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Directional Tools Used for Measurements

Three features of a directional wellbore are measured at given points within the well: (1) measured depth, which is the distance from the surface to a given point, measured along the wellbore; (2) angle of inclination from the vertical (drift angle or deviation angle); and (3) drift direction, or the directional path of the wellbore. These parameters are the basis for calculations of the position of each point in the subsurface, and all this information is included in a directional survey. Drift angle and drift direction are measured by a survey tool conveyed in drill pipe or by wireline, and MD is determined by length of drill pipe or wireline. The various tools that are used fall into two categories: magnetic and nonmagnetic.

Magnetic Surveys

Magnetic is a generic term for describing several survey tools that use a magnetic compass for direction and therefore must be run inside a special nonmagnetic drill collar to negate the effects of the drill pipe. An example of such a survey is the single-shot magnetic survey. This device records, on a heat-resistant film disc, the magnetic direction and inclination angle of the wellbore at specific depth intervals. A multishot survey uses a filmstrip to record several readings of hole angle and direction at different depth intervals. Newer wireline tools provide real-time survey data at the surface. Measurement accuracy of magnetic survey tools has been improved within the last 30 years. Modern magnetic tools are used for measurement-while-drilling (MWD) surveying. These surveys provide real-time data for more efficient directional drilling.

Nonmagnetic Surveys

Nonmagnetic survey tools are of two types: those with no direction-finding device and those with a gyroscopic mechanism for determining direction. A drift indicator tool (e.g., a Totco tool) measures only drift angle and is usually run in vertical wells or shallow vertical sections of deviated wells where directional information is not required. This tool generally consists of a housing or barrel, a motion indicator, a timer, a punch, and a printed paper disc. The unit is either run on a wireline or dropped on a drill bit. When the motion sensor determines that the tool is no longer moving, the timer is activated, and after a predetermined interval of time the punch is released. The punch, which is allowed to swing freely and act as a plumb bob, drops vertically and punches a hole in the paper disc, which is marked in degrees. Figure 3-16 shows a Totco disc scaled to a maximum of 8 deg. The hole punched in the disc indicates an inclination angle of 4.5 deg, but the drift direction is unknown.


Figure 3-16 Example of a Totco survey. Well deviation angle is 4.5 deg.

Gyroscopic survey tools are widely used and are capable of providing more accurate data than magnetic survey tools. Because the magnetic compass is replaced by a gyrocompass, the system can be run in both cased and uncased holes and run where cased holes are nearby, as in a platform well cluster. The gyro system can be set up as a single-shot or multishot instrument. Conventional tools, referred to as free gyros, are less accurate than the newer rate gyros, which are of different construction. A gyro survey is sometimes run after an MWD magnetic survey to provide additional information for well path determination. Gyroscopic tools have recently been developed for use while drilling (gyroscope while drilling [GWD]) (e.g., Weston et al. 2014).

Survey Frequency

Survey frequency can affect the accuracy of a survey, especially if the well is drilled with the traditional mud motor and bent housing. Deviated wells drilled using a mud motor and bent housing are drilled using a combination of sliding and drilling. During sliding, the bent sub is oriented in the direction the driller wants to turn the well, and the mud motor rotates the bit while the drill pipe is not rotated. After a short section of hole is drilled this way, the drill pipe is rotated and additional hole is drilled. In this system, sliding and then drilling will result in a slightly different TVD than drilling and then sliding, even if the MD, borehole inclination, and borehole azimuth are identical at the beginning and end of the hole section (Jamieson 2012). Wells drilled with rotary steerable assemblages may be less susceptible to errors due to survey frequency, but it is still best practice to survey deviated wells at frequent intervals, generally every 100 ft or less.

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