Study On Wellbore Stability Of Igneous Rock Intrusion In Sangtamu Formation In Shunbei NO.1 Well Area

The Ordovician reservoirs in Shunbei area belong to the north margin structural belt of Shuntogol with an area of 3126.22 km2.The proven oil reserves in the adjacent blocks are 14.29 million tons.Therefore,geological analysis shows that the reservoir-forming conditions in Shunbei area are superior and there is potential for exploration and development.However,after exploration in Shunbei NO.1 well,it is found that there are some complex conditions such as wall collapse,drilling sticking,drilling fluid leakage and overflow in the igneous rock formation of Santamu Formation of Ordovician in Shunbei block,which makes the drilling operation slow,long cycle and low efficiency,and seriously retards the drilling development progress of Shunbei Block reservoir.In order to effectively solve the problems of wellbore collapse and drilling fluid leakage in igneous formation of Sangtamu Formation in Shunbei Block,this paper firstly combines the drilling history data of completed wells in Shunbei Block to sort out and analyze the drilling technical difficulties and determine the main research ideas and contents.Secondly,through laboratory experiments,the parameters of composition,macro-micro-structure,mechanical properties and pore-permeability characteristics of igneous rock intrusions in Sangtamu Formation of Shunbei Block are determined,and the mechanism of wellbore instability is determined.Thirdly,the wellbore stability evaluation model of igneous rock in Sangtamu Formation of Shunbei Block is established from the point of view of mechanical mechanics.The influencing factors of wellbore instability are analyzed based on the mechanical properties of igneous rocks,pore-permeability characteristics and in-situ stress state.Finally,the anti-sloughing drilling fluid technology is determined according to the microstructure and physical and chemical properties of igneous rock.The key treatment agents of drilling fluid are optimized experimentally,and the anti-sloughing drilling fluid system is established.Afte the performance evaluation of drilling fluid system earring out,the test well is used to evaluate the anti collapse effect of the anti sloughing drilling fluid.The research results show that:(1)The development of micro-fractures and strong hydration dispersion of igneous rock in Sangtamu Formation in Shunbei NO.1 well area are the main causes of wellbore collapse and drilling fluid leakage.The invasion of drilling fluid into formation will enhance the failure probability of weak fracture surface.The longer the immersion time,the deeper the drilling fluid invasion depth,wicth leads the higher the wellbore collapse pressure.(2)The anti-sloughing drilling fluid technology is based on strong inhibition and strong plugging,and the potassium amine polysulfonate drilling fluid system and formulation are optimized.The performance test results meet the drilling requirements of Sangtamu igneous formation,and can effectively reduce borehole wall collapse and drilling fluid leakage to ensure safe and efficient drilling;(3)The anti-sloughing drilling fluid technology in this paper has a remarkable practical application effect.[t can greatly shorten the well construction period on the premise of enhancing the stability of the wellbore,and can be used as the preferred drilling fluid technology in Shunbei Block.In a word,this paper has positive significance for ensuring the safe well construction,shortening the drilling cycle,reducing the drilling cost and improving the development efficiency in Shunbei Block.