Research On Geophysical Characterization Method Of Tight Gas Reservoir

In recent years, with the decrease of the conventional oil and gas resources and substantial growth of energy demand in the world, conventional oil and gas can’t completely meet the demand of market, hence the contradiction between energy supply and demand is more and more obvious. China has became a net importer of more and more resources, and the demand for imported energy is increasing year by year. In this situation, the exploration and development of unconventional oil and gas has received considerable attention. Tight sandstone gas is one of unconventional oil and gas resources, which is widely distributed in the Ordos, Sichuan, Tarim, Qaidam, Songliao and Junggar basins in China, mainly concentrated in the clastic reservoir in Upper Palaeozoic in the basin of Ordos and Upper Triassic Xujiahe Formation in the basin of Sichuan. Because of its great resource potential, tight sandstone gas is receiving more and more attention.Tight sandstone reservoir in China has typical characteristics with low porosity and low permeability, high heterogeneity, larger range of effective reservoir thickness and poor continuity. The porosity is generally lower than12%, and permeability is generally less than0.1×10-3μm2. The complicated geological conditions increase the difficulty of tight sandstone gas reservoir prediction.Aiming at the practical difficulties of the tight sandstone exploration, this paper systematically studied the large amount of information about the tight sandstone reservoir. Basis on a comprehensive understanding of the tight sandstone reservoir characteristics, this dissertation makes a thorough analysis and study of geophysical characterization methods of the tight sandstone reservoir. Wave impedance inversion is one of the main methods used in geophysical reservoir characterization, and it usually predict reservoir using wave impedance from seismic inversion. However, seismic attribute from wave impedance inversion is unitary, therefore it is unable to predict tight gas reservoir effectively. AVO method is a kind of geophysical methods based on strict knowledge representation and clear geological significance, the application of which is based on the change of Poisson’s ratio. Domenico theory and Boit three-phase flow theory have confirmed that a small amount of gas can cause a rapid decline in poisson’s ratio. Considering the relationship between reflection coefficient and incident angle, the method can get more attribute parameters from the pre-stack seismic data (P-wave velocity, S-wave velocity, Density, Poisson ratio), so it can improve the precise of reservoir characterization. Besides, in the process of inversion, actual wells are not involved, so the inversion result will not affected by well data. Experience from previous tight gas reservoir research in test area also shows that prestack AVO method has achieved good results in tight gas reservoir prediction. So whether in theory or in practical application, AVO method should be a suitable characterization method of tight gas reservoir.On the basis of this, in this paper,taking the tight sandstone reservoir in Jiannan area as an example, we predict the hydrocarbon by applying AVO method. Xu-6formation belongs to typical delta front sedimentary facies, and has the characteristics of lithology dense, high heterogeneity, gas and water cannot separate completely.Based on the comprehensive analysis and study of seismic and geological data, according to the geological characteristics of the area, we make full use of more information contained in prestack seismic data. On the Basis of rock physics analysis, AVO forward modeling, we predict the tight gas reservoir by implementing AVO inversion and attribute analysis. Firstly, through the research of petrophysical properties, we understand the elastic parameters characteristics of tight gas reservoir, establish the relationship between elastic parameters and reservoir parameters, and determine the sensitive parameters which can reflect the hydrocarbon. Then by means of studying the forward modeling, we receive the response characteristics of tight gas sandstone. Through the fluid replacement, simulating the response characteristics of different saturation conditions, we study the effect of fluid changes towards AVO attributes. Through AVO inversion, we extract several AVO attributes. By analyzing abnormal response and log interpretation results, in combination with the geological significance of attributes, rock physics and the forward model research results, attributes parameters which reflect hydrocarbon can be finally determined. Finally using multi-attribute comprehensively predict the plane distribution of tight gas reservoir, and simultaneously combine with the well logging interpretation, the well testing results to analyse the reliability on the result.Through the above several aspects of research, we obtained the following conclusions:(1)Rock physics analysis shows:Being filled with gas, the acoustic impedance of the tight sandstone decreases and can’t be separated from the acoustic impedance of the mudstone; Poisson’s ratio of the tight sandstone decrease, significantly lower than the poisson’s ratio of tight sandstone and mudstone.(2) AVO forward model study of the actual wells shows:the AVO abnormal response of gas-bearing sandstone belongs to the class III and the characteristic is that reflection amplitude negative increases with incident Angle increase. The response characteristic of water-bearing sandston is that reflection amplitude negative decreases with incidence Angle increase. Fluid replacement simulation shows that with the increase of gas saturation, P value and G value all decrease. Rock physics analysis and forward modeling study demonstrate the feasibility of AVO method for tight sandstone reservoirs.(3) Through the contrast and analysis of the whole area, AVO attributes parameters that reflect reservoir hydrocarbon are finally determined:Poisson’s ratio, Fluid factor and P-wave velocity. Use these three attributes comprehensively predict the plane distribution of tight gas reservoir, according to the well logging interpretation and the testing results, illustrate that AVO predicting results is reliableThrough this research, combining the conclusion from previous tight gas reservoir research in test area, we finally confirm that prestack AVO method is suitable for characterization study of tight sandstone reservoir, therefore it can provide foundation and guidance for the distribution of tight gas reservoir.