Sealing Mechanism of the Roof and Floor for the Wufeng-Longmaxi Shale Gas in the Southern Sichuan Basin

Abstract

Exploration activities revealed that high-efficient sealings of the roof and floor for the Wufeng-Longmaxi (WF-LM) gas-bearing shales were the indispensable conditions for shale gas preservation and enrichment in the Sichuan Basin. However, little attention has been paid to their sealing mechanism and sealing model. In this paper, systematic core samplings (Ning-A and YY-B) and a series of supporting experiments (geochemical and rock components analysis, physical properties measurements, pore structure characterization, and sealing capacity tests) were carried out for further analyses and discussion. Results show that the roof and floor have four-level vertical sealing capacities for the WF-LM shale gas, where the direct roof is the argillaceous shale and silty shale of the LM1–2 and LM1–3 members (the middle and upper members of the first part of the Longmaxi Formation) and the floor is the nodular limestone of the Linxiang Formation (LX Fm.). Heterogeneous lithofacies and steady thickness of the roof and floor lead to strong lithologic sealing capacities. The roof and floor are characterized by higher density and water saturation, lower porosity and permeability, lower pore volumes, and poor pore connectivity, resulting in developing larger capillary pressure at the interface and thereby forming strong petrophysical sealing capacities. Based on the sealing model, the great sealing capacities are attributed to the well-developed laminar fabrics, lower TOC content, less (organic matter) OM-related pores, and the compacted clay mineral fragments reoriented into lamellar ones, as well as the unconnected clay-related interparticle pores in the roof shales and high breakthrough pressure in the floor. This study will provide guidance for evaluating the preservation condition and sweet spot prospecting of shale gas exploration in the Sichuan Basin.

Asymptotic Model of Breakthrough Pressure in Partially Saturated Porous Media with Microvisualization Step-by-Step Breakthrough Experiments

“…In this experiment, PEG200 was chosen for the wetting phase with its viscosity of 42 mPa·s . Capillary pressure at the gas–liquid interface is the main factor preventing gas from breaking through the reservoir caprock core porous media and leaking. ,,− Air, CO 2 , CH 4 , and CH 4 –CO 2 mixture were usually used as gas phases in breakthrough experiments. To reduce the operation difficulty, the air was applied as the gas phase to perform breakthrough experiments since the influence of the interface effect on gas breaking through porous media was only considered in this work.…”

“…Gas breakthrough behavior in partially saturated porous media (PSPM) has inspired extensive attention in hydrocarbon enrichment and exploration, − CO 2 geological sequestration, − CO 2 flooding, deep geological treatment of high-level radioactive contaminants, , etc. These practices involve low and ultralow permeability reservoir caprock core porous media, which exhibit significant sealing ability …”

“…When the sealed gas pressure under the reservoir caprock is higher than the caprock core capillary pressure, the gas will break through the caprock core’s restraint and spill into the biosphere. Then, it will lead to an easy leak of the hydrocarbon and sequestered CO 2 , and destruct the reservoir. , The sealing ability can be featured by the minimum pressure at which the nonwetting phase breaks through partially wetting-phase-saturated porous media to form a complete path and can flow continuously. , In previous studies, uniform terms were used to describe this characteristic pressure, such as threshold displacement pressure, , displacement pressure, ,, breakthrough pressure, ,,,− minimum displacement pressure, − etc. This paper adopts breakthrough pressure.…”

Dynamic Accumulation of the Quaternary Shale Biogas in Sanhu Area of the Qaidam Basin, China

“…For example, compared with the Antrim shale in the United States and Yangxin shale in the Jiyang Depression of Shandong Province, the types and sizes of reservoir spaces have different characteristics [15]. The reservoir spaces of Antrim shale and Yangxin shale gas reservoirs in the Jiyang Depression are dominated by natural fractures, while the pores of shale in the Sanhu area are relatively developed (porosity can reach 10~35%) and the physical properties are better, which are more suitable for the formation of shale gas (Table 1) [16,17]. Therefore, the study and analysis of the formation conditions of biological shale gas reservoirs are of the most practical significance for the increase of natural gas reserves and production in the Qinghai Oilfield [18][19][20].…”

Dynamic Accumulation of the Quaternary Shale Biogas in Sanhu Area of the Qaidam Basin, China

“…For example, compared with Antrim shale in the United States and Yangxin shale in Jiyang Depression of Shandong Province, the types and sizes of reservoir spaces have different characteristics [15] . The reservoir spaces of Antrim shale and Yangxin shale gas reservoirs in Jiyang Depression are dominated by natural fractures, while the pores of shale in Sanhu area are relatively developed (porosity can reach 10%~35%) and the physical properties are better, which are more suitable for the formation of shale gas [16,17] . Therefore, the study and analysis of the formation conditions of biological shale gas reservoirs are of the most practical significance for the increase of natural gas reserves and production in Qinghai Oilfield [18,19,20] .…”