The Fine Reservoir Description Of Low/Ultra-low Permeability Lithologic Reservoirs

Chang81reservoir of Yanchang formation in Huaqing Area in Ordos Basin is a typical low/ultra-low permeability sandstone reservoir with the characteristics of mainly composed of delta-front sediments, stable formation, good inheritance structure, strong diagenesis, complex micro-pore structure and percolation characteristics, strong non-homogeneous, and excellent "source and facies and dynamic"conditions. Under this background of low/ultra-low permeability, the reservoir has the following characteristics:yielding oil wells and yielding water wells very different between adjacent wells in the same reservoir, and the distribution of oil and water is complex in the reservoir. The relatively high permeability areas have great significant difference. Sometimes, the better reservoir development effect will probably grow less than low growth, and so on. Therefore, it is imperative to give a fine reservoir quantitative and qualitative description and a comprehensive evaluation about the combination of space coupling effect the relations between the every key factor features.Based on the abundant physical geological data and experimental analysis data of several typical samples, the research gives a fine reservoir description by studying the analyzed multi-parameter data from quality to quantity methods and multi-factor of coupling effect the relations between micro and macro.The main conclusions of this paper are presented as follows:(1) Generally speaking, submerged distributary channel sand body in delta front subfacies is the major reservoir in the study area. The sand body is well developed with the stability formation and low amplitude nosing structure with good inheritance. At the same time, the sand body is of medium and somewhat low compositional maturity, medium texture maturity, poor physical property, strong non-homogeneous and strong diagenesis.(2) The experimental results of high pressure Hg injection and constant pressure mercury experiment demonstrate some features of the study area,complexity pore structure, strong heterogeneity, the comparability size of the porous media and fracture profile irregularities of the porous. Micro-pore texture is largely controlled by throat radius in the reservoir, which is small and has large variation.(3) The result of Nuclear magnetic resonance (NMR) test and Oil-Water Two-Phase Flow test experiment shows that the reservoir is of low saturation of mobile fluid, low oil relative permeability, narrow two-phase region and strong Oil-Water Two-phase interference. The micro-filtration characteristics is controlled by diagenesis and micro-pore structure in the reservoir.(4) Chang81reservoir of Yanchang formation is the typical low/ultra-low permeability lithologic sandstone reservoir, and the distribution of oil and water is controlled by "source and facies and dynamic"conditions, micro-pore structure. and micro-filtration characteristics.(5) Based on the characteristics of the flow units, diagenetic facies, micro-pore texture and micro-filtration, four types(Ⅰ,Ⅱ,Ⅲ,Ⅳ) of favorable reservoir zones were divided in the Chang81reservoir, the main favorable zones are the firs(Ⅳ)t and the second(Ⅱ).