Unconventional resources encompass a wide range of reservoir types. The art of designing and engineering drilling fluids for unconventional resources should not just enable rotary drilling, but also prevent wellbore instability, protect production, and facilitate formation evaluation. In some cases, geosteering, based on logs or cuttings analysis, is part of the drilling process. In addition, many unconventional wells have trajectories that require excellent lubricity to drill. Drilling fluids must be tailored to achieve the goals for each well.
Examples of how drilling fluids have been developed for low-porosity microfractured gas sand, oil shale, and tar sands are presented. In low-permeability gas sands in South Asia, preserving natural fractures in the near wellbore region was critical to obtaining maximum production. In Canadian tar sands, preventing hole washout in the production interval was critical for obtaining maximum production. In the Texas oil shale, replacing oil-based drilling fluid with water based alternatives had implications for formation evaluation, wellbore stability and well length in the production interval. Emphasis is placed on the design process for developing drilling fluid systems that contribute to meeting critical objectives for each project.
Many hydrocarbon resources are drilled as part of the process leading to production. Rotary drilling methods require drilling fluids to remove rock cuttings and control subsurface pressures. Often the drilling fluids used for unconventional resources have been selected from the array of fluids that have been used to drill conventional oil and gas wells simply on the basis of expense and availability. A more thorough process of developing drilling fluid systems for specific projects can result in improvements in meeting project objectives.