Pore Pressure Study On Low-porosity And Low-permeability Gas Reservoir In Xihu Sag,East China Sea

Drilling practices in the low-porosity and low-permeability gas reservoir in Xihu Sag indicate a highly development of overpressure,but the responses of overpressure from the well logs and seismic data are not so obvious that lead to obstructions to accurate pore pressure prediction.Thus it's necessary to figure out the pore pressure of low-porosity and low-permeability gas reservoir,so as to precisely predict and monitor the pore pressure.From the perspective of theoretical analysis and engineering practice,this paper studies the pore pressure prediction and monitoring methods for low-porosity and low-permeability gas reservoir.The main achievements are as following:According to the formation lithology,reservoir physical properties,fault tectonic evolution,and hydrocarbon migration and accumulation characteristics,the research of pore pressure distribution characteristics in Xihu Sag and the reasons are conducted.These results show that the distribution of oil and gas is determined by the mutual coordination effect of fault evolution with hydrocarbon generating and expelling process.In addition,there developed a variety of different types of fault in Xihu Sag,and the closure property between these faults also show a great diversity.Therefore,the vertical and horizontal distributions of pore pressure in Xihu Sag present great complexity and difference.Combined with the engineering practices in Xihu Sag,the overpressure generation mechanisms are judged by velocity-density crossplot,velocity-effective stress crossplot and velocity-resistivity-density curves.The results show that the overpressure generation mechanisms may be misjudged if simply according to the logging data.Combined with the low-porosity and low-permeability gas reservoir characteristics and the pore pressure distribution,the results show that the overpressure generation mechanisms of Xihu Sag low-porosity and low-permeability gas reservoir are mainly the generation and charging of hydrocarbon.The undercompaction and tectonic extrusion may not be the reasons.The pore pressure in Xihu Sag is respectively analyzed by Eaton method and Bowers method,and the results indicate an underestimation of overpressure.The reasons include the weak response of acoustic velocity to the low-porosity and low-permeability overpressure and the special pore pressure distribution.So the tranditional pore pressure prediction methods are not suitable to the low-porosity and low-permeability gas reservoir.The overpressure preservation mechanism is studied,and a method to study the existence possibility of overpressure from the analysis of the overpressure releasing pathway is proposed.On the basis of structure characteristic in Xihu Sag,it is considered that the fault and unconformity plane are the main factors influencing the overpressure preservation.Based on BP artificial neural network,the relationship between fault type and overpressure is analyzed and a method to determine the existence possibility of overpressure is established.A method to study the overpressure upper limit from the analysis of the overpressure releasing power is proposed.The pore pressure upper limit based on the hydraulic fracturing and fault stability is respectively studied,and the influence factors are also discussded.Eventually,the method of using seismic data to predict the longitudinal distribution of pore pressure upper limit is established,which provides a quantitative reference index for the determination of parameters in pore pressure prediction.According to the engineering practices in Xihu Sag,the applicability of the Eaton method based on d_c exponent and the standard drilling penetration rate method to the low-porosity and low-permeability formation is studied.The results show that the Eaton method based on d_c exponent is not suitable for the overpressure monitoring in the low-porosity and low-permeability formation,which is casued by the unloading.The same to the standard drilling penetration rate method,which doesn't take the effect of confining pressure into account when drilling the low porosity and low permeability formation.By optimizing and modifying the Young's ROP equation and considering the influence of confining pressure on the drilling resistance strength,the pore pressure monitor model which is suitable for the low-porosity and low-permeability formation is established.The convergence analysis and engineering practices show that the model is applicable to the low-porosity and low-permeability formation.