Inventory Verification in Underground Gas Storage Rebuilt from Depleted Gas Reservoir: A Case Study from China

Inventory verification is one of vital tasks in underground gas storage (UGS) management process. For one reason, it is possible to know exactly how much natural gas is actually in the gas storage and ensure that it can be produced and supplied to the market in winter season when needed. For another, possible natural gas leakage can be discovered in time by inventory verification, to ensure the safe and economic operation of the gas storage. HTB UGS is a gas storage facility rebuilt from a depleted gas reservoir in China, which has been commissioning in June 2013. After 7 years injection-withdrawal cycles, we calculated and analyzed the inventory of this gas storage. First and foremost, we analyzed the data of 13 observation wells, including monitoring of gas-water interface, caprocks, and faults of the HTB UGS. In addition, we carried out core experiments in the laboratory to simulate the multi-cycle injection and withdrawal of gas storage, and analyzed the microscopic pore seepage characteristics of the reservoir during the UGS operation. Next, based on the operating pressure test data of the gas storage, we corrected the formation pressure and calculated the effective inventory. Furthermore, combined with the simulation results that we have carried out in the previous period, the effective inventory of HTB UGS was comprehensively evaluated. The result shows that: 1) The complete monitoring system indicates that the HTB UGS has no gas escaping from the storage field through faults, caprocks or wellbore. 2) The experimental result shows that in the process of gas withdrawal, various forms of natural gas such as jams and bypasses in some areas of the reservoir cannot participate in the flow, leading to this part of natural gas cannot be used. 3) Inventory calculation shows that as of the end of gas withdrawal in March 2020, the book inventory of HTB UGS is 99.8×108m3，while the effective inventory is 91.8×108m3 and the working gas is 39.9×108m3. 4) By acidification or other measures to improve the geological conditions, intensifying the well pattern and extending the gas production time, HTB UGS can increase its effective inventory. With the great efforts in constructing underground gas storage in China and the market-oriented operation of UGS, inventory verification of gas storage will become increasingly important. The inventory analysis method established in this article can provide a certain reference.

Analysis of Reservoir Fluid Migration in the Process of CO2 Sequestration in a Partially Depleted Gas Reservoir

This paper addresses problems of reservoir fluid migrations in the process of CO2 sequestration in a partially depleted petroleum reservoir. A detailed analysis of the migrations is required to obtain fundamental characteristics of a sequestration structure, including estimation of its sequestration capacity and leakage risks. The paper presents a general discussion of the relevant mechanisms and their contributions to the analysed issues. The proposed approach to solve the problems relies on the usage of numerical structure modelling and simulations of the sequestration processes on numerical models of the structure. It is applied to a selected geological structure comprising a partially depleted gas reservoir. The modelling includes key types of reservoir fluid migrations: viscous multiphase transport and convection transport. It also takes into account other phenomena that affect fluid migrations including injected gas solubility in the formation water and gas trapping by capillary forces. Correspondingly, the leakage risks are associated with distinct leakage pathways (beyond the structural trap, to the caprock, via activated fractures). All these cases are separately modelled and their detailed characteristics are presented and discussed. The final results of the fluid migrations and their consequences for the leakage events are discussed and some generalized conclusions are drawn from the approach employed in the study.

The Feasibility Analysis of an Underground Natural Gas Storage Using a Depleted Offshore Gasfield

Objectives/Scope Compared with the underground nature gas storage (UNGS) onshore, the offshore UNGS is further from residential areas and industrial areas, which can shave the peak of natural gas more safely. However, the investment of offshore engineering is higher. Offshore UNGS with reusing offshore depleted gas reservoir construction can not only solves the problems of offshore engineering structure and wells abandonment but also greatly reduces the investment. Methods, Procedures, Process According to the experience of UNGS onshore, the following factors were considered: reservoir buried depth, working gas volume, distance from land, etc. Based on one depleted gasfield in Bohai Bay, the feasibility analysis of an UNGS was carried out, the impact of the offshore UNGS on the environment, the sealing of the underground gas trap and the integrity of the wellbore were evaluated, and the result proves that the sealing of the underground gas trap of the gas field was good. The natural gas pipeline network subsea can provide transportation for the UNGS and compatible to be transformed into the offshore UNGS. However, for one thing, the poor wellbore integrity conditions and imperfect well pattern constraint the reusing, for another, the capacity of offshore facilities need to be improved. Results, Observations, Conclusions According to the economic evaluation, the investment of offshore depleted gas reservoir reusing as UNGS can be reduced by 56% compared with the construction of a new onshore UNGS, and 32% compared with the offshore gas field abandon. A new idea is provided for peak shaving of natural gas, greatly reducing the investment in gas storage construction. Experience has been accumulated by the feasibility analysis of offshore depleted gas field UNGS and that has a bright future. Novel/Additive Information The limitation of natural conditions on project construction, contingency plans for force majeure such as sea ice, storm and earthquake, etc are necessary to be considered, and its implementation still needs the support of the government and relevant non-governmental organizations

Utilization of CO2 as Cushion Gas for Depleted Gas Reservoir Transformed Gas Storage Reservoir

Underground gas storage reservoirs (UGSRs) are used to keep the natural gas supply smooth. Native natural gas is commonly used as cushion gas to maintain the reservoir pressure and cannot be extracted in the depleted gas reservoir transformed UGSR, which leads to wasting huge amounts of this natural energy resource. CO2 is an alternative gas to avoid this particular issue. However, the mixing of CO2 and CH4 in the UGSR challenges the application of CO2 as cushion gas. In this work, the Donghae gas reservoir is used to investigate the suitability of using CO2 as cushion gas in depleted gas reservoir transformed UGSR. The impact of the geological and engineering parameters, including the CO2 fraction for cushion gas, reservoir temperature, reservoir permeability, residual water and production rate, on the reservoir pressure, gas mixing behavior, and CO2 production are analyzed detailly based on the 15 years cyclic gas injection and production. The results showed that the maximum accepted CO2 concentration for cushion gas is 9% under the condition of production and injection for 120 d and 180 d in a production cycle at a rate of 4.05 kg/s and 2.7 kg/s, respectively. The typical curve of the mixing zone thickness can be divided into four stages, which include the increasing stage, the smooth stage, the suddenly increasing stage, and the periodic change stage. In the periodic change stage, the mixed zone increases with the increasing of CO2 fraction, temperature, production rate, and the decreasing of permeability and water saturation. The CO2 fraction in cushion gas, reservoir permeability, and production rate have a significant effect on the breakthrough of CO2 in the production well, while the effect of water saturation and temperature is limited.