A comprehensive review of laboratory, field and modelling studies on drilling mud rheology in high temperature high pressure (HTHP) conditions

As the demand for hydrocarbons increases, the search frontiers for petroleum and natural gas deposits keep being pushed towards unconventional terrains. An example of an unconventional terrain where oil and gas deposits can be found is the high temperature and high pressure (HTHP) environment. While HTHP wells can hold a wealth of resources, they are difficult and expensive to complete. One area of difficulty is in optimizing the rheology of muds used for drilling wells in these environments. As a result, a plethora of laboratory, field and modelling studies have been conducted on mud rheology under HTHP conditions. Despite the huge number of studies on this subject, bibliometric analysis indicates that in the last five decades, there is no comprehensive, up-to-date review on mud rheology under HTHP conditions. This work is therefore an attempt to curate the laboratory, field and modelling studies on mud rheology in HTHP conditions with a view to highlighting recent advances on the subject and identifying grey areas where more studies should be directed. The contributions of the current study for which novelty is claimed include: a comprehensive curation and critique of HTHP mud rheological models, laboratory studies, HTHP mud cost analysis and a detailing of learnings from specific field experiences on HTHP mud rheology. A conscious effort was made to present the review in a straightforward manner with enough theoretical background so that readers in and outside of the drilling community can benefit. Although the authors have striven for completeness, it is essentially an impracticable undertaking to be exhaustive. Results from the review indicate that while the relationship between mud rheology and temperature and pressure in HTHP wells remains complex, additives with high thermal and rheological stability are available and can be used in HTHP drilling. The real essence of this work is not to create an encyclopaedia of mud rheology experiments but to facilitate the understanding of the behaviour of various mud types under HTHP conditions. Better still, it could serve either as a useful reference for the academic community or as an added preparatory guide and checklist to wellbore drillers planning to drill in HTHP environments. •HTHP mud rheology modelling, experiments, field and cost studies have been reviewed.•No universal model for HTHP mud rheology prediction exist.•A critique of the rheological models and laboratory studies has been done.•Field stud