Study On Roof Stability Of Salt Cavern Gas Storage Under Different Factors

The gas storage usually has a complex casing structure,and the roof will retain a certain thickness of salt layer under the mudstone caprock.The top of the cavity to the casing shoe is generally a bare well.Under the influence of improper setting of the thickness of the reserved salt layer on the roof,the existence of interlayers in the roof position,and the unreasonable operating pressure of the gas storage,the roof structure of the salt rock gas storage is prone to damage,resulting in the leakage of natural gas from the wellbore damage and overflowing along the formation fracture joints.Therefore,in order to ensure the safety and stability of the roof structure of the gas storage,the paper studies the mechanical stability of the casing-cement ring-surrounding rock structure cavity and the cyclic injection-production stage of the gas storage roof under different factors through theoretical analysis and numerical simulation.On this basis,the structural parameters of the roof of the gas storage are optimized to improve its safety stability and gas storage efficiency.The main results are as follows:(1)The theoretical solution of the two-dimensional stress state of the elastic model of the casing-cement sheath-rock mass composite structure of the gas storage is derived by using the thick-walled cylinder theory.The stress distribution and displacement of the elastic and plastic zone of the casing-cement sheath-rock mass structure are further derived,and the influence of different factors on the stress state of the cement sheath structure is analyzed,which provides the corresponding theoretical support for the three-dimensional numerical simulation calculation of the gas storage roof structure.(2)The influence of the thickness of the reserved salt layer at the roof,the height of the bare well under the casing floating shoe and the internal pressure on the static stability of the roof structure after the cavity is studied.With the increase of the thickness of the reserved salt layer and the height of the bare well,the deformation of the roof structure,especially the casing floating shoe,decreases.The compressive stress of roof cement ring and surrounding rock decreases,and the stress of casing changes from tension to compression.The effective strain of casing,cement ring and surrounding rock decreases.With the increase of operating internal pressure,the deformation of roof structure decreases.The compressive stress of cement ring and surrounding rock increases,and the tensile stress of casing decreases.The effective strain decreases.From the top of the cavity to the upper boundary of the roof,the deformation and effective strain of the surrounding rock increase gradually,but decrease significantly at the casing shoe.The deformation and effective strain of cement sheath structure increase gradually.The increase of the thickness of the reserved salt layer,the height of the bare well and the internal pressure of the casing strengthens the stability of the roof structure of the gas storage.(3)The influence of the thickness of the reserved salt layer at the roof,the height of the bare well and the lower limit of the injection-production internal pressure on the long-term stability of the roof structure under the action of cyclic gas storage is analyzed.The deformation,maximum principal stress and effective strain of the casing,cement ring and surrounding rock of the gas storage roof increase with the duration of the injection-production operation,but the change rate slows down year by year and tends to be stable.With the increase of mudstone caprock thickness and bare well height,the deformation of surrounding rock,casing,cement sheath,especially casing floating shoe decreases.The compressive stress of the cement ring and surrounding rock at each height of the wellbore increases.The effective strain of casing,cement ring and surrounding rock decreases.With the increase of the lower limit of injection-production internal pressure,the deformation of roof structure decreases.The compressive stress of casing,cement ring and surrounding rock increases.The effective strain tends to decrease,and the effective strain of the surrounding rock in the bare well section is much more affected by the above factors than the casing and cement sheath.The increase of the thickness of the reserved salt layer,the height of the bare well and the lower limit of the injection-production internal pressure strengthens the long-term stability of the roof structure of the gas storage.There are 89 figures,6 tables,and 84 references.