Flow Unit Research Of Low Pressure Gas Reservoir With Low Porosity Low Permeability

Reservoir fluid units are kinds of reservoir rock mass distributed continuously in three-dimension space at various level from macro view to micro view which have similar rock characteristics influencing fluid flow and filtering attributes of the fluid itself.They are reservoir genetic units resulted from multiple geological functions include sedimentary process,diagenism and after reconstruction .The study and division of flow units could be used to judge reservoir reasonably and further predict the distribution of reservoir and residual oil /gas.Based on depth survey of reservoir flow units research methods, systematic research of reservoir flow units is carried out in this paper including the division method research,synthetical appraisal,distribution pattern and effect factors of flow unit and correlation analysis between flow units and oilfield production performance.The paper takes Shiheziâ…¢formation for an example of flow units research which is based on the throghout survey of division measures home and abroad,according to factual geological characteristics,current state of exploitation and exhaustive well log data from former research results proposes flow units division of Xiashihezi gas reservoir with the method of mutiple-parameters cluster.In the cluster process, 6 parameters are selected including microfacies type of deposition,shale content,porosity,permeability,reservoir attribute index RQI ,3 types of flow units are finally categorized. A discrimination regression method is used to examine the credibility of cluster result which turns out to be 95.5%.According to the analysis result of flow units division,it could be found that class 1 flow unit belongs to the weak flow unit type which has developed clay stone interbed ,low porosity and permeability ,fine pore and throat size;class 2 flow unit belongs to moderate flow unit type that has moderate-developed mudstone interbed distributing on both sides of the stream channel,moderate throat size and enhanced heterogeneity;class 3 flow unit is the best type with strong heterogeneity .Among the four vertical sublayers H32-1,H31-1 have larger proportion of wells belonging to flow units, H31-2,H32-2 contain a relatively small proportion of flow units while a large proportion of nonreservoir such as mudstone.The division result of flow units shows that the contributing factors and effective affecting factors are the interaction of mutiple elements including sedimentary process which is the determinative factor in flow units division and diagenism that plays a direct role in transformation. Deposit microfacies reflect the micro environmental of deposition which is the minimum unit with unique deposition characteristics in terms of lithology,rock texture,thickness,rhythmicity and certain plain distribution pattern.So microfacies of deposition form the environment with minimum level of sand body which controls not only the heterogeneity of sand body but the oil and water movement pattern underground.The same type of flow unit has similar infiltrating features in which the porosity and permeablity vary within certain limit while the difference of porosity and permeability is remarkable among different flow units.Of the three types of flow units,classâ…¢flow unit posesses the best infiltrating capacity and is the flow unit with best oil and gas occurrence,the microfacies of deposition is mainly composed of point bat,river bed detention and the lithology component include conglomerate,conglomeratic sand,the intergranular space is developed;classâ…¡flow unit is the secondary flow unit for oil and gas occurrence,the microfacies of deposition is mainly composed of crevasse splay and river channel and the sand body includes medium-size sand and gritstone;classâ… flow unit is the worst for oil and gas occurrence,the microfacies of deposition is mainly composed of flood plain,crevasse riverbed,The porosity and permeability of original sand changed by diagenism , the developed secondary pore also adds to the heterogeneity of reservoir. In this region ,the larger the permeability max-mean ratio and max-min ratio are ,the better the flow units are, among which class 2 and class 3 flow units have relatively larger permeability max-mean ratio and max-min ratio ,at the same time,the stronger the heterogeneity is,the better property the flow units have.The difference of flow protential among 3 types flow units is huge. Overall,different flow units are well corresponding to various open-flow potential per meter of effective thickness,that is the open-flow capacity per meter of effective thickness of class 1 flow unit is lower than that of class 2 flow unit which is subsequently lower than that of class 3 flow unit. Meanwhile, fracturing technology also has some effect on flow potential,the larger the formation capacity and fracture treatment size are,the higher the open-flow potential is,in class 1 flow units,some wells have low permeability while high in other flow capacities thus the flow potential is large;in class 3 flow unit,some wells have good porosity and permeability while other flow capacities are low thus the deliverability is low .The research of flow units division in this region could help to deepen theunderstanding of the distribution pattern of reservoirs in plain or vertically.3 blocks are divided according to well density, block 1 locates in the west corner with the density of 7.64 wells per square kilometer and inter-well interference exists in this region;block 2 locates in the northern part with the density of 2.38 wells per square kilometer;block 3 stretches to the north and middle part of east border with low density of 0.28 wells per square kilometer. In a word, class 2 and class 3 flow units account for a less proportion of area than that of class 1, class 2 has the relatively more wells and largest well density while class 3 has a medium size of well density.Different flow units demonstrate various production characteristics, the wells of class 1 flow unit have low production and weak gas supply capacity due to inefficient producing energy; the ones of class 2 flow units have efficient producing energy but influenced by liquid loading at the bottom of the wells,the production fluctuates much;the wells of class 3 flow units have efficient producing energy and could maintain high producing capacity for a long period. After all, the division of flow units could help to predict production performance and provide theoretical basis for further development.