Classification and evaluation of low-permeability sand reservoir based on micro-pore structure

The undeveloped low-permeability oil reserves of the Liushagang Formation in the Beibu Gulf Basin are large, but the reservoir quality is poor. The existing classification standards for low-permeability reservoirs cannot meet the needs of fine reservoir evaluation and offshore development.Based on the experiments of constant rate mercury injection, high pressure mercury injection, NMR and seepage mechanism, this paper comprehensively analyzed the micro-pore structure and seepage characteristics, and optimized six kinds of reservoir evaluation parameters: reservoir quality index, fractal dimension of pore structure, mainstream throat radius, mean throat radius, percentage of movable fluid and change rate of threshold pressure gradient, meanwhile, the classification limit of various characterization parameters were determined by integrating with the reservoir quality index, reservoir seepage ability evaluation factors and specific productivity index constraint parameters. A comprehensive evaluation criterion for low-permeability oil reservoirs is established based on the Grey Relational Analysis method, which is divided into Ⅰ, Ⅱa, Ⅱb, Ⅲ and Ⅳ from good to bad.Carried out the grading evaluation of low-permeability reserves and the research results provide a theoretical basis for the analysis of the development potential, the research direction of the low-permeability reserves in Beibu Gulf Basin.The results show: types Ⅰand Ⅱa are relatively high-quality low-permeability reserves and have been put into large-scale development.Types Ⅱb and Ⅲ are extra-low-permeability reserves and are difficult to develop, which need to be formulated for experimental development by reasonable technical measures and development models.