Risk Evaluation For Safety Of Hydrocarbon Exploitation And Geologic Storage Of Natural Gas In Fault Realated Traps

The fault related trap or fault trap is one of basic cells where hydrocarbon accumulates.Its seal integrity is directly related to the risk of petroleum exploration,exploitation and geologic gas storage in the depleted hydrocarbon pools.First,we studyed the architectures of fault zones developed in clastic sequences using core samples provided by two wells including core zone and damage zone in the main borehole,and analysised controls of hydrocarbon preservation such as permeability and strength of fault zones.And then we established permeability and strength model of fault zones in clastic sequences according which a methodology for quantifying the risk of fault trap seal integrity was made.There are two projects in this study including?Safety exploitation of remaining oil in fault traps in Xingshugang oil field in Daqing placanticline?and?Geological gas storage in depleted pools in fault traps in Dagang oil field?,and conclusions we get is followings.The fault zones at both boreholes of the two wells consist of a strongly deformed core zone that includes fault breccia and clay smear with the characteristic of low permeability and low strength surrounded by less deformed rocks as damage zone.Microstructures such as micro-scale faults,fractures and deformation bands developed in damage zone which density is decreased with the increasing distance to core zone,some of the micro-scale faults show the seal capacity to some extent.The development of fractures and deformation bands is mainly controlled by the porosity of wall rocks.Fractures with high permeability and low strength develop in low porosity rocks.As the porosity goes up,the type of microstructures transfers from fractures to deformation bands,and the critical porosity transition is almost 10~14%.Deformation bands are characterized by low petrophysical properties and high strength effected by cataclasis and strain hardening during deformation,and they could make petrophysical properties of reservoirs more heterogeneous and reduce the fluid flow efficiency.Combined with the evaluation methods of fault membrane seal,fault stability and hydraulic seal capacity of cap-rock,we improved the traditional juxtaposition seal model and assessed the vertical fault seal capacity using the thesis of membrane seal.And then we established the risk evaluation methodology of current seal integrity of fault related traps based of permeability and strength structures of fault zones.We evaluated the risk of fault traps for the two projects aforementioned using the risk evaluation methodology of current seal integrity of fault related traps.In the first project of waterflood exploitation of remaining oil in fault traps in Xingshugang oil field,the water injection wells position we designed should be out of the damage zones and keep more than safe distance to faults(see the equation of 4-11)to reduce the risk of casing damage.Directional wells should be parallel to fault surface and almost horizontal in damage zone of the target layer to improve the exploitation efficiency.In the second project of?Geological gas storage in depleted pools in fault traps in Dagang oil field?,the results suggest that B814block and Banzongbei block composed a one storage facility because of a leak bounding fault named B816 fault.The largest amount of injection gas can be up to 13.94×10~8m~3 under condition of seal integrity of the fault trap.We introduced a safety model during hydrocarbon production as follows combined with fault related traps seal integrity controls of petroleum exploitation and geologic gas storage.Wellhead pressure should be lower than hydraulic seal pressure of cap-rock regardless of pressure decrease in wellhole,and the additional fluid pressure on fault surface should be lower than fault membrane seal pressure.