Extreme Efficiency from High Performance Water-Based Mud Under HPHT Conditions

Abstract

Abstract While drilling a campaign of three exploration wells, challenging conditions were encountered which included high temperatures up to 180°C, highly reactive interbedded shales/silts, high formation pressures which required fluid densities up to 2.35 SG and narrow drilling margin between pore and fracture gradients. These conditions posed a host of technical, logistical, and cost challenges to operator's activities and required an accurate drilling fluid design to maximize operational efficiency and to minimize the risks related to drilling in such an extreme environment. Technical demands on the fluid were particularly critical since the reactive shale formations had historically proved to be difficult to inhibit when drilled with water-based mud (WBM) exhibiting swelling, tight hole, failures in open-hole wireline logging operations, bit-balling and accretion that could have resulted, among other issues, in low rates of penetration (ROP). In order to achieve the well objectives high mud rheology readings, caused by the formation nature coupled with equivalent circulation density (ECD) constraints due to the high mud weight required to address the high pore pressure, were therefore a key limitation to overcome. A tailored drilling fluid program was proposed which consisted of an inhibitive, high-performance water-based mud (HP Fluid) that could be converted to a high temperature, high performance water based mud (XP Fluid) while drilling, prior to entering the deeper and hotter sections of the well. The specifically engineered fluid was optimized after each well. A combination of monovalent salts at high concentration was used to guarantee inhibition and reduce solids loading in conjunction with a dedicated polyamine shale inhibitor and fluid loss additives to minimize API/High-Pressure High-Temperature (HP/HT) filtration and filter cake thickness, with the aim of reducing water invasion into the shale. Throughout the drilling campaign graphite was used to minimize fluid invasion and fracture propagation and ROP enhancer was continuously injected using dedicated pumps to act as an anti-balling and anti-accretion additive. In addition to an extensive laboratory testing campaign and optimization of the mud formulations new equipment and several optimized drilling practices were introduced on the project taking in account the lessons learned from previous wells and experience on similar HP/HT fields which contributed to the improvement in drilling performance achieved well by well. The drilling targets were achieved safely, on time and with good overall fluid performance, which either reduced or eliminated many of the challenges seen in offset wells, including: no barite sag, rheology stability, and stable long-term mud properties and wellbore conditions even during extended periods of formation log acquisition. This paper covers the design, execution and accomplishments of the water-based drilling fluids employed on three HP/HT wells drilled, together with all the lessons learned captured, highlighting the evolution of these systems to reach a step-change in terms of performance in such a harsh environment.