Geomechanics Aided Successful Well Delivery Through Abrasive Rock While Adding New Unconventional Reserves

Abstract

Abstract Deep wells drilled down to 4000 m to 4500 m true vertical depth (TVD) in the offshore Kutch-Saurashtra rift basin encounter more than 1500 m of abrasive Deccan Trap volcanics in a 12.25-in. section with target stiff Mesozoic sandstone formations in an 8.5-in. section. Weathered basalt flows, fractured sandstones, tightly cemented siltstones, pyritic shales, abnormal pressures, and complicated transverse isotropic layers test the limits of well construction and engineering design at high-pressure, high-temperature (HPHT) conditions. This reduces the rate of penetration (ROP) and sidetracks with premature termination of wells. Traditional prognosis methods fail to predict the abnormal pore pressure regimes and stress anisotropy created by the disturbed tectonic history and complex geological setting. The operator faced unpredictable flow events and wellbore instability incidents such as cavings, tight pulls, breakouts, and equivalent circulating density fluctuations during drilling. Apart from the drilling and completions challenges, the wellbore instabilities affected openhole logging and coring operations, leading to inadequate formation evaluation. In this paper we present an integrated approach to using geomechanical analyses for determining the mud-weight window, drilling bottomhole assembly (BHA), and optimizing mud chemicals. An anisotropic Mechanical Earth Model (MEM) was built using both horizontal and vertical elastic properties to estimate an accurate stress profile that can guide mud loss zones and completion quality. Engineered drilling bits based on the estimated rock mechanical and stress analysis were selected to improve effective ROP through the more abrasive and compacted rocks. High risk zones were flagged inside the Deccan Trap for mud loss while look-ahead mud weight design for the Bhuj and Jhuran formations were optimized by considering plane of weakness mode of failure. Dynamic hydraulics simulation was conducted for the tripping speed of the Casing and BHA. The casing run-in speed was optimized across the Deccan Trap by pumping lost-circulation material (LCM) additives to mitigate losses. This helped to set casing until the 12.25-in. section at total depth (TD) with a narrow mud-weight window of 0.2 to 0.3 ppg. A mud weight of 12.7 to 13.0 ppg was used initially in the 8.5-in. section based on the look-ahead model and was proactively increased to 13.7 ppg to minimize nonproductive time (NPT) with very few borehole breakouts or fracture plane slippage. This result was quite different from offset wells that were drilled with only 11.5 to 12.2 ppg mud weight, which resulted in many tight-hole and stuck-pipe incidents. Bits were changed to manage mean stress, which was expected in the range of 15,000 to 17,500 psi, with formation strength ranging 9,500 to 23,000 psi. The 8.5-in. section was drilled successfully with fewer bit trips, and the hole condition was in better shape compared to the offset wells. The formation evaluation and completion quality review led to the successful discovery of four new zones with minimal near-wellbore damage. Despite the extreme conditions, there was improvement in the instantaneous ROP by 15 to 20% while drilling an additional 250 m of abrasive formation without any wellbore instability.