Application of fuzzy mathematics in risk evaluation in geothermal resources exploration in karst reservoir: A case in mountain type karst thermal storage in Chongqing

Geothermal resources of karst reservoir are rich in Chongqing, belonging to the mountain type. The karst thermal reservoir, stripped in shape and distributing independently in the thermal field, is located in both wings beside 23 high uplift anticlines and 1-2 km from the axis of the anticlines in the city. The geothermal resources exploration has a long history, but there are risks. In this study, in order to quantitatively or semi-quantitatively evaluate the risk of the exploration a risk assessment model is established with the fuzzy comprehensive evaluation method. The model contains 9 risk factors: the opening degree of thermal reservoir structure, thermal storage structure, alley cutting degree, geophysical prospecting effect, buried depth, thickness of geothermal reservoir, thermal reservoir strata dip angle, thermal anomalies and thermal reservoir layer. Each risk factor can be divided into three levels, including small, medium and big risk level. Quantitative processing is carried out for this model. Comprehensive evaluation of geothermal resource exploration risk with fuzzy mathematics on 23 high uplift anticlines, 46 high mountain uplift karst geothermal fields (a relatively independent geothermal field on each legs) and 79 geothermal units in this city was performed. The evaluation results indicate that there are 18 units in small risk, 42 units in medium risk and 19 units in high risk. The geothermal field with small risk generally has large heat reservoir catchment’s area and good recharge runoff circulation conditions, with roof heat reservoir being buried 1 000-1 800 m deep, thermal reservoir thickness bigger than 300 m, thermal reservoir strata dip angle being 25-40°, and deep fissure developed, such as the evaluation unit of Xishan anticline at the northern section of the west wing. The geothermal field with high risk has smaller catchment’s area and poor recharge runoff circulation conditions, with roof heat reservoir being buried more than 2 500 m deep, thermal reservoir thickness smaller than 200 m, thermal reservoir strata dip angle <20° or ≥40°, and deep fissure undeveloped, such as the evaluation unit at the southern section of the Xindianzi anticline. The geological characteristic of geothermal field with medium risk is between the two. The evaluation results have nicely reflected the risk state in exploration for the geothermal resources in the karst heat reservoir in Chongqing.