Pre-drill pore pressure prediction using seismic velocities data on flank and synclinal part of Atharamura anticline in the Eastern Tripura, India

摘要

The Tripura state went through extensive geological tectonics that resulted in the creation of complex structural styles with different fault types, lineaments, and plate boundaries, which in turn caused possible zones with over-pressured formations characterized by higher seismic amplitude signatures. Without accurate estimates of pore pressures, drilling through these hazardous zones is very troublesome and could jeopardize the whole drilling rig site. Pore pressures are easily predicted for sediments with normal pressure gradient. The prediction of pore pressure for the abnormally pressured (i.e., overpressured) sediments is more difficult and more important. Understanding of the pore pressure is a requirement of the drilling plan in order to design a proper casing program. With balanced drilling mud, overpressured formations, and borehole instability will be effectively controlled while drilling and completing the well. Well control events such as formation fluid kicks, loss of mud circulation, surface blowouts, and subsurface kicks can be avoided with the use of accurate pore pressure and fracture gradient predictions in the design process. In this study, transform models using modified Eaton’s method were used to predict pore pressures from seismic interval velocities. Corrected two-way travel times and average velocity values for 28 sorted common depth points were input into the transform for pore pressures prediction predicted pore pressures show a reasonable match when plotted against formation pressure data from the offset wells namely AD-4 trend, Agartala Dome-6. Ambasa trend, Kathalchari trend, Kubal, Masimpur-3, Rokhia structure-RO1, and Tichna structure-TI1. In this study, it is observed that overpressure starts at shallow depths (1,482–2,145?m) in synclinal section while in flank section it starts deeper (2,653–5,919?m) in Atharamura anticline. It is also observed that the most of wells showing pressure match are located in the western side of the Atharamura. The maximum predicted pore pressure gradient observed in this study is 1.03?psi/feet in both synclinal and flank sections of Atharamura anticline. Based on our observations, it is interpreted that Tripura region is characterized by single pressure source and the pressure is distributed evenly in all the anticlines in this region.