Relationship between hydraulic fracture and natural gas accumulation in over-pressured basin:A case study on the LD-A Gas Field in the Ledong slope belt of the Yinggehai Basin
The successful discovery of the LD-A Gas Field in the Ledong slope belt of the Yinggehai Basin confirms the huge hydrocarbon resource potential in high-temperature and high-pressure lithologic trap.The reservoir pressure coefficient in this gas field can reach 2.3,and hydraulic fracture induced by fluid over-pressure plays an important role in controlling the migration and accumulation of natural gas.Therefore,the key to the further study of natural gas accumulation in the Yinggehai Basin is to determine whether any hydraulic fracture occurs under the over-pressure conditions and what its evolution process is.In this paper,the vertical migration system of natural gas is clarified.Then,with the rock failure criterion as the theoretical basis,a hydraulic fracturing mode is determined by using the in-situ stress and the tensile strength of rocks,and the conditions of hydraulic fractures in the formation are clarified.Finally,combined with the over-pressure evolution process,the controlling effect of dynamic evolution of hydraulic fractures on the process of natural gas accumulation is analyzed.And the following research results are obtained.First,faults,sub-earthquake faults and fractures together constitute the vertical migration system of natural gas in the LD-A Gas Field.Hydraulic fractures induced by over pressure control the opening and closing of the migration system and its effectiveness,which determines whether natural gas can be enriched in a large scale.Second,in the LD-A Gas Field,tensile hydraulic fractures are developed in middle-deep layers.The top Huangliu Formation and the top Meishan Formation are currently in the state of hydraulic sealing and hydraulic leakage,respectively in the LD-A1 gas region,and both in the state of hydraulic sealing in the LD-A2 and the LD-A3 gas regions.Third,two types of hydraulic fracture evolution are identified in the LD-A Gas Field,including sealing-fracturing-sealing type(LD-A1 and LD-A2 gas regions)and continuous sealing type(LD-A3 gas region).Different evolution processes of hydraulic fractures lead to different charging and enrichment of natural gas in sand bodies at different horizons.In conclusion,the shallow high-quality sand bodies in the second Member of Pliocene Yinggehai Formation and the Huangliu Formation of Upper Miocene are favorable natural gas enrichment areas in the LD-A1 and the LD-A2 gas regions,while in the LD-A3 gas region,breakthroughs shall be found in the deeper Meishan Formation.
Yinggehai BasinLedong slope beltOver-pressureHydraulic fractureMigration systemNatural gas accumulation
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