Reservoir gas losses at UGS facilities in aquifers

Experience shows that in the process of creating and long-term cyclic operation of underground gas storage facilities (UGS) in reservoirs of aquifers, reservoir gas losses can occur. Losses can reach tens of percent of the injected volumes of gas into the reservoir and have a significant impact on the reliability and safety and efficiency of operation of UGS. In this regard, the issues of the organization of field control and methodology for assessing reservoir gas losses at UGS are relevant. The article proposes the structuring of the gas injected into the reservoir into possible reservoir components, taking into account the state of the gas phase and participation in filtration mass transfer processes. The main factors determining the formation of different reservoir components are given. The basic concepts are considered and the definition of reservoir gas losses at UGS in aquifers is given. The main features of one or another type of reservoir gas losses are shown. Reservoir losses from free gas currents from an artificial gas deposit, which can occur as a result of vertical leaks from the storage facility and lateral gas escapes through the reservoir beyond the trap, are briefly described. Formation losses associated with gas adsorption by rocks, gas phase transitions and gas saturation of low-permeable sections of reservoirs; gas dissolution in invading reservoir water and its convective-diffusion entrainment by displaced water into the aquifer region of the reservoir are also considered. Using the example of a UGS created in an aquifer, the system of geological and commercial monitoring of a subsurface area within a mining branch in conditions of vertical interplastic flows of free gas is considered. It is shown that the implemented system of observation and control wells allows for adequate monitoring of the gas storage facility and control of the tightness of UGS throughout the section above the storage facility. The following components of reservoir gas losses at the storage facility are considered: dissolved gas in residual water within the gas reservoir; gas adsorbed by rocks within the gas reservoir; dissolved gas diffused from the gas reservoir into the contact aquifer region of the reservoir; dissolved and free gas in the control horizons. Using a geological model of the formation, as well as the results of modeling the convective-diffusion transfer of dissolved gas into the aquifer region of the formation, the assessment of the components of reservoir gas losses in a direct way by their locations is given.