Deviation Survey Reference

It is important in the 3 dimensional world to know the reference from which all the measurements taken. To this end, each survey must have a coordinate system associated with it at the survey level. In addition, each individual point can have its own coordinate system associated with it.

Positioning and Coordinate Systems

The oil industry has evolved from relative positioning (i.e., the target is 1200' from the surface location along N 48.6° E) to absolute positioning (i.e., the target is located at UTM 6,234,345.67 m N and 474,628.34 m E). The need to interchange meaningful data with others, government regulations, the requirement to locate the blow out wellbore when the surface rig has cratered, and many other equally important reasons require that the Directional Driller of today understand far more about positioning and coordinate systems.

The problem

The earth is a sphere. Actually, it is an oblate spheroid (a squashed sphere). The radius of the earth at the North Pole is about 13 miles shorter than the radius at the Equator. If the earth was the size of a billiard ball, the human eye could not tell the difference; however, when it comes to modeling the size and shape of the border of a country or an oilfield lease, this 13 miles causes many problems for the geodesist (a scientist who studies the shape of the earth).

The maps and drawings used in directional drilling are flat. Plotting data which lies on the surface or subsurface of a sphere onto a flat map is impossible without compromises and the introduction of controlled error. The science of geodesy and cartography (map making) are drawn upon heavily to provide a complex, yet straight forward method for the Directional Driller to represent and plot his surveys and wellplans.

Geographic Coordinates (Latitude and Longitude)

To identify the location of points on the Earth, a graticule or network of longitude and latitude lines has been superimposed on the surface. They are commonly referred to as meridians and parallels, respectively. Given the North and South Poles, which are approximately the ends of the axis about which the Earth rotates, and the Equator, an imaginary line halfway between the two poles, the parallels of latitude are formed by circles surrounding the Earth and in planes parallel with that of the Equator. If circles are drawn equally spaced along the surface of the sphere, with 90 spaces from the Equator to each pole, each space is called a degree of latitude. The circles are numbered from 0 at the Equator to 90 North and South at the respective poles. Each degree is subdivided into 60 minutes and each minute into 60 seconds of arc.

Meridians of longitude are formed with a series of imaginary lines, all intersecting at both the North and South Poles, and crossing each parallel of latitude at right angles, but striking the Equator at various points. If the Equator is equally divided into 360 parts, and a meridian passes through each mark, 360 degrees of longitude result. These degrees are also divided into minutes and seconds. While the length of a degree of latitude is always the same on a sphere, the lengths of degrees of longitude vary with the latitude.

At the Equator on the sphere, they are the same length as the degree of latitude, but elsewhere they are shorter.

Local Coordinate Systems

In most cases, the Directional Driller will use a system of local coordinates for day-to-day activities. This local system depends upon and has a direct relationship with all the concepts presented thus far in this chapter. Many assumptions are often made in defining local coordinate systems that are not obvious, but are very important. Care must be used in specifying local coordinate systems so that all implicit and explicit relationships to “legal” coordinate systems are preserved.

The Local Coordinate System must have its origin at a point that can be positioned in the “legal” coordinate system. This point should be referred to as the Structure Reference Point, if the local coordinate system applies only to a single structure (platform/rig) or as the Field Reference Point, if the local coordinate system is used over the entire field. The term Reference Point will be used in this chapter to mean either or both. The Reference

Point has a location in the a “legal” coordinate system, and it has a location of (0,0) in the newly defined Local Coordinate System. This reference point has only North and East coordinates defined. An additional reference, the Vertical Reference Datum, must be defined in order to measure depth, either TVD or MD. Common examples of a Vertical Reference Datum are RKB, MSL, LAT, mud line, etc. If necessary, a separate Vertical Reference Datum can be defined for each MD and TVD.

Unless specifically defined otherwise, a Local Coordinate System has each of its axis oriented parallel to the corresponding axis of the "legal" coordinate system in which its Reference Point is defined. Obviously, there must be a defined unit of length, however, this is normally dictated by the customer's preference or governmental regulation. By definition, a Local Coordinate System is a grid system and has to use a Grid North in order to be plotted correctly. Only on a plot drawn using Grid North, can distances and angles be measured directly. If True North or Magnetic North is used to plot directional survey data, the relationships between lines and points on the plot are not linear and therefore cannot be measured directly with a compass or ruler. Quite often, the error (distortion) is small, but this is not something that is readily apparent and cannot be left to individual judgment.

Often, it is necessary to convert location data from one local coordinate system to another. A good example is the slot pattern of a multiwell platform which is usually defined on the "as built” drawings of the construction company. The slot locations on a drawing are usually defined relative to a drawing local reference system, which has its own origin and reference North. It is up to the planner to determine the amount of translation (moving the pattern in N, E) and rotation (moving the pattern around a point) required to allow the slots to be located in the Directional Driller's local coordinate system. In order to translate the data, at least one point on the "as built" drawing must be locatable in the “legal” coordinate system or the Directional Driller's coordinate system. Likewise, to rotate the pattern into the Directional Driller's local coordinate system, a reference line on the “as built” drawing has to be related to a reference line in the Directional Driller's or “legal" coordinate system. These reference lines should be referred to as Structure Reference Lines. An analogous discussion can be made for relocating Targets from a geophysical or reservoir based reference system to the Local Coordinate System.

Magnetic declination correction is the angle between Magnetic North and True North. Values of magnetic declination change with time and location. As the movement of Magnetic North is constant and predictable, magnetic declination can be calculated for any given point on the earth at any given time. Charts depicting the various declinations and rate of change (usually expressed as an annual change) are widely used. An Easterly declination is expressed as a Positive value and a Westerly declination is expressed as a Negative value. Although converting from one reference to another appears to be a simple task, considerable care is needed, depending on the relative directions of convergence and magnetic declination.