Study On Formation Mechanism And Accurate Prediction Method Of Marine Formation Pressure System Considering Diapir Structure

The Yinggehai Basin in the South China Sea is located at the intersection of the Eurasian,Pacific and Indo-Australian plates.The water depth varies greatly,the diapir structure is abnormally developed,and the characteristics of high temperature and pressure are obvious.Under the influence of marine sedimentary environment and diapir structure,the formation abnormal high pressure is complicated,showing the characteristics of multi-source and multi-mechanism.The properties of the formation are different,shallow rocks have low degree of consolidation,showing the mechanical characteristics of saturated soil,deep temperature is high,fractures are developed,and the characteristics of formation breathing effect are obvious,which seriously affects the analysis of well loss.Which make it difficult to predict formation pressure system including pore pressure,collapse pressure,leakage pressure and fracture pressure,and cause frequent downhole complications such as overflow and leakage when drilling into narrow density window area,which seriously affects drilling time.Therefore,it is of great significance to carry out research on formation pressure system formation mechanism and accurate prediction method,determine the safe density window and analyze the distribution characteristics of the window for ensuring the safe and efficient drilling of oil and gas in marine sedimentary basins with developed diapir structures.The main research contents and conclusions of this paper are as follows:(1)Influence mechanism of diapir structure on formation pressure system in Yinggehai BasinBased on the regional geological characteristics of Yinggehai Basin,the formation mechanism and morphological characteristics of the diapir structure are analyzed,and the influence of the formation and evolution of the diapir structure on the formation of abnormal high pressure,drilling fluid leakage,borehole collapse and fracture is studied,and the influence mechanism of the diapir structure on the formation of formation pressure system is defined.The results show that the diapir structure of Yinggehai Basin is the result of rapid subsidence,fine sediment filling,strong overpressure development and regional tectonic movement.According to different evolution stages,the diapir structure can be divided into turtle-back type,spike type,pierced type and depositing type.The development of associated fault-fissure and up-invasion of hot fluid are two major characteristics in the formation of diapir structures.The deep overpressure transmits upward through the associated fault-fissures and accumulates into high pressure in the shallower formation,and the thermal fluid activity reduces the threshold of hydrocarbon generation in the shallower source rocks,promotes the evolution of clay minerals,causes hydrothermal expansion,and enhances the shallow high pressure.The development of an associated fault-fracture system damages the rock integrity,and thermal fluid activity changes local geothermal conditions,increasing the risk of loss,wall fracture and collapse during drilling.(2)Multi-source and multi-mechanism formation pore pressure prediction method based on machine learningBy analyzing the mechanical relationship,response characteristics of logging data,natural gas isotope sources and measured pressure data in the process of formation subsidence and accumulation,a comprehensive discriminant method for the cause of abnormal high pressure is formed.Considering the nonlinear mapping law between acoustic velocity,porosity,mud content and density and effective stress,the non-parametric multivariate transformation relationship of effective stress was constructed,and the algorithm structure was optimized.A prediction model of multi-source and multi-mechanism formation pore pressure based on machine learning was established.The results show that due to the influence of specific functional form,lithology and genetic mechanism,conventional parametric prediction method has low accuracy for multi-source and multi-mechanism abnormal high pressure prediction.The prediction model based on machine learning is not limited by these factors,and avoids the tedious coefficient correction process.It has the characteristics of high precision and strong adaptive ability,and can accurately and reliably predict pore pressure.(3)Calculation method of leakage pressure considering the characteristics of formation breathing effectConsidering the influence of marine sedimentary environment and diapir structure,the induced mechanism of formation breathing effect in shallow and deep offshore drilling was determined,and the mechanism models were established respectively to analyze the influence of different factors on formation breathing effect.The strength coefficient to judge the influence of formation breathing effect on the leakage pressure is put forward,and a calculation method of the leakage pressure considering the characteristics of formation breathing effect is formed.The results show that the shallow formation of Yinggehai Basin is mainly permeability-induced breathing effect,while the deep formation is fractured respiration.Formation physical properties,drilling fluid properties and fracture size will significantly affect the characteristics of fluid loss and flowback,thus affecting the prediction of loss pressure.If the formation breathing effect is ignored,the prediction result of formation leakage pressure will be higher.(4)Determination and distribution characteristics of safety density window in Yinggehai BasinAccording to the characteristics of weak cementation,high porosity and permeability of shallow rocks and high temperature and pressure and fracture development of deep rocks,the collapse pressure and fracture pressure calculation models based on saturated soil mechanics theory and thermal-fluid-solid coupling theory were established respectively.Combined with the calculation results of pore pressure and leakage pressure,the safety density window was determined and the distribution characteristics of the window were analyzed.The results show that the prediction width of the safe density window is too large if the formation breathing effect is ignored,which is not conducive to safe drilling.Under the influence of marine sedimentary environment and diapir structure,the pore pressure and collapse pressure of deep formation are higher,and the leakage pressure and fracture pressure are lower,and the safety density window is narrower.The window width of a single well increases first and then decreases with the increase of well depth.In this paper,the formation mechanism and accurate prediction method of formation pressure system including formation pore pressure,collapse pressure,leakage pressure and fracture pressure are studied in Yinggehai Basin where the diapir structure is developed.The safe density window of drilling is determined and the distribution characteristics of the window are analyzed.The research results can provide some theoretical guidance for oil and gas drilling safely and efficiently in Marine sedimentary basins with diapiric structures.