Research on Engineering Solutions for Presalt Formation Development and Depleted Salt Cavern Utilization in China and Middle Asia

摘要

How to efficiently develop pre-salt oil and gas reservoirs has become a global engineering issue.Thick saltgypsum formations are widely found in Middle Asia,where KazakhstanKenkyak Oilfield used to be an unconquerable challengein oil industry.Some regions are also well-known for their rich experiences in salt mining and many wells that have been abandoned due to distribution of thick salt domes are still found in old rig sites,hence people are trying to develop those depleted salt caverns to be underground gas(oil)storages.Salt-gypsum samples were collected from Kenkyak Oilfield to study their creep behaviors.A finite element model of mechanics was set up to analyze effects of mud density ranges on salt-gypsum borehole sizes with different in-situ stresses,in order to determine the most appropriate mud density to drill salt-gypsum intervals.An investigation was finalized to update information of depleted salt cavern utilization in the world to study relevant accidents in China and abroad.Successful experience of building Jintanunderground gas storages in China was analyzed and the first-order second moment shown performance function that is used to determine the volume convergence failure probability of salt underground storages was established,so that it is easier to reveal the relationship between volume convergence failure probability and internal pressure of gas and time.Engineering challenges arising from pre-salt formation development are mainly plastic creep of saltgypsum intervals,complexity of pressure systems,as well as instant and stable creep.Salt domes that are as thick as 3000m(at the depth of 700m – 3700m)are widely found there,while conventional casing programs that used to be adopted during the former Soviet period greatly increase drilling costs.Based on the analysis of formation profiles and pressure systems,we combined the surface casing interval and the first technical casing interval,reaching the Carboniferous target with near-balance drilling techniques and 3 casing intervals.Some underground gas storages in USA or Europe collapsed or were ever faced by gas seepage,cavern shrinkage,surface subsidence or ecological disasters because of abnormal cavern shapes,interlayer collapse,bottom uplift even pillar destruction,etc.Natural gas is injected with high pressure into the cavern and then stored with constant pressure in it.Gas pressure is released when it is collected from underground but the salt cavern keeps running in low pressure eventually.In long-term constant internal pressure and short-term low pressure operation period,the salt cavern’s volume convergence failure probability decreases with an increasing internal pressure.Besides,surrounding rocksare more capable of resisting to deformation and rock displacement if the internal gas pressure is improved.Drilling costs are saved due to casing program simplification,while ROP is improved due to cordwood turbo drill tools in Kenkyak Oilfield.As for Jintan underground gas storages,salt creep accelerates when caverns run in constant high pressure,but salt is more sensitive to internal pressure when caverns are operated with constant low pressure.The cavern’s creep is also affected by the gas injection/discharge speed.