Research On Prediction Method Of Abnormal Pressure Of Middle Cambrian Gypsum Salt Layer In Tazhong Block

Cambrian oil and gas in Tazhong area is an important field of oil and gas resources exploration and development in Tarim Basin.The Middle Cambrian formation have complex structure and lithology.Gypsum,salt and mud is widely developed.In this formation,abnormal high-pressure water layer is often drilled during drilling.Due to the unclear causes of pore pressure in gypsum-salt formation,the existing pore pressure prediction methods have certain limitations.They have low pressure prediction accuracy and unreasonable drilling mud density design.It results in downhole complexity and frequent accidents,which seriously restricts safe and efficient drilling.It is particularly important to accurately predict the abnormal pore pressure of Middle Cambrian.In view of the above problems,based on geological and engineering information,the causes of abnormal pressure in the Middle Cambrian gypsum-salt layer is defined.The abnormal pressure prediction model based on pore elasticity is established.the abnormal pore pressure prediction method of gypsum-salt layer based on earthquake is put forward.Field verification is carried out.The specific research results are as follows:(1)Based on geological,logging and engineering information,the lithology distribution map,logging curve and acoustic velocity density intersection map of drilled wells in Tazhong block are drawn.The characteristics of high acoustic time difference,high porosity,low density and associated with gypsum-salt rock of abnormal pressure formation in Tazhong area are found.It is clear that the abnormal pressure of Middle Cambrian formation has the characteristics of under compaction,structural compression Gypsum-salt rock diagenesis and high-pressure fluid infiltration.(2)Based on Biot’s hypothesis of pore elasticity,the relationship between formation pore pressure and formation fluid and rock bulk modulus is found.And a new prediction model of Cambrian pore pressure in Tazhong area is established.Compared with the traditional method,the new method inherits the advantages of high accuracy of traditional method in predicting pore pressure of normal compacted and under compacted formation.The prediction of pore pressure in the abnormal pore pressure formation of Cambrian in the Tazhong area also has high accuracy.(3)Combining seismic data with physical model,a process of predicting threedimensional pore pressure by using logging data and seismic data through geophysical methods such as interpolation and inversion is proposed.The spatial distribution characteristic map of pore pressure of Cambrian formation in Tazhong area is drawn.It is found that there are two abnormal high pore pressure zones in the Cambrian formation,which are obviously distinguished from the normal pore pressure formation.And it is also found that the horizontal distribution characteristics of the abnormal high pore pressure gradually decreases from north-west to south-east.Through field drilling verification and comparison,it is found that the accuracy of pore pressure predicted by the new method in different well depths of Cambrian formation is generally more than 94% in well ZH1.It provides an effective means for the prediction of abnormal pore pressure in Middle Cambrian gypsum-salt formation.