WELLOG                                 DEVIATION SURVEY


Revised 11-07-2016

© 2004 - 2016 WELLOG

All Rights Reserved 




During the course of drilling a well, the direction of the drill may change.  The causes vary, however, the effect of changing formation hardness and drilling technique are some of the major factors. Borehole deviation is defined as the angular change from vertical.


In certain situations, the direction of drilling is deliberately changed in order to intersect a certain point below the surface. Directional drilling methods are used for this application.


Borehole deviation is measured in angular degrees from vertical often called inclination. When the inclination is known, true vertical depth can be determined from the top of the hole. True Vertical Depth (TVD) can be determined from deviation information.


Exploration drill holes may be drilled at a specified inclined angle to intersect a target such as a mineralized ore body.




Early measurements of deviation were performed using piezo-pendulum sensors and potentiometer-pendulum sensors. Other deviation tools use a one-shot photographic method or locking compass mechanism.


More recent electrolytic technology uses a small fluid filled container with a geometric configuration of fluid level sensors.


Ask about the WELLOG basic x-y tilt sensor tool with RS-422. WELLOG provides the interface!


Improved techniques using new technology include the following methods:




Recent advances in semiconductors use surface micro-machined Poly-silicon springs also known as MEMS technology. These devices have given rise to a family of low cost solid-state 2 and 3 axis accelerometers. Accelerometers can measure gravitational acceleration forces from tens and hundreds of g’s down to fractional g forces.  When used in fractional g applications, accelerometers can measure angular displacement or tilt from the vertical. WELLOG has recently developed MEMS based accelerometer sensor systems that can detect fractional changes in borehole angle with increments of .1 degree or less. Measurement of G force is non-linear. Our sensor systems “linearize” the sensor output into a linear x-y axis measurement of degrees. Single chip integrated circuit 3-axis accelerometers are embedded in WELLOG deviation tools.




Recently, 3 axis solid-state integrated circuit magnetic field sensors called magnetometers have come into the market. They provide accurate, high resolution electronic compass and x-y axis tilt information. A magnetometer can be incorporated into a digital electronic compass for azimuth information. Azimuth information is logged with repeatable accuracy of less than one degree. Magnetic azimuth and magnetic x-y tilt information may be used for borehole directional surveys. 


Ask about the WELLOG Mini-Mag heading sensor.


Tilt compensated compass for open-hole boreholes:


Ask about the WELLOG ez-Mag vertically oriented tilt compensated 360 heading and 90 degree x-y tilt sensor with RS-422, RS-485 or RS-232.


Ask about the WELLOG Tractor-Mag Tilt compensated 360 degree heading and 90 degree x-y tilt with RS-422, RS-465, RS-232 or USB.


View an example of the default NEMA format data produced by this system here.


As always WELLOG provides the interface!




Solid-state gyroscopes use a technology referred to as iMEMS. Three axis solid-state gyroscopes can provide direction in steel cased holes and in regions that are affected by magnetic mineralization. Rate sensing integrated circuits offer capabilities in applications where magnetometers are unreliable.




Combinations of sensors measure tilt by measuring gravitational acceleration using a x-y accelerometer and measure direction of tilt with reference to magnetic north using a magnetometer. Magnetometers are reliable in most cases. Magnetometers can become unreliable around machinery or when they are inside steel well casing. When used in COMBINATION with rate gyros and accelerometers, all three sensors provide input to a high speed processor that maintains continuous orientation.


Applications within steel casing or areas having anomalous magnetic fields:  (NO PROBLEM!)


Rate Gyros offer a solution to the problems associated with magnetometers. A combination x-y tilt sensor using an accelerometer and a rate sensing gyro provides a robust deviation measurement platform.




In the best case, a combination of  both a 3-axis rate gyroscope and 3-axis accelerometer sensor package provides a complete sensor package. This combination results in measurement of NINE degrees of freedom (DOF). The most advanced product in our line of deviation instruments is a “WELLOG Navigator”. This borehole surveying package can be combined with our V30 logger surface well logging system and USB interface to a notebook PC.


Ask about the WELLOG Navigator with RS-485 wire line driver and USB input to PC. It creates a standard text file ready for immediate viewing.


Nine Degrees of Freedom:


Recent improvements in navigation boards have resulted in a system referred to as an INERTIAL MEASUREMENT UNIT (IMU). The IMU integrates a 3-axis Gyro, 3-Axis Magnetometer, and 3-Axis Accelerometer.


WELLOG is developing a wide range of IMU boards in airborne, surface, marine, and down-hole applications.





Deviation tools are placed on a stand that is calibrated in degrees. As the tool is tilted from vertical, thru each angular position, the response is noted. Because all logging tools may change performance during logging, it is important to perform a field calibration test before and after every logging job.




The result of a sequence of measurements of x-y tilt and directional heading information is combined into a polar or rectangular plot for presentation. A rectangular presentation affords a 3 dimensional view of the well or borehole in 3 dimensional space.


A polar plot is a presentation of the direction of the well or borehole from the view of an observer looking down hole from the location of the well at the surface. This presentation resembles a target centered at the location of the well at the surface and having concentric rings representing the displacement and direction (in degrees) of displacement of the hole at given depths.




No one computational method is always the “right” method, and the right method to be used can be chosen from several methods.


Inclination (j) is taken to be the angle of the well course from the vertical. Azimuth (q) is taken clockwise from geographic north. In order to maintain a right handed coordinate system, coordinate x increases to the north and y increases to the east.


The relations between measured angles and rectangular coordinates are:


                        dx = sin j  cos q dl = a dl

                        dy = sin j sin q dl = b dl

                        dz = cos j dl = g dl



Where:            inclination = j   Azimuth = q


The direction cosines a, b and g = defined in the above equations represent the cosines of the angles between the well course and the x, y, and z directions. The simplest method of computing a well course called the tangential method, is to fit a series of straight-line segments to the well course and establish the direction of each segment by the lower of each pair of stations. The method is an extension of the above equations applied to data from two sequential stations labeled a, b so that


                        Dx = sin jb cos qb Dl = ab Dl

Dy = sin jb sin qb Dl = bb Dl

Dz = cos jb  Dl = gb Dl


where the incremental distances are the differences between values taken at a and b, for example, Dx = xbxa. It is generally held that it is less accurate than other methods and its use is not recommended (Wilson, 1968).


In more satisfactory methods, some means of averaging the measurements taken at two adjacent stations is utilized. The tangential method can be improved by averaging the directional cosines of stations a and b. The result is referred to as the balanced tangential method


                        Dx = ˝ (aa+ ab) Dl

                        Dy = ˝ (ba+ bb) Dl

                        Dz = ˝ (ga+ gb) Dl


From: “Well Logging For Physical Properties”, Joseph R. Hearst, Philip H. Nelson, McGraw-Hill, inc., 1985




Deviation can be measured in several modes. The three modes are single shot, multi shot and continuous. The more measurements, the more accurate the final analysis of the directional survey will be.




WELLOG is constantly improving the process of logging wells. The logging process can be tedious, and susceptible to human error. Often, well logging is performed after long periods without rest or under conditions that make measurement difficult. Reducing the complexity of setting up and operating logging systems can be part of the design process.


KEEPING IT SIMPLE:  Here’s an illustration of the calculations in MS Excel : borehole navigation.xls and a 3D view.




WELLOG has made great progress in making well logging easier and more reliable. New developments in memory logging tools are examples. Making tools that are self contained and wireless is an example.


NEW DEVELOPMENT:                 NEW! World’s smallest radio-modem memory deviation logging tool!


WELLOG has developed what could possibly be the first of its kind commercially available microprocessor based deviation logging tool.

This tool measures course and tilt to less than one degree using solid-state accelerometers. The tool is battery powered and rechargeable.

It is a memory logging tool containing a radio-data modem for downloading memorized logging data.


OTHER APPLICATIONS include slope stability alarms in mining. Avalanche detection. Unmanned Aerial Vehicles.


This tool employs a single navigation platform with NMEA data output. It’s available in a 1 ˝ inch “slim hole” tool housing memory mode or GO style 4 conductor tool head for wire line.






Easy to use.




WELLOG has developed deviation tools using the most advanced solid-state devices available. In the process, the design time has been short, the cost of the components has dropped considerably because of competition in the world market. Getting the latest technology can save you money! Ask WELLOG about the competitive cost savings available on these tools.