Abstract
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.
Highlights
A pre-drill prediction of pore pressure is an integral part of the well planning and formation evaluation process
An improper understanding of the subsurface geology and the formation pressures may result in fracturing the formation if the mud weight is too high
Result and discussion In Atharamura anticline the overpressure starts in shallow depths (1,482–2,145 m) in synclinal section part while in flank part it starts deeper (2,653–5,919 m) as shown in the Table 1
Summary
A pre-drill prediction of pore pressure is an integral part of the well planning and formation evaluation process. An accurate estimation of formation pore pressure is a key requirement for safe and economic drilling in overpressured sediments. In normal pressured formations, porosity decreases with depth as the pore fluids are expelled out of pores due to the increasing overburden weight. Kathalchari trend, Kubal, Masimpur-3, Rokhia structure-RO1, and Tichna structureTI1 to estimate reliable surface fluid flow as well as to get better understanding of the pore pressure succession in this region. The general elevation of the region rises eastward from few tens of meters in the area adjoining Bangladesh plains in western Tripura to about 1,800 m in eastern Mizoram bordering the Chin Hills of Burma The sedimentation in this basin probably started with a breakup of the Gondwana land in Jurassic and Cretaceous and had been almost continuous since . In Atharamura anticline middle Bhuban is exposed to the surface and increases the possibilities of overpressure at the shallow depth
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