Geosteering: Part II – Data

The most important first

Perhaps the single most important piece of data used during geosteering operations are the measured while drilling (MWD) surveys.

Why?

You need to know where your well is being drilled (i.e. within the lease boundaries).  You must also know if your well is hitting its targets.  To find out if it is being drilled according to plan, you must perform the first correlation: matching the active well’s path to its intended path.

Why else?

You cannot properly correlate data without having accurate and complete survey data for both your offset well and your active well.  For example, just a few feet of inaccurate TVD data can land you above or below your target line.

Devil is in the details

Nearly all of the data used in geosteering operations are termed “curve” data.  This is data which are represented on a log graph against either measured depth or time (or both).  However, there is at least one piece of non-curve data that is commonly used.

Curve Data
  • Gamma – the radioactive gamma count
    • Direction
      • Radial – measures entire circumference of the wellbore
      • Azimuthal – measures at a specific angle within the wellbore
    • Capture
      • Spectroscopic – measures value of gamma ray sources
      • Composite – measures total magnitude of gamma ray sources
  • Gas – the expelled formation gases
    • Total – total concentration of flamable gases in the C1-C4 range (usually expressed in terms of units relative to atmospheric concentration)
    • Chromatograph – breakdown of individual C1-C4 concentrations, and perhaps some other individual gas components (i.e. CO2, H2S, etc)
  • ROP – rate of penetration of the drill bit
    • ROP – feet/hour
    • Drill Rate – minutes/foot
  • Lithology – cutting descriptions and lithological percentages
Non-Curve Data
  • Seismic data – seismic cross-sections (although there is seismic while drilling, I won’t get into that here since it has other purposes, such as well control)

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