Research On Niche Ant Colony Algorithm Of Parameters Inversion In Complex Media

Because of the scarcity of resources in today's world, especially the increasing demand for oil and gas resources, the theoretical study and practical application in oil and gas exploration have been the concern of researchers around the world. In particular, resource distribution in our country is more decentralized and more concealed, which increased the difficulty of oil and gas exploration. Therefore, parameters inversion problem in complex media plays more and more important role in the practical application. For the actual underground media, most of reservior are made of two parts including fluid and solid, namely two-phase media, such as standatone reserior and carbonate reservior. The single-phase medium theory which is too idealistic has a certain gap with the real world. The discription that two-phase medium theory made for actual stratum structure and stratum properties is more accurate. Thus, two-phase medium theory has been widely used in geophysical research such as earthquake engineering, geophysical exploration and soil dynamics. The research focus is also biased in favor of two-phase medium theory gradually. For carrying out exploration in oil and gas reserviors, we need to know the reservior parameters, such as porosity, permeability, saturation and so on, to predict the stratum structure. However, we have to adopt the geophysical inversion in order to obtain these parameters generally.Based on the analysis and summary of several traditional inversion methods, this thesis introduces the basic theory and research status of ant colony algorithm, Biot model and BISQ model in two-phase medium in detail. Taking into account the reservior parameter inversion problem is extreme value problem of nonlinear milti-modal function. In order to prevent"preneture"and avoid a global optimal falling into a local optimal, we proposed the niche ant colony algorithm based on the fitness sharing by improving the niche ant algorithm. First, we use the improved niche ant colony algorithm in multi-modal function optimization problem, and we compare the results obtained by niche ant colony algorithm with the results found by niche genetic algorithm. It can be shown that the former has higher effectiveness and superiority in both local optimization and global optimization. Second, we perform the inversion of reservior parameters such as porosity and permeability by tne niche ant colony algorithm based on the Biot model and the unsaturated porous media BISQ model, and we get better inversion results.In the parameter inversion based on the Biot model, according to the analytic solutions of transient displacement response of one dimensional wave equation in two-phase medium and the true value of the two-phase medium material, we can obtain accurate surface displacement and fluid displacement, which is the additional condition of inversion. Then, calculate the corresponding surface displacement and fluid displacement in the inversion intervals of pre-inversion parameters based on the analytic solution. The objective function is the sum of absolute value of the two types of displacement differences. The parameters value when the objective function attained the minimum are the inversion value of parameters we require. In the parameter inversion based on the BISQ model, we calculate the actual value of transverse and longitudinal waves according to the high-dimensional unsaturated porous media BISQ forward model and the true value of each parameter. Then, calculate the theoretical predicted velocity of transverse and longitudinal waves in the inversion intervals of pre-inversion parameters under the forward model. The objective function we selected is the sum of absolute value of the two kinds of velocity. The inversion results are just the parameters value when the objective function attained the minimum.In this thesis, the niche ant colony algorithm adopted is applied to paraneters inversion for the fisrt time in geophysics, the results achieved is very well and the inversion accuracy is very high. In the parameter inversion based on the Biot model, the relative error of single parameter inversion are all less than 0.4 percent, and the relative error of two parameters (porosity and permeability) inversion can reach about 7 percent. In the parameter inversion based on the BISQ model, the relative error of single parameter inversion can be maintained at less than 0.08 percent, the relative error of two parameters (porosity and saturation) inversion can attain the same accuracy with single parameter inversion, and the inversion effect of three parameters (porosity, solid density and fluid density) inversion is a bit weaker, but the relative error can still maintain less than 4 percent. In the process of inversion, we found the demand for the number of the ants in the algorithm is not high, as long as very few individuals, the inversion results can achieve higher accuracy. It represented the effectiveness and the superiority of the improved niche ant colony algorithm applied to the parameter inversion in complex media. Therefore, the inversion algorithm we adopted in this thesis is expected to be widely used in oil and gas reservoir dynamic monitoring development, static description and the theoretical study and practical application of earthquake.