Research On Changes Of In-situ Permeability Near Wellbore In Unconsolidated Sandstone Reservoir With Sand Management

The rock matrix and solid particles of unconsolidated sandstone reservoir are cemented weakly.Reasonable sanding strategy allows partial fine silty sand and clay being produced with crude oil output by optimizing the sand control precision,combining with the control of production well proration.On the premise of meeting the requirements of sand carrying capacity in wellbore and sand processing capacity on offshore platform,it delays the formation plugging and resolves the contradiction between sand control and production.The initiation,migration and deposition of the solid particles will lead to the dredging or clogging of the micro pore tunnels,which will affect the seepage process and seepage law.As a key science problem of nation's 12th Five Year Plan in oil field development(Study on improving single well productivity of multilateral wells with sand production by comprehensive reservoir engineering methods),based on physical simulation test and the flow simulation in micro pore network model,this paper studies the sand control and production strategy of wells in unconsolidated sandstone reservoir and the micro mechanism of sand production management,analyzes permeability changes and permeability distribution affected by compaction and sand migration near wellbore.The research results are applied to sand producing wells of an unconsolidated sandstone reservoir in Bohai Bay.The major contents and achivements of the dissertation are summerized as follows:1.Rock physical properties,main typies of removable sand near wellbore,sand migration in producing process,and sand production management of unconsolidated sandstone reservoir were analyzed.The particulate processes and the affecting factors of fluid prop-carrying capacity were studied based on observing how fluid mixed with sand flows in three-dimensional porous media.The experiment scheme to test how sand migration affects permeability was designed by previous study.2.Penneability changes in diffirent conditions were characterized by core test.The main controlling factors and change patterns of penneability were analyzed.Compaction characteristics of unconsolidated sandstone reservoir and the way permeability changes caused by sand migration were studied.Dredging and clogging determinants of sand migration were summarized.Permeability improvements with different sand production amount were compared.All of these provided a quantitative basis for optimizing the parameters of sand control in sand production management.3.Static digital cores of core samples were obtained by image recognition of micro computerized tomography(?-CT)scanning and image-base three-dimensional(3D)reconstruction.Through the steps of cube division,porosity and connectivity calculation,the static digital core was evolved into micro pore network model.According to the mechanical balance in each connected pore tunnel,the pore tunnels that contain flowing fluid in different driving pressures were sorted out,dynamic apparent permeability of the pore network model in different driving pressure was calculated based on these pore tunnels.Then the nonlinear characteristics of effective permeability at low velocity were studied.4.Upscale the micro pore scale model to reservoir scale by introducing the nonlinear dynamic permeability based on micro pore network flow simulation into continuous flow simulation in unconsolidated reservoir.By taking the characteristics of the initial structural deformation and the volumetric deformation during later diagenesis stage of unconsolidated sandstone,physical property of reservoir,fluid property and production well proration into account,the initial pressure and velocity field near wellbore was calculated.The motion state of movable sand particles in radial direction can be obtained on the basis of the critical velocity that sand start migrating.Based on the sand concentration diffusion model and the sand deposition model,combining with the particle size distribution and the precision of sand control screen,the distribution of dynamic permeability near wellbore was calculated.The pressure and velocity field can be updated in real time,as well as the maximum sand production rate and the production capacity of the sand producing well.5.Take the porosity change as the solid phase flow coupling node to couple rock skeleton and sand migration.Take the boundary condition of dynamic digital core as the scale coupling node to couple the flow in micro pore network and the radial flow in macroscopic reservoir.A multi-scale fluid solid coupling numerical simulation method was established to simulate the variation and distribution of permeability near wellbore with sand production management in unconsolidated sandstone reservoir.Meanwhile,a solid-liquid two-phase reservoir numerical simulation software was developed.6.A case based on a sand producing well of an unconsolidated sandstone reservoir in Bohai Bay was studied.The analysis method established in this paper was applied in the case by calculating permeability distribution near wellbore with different production pressure and sand control screen in different precision,optimizing production pressure drop and sand control precision under the condition of maximum allowable sand production,to maximize oil well productivity.