The wellpath is defined by a survey which gives the positions of the well at a number of points along the wellbore.
The position is defined with the co-ordinates in the North, East and Vertical directions (N,E,V). In addition Along-Hole Depth (AHD), inclination and azimuth of the survey station are included as most surveys measure inclination and azimuth to calculate the co-ordinates. Inertial Navigation Systems measure the co-ordinates directly and calculate the inclination and azimuth..
Wellhead location
The location of a wellhead is given as co-ordinates. These can be geographic in terms of degrees of latitude and longitude or in grid co-ordinates (e.g. Universal Transverse Mercator UTM co-ordinates). Land wells are usually surveyed by traditional triangulation techniques from existing benchmark stations. In remote areas with no reliable benchmarks, a Global Positioning System (GPS) using satellite navigation may be used. Offshore, there are a variety of options depending on the distance from shore and area of operation.
Survey references
Depth
·Along-Hole Depth (AHD), which is measured;
·True Vertical Depth (TVD), which is calculated (except in the case of Inertial Navigation Systems, where TVD is measured).
The Rotary Table (RT) elevation is used as the working depth reference. For land wells normally the position of the top cellar is surveyed prior to starting drilling. To be able to relate back to the well, a fixed level shall be defined usually to a flange of the wellhead that remains for the life of the well.
Offshore the distance from the observed sea level to the rotary table is measured and thereafter corrected for the variation with respect to the Mean Sea Level (MSL).
Inclination
The angle (º) between the local vertical and the tangent to the well bore axis at a particular point.
Azimuth
The angle (0 - 360º) between the horizontal component of the direction of the wellbore, measured in a clockwise direction, from the reference.
There are three azimuth reference systems:
1· Magnetic North (MN): This is the direction of the horizontal component of the Earth's magnetic field. A magnetic compass will align itself to these lines.
2· True (Geographic) North (TN): This is the direction of the geographic North Pole, on the axis of rotation of the Earth. The direction is shown on maps by the meridians of longitude. North Seeking gyro tools and Inertial Navigation Systems measure to this reference.
3· Grid North (GN): The meridians of longitude do not produce a rectangular grid system. The grid lines on a map form a rectangular grid system, the Northerly direction of which is determined by one specified meridian of longitude, called Grid North. In the Universal Transverse Mercator co-ordinate system the world is divided into 60 zones of 6 degrees of latitude, in which the central meridian defines Grid North. Grid North and True North are only identical for the central meridian.
Azimuth reference system conversions
Azimuths must be quoted in the same reference system. This is usually the Grid North system. In practice, azimuths are often measured in other systems and two conversions normally have to be applied to the measured azimuths:
1 Grid convergence: Grid convergence converts azimuth values between the Grid North and the specified True North system. By definition, the grid convergence is positive when moving clockwise from True North to Grid North, and negative when moving anti-clockwise from True North to Grid North. Close to the Equator the convergence is small and it increases with increasing latitude.
·Grid correction: Grid correction is by definition the negative of grid convergence.
2 Declination: Declination converts azimuth values between the Magnetic North and True North systems. By definition, the declination is positive when moving clockwise from True North to Magnetic North, and negative when moving anti-clockwise from True North to Magnetic North.
Values of declination change with time and location and those representative of the parameters at the time of drilling should be used.
When magnetic survey results are stored, the declination and the date must be included.
Cross-borehole references
For steering purposes the BHA, needs to be oriented. This requires measuring the angle between a reference direction in the cross-borehole plane (high-side) and a reference point marked at the outside of the bent sub (scribe line). The positioning of the survey tool in a fixed orientation is performed using a key seat or mule shoe of the Universal Bottom Hole Orienting (UBHO) sub. The angle difference between the scribe line of the UBHO and the bent sub scribe line is the toolface offset.
The survey tool measures the toolface, the angle between its fixed orientation and high-side. The angle between the scribe line of the bent sub and high-side is the sum of the toolface offset and toolface.
High-side is the upward direction perpendicular to the borehole axis in the local vertical plane. For vertical wells, high-side is undefined.
Toolface is the rotation angle (º) of the tool about the borehole axis measured in a clockwise rotation from high-side. Toolface can be measured using a plumb bob or by accelerometers. Below 5º inclination toolface is not accurately measured. In these situations a magnetic or a gyroscopic toolface can be used. These toolfaces are identical to magnetic or true azimuth.
Vertical section (projected horizontal displacement)
The vertical view of the wellpath represents a graph of True Vertical Depth (TVD) versus horizontal displacement projected, known as the Projected Horizontal Displacement (PHD), onto a single plane. Where wellpath direction is more than about 20º from the target plane, the distortion of the wellpath projected becomes excessive and the plot is difficult to understand.
There are a number of solutions to the problem of graphical representation. The two most practical yield:
·Multiple vertical section views: Additional vertical views projected into the planes of the major hole directions.
·Stretch plot: This plot looks similar to the vertical section plot, but the horizontal scale is the True Horizontal Displacement (actual displacement in the horizontal plane). A stretch plot represents the horizontal displacement of the wellpath as if it were stretched out along one constant azimuth. It has the advantage that the plot is independent of azimuth and the true inclination of the hole can be seen.
Wellpath calculations
With the exception of Inertial Navigation Systems which measure true displacement, a directional survey measures inclination and azimuth at a number of survey stations at specified along-hole depths. The measured values are used to calculate North, East and True Vertical Depth co-ordinates within the specified reference systems. Additionally, the dogleg severity and Projected Horizontal Displacement (PHD) may be calculated.
There are a number of different calculation methods, however the Minimum Curvature is the standard generally used.
Minimum curvature calculation: The increments for each station in a survey are summed to give the co-ordinates for the full survey. The azimuth and co-ordinate data may be expressed in Magnetic, True or Grid North but must be consistent throughout the survey. The calculated dogleg severity is the dogleg angle per characteristic length of borehole, e.g. degrees/10 m:
Borehole position uncertainty
The borehole position uncertainty is the limit of the actual possible positions of a survey station from its calculated position. It is a result of uncertainties resulting from the survey. The magnitude of the position uncertainty describes a three-dimensional ellipsoid defined by the type, quality and uncertainty model of the survey made.