A Caprock Evaluation Methodology for Underground Gas Storage in a Deep Depleted Gas Reservoir: A Case Study for the X9 Lithologic Trap of Langgu Sag, Bohai Bay Basin, China

Abstract

The evaluation of caprocks’ sealing capacity is exceedingly important for depleted gas reservoirs to be reconstructed into gas storage. In this paper, based on the physical sealing mechanism of caprock, four aspects of ten indexes of caprock quality evaluation were firstly selected, and the related classification standards were established. Secondly, based on the rock mechanical sealing mechanism, elastic and plastic indexes were selected to characterize the mechanical brittleness of caprock, and a brittleness evaluation method of caprock based on complete stress-strain curves was established. Then, a systematic comprehensive evaluation model (including 5 aspects and 12 evaluation indexes) for the sealing capacity of gas storage caprock was proposed, and the analytic hierarchy process (AHP) was used to determine the weight of the 12 indexes in the evaluation model, and the formula for calculating the suitability of the caprock sealing capacity was established. Finally, the geological data, laboratory, and field test data, including X-ray diffraction, poro-permeability test, displacement pressure, and tri-axial compression test, were used for the caprock sealing capacity evaluation of the X9 depleted gas reservoir, and the result from this model showed that the caprock quality is suitable for underground gas storage.

Exploring the evidence of Middle Amazonian aquifer sedimentary outburst residues in a Martian chaotic terrain

“…The presence of an expansive, ancient evaporite stratum beneath the mudstone unit could have served as a gas-impermeable lithological boundary 98 – 100 . This configuration sheds light on a potential gas trap mechanism.…”

“…The stratigraphic configuration in Hydraotes Chaos, characterized by mudstones superimposed on evaporites, mirrors terrestrial mega gas fields. On Earth, argillaceous materials and evaporites commonly serve as caprock materials for gas fields, gypsum salt rock exhibiting the most effective sealing performance 98 . We propose that the mudstone's impermeability could have facilitated the sequestration of volcanic effluents, including carbon dioxide and methane, potentially catalyzing clathrate genesis.…”