Hydraulic interactions of subsurface reservoirs used for excess energy storage
Authors: Ma Jianli , Kempka Thomas , Chabab Elena , Li Qi , Kühn Michael 
Published: March 2020

&lt;p&gt;Excess electricity produced from renewables can be converted into CH&lt;sub&gt;4&lt;/sub&gt; by consuming CO&lt;sub&gt;2&lt;/sub&gt; and H&lt;sub&gt;2&lt;/sub&gt; by means of the Power-to-Gas (PtG) technology&lt;sup&gt; [1]&lt;/sup&gt;. Previous work indicates that subsurface storage of CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; can meet the projected energy storage requirements &lt;sup&gt;[1] [2]&lt;/sup&gt;. However, gas mixing occurs if both gases are stored in the same reservoir &lt;sup&gt;[3]&lt;/sup&gt;, and energy is lost if CH&lt;sub&gt;4&lt;/sub&gt; is used as cushion gas when both gases are separately stored in different reservoirs &lt;sup&gt;[2]&lt;/sup&gt;. Therefore, an innovative approach to overcome the limitation of aforementioned storage schemes is introduced in this study. For that purpose, the focus is on a double reservoir setting in one anticline system as it is commonly found in, e.g., the Northern German Basin. Here, the confining layer and preexisting or artificial hydraulic connections between the two reservoirs enable the operator to reduce energy losses and avoid gas mixing. We have elaborated a numerical multiphase flow model including the wellbore systems and reservoirs to study the fluid flow and beneficiary effects of pressure interaction between both reservoirs. Based on the geological and operational data of our regional showcase in Germany &lt;sup&gt;[4] [5]&lt;/sup&gt;, the energy storage efficiency is quantified, and the potential benefits of the proposed storage scheme are evaluated. It shows that the production of CH&lt;sub&gt;4&lt;/sub&gt; increases by 68% over twenty years of injection and production. Furthermore, the factors that affect storage efficiency are analyzed to provide information for the optimization of PtG-based subsurface energy storage systems. The simulation can be applied to different geological systems and for parameter sensitivity studies to reduce energy losses and improve storage efficiency.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Keywords:&lt;/strong&gt; Power-to-Gas; Subsurface gas storage; Carbon dioxide; Methane&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;[1] K&amp;#252;hn M, Nakaten N, Streibel M, Kempka T. CO&lt;sub&gt;2&lt;/sub&gt; Geological storage and utilization for a carbon neutral &amp;#8220;Power-to-gas-to-power&amp;#8221; cycle to even out fluctuations of renewable energy provision. Energy Procedia. 2014; 63:8044-9.&lt;/p&gt;&lt;p&gt;[2] Ma J, Li Q, K&amp;#252;hn M, Nakaten N. Power-to-gas based subsurface energy storage: A review. Renewable and Sustainable Energy Reviews. 2018; 97:478-96.&lt;/p&gt;&lt;p&gt;[3] Ma J, Li Q, Kempka T, K&amp;#252;hn M. Hydromechanical response and impact of gas mixing behavior in subsurface CH&lt;sub&gt;4&lt;/sub&gt; storage with CO&lt;sub&gt;2&lt;/sub&gt;-based cushion gas. Energy &amp; Fuels, 2019; 33 (7), 6527-6541&lt;/p&gt;&lt;p&gt;[4] Streibel M, Nakaten N, Kempka T, K&amp;#252;hn M. Analysis of an integrated carbon cycle for storage of renewables.&amp;#160;Energy Procedia&amp;#160;40 (2013): 202-211.&lt;/p&gt;&lt;p&gt;[5] K&amp;#252;hn M, Streibel M, Nakaten N, Kempka T. Integrated underground gas storage of CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; to decarbonise the &amp;#8220;power-to-gas-to-gas-to-power&amp;#8221; technology.&amp;#160;Energy Procedia&amp;#160;59 (2014): 9-15.&lt;/p&gt;