Drilling a TAR formation has never been easy due to the sticky nature of the material. TAR sands are sandstone formations saturated with bitumen or heavy oil and are soft and poorly consolidated. The sandstone is usually water-wet in the formation with the sand grains held together mainly by the bitumen. Weathering and biological process results in loss of the lighter oil fractions, leaving behind the very viscous, solid or semi-solid heavy residues. Bitumen deposits are found in more than 70 countries globally, with large outcrops in Venezuela and Canada.
Drilling TAR sections with conventional drilling fluid systems, either water-based or oil-based, has proven ineffective. Some factors that must be considered are wellbore enlargement, accretion, screen blinding, erratic torque and drag, larger cuttings and temperature conditions. TAR tends to be mostly hydrocarbon-based and it swells, softens or dissolves in oils or in synthetic base-fluids that the industry uses for oil Based Muds (OBM) or Synthetic Based Muds (SBM). Drilling with OBM or SBM tends to lower the mechanical strength of the TAR, exacerbating any tendency to mobilize into the wellbore. A conventional water-based mud with basic inhibition and minimal reduction in pore pressure transmission may not be helpful in mitigating the challenges associated with wellbore stability, accretion, torque and drag complications.
A High-Performance Water Based Mud (HPWBM) was designed to drill these types of complicated intervals, nearly matching the drilling performance of oil-based mud (OBM) and delivering a gauge hole. This paper presents a successful drilling through a TAR section using a HPWBM after failures using OBM. The previous attempts were carefully studied to identify the challenges associated during drilling and considered while designing this HPWBM.
The TAR interval was successfully drilled without any drilling fluid related Non-Productive Time and completion set on bottom without any problems. This approach for drilling fluid chemistry provides an environmentally sound replacement for the current drilling fluids to drill TAR sections.