Design of safe well on the top of Atharamura anticline, Tripura, India, on the basis of predicted pore pressure from seismic velocity data

Jwngsar Brahma,Anirbid Sircar

doi:10.1007/s13202-018-0440-9

Abstract

Pore pressure prediction is an essential part of wildcat well planning. In India, Tripura sub-basin is characterised by huge anticlines, normal faults and abnormally pressured formations. These factors push the wildcat well planning in this area into wide margin of uncertainty. Pore pressures were predicted from seismic velocities by using modified Eaton’s method over the synclinal and flank part of Atharamura to understand the pressure succession towards the anticline. These predicted pore pressures on the flank part lead to a reasonable match when plotted with offset well-measured pore pressures. To reduce the uncertainty, fracture pressures were established by various methods such as Hubbert and Willis method and Matthews and Kelly method from predicted pore pressures. But the fracture pressures were predicted with available horizontal stress correlations due to lack of Poisson’s ratio curve for the study area. The mud pressure required to drill the well is calculated using median line principle, and hence drilling mud window is established by assuming virtual tight conditions. The plot of equivalent circulation density versus depth suggests that well can be drilled with two casing policy. But it is found that adding one more casing pipe will ensure the safety of well. Casing pipes were designed on the basis of collapse pressure, burst pressure and tensile load. Finally, a well plan which includes pore pressure, fracture pressure, drilling mud policy, casing policy and kick tolerance graph were proposed to give clear picture on well planning on the top of the anticline in pore pressure point of view.

Highlights

The systematic study of the fluid characteristics of subsurface formations is a critical importance in the well planning and formation evaluation
In normally pressured formations, pore pressure will be equal to the hydrostatic pressure to the corresponding depth, when exceeds or lags the hydrostatic pressure gradient, over-pressure or sub-pressure develops (Chillingar et al 1995)
Under good sealing and low-permeability conditions, this phenomenon can cause over-pressure (Huffman and Bowers 2002). This cause has been postulated as a possible cause of high (17 ppg) pore pressure recorded in the high-pressure/ high-temperature (HP/TP) reservoirs in the deep Jurassic clay stones and sandstones of the central North Sea

Summary

Introduction

The systematic study of the fluid characteristics of subsurface formations is a critical importance in the well planning and formation evaluation. Fluid trapped in the pore spaces escapes and porosity reduces to balance the overburden weight so as to maintain the normal hydrostatic pore pressure Even this equilibrium (pore water expulsion) is distributed by the rapid sedimentation (Rubey 1927) of absence of permeable pore networks, abnormal or over-pressure occurs. Expansion of pore fluids at deeper depths is the main over-pressure causing mechanism as fluid volume increases under the temperature conditions (Bowers 1995). Under good sealing and low-permeability conditions, this phenomenon can cause over-pressure (Huffman and Bowers 2002) This cause has been postulated as a possible cause of high (17 ppg) pore pressure recorded in the high-pressure/ high-temperature (HP/TP) reservoirs in the deep Jurassic clay stones and sandstones of the central North Sea. Tectonic plays main role in the over-pressure development as the validity of over-pressure depends on the seal integrity which can be offered by tectonics.

D O L AI

Result and discussions

Conclusions