Study On Borehole Stability Of Shallow Extended Reach Well In Liuhua Gas Field

Nowadays,extended reach horizontal well technology has been widely used in offshore,offshore and marginal oil fields with complex topography.The larger water vertical ratio increases the contact area between the extended-reach well and the reservoir,which greatly saves the development cost of the field while increasing production.However,due to the large horizontal displacement and water-vertical ratio of shallow extended-reach horizontal Wells,the target reservoir is buried shallowly and the rock strength is low,which is easy to collapse in the drilling process.In the Pearl River Mouth Basin,located north of the South China Sea,lies the Liuhua block,which is in a deep-water self-operating area.The oilfield structure is regulated by regular faults.During the field drilling process,the Hanjiang Formation to Zhujiang Formation horizon is mudstone stratum,which is easy to hydrate with drilling fluid and lead to expansion and collapse,and the blockage and stuck occur frequently,which seriously delays the development process of Liuhua block.Therefore,in view of the collapse problem of reservoir section in Liuhua block,this paper takes two exploration Wells as examples to study the reservoir wall stability of Liuhua gas field.The Liuhua block reservoir mud shale’s fundamental physical properties were initially comprehended from a microscopic perspective,and the safety evaluation principle was applied to devise an analytic hierarchy procedure.the structural model of influencing factors of borehole stability,construct the judgment matrix,calculate the weight of various factors affecting borehole stability,and finally get the main influencing factors of borehole stability so as to study the mechanism of borehole instability.The field logging data and laboratory rock mechanics experiment results yielded the rock mechanics parameters of Liuhua block,which were then used to establish the pore pressure and in-situ stress profiles.This provided the foundation for further research into borehole stability.By analyzing the distribution law of wellbore stability risk for extended reach wells,the change law of formation rupture pressure and collapse pressure with well trajectory,and the pressure profile of two exploration Wells,a safe mud density window of shallow extended reach horizontal wells can be obtained.Finally,a calculation model of formation rupture pressure,collapse pressure,and leakage pressure is determined.In addition,as the shale reservoir is prone to hydration,considering the influence of the mechanical-chemical coupling,the hydration collapse cycle study is carried out,the water absorption law of the mud shale in the reservoir section of Liuhua block is analyzed,the calculation model of hydration collapse cycle is determined,and the chart of hydration collapse cycle is drawn,according to which the safe drilling cycle can be determined under the conditions of using drilling fluids of different densities on site.Results demonstrate the low rock strength of Zhujiang Formation in Liuhua block,thus posing a high risk of complex drilling scenarios.The Yimen mixed layer of clay minerals has little content,and the porosity of the rock is developed,making hydration with water-based drilling fluid easy and potentially leading to the collapse and instability of the borehole wall.Comparing the calculated three-pressure profile with the field drilling fluid,it can be seen that the low density of drilling fluid is the main reason for the sustained collapse.The research results,thus,serve as a significant reference for drilling engineering design and the application of mud density on the premises.