For this blog entry I continue to quote from a White Paper prepared by Baker Hughes and available on our website. The complete paper can be obtained here. I will present it here in three parts.
To supplement the natural barriers to fluid migration and further protect underground drinking water sources, manmade barriers are created during the well construction process. First, reservoir analysis can help determine the best location for each well to maximize field efficiency and minimize surface impacts. Next, as the well is drilled, proper steel casing must be run and an engineered cement slurry is pumped to protect the environment and the well.
Drilling speed and accuracy increase efficiency and wellbore stability while minimizing requirements for drilling fluid (referred to as mud). Wellbore stability is particularly critical for ensuring long-term zonal isolation with openhole multizone completion technologies. Adding real-time diagnostics to the drilling program helps engineers on location refine mud programs, well paths, and casing plans.
Well casing consists of a series of metal pipes. Casing strengthens the borehole, ensures that no oil or natural gas seeps out of the well as it is produced, and ensures other fluids do not reach the wellbore from the formation. Typically, several casing strings are run concentrically to provide added protection and stability, especially across drinking water zones.
Casing is selected based on the subsurface characteristics of the well, including the diameter of the well and the pressures and temperatures the well is expected to experience over its full producing lifetime. However, casing must be properly run into the borehole to ensure it meets all of its specified ratings. When the casing is in place, proper well cementing is essential to protect subsurface aquifers and to prevent gas migration for the full expected lifetime of the well.
The most important step in achieving long-term zonal isolation is to know what is expected of the cement sheath over the lifetime of the well. This long-range thinking is a step-change in the traditional well construction/cement design model, made necessary by modern well and stimulation design requirements.
Although some of these requirements have existed since the early days of the oilfield, the challenges (temperatures, pressures, well trajectories, enhanced oil recovery programs, etc.) are often more extreme today than encountered in the past. Ideally, expert cement engineers are typically included in the development of an integrated well and field development program to fully understand the reservoir and stresses the well may experience over its lifetime. Engineering and simulation software, such as BJ Services CemFACTSTM and IsoVisionTM applications, aid in producing a design that suits the well while ensuring zonal isolation for the life of the well. Mechanical considerations should, of course, include the traditional measure of compressive strength, but in many cases will also need to consider the tensile and flexural strength, Young’s modulus, Poisson’s ratio, fracture toughness and corrosion resistance.
Because of the cement’s critical role in protecting drinking water sources, cementing the surface casing is highly regulated, often to the point of prescribing minimum cement thicknesses, depths, and other parameters specific to the well location. The regulation of surface casing installation and cementing practices are enforced by state and/or federal agencies. Cementing must meet or exceed these regulations, but Baker Hughes recommends additional standard best practices to ensure lifetime zonal isolation:
• Casing or liners should be prepared with cable scratchers to aid in hole conditioning and with centralizers adequate to achieve a minimum standoff of 70%.
• The use of both top and bottom cementing plugs is strongly recommended. The top plug is almost universally used, but the bottom plug is also important because it mechanically separates drilling mud from cement slurry inside the casing, minimizing contamination.
• Proper mud conditioning is the most important operational factor for ensuring cementing success. Pipe movement (when possible) is the best means of improving mud displacement and hole cleaning.
After the pumping operation, cement is allowed to set and then can be tested with pressure, a cement bond log, or more sophisticated tools to ensure its integrity and bond with the casing and formation. Interpreting the data requires considerable expertise, as many of these logging systems are sensitive to even small variations in eccentricity, borehole fluids, or formation rocks.
Some segments of some wells may not require cementing to maintain zonal isolation. When using a horizontal openhole completion, such as Baker Hughes’ time-saving FracPointTM system, a segment of the well’s payzone is left uncemented and zonal isolation is achieved by extruding packer elements from the casing system out to the borehole wall. Isolation is tested with high pressure. Fluid migration risk is minimal because the packers provide excellent isolation, the openhole segment is thousands of feet from the nearest drinking water source and the casing is carefully cemented above the openhole section.