Directional Surveying Measurements And Sensors

In order to guide a wellbore to a desired target, the position and direction of the wellbore at any particular depth must be known. Since the early days of drilling, various tools have been developed to measure the inclination and azimuth of the wellbore.

To calculate the 3D path of the wellbore, it is necessary to take measurements along the wellbore at known depths of the inclination (angle from vertical) and azimuth (direction normally relative to true north). These measurements are called surveys.

To compute the wellpath between two surveys, various mathematical constructions have been proposed. The most common modern method assumes that the wellpath forms a perfect arc between two surveys. This model is called minimum curvature. The calculations can then be done to calculate the position of the second survey if the position of the first survey is known.

There are errors inherent in these calculations-first, survey instruments are only accurate to a certain degree, and second, though a perfect arc is assumed, this is unlikely to be the case. The calculated wellpath will be more accurate if surveys are taken close together (every 20' -30') and if the well is not highly deviated.

One of the earliest tools used to document the wellpath was a bottle of acid that was lowered down the well. After half an hour, the acid would etch a mark on a copper cylinder inside the bottle. When the bottle was pulled back to the surface, the inclination of the well at the depth where the bottle was left could be measured by measuring the angle of the etched mark.

The acid bottle could be called a type of "single shot" tool because it only takes one measurement each time it is run in the well. There are two other single shot tools that are still used today, the acid bottle having been consigned to the history books.

The first of these tools is called a totco tool. It is a mechanical tool that takes a reading showing the inclination of the wellbore, but not the azimuth. It is used in vertical wells to check that the well is within a few degrees of vertical. While drilling, the tool can be run on a wire to the BHA and recovered as soon as the survey is taken. A clockwork timer determines when the survey is taken. When the drillstring is tripped out, the totco survey tool can be dropped down the drillstring (with the timer set to give enough time to fall to the bottom) and will be recovered when the BHA is back at the surface. The survey is recorded by punching a hole in a paper disk.

The second single shot tool is the magnetic single shot (MSS). This tool measures both inclination and magnetic azimuth. The azimuth is normally converted to true north before being used in calculating the well path. For this tool to measure azimuth, it cannot be placed inside a normal steel drill collar. Normal drill collars have their own magnetic field that renders the azimuth reading magnetic compass unreliable. A special steel compound called "Monel" is used to make non-magnetic drill collars and stabilizers, which have to be placed in the BHA to allow the MSS to be used. In a directional well, MSS surveys will be taken more frequently when the well is being deviated (in the build up sections) and less frequently when the well direction is not being changed intentionally (in the tangent section) .The MSS has interchangeable measuring units that allow inclinations up to 90° to be recorded onto a film that is developed on the surface.

A MSS can also record the orientation of the tool face. When the well is kicked off using a jetting assembly or a downhole motor, the assembly can be orientated in the correct direction by running a MSS to see the tool face azimuth, then the drillstring can be turned at the surface to correctly align the tool face.

The totco and MSS surveys are routinely used while drilling with rotary assemblies. There are also survey tools that allow surveys to be taken in "real time" and display the data to the driller. These are called measurement while drilling (MWD) survey tools.

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    How to read a mechanical totco survey tool?
    7 years ago

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