Drilling Offshore Wells with High-Performance Water-Based Mud (HPWBM) in Extreme High-Pressure / High-Temperature (HP/HT) Conditions
Authors: Affede L. , Giubertoni M. , Ciuca A. , Bertoldi L. , Vieira L. Pinto , Carpineta G. 
Published: October 2023

During drilling of three exploration wells challenging conditions were encountered including temperatures up to 180°C, interbedded highly reactive shales/silts, formation pressures which required Mud Weights (MW) up to 2.35 SG and narrow margin between pore and fracture gradients, posing a host of technical, logistical, and cost challenges to Eni activities. These conditions required an accurate drilling fluids design to maximize operational efficiency and to minimize the risks related to such an extreme environment.

Technical demands 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, sticky wireline runs, bit-balling and accretion that could have resulted, among other issues, in low penetration rates (ROP). The formation nature coupled with Equivalent Circulation Density (ECD) constraints due to the high mud weight required to cope with high pore pressure, which also caused high mud rheology readings were therefore the main limits to be overcome to achieve the well objectives.

A tailored drilling fluid program was thus proposed which consisted of an inhibitive High-Performance Water Based Mud (HPWBM) that could be converted to a High Temperature-High Performances Water Based Mud (HT-HPWBM) while drilling, to cross the deeper and hotter sections of the well. This fluid was specifically engineered and optimized after each well in order to contain high concentration of a combination of monovalent salts to guarantee inhibition and reduce solids loading, dedicated polyamine shale inhibitor and fluid loss additives to minimize API/High-Pressure High-Temperature (HP/HT) filtrate and filter cake thickness with the aim to reduce shale water invasion throughout the drilling campaign, graphite to minimizes fluid invasion and fracture propagation and Rate Of Penetration (ROP) enhancer continuously injected using dedicated pumps to act as anti-balling and anti-accretion additive. In addition to lab testing campaign and mud formulations optimization several drilling operations practices and equipment were introduced on the project taking in account the lessons learned gained by previous wells and experience on similar HP/HT fields contributing on the optimization of 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 formation logs acquisitions.

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 performances in such a harsh environment.