluid dynamics law in the porous medium, a slippage model of uniform porous media is introduced with HNPs-adsorbed boundary, and the relative formulas between slip length and permeability is derived. This model combines macroscopical effect with microscopical features , provides a theoretical guidance and makes a good interpretation to the drag reduction mechanism of HNPs.3) Based on the characteristics of the porous structure of reservoirs and the surface characteristics of HNPs, microcosmic force analysis of HNPs in reservoir microchannels is performed, and some formulas are deducted to calculate the relative microscopic interaction energy of HNPs with porous walls. The results show the electrostatic attraction is the main factor for HNPs to overcome resistance force, and multi Hydrogen bonding attraction is a key role for HNPs to adsorb firmly on rock porous walls, which can provide a theoretical explaination for HNPLs to be formed instead of hydrated film on porous walls in reservoir.4) core adsorbing HNPs method and observation method by Scanning Electron Microscope (SEM) , are used to study the adsorption character and distribution state of HNPs on core surfaces. The results show that the adsorptive power of HNPs SiO₂ is stronger than that of HNPs TiO₂ or ZnO on core surfaces, furthermore the influence of the hydrated film on absorption character is of lesser significance than that of the influence of surface roughness of microporous walls. With walls of increasing roughness, the HNPs-adsorbed layers increase in thickness correspondingly.5) Both core adsorbing HNPs method and pressed slice with nanoparticles method are adopted to construct micro- and nano-structural surfaces with HNPs directly. The contact angles and rolling angles of water drops on these surfaces are tested, and SEM or FESEM is used to observe the microstructure of them. The results show that these surfaces are irregular micro- and nano-structural, air-solid compound interfaces and their wettability is changed from strong hydrophobic to superhydrophobic. It suggests that the adsorption of HNPs changes the wettability of core surfaces from hydrophobic to superhydrophobic. Some rules impacting on dewetting phenomenon are studied and the mechanism of wettability change are revealed on the core surface by numerical calculation.6) Several samples of injection-argumented HNPs are developed based on the drag reduction mechanism proposed in this paper, and core displacement experiments are used to evaluated drag-reducing effect. The results show that the drag-reducing effect of these samples is evident, which are the HNPs SiO₂ modified by two kinds of hydrophobic material and the one, named ShU2, with two kinds of diameter, and the water-phase effective permeability of cores is increased by 24%～59%.7) Core displacement experiments are conducted to determine a pretreatment technique, and to optimize injection volume of HNPs fluid, concentration of HNPs, shut-in time and other parameters under the conditions of reservoir temperature and pressure. Based on the experimental results, the principles to choose the well for filed trial are established, and the technical parameters of filed trial are optimized. Trial projects of three wells were designed, and filed trials of two wells were implemented. The results showed that the injection pressure decrease by 4 to 12.5 MPa while previous water injection rate is maintained.8) Based on the successful simulation of water flow through the HNPs-adsorbed capillary, Lattice Boltzmann method is introduced to simulate the results of HNPs-adsorbed core displacement experiment, and then slip length was deduced on the microporous walls of HNPs-adsorbed core samples.