--> Reducing Drilling Risks In J Bend Wells Targeting Basement In Tectonic Area Through Geomechanical Solutions

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Reducing Drilling Risks In J Bend Wells Targeting Basement In Tectonic Area Through Geomechanical Solutions

Abstract

Introduction: J-bend development wells are being drilled in the structurally complex field of North-East, India. They are threatened by considerable risk of wellbore instability resulting in severe hole problems which ultimately caused NPT and LIH incidents in the field. For drilling optimization, key is to predict optimal stable mud weight window especially in deviated 8.5” section with deep target sandstone reservoir overlain by different shale formation of varying rock strength and pore pressure in same open section. Methods, Procedures, Process: This paper describes how geomechanical information was utilized for the decision-making process and successful completion during J bend well design and drilling operations in Baghjan field. The geomechanical modelling helped select specific mud weights and to plan casing points in order to drill the critical deviated section over the 800m-1000m. Key drilling risks in each formation were identified and accordingly mud weights were suggested with additives and BHA type to be used for drilling optimization. A Mechanical Earth Model was constructed to represent the state of pore pressure, stress and mechanical properties of the overburden and reservoir. The model incorporated data from a number of sources including open hole logs, well testing, leak-off tests and drilling records collected from the earlier vertical wells. Estimated rock properties and stress profile were subsequently validated using wellbore stability analysis against field observation like stuck pipe, tight hole, kicks and cavings. Sonic compressional velocity has been used to build pore pressure profile and further calibrated with direct pressure measurements. Results, Observations, Conclusions: The geomechanical analysis helped to take several critical decisions in the well design: 12.25in section should be drilled with 71pcf-73pcf to minimize breakouts in shale layers of Barail formation at 25deg well deviation even formation pressure is hydrostatic. 9 5/8in casing shoe should be set as deep as possible inside Kopili shale to minimize exposure of shale for longer duration. Pore pressure increases from 63pcf to 74pcf inside Kopili formation and further reversal trend has been observed from Prang formation to target sandstone Langpur formation (68pcf-70pcf). Mud weight needs to be in range of 76pcf-81pcf to minimize breakouts in Kopili shale layers while drilling at 30deg well deviation. Differential sticking incidents are evident in permeable sandstone due to thick mud cake formation based on offset wells and hence fluid loss needs to be maintained while drilling J bend well. Depending on well azimuth, mud weight can vary 3pcf-4pcf due to minimum and maximum horizontal stress contrast. Novel/Additive Information: Although early vertical wells had suffered similar wellbore instability incidents, the J bend wells engineered with the geomechanics information and better hole cleaning were drilled from surface to TD without major drilling incidents. It helped to save almost 6-7days as compared to planned schedule.