Study On Formation Pressure Prediction Method Based On Rock Physics Theory

The study of formation pressure can reflect the characteristics of subsurface lithology,age,buried depth,porosity and fluid,etc.Accurate pressure prediction is very important for judging the direction of oil and gas migration and accumulation,determining the effect and law of reservoir formation,and selecting the favorable areas for exploration and development.Most of the traditional prediction methods of formation pressure are based on the empirical relationship of mudstone compaction law,and directly establish the function relationship between formation velocity and formation pressure,ignoring the influence beyond velocity.In this paper,based on the contact medium model,Digby model and Gassmann model,a shear wave velocity prediction model considering pressure variation is established by comprehensive use of rock physics experiment,well logging data and seismic data.On this basis,an ergodic search inversion correction method for solving formation pressure is further proposed.First of all,from the rock physics theory to establish the appropriate contact medium model,rock density,bulk modulus and shear modulus and mineral composition,porosity,parameters such as pressure,temperature,flow,considering the pressure change Digby model shear velocity prediction model is established,and verified core rock physics experiment.Two Wells with measured shear wave data in the Bozhong 19-6 large condensate gas field in the Bozhong depression were selected to build forward velocity model.Considering the change of modulus under different pressures,the velocity error of the shear wave obtained was small.Secondly,on the basis of the established shear wave velocity prediction model,the equivalent model of two-layer medium was established by using the well logging data of gas-bearing reservoir in Well B1 in the study area to simulate the state of rock under different pressures and saturation.It is found that the shear wave velocity changes significantly with the change of pressure,and almost does not change with the change of saturation.Compared with the compressional wave velocity,the shear wave velocity is not affected by the change of saturation and is affected by more single environmental factors.For rock assemblages with different pores and velosities,Digby+Gassmann fluid replacement model is established in this paper to correct formation pressure by traversal search inversion under shear wave velocity constraints.In the study area,the pressure was calculated from the Wells with measured pressure in the abnormal high pressure sandy conglomerate reservoir and the surrounding atmospheric pressure formation.The normal compaction trend line was established by using the actual acoustic logging data.The pressure was predicted by the traditional balanced depth method,Eaton method and the proposed method.The applicability of this petrophysical inversion method in abnormal pressure reservoir and atmospheric pressure formation is proved by the comparative analysis of measured pressure.Finally,the power function relationship between pressure and shear wave impedance was established in the study area,and the prestack simultaneous inversion was carried out to calculate the three-dimensional formation pressure data body in the whole area,which verified the accuracy of the petrophysical inversion method.