A nanometer (10-9m) structured particle material, generally so defined that the diameter of the particle is no more than 100nm, has some special physical efficacy in its surface, small size and other properties. One kind of polysilicon with sizes ranging from 10~500nm, and considered as nanometer or sub nanometer sized powder, was used in oilfields to enhance water injection by changing wettability of porous media. The mechanism of enhancing water injection is through improving relative permeability of the water-phase by changing wettability induced by adsorption of polysilicon on the porous surface of sandstone. On the other hand, the adsorption on the porous surface and plugging at the small pore throats of the polysilicon may lead to reduction in porosity and absolute permeability (K) of porous media for pore sizes from 100 to 1000,000nm. Thus the degree of success in well treatment is determined by the improvement of effective permeability of the water-phase. In this paper a mathematical model, which was combined with the study of experiments in the laboratory, is presented and a simulator is developed to simulate water injection dynamics under the conditions of polysilicon injection. The simulator can accurately simulate the process of migration and adsorption in the pore bodies and blocking at the pore throat of the polysilicon in the sandstone. A series of numerical simulation runs was conducted to study the effect of a wide range of parameters, such as the sandstone with different permeabilities, concentration of the polysilicon, injection volumes, and others. The effective permeabilities of the water-phase measured by a number of core flooding experiments are matched well by the numerical results. Since April 2000, nine well treatments with solvent slugs of suspended polysilicon particles in several oilfields in China was shown to be successful and the average injection rate increased 5 times after treatments. Introduction A nanometer particle, generally defined as its size from 1 to 100nm and invisible with the naked eyes or ordinary microscope, is referred to as a nanometer scaled ultra fine particle in its size which is larger than an atom cluster and smaller than ordinary micro-powder. Nanometer technology originated at the end of the 1980's and is developed into a new high technology, by which new materials can be formed by rearranging atoms or molecules. A nanometer structured particle material has some special physical effects in its surface, small size, quantum size and macro-quantum tunnel1. Nanometer particle material has a large specific surface area, which increases rapidly with the decrease in diameter of particle. The large surface area leads to an increase in the proportion of atoms on the surface of the particle, which results in an increase in surface energy. The deficiency of atomic coordination and high surface energy leads to the unsteady, high activity of atoms on the particle, the increase in tendency of combination with other atoms, and the appearance of active cores. Non-chemical equilibrium and coordination of non-integer numbers lead to considerable difference in chemical properties and chemical equilibrium systems for nanometer powder. Analogously, sand rock, which is composed of grains with different sizes, is porous media deposited under the combination of consolidation and compaction throughout a long geological period, and also has large specific surface area. Since the property of the surface of minerals determines the wettablity of porous walls, and the wettability of reservoir rock governs, to a great extent, the location, flow, and distribution of oil, water, and gas in a reservoir, the distributive characteristics, relative permeability of water and oil and flow dynamics of fluids in porous media can be changed by modifying the wettability of porous walls. Accordingly, the process of development of a reservoir can be improved by wettability modification.
Sand fines release and migration is a universal problem in the production of oil from unconsolidated sandstone reservoirs, which can result in both sanding problems and profound effects on oil recovery. A new three dimensional (3D) field scale mathematical model, differing from those used for conventional oil reservoir numerical simulator in that both the advanced theories of sand particle release and migration, is presented. And the model is solved by a finite-difference method and the line successive over relaxation (LSOR) technique. A numerical simulator is written in Fortran 90 and VC++ and it can be used to predict (1) the sand content in produced liquid, (2) the porosity and permeability changes caused by sand release and migration in formation, and (3) well production performances and residual oil distribution. A series of runs of oil field examples with five-spot patterns were made on the numerical simulator. The results show that sanding problems in the oil formation can accelerate the heterogeneity of the reservoir rocks, and has an obvious influence on production performances: water-drive process, water-cut trends, and oil recovery. In a conclusion, the new simulator improves the ability and accuracy for numerical simulating the development of the unconsolidated sandstone reservoirs. Introduction Sanding problems during oil production from unconsolidated sandstone reservoirs, such as Gulf of Mexico reservoirs[1] (Deskin et al., 1991), the Southeast Pauls Valley Field, Oklahoma, and oil fields around the Bohai Gulf, China, may lead to much adverse influence on the production facility. Several papers (Anne et al., 1997[2], Davies et al., 1997[3], Tom et al., 1995[4], Vásquez et al., 1999[5]) have reported the production facility damage and completion difficulty as a result of sanding. Anne et al. (1997[2]) have also studied the relations between water breakthrough and sand production. In fact, sand production can lead to other severe problems such as formation collapse and effects on improving oil recovery. The sanding process includes three steps: first, sand particles are released from the surfaces of porous media when the critical colloidal or hydrodynamic conditions are satisfied (Ju et al., 2002[6]); second, sand particles migrate in pores with flowing fluids; finally, particle deposition on pore surfaces or capture at pore throats may occur in the process of sand migration. Therefore, the phenomena of particle release, migration and retention must be considered in the mathematical model for sanding. According to the literature concerning sanding of formations (Gruesbeck et al., 1982[7], Khilar et al. 1983[8], Liu and Cvian, 1994[9], Ohen, et al., 1990[10], Sharma and Yortsos, 1986[11]), two kinds of models, the macroscopic mathematical model and the microscopic network mathematical model, are classified. The first kind of model, such as Gruesbek and Collins's model[7] (1982), Khilar and Fogler's model[8] (1983) and Ohen and Civan's model[10] (1994), is based on the theories of the flow in macro-continuous porous media and sand particle release and migration. However, the second kind describes the flow characters of fluid and fines migration in micro-networks. Sharma and Yortsos'model[11] (1986) belongs to the microscopic work mathematical model. Unfortunately, the microscopic work mathematical model has some limitations in that it strongly depends on probability, and its numerical solving process uses tremendous amounts of computer time. Consequently, the theories of macro-continuous porous media and fines migration are used to develop a three dimensional (3D) mathematical model for fines migration in oil formation in this paper. Currently, the major approaches to study the sanding process in permeable formations are physical simulation in the laboratory and mathematical simulation. This paper focuses on the mathematical model to simulate sand release and migration process and behavior, the solution methods of the model, and the development of an oil reservoir numerical simulator.
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