THD LOGS If Technical Hole Deviation (THD) in general is new for you, we kindly suggest you first read this. This document contains the following sections. TECHNICAL HOLE DEVIATION LOGS INTRODUCED A powerful, visual way in which to present Technical Hole Deviation is with a well log. A "Technical Hole Deviation Log" is a well log that conveys the directional well plan and the actual associated geometric deviations. THD logs equip the operator with a superior mechanism for monitoring and/or "grading" directional control performance, and they provide directional drillers with critical information for rationalizing tool settings while drilling. As would be expected, THD values are plotted versus actual measured depth. The Vertical Technical Hole Deviation Log addresses deviations in the "vertical" sense. It presents planned and actual values of wellbore inclination and dogleg-severity, and THD components msVD, RCVD, msID, and RCID. The Horizontal Technical Hole Deviation Log addresses deviations in the "horizontal" sense. It presents planned and actual values of wellbore azimuth and dogleg-severity, and THD components msHD, RCHD, msAD, and RCAD. THD logs can be created prior to drilling in order to present the plan, just like standard directional plots. Then, as drilling commences, the "actual part" of the THD logs are plotted and the directional control performance unfolds. The footer section of a THD log contains standard vertical section and plan view directional plots. Regardless of the planned well path linear/curved 3D/2D complexity, the target path is always a single vertical green zero line on the Technical Hole Deviation log tracks. THD WELL LOG HEADERS Besides measured depth, the commonality between the Vertical THD Log and the Horizontal THD Log is the inner left track. Here, planned and actual dogleg-severities (DLS) are graphed. Circular unfilled markers on the actual DLS curve convey depths with directional surveys. Circular filled markers on the planned DLS curve convey critical-point changes in the well plan (e.g., curved to straight). Vertical Technical Hole Deviation Log Header The upper THD log header section contains basic summary statistics. These include: - Excess Measured Depth - amount of actual hole drilled subtracted by the corresponding planned measured length of drill hole. This value typically indicates the "cost" of oscillating about the planned path. However, in special cases this value can be negative. Such may be observed, for example, when dropping a wellbore to vertical with actual dogleg-severities exceeding planned dogleg-severities.
- Average Absolute msVD - average absolute value of vertical deviation (msVD) associated with real survey stations over the length of drill hole for which the THD log was created. A smaller value usually indicates better directional control performance.
- Average Absolute msID - average absolute value of inclinational deviation (msID) associated with real survey stations over the length of drill hole for which the THD log was created. A smaller value usually indicates better directional control performance.
- THD Grid Values - conveys the grid values of the THD tracks, as a result of auto-scaling. For the example header below, the "20~2" means the grid value for the msVD track is 20 feet, and the grid value for the msID track is 2 degrees.
The outer left track graphs planned and actual wellbore inclination. Hole deviation in the "vertical" sense is displayed on the two right tracks. The inner right track displays vertical deviation (msVD-blue) and the relative change in vertical deviation (RCVD-red). The outer right track displays inclinational deviation (msID-blue) and the relative change in inclinational deviation (RCID-red). A zero line (green) centers each of the two THD tracks. For the actual drill path to follow the planned path in the vertical sense, msVD, RCVD, msID, and RCID must trace their respective zero lines. Horizontal Technical Hole Deviation Log Header The upper header section of the Horizontal THD Log contains the same Excess Measured Depth values and other basic summary statistics including: - Average Absolute msHD - average absolute value of horizontal deviation (msHD) associated with real survey stations over the length of drill hole for which the THD log was created. A smaller value usually indicates better directional control performance.
- Average Absolute msAD - average absolute value of azimuthal deviation (msAD) associated with real survey stations over the length of drill hole for which the THD log was created. A smaller value usually indicates better directional control performance.
The outer left track graphs planned and actual wellbore azimuth. Hole deviation in the "horizontal sense" is displayed on the two right tracks. The inner right track displays horizontal deviation (msHD-blue) and the relative change in horizontal deviation (RCHD-red). The outer right track displays azimuthal deviation (msAD-blue) and the relative change in azimuthal deviation (RCAD-red). A zero line (green) centers each of the two THD tracks. For the actual drill path to follow the planned path in the horizontal sense, msHD, RCHD, msAD, and RCAD must trace their respective zero lines. PROJECTIONS Forecasts of the msVD and msHD profiles are shown as thick black lines that extend from the last survey station. These THD projections are helpful for directional drillers while they rationalize directional tool setting adjustments, during drilling operations. EXAMPLE THD WELL LOGS THD well logs for three wells are presented below. These logs were created with SES and then converted to JPEG graphics files. Click a thumbnail to enlarge. All data are from real wells. | 3D Sidetrack Horizontal | 3D Sidetrack Horizontal | 2D Sidetrack Directional | | 3 in. / 1000 ft | 5 cm / 300 m | 1.5 in. / 1000 ft |  |  |  |  |  |  |
| 
