Pore Structure Characterization Of Low Permeability Sandstone Reservoir And Application

As for extensive micron and sub-micron pore throat system, there’re two main problems during the exploration and development process of low permeability sandstone reservoir. One is the serious water blocking damage. The other is the low accuracy of reservoir evaluation based on resistivity log interpretation. These problems restrict the large-scale and efficient development of low permeability sandstone reservoir. So far, the traditional experimental and theoretical studies have failed to obtain a satisfied break. And this is mainly owing to insufficient recognition of the essential contact and acting mechanism between the internal factors of reservoir (include microstructural characteristics and properties of the pore fluid distribution) and macroscopic representation (such as permeability damage and resistivity). But the present experimental conditions cannot regulate these microeconomic constraints factors into quantification research. Therefore, the technology of characterizing the micro pore structure of low permeability sandstone has become a key of the efficient development of this kind of reservoirs. Based on the microscopic pore structure model, developing numerical simulation methods and a better understanding of the microscopic mechanism, the technique has important theoretical and practical value in supporting logging interpretation and evaluation and directing the process of the injection of well fluid.This paper was based on micro-CT scanning experiments on low permeability sandstone. By means of advanced image processing techniques, the author created micro three-dimensional refined models of pore structure and researched on the quantitative characterization of pore-structure parameters (included digital core and simplified characterization equivalent pore network model). Based on these digital models, several experiments were conducted, including single-phase flow simulation at pore scale, liquid trapping static simulation, dynamic virtual "spontaneous imbibition" and digital rock electricity tests. Taking advantage of the flexibility of numerical simulation methods, the paper discussed the impact of internal factors (such as wettability, saturation and microscopic pore structure parameters) on the macro water blocking damage and the rock electrical properties. The main results and understandings are listed as follows:(1) Establish a set of technique of refining pore structure modeling suitable for low permeability sandstone. By comparing the porosity error of four kinds of threshold segmentation algorithm results, the author obtained a method of image segmentation based on digital image binarization according to experiment-measured porosity; using graphics-based distance transform skeleton extraction algorithm to obtain the axis of the pore space, the pore network model can reserve real topology of pore space to the greatest extent; comparing the statistical distribution of the relevant parameters, the author verified the equivalence significance of the simplified characterization pore network model and the digital cores.(2) Established a new method to calculating absolute permeability, by means of data exchange between Avizo and Comsol software, realizing the single-phase fluid flow process simulation in real pore space. By comparing with porosity-permeability data of lab experiments, the author verified the reliability of the method and the accuracy and representative of the digital model.(3) Based on digital core theory, the author used mathematical morphology algorithm to achieve static simulation of the gas-water two-phase distribution under different pore space saturation. The author also formed intuitive understanding of "liquid trapping" process which is one mechanism of the water blocking at the same time.(4) By means of the pore-scale flow simulation method, the author carried out dynamic virtual "spontaneous imbibition" experiment based on the equivalent pore network model. The results were consistent with capillary self-absorption physics experiments, which validated the numerical simulation algorithm. The author defined the relative permeability index as an evaluation index to describe the dynamic changes of gas-phase permeability curves under single variable. Based on that, the author quantitatively linked the internal factors with water block damage degree. Thereby the author obtained a regularity understanding:for the low permeability sandstone reservoirs in this study, as long as changing the capillary environment from strong water-wettability to weak gas-wettability, the gas-well productivity will be significantly improved. Larger difference between initial water saturation and bound water saturation causes more serious water blocking damage; the shape of gas phase permeability curves has a great relation with the uniformity degree of the distribution of rock pore and throat.(5) As for some low permeability sandstone that beyond micro-CT effective resolution capacity (porosity between8%-12%and permeability less than0.3×10-3μm2), random pore network models that satisfied certain constraints were constructed to investigate the influence of microscopic pore structure factors on the degree of water blocking damage. The results showed that:the degree of water blocking damage has a negative correlation with the coordinate number, while a positive correlation with the pore-throat ratio; under the conditions of this study, the formation composited of medium-sized throats is more prone to water blocking. Therefore, the study suggests that there exists a critical throat radius making the water blocking most serious.(6) Three-dimensional simulation technology on pore scale of rock electrical property is also applicable of complex reservoirs like low permeability sandstone. Micro factors, such as wettability, pore structure parameters et al., have an important influence on rock electrical characteristics. The resistivity of water-wet reservoir is lower than gas-wet reservoirs. And the difference is more obvious at low water saturation phase. Among microscopic pore structure parameters, throat radius and pore-throat ratio showed a well positive correlation with resistivity index, while the resistivity index decreases with the coordination number increases; this effect is particularly prominent under low water saturation. The pore structure and wettability mainly control the distribution of pore space fluid and current conduction pathway.