Well Integrity and Well Control

Nonaqueous drilling fluids have been used extensively by the industry, particularly in complex drilling scenarios. They carry some concerns, however, related to environmental issues and higher initial cost, potential for lost circulation, effect of pressure and temperature on drilling-fluid density, and capacity to solubilize natural gas compared with water-based drilling fluids [water-based mud (WBM)]. The latter three concerns have implications for well integrity.

It is generally accepted that the risk of induced fracture (and consequent lost circulation) is greater when using nonaqueous fluids than when using WBM. The formation-breathing phenomenon is also more pronounced with nonaqueous drilling fluids compared with WBM.

Nonaqueous drilling fluids are more sensitive to pressure and temperature changes than WBM is. Thus, the density and viscosity of these fluids inside the well are more difficult to establish, increasing uncertainties about bottomhole and casing-shoe pressures. Because they are more compressible, longer periods are expected for shut-in pressure stabilization, for response delay of drillpipe pressure after choke adjustment, and for flowback to stop after the mud pumps are shut off.

The gas solubility in nonaqueous drilling fluids makes the kick indicators less pronounced for its detection and increases the risk of rapid gas evolution where the pressure inside the well drops below the bubblepoint pressure. However, the solubility properties of these fluids can lead to favorable conditions also: If pressure remains above the bubblepoint as the kick is circulated out of the well, there will be no free gas and, consequently, the magnitude of pressure will be smaller. Also, no free-gas migration will occur in static conditions.

All these issues have been studied extensively, especially those related to change in fluid density and gas solubility. In the list of papers recommended for additional reading, two address gas-solubility aspects in nonaqueous drilling fluids.

This Month's Technical Papers

Use of Managed-Pressure Drilling Requires Adjustments To Bridge Gap to Well Control

Pressure Measurements Plus Simulation Help Differentiate Between Downhole Events

Design and Surveillance Tools Help Lower Integrity Risks for High-Temperature Wells

Recommended Additional Reading

SPE 180039 Kick-Detection Capability of Oil-Based Muds in Well-Control Situations by Harald Linga, SINTEF Petroleum Research/DrillWell, et al.

SPE 185470 Thermodynamic Behavior of Olefin and Methane Mixtures Applied to Synthetic-Drilling-Fluids Well Control by D.C. Marques, Consultant, et al.

SPE 180292 Mathematical Modeling of Gas in Riser by Ulf Jakob Flø Aarsnes, IRIS/DrillWell, et al.

SPE 181672 Multiphase Well-Control Analysis During Managed-Pressure-Drilling Operations by Z. Ma, The University of Texas at Austin, et al.

Otto Luiz Alcantara Santos, SPE, is an independent consultant and instructor of well-control and advanced well-construction technologies. During almost 40 years, he worked as a Petrobras well-construction engineer. Santos holds a BS degree in civil engineering and an MS degree in petroleum engineering from the Colorado School of Mines and a PhD degree in petroleum engineering from Louisiana State University. He was also a faculty member at The University of Tulsa in 1994. Santos has written several technical papers on well-construction technology, especially on well integrity and directional and horizontal drilling. He is the author of the book Well Control in Drilling Operations and coauthored the book Directional Drilling. Santos is editor of the SPE book Drilling and Production Operations in HPHT Wells. He was an SPE Distinguished Lecturer for 2009–2010. Santos is the current program chairperson of the SPE Bahia/Sergipe Section and has served or is currently serving on several SPE committees. He is also a member of the JPT Editorial Committee. In 2010, Santos received the International Association of Drilling Contractors Exemplary Service Award. He can be reached at

Well Integrity and Well Control

Otto Luiz Alcantara Santos, SPE, Consultant

01 January 2018

Volume: 70 | Issue: 1

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