“ Four properties ” geological evaluation model and resource type classification of non-well type UCG site and layer selection in Guizhou Province

In order to establish a scientific and systematic “geology-engineering” integrated geological risk assessment model, so as to solve the problem in the site and layer selection of underground coal gasification (UCG) project, a new hierarchical structure model of geological evaluation elements including 4 first-level indicators, 10 second-level indicators and 26 third-level key geological parameters is established using the FAHP fuzzy analytic hierarchy process based on the four characteristics of 'feasibility of furnace construction, easy control of process, safety of gasification and economy of development'. The model introduces the concept of “threshold”, and implements “one-vote veto”when exceeding the thresholds of the four indicators including fault index, Oya expansion degree, bond index and distance of overlying aquifer, and pays full attention to the key role of specific geological parameters in the UCG control. The weight vector of each index is determined by “pairwise judgment matrix”. According to the membership function, the membership degree of the index layer is determined. The corresponding geological evaluation techniques and methods are put forward, and the algorithms of weight product, comprehensive weight product sum and optimal three-segment segmentation are established, so as to quantitatively analyze the indicators involved and accurately assess the type and level of resources. In addition, based on the mixed coding method of matrix structure, a UCG resource type representation system of “four properties and four levels”five-bit coding is proposed. Finally, a scientific and systematic geological evaluation system of UCG site and layer selection is formed, which is based on the understanding of “four properties”, “algorithm” as the skeleton, and “four properties and four levels” five codes as the soul. This evaluation system can play a key role in the frontier exploration of risk identification mechanism related to UCG production process with multi-field coupling. It can effectively overcome the problem that the existing UCG risk identification theory is limited to geological unilateral and lacks engineering connection. In order to realize the UCG quantitative site and layer selection in a large area, it provides an efficient solution for the strategic planning of underground coal gasification at the provincial and even national levels. The study puts forward the suggestions for the pilot test area of typical geological conditions that can be implemented step by step, and effectively promote the industrialization process of underground coal gasification.