4.1 Basic Definitions

This section provides a brief overview of these real-world objects and their purpose in drilling and well-related operations. These real-world objects are represented as a data objects in WITSML. Each data object is discussed in more detail later in this chapter.

A well in the oil and gas industry is a boring in the Earth that has two main purposes:

• Exploration of the subsurface to determine viability of economic development and production of hydrocarbons.
• Production of hydrocarbons to the surface and conducting of a variety of operations and services for the safe and efficient operation of the wells.

Each well has a unique surface location on the Earth. Knowledge of this location and reference to it are vital for accurate and safe well-related operations and field development. The well is located on the Earth’s surface (or in space) using a coordinate reference system (CRS).

To maximize operational efficiency and increase subsurface access, a well may contain multiple wellbores, which are the actual boreholes that comprise the well. A wellbore represents the path from surface to a unique bottomhole. While drilling a wellbore, measurements called surveys are taken at various locations as drilling progresses. These measurements determine the shape and location (curve) of the wellbore, which is referred to as the wellbore’s trajectory. The individual locations where the measurements are taken are called trajectory stations or survey stations.

Like any tools, the tools use to conduct surveys are subject to inaccuracies. Errors in a survey can have significant impacts on the HSE, operations, and financial success of well. As such, tool error models are used to quantify the inaccuracies of a tool, so that associated risks can be defined.