Today, most of wellbores is being surveyed by MWD/LWD systems. MWD systems (Measurement While Drilling) determine the trajectory of the wellbore relative to the gravitational and magnetic fields of the Earth. That is why these tools are not capable of measuring azimuth near sources of magnetic interference, such as cased wellbores, magnetic rocks, or near adjacent wells (less than 30 meters). In case there is impossible to use MWD with magnetic sensors, Gyro While Drilling (GWD) tools are used. Atlas Drilling has the technology and unique experience in gyro surveying – high-precision gyro surveying, memory drop gyro, orientation, gyro while drilling.
The drilling contractor builds the wellbore according to the project, in which the target is indicated with a certain spread – the circle of tolerance, usually with a radius of 25-50 meters. But often the actual errors of the MWD go beyond the tolerance range. Next, we review the reasons leading to the deviation from the planned trajectory of the MWD systems.
Despite the simple design of MWD magnetic inclinometers, determining the spatial position of a well is not a trivial task. The magnetic field measured by the MWD systems, in addition to the main magnetic field, contains the values of geomagnetic anomalies, short-term variations in the geomagnetic field, and the drift of the North Magnetic Pole.
In addition, the measurements of MWD systems is affected by local magnetic fields, magnetization (drilling tool, drilling fluid and wellbore) and solar activity. The combination of these factors is called magnetic interference. Interference introduces an additional error in tool readings and significantly reduces the accuracy of measuring the trajectory of the wellbore.
The magnetic field has a very complex physical model. As a result, measurements of MWD and EMS tools have survey errors
Bobserved (RED) – MWD registration magnetic field consist of:
- Bm (GREEN) – Main magnetic field
- Bc (BLUE) – Local crustal magnetic field
- Bint (YELLOW) – Drillstring interference
Drilling in West/East direction
When drilling in the West/East direction, i.e. at magnetic azimuth close to 90° and 270°, the measurement error of magnetometers also increases. This is due to the fact that the horizontal component of the magnetic field in these directions is smaller and the tool becomes more sensitive to interference. Under these conditions, even a small variation in the magnetic field leads to errors in determining the azimuth up to several degrees. Such errors can significantly deviate the wellbore from a planned trajectory.
Drilling at high latitudes
Survey measurements in high latitudes – in the Arctic and in the Far North – are subject to the influence of distorting factors to a greater extent than in middle latitudes. The azimuthal measurements are affected by more powerful short-term perturbations of the geomagnetic field and magnetic storms caused by solar activity. The relative proximity of the North Magnetic Pole also contributes to an increase in the error in determining the magnetic declination angle. In addition, the azimuth measurement error because of the north magnetic pole drift is additionally 1.5-2°/year.
High-precision gyroscopy to solve the MWD accuracy limitations
The problem of measurement accuracy of MWD systems is relevant due to the complex and imperfect physical model of the geomagnetic field. At the same time, when drilling at high latitudes (latitude more than 60 degrees) and/or on the West/East direction, magnetic interference introduces even greater uncertainty into the readings of magnetometers. As a result, even the most advanced and accurate MWD systems are prone to errors. These errors do not allow accurate trajectory measurements to be made and may cause the actual trajectory to be out of tolerance even if all MWD measurement quality policies are followed.
To improve the accuracy of survey, high-precision gyro can be used. They have a number of advantages over MWD: immunity to magnetic field, high measurement accuracy and the ability to obtain independent survey data, because the high-precision gyroscopes operate on a different physical model.
To solve the problems of insufficient accuracy of well positioning when drilling at high latitudes and West/East direction, Atlas Drilling offers the following solutions: a Small-sized Memory Drop Gyro, a High-precision Small-sized Gyroscope on wireline/coiled tubing/tractor and Gyro While Drilling (GWD, GMWD) or Gyro Steering.