Analysis On Characteristics Of Gravel Stratum And Mechanism Of Borehole Wall Instability In BZ Block

The BZ block has a massive gravel layer,which has high abrasiveness and poor drillabilityim,and the ROP(rate of penetration)in there is very slow.The use of gas drilling technology at the site has greatly increased the ROP,and there have been complex cases and accidents such as the collapse of the wellbore of the formation and the loss of blocks.This paper analyzes the stratum characteristics of the gravel layer in the BZ block and the instability mechanism of the wellbore,and establishes a set of wellbore stability evaluation methods suitable for this area,and using the PFC simulation to analyze the failure mechanism of the surrounding rock in the gravel layer of the Xiyu Formation,Kuche Formation and Kangcun Formation of BZ1 well and the failure characteristics of wellbore instability.The main research results obtained in this paper are as follows:(1)Analyze the distribution characteristics of BZ block stratum.Analysis and comparison of ground outcrops on the gravel layer above the BZ block and mineral composition and content of intergranular fillers,and analysis and processing of geological and hydrological data,logging interpretation methods and logging data.This paper summarizes the vertical and vertical variations of rock mineral components in gravel layers of BZ block,the vertical and horizontal distribution of gravel particles,the degree of compaction of strata and the output of formation fluids.(2)Get the geomechanical parameters of BZ block.Using the acoustic emission method combined with on-site logging data to obtain the distribution of ground stress in the BZ area;Analysis of rock elastic parameters,mechanical strength,and their variations in rocks of BZ Gravel by ssing hammer test and logging data.The effect of water-based solution soaking on rock mechanics parameters was tested,and basic parameters were provided to evaluate gas well borehole stability.(3)Analyze gravel layer microstructure and hydration effect.Analysis of gravel microstructure and spatial distribution by scanning electron microscope and cast thin sheet test.start an experiment on the effect of water-based liquid on the mechanical parameters of weak layer surface of gravel layer,study its effect on rock mechanical properties and borehole wall stability.(4)Study on collapse mechanism of borehole wall in gravel gas drilling.Combining the characteristics of gas drilling fluid circulation medium feature and rock properties and stress distribution in the near wellbore during drilling process.Based on the logging data,the bedrock collapse pressures at three depths of 1500m(Xiyu group),3000m(Kuche group)and 4500m(Kangcun group)of BZ1 well were calculated.(5)Study on simulation of rock failure mechanism in gravel layer.Based on the gravel distribution characteristics of the three strata in the Xiyu Formation,the Kuche Formation and the Kangcun Formation,and based on the related theories of soil-rock mixture,using the PFC numerical simulation platform,analyzing the uniaxial compression characteristics of simulation conglomerate cores and the surrounding rock damage characteristics of gravel layer,providing theoretical reference for the stability control technology of gravel layer borehole.