Contractors using trenchless drilling technology for their projects may run into drilling deviations somewhere along in the project. A thorough knowledge of what lies beneath can be obtained by conducting a geotechnical investigation and gathering data from the local municipality. This data is an essential first step to plan a bore path in a trenchless project irrespective of the method used. Catastrophic problems can arise if major deviations from the bore path take place during the boring operation. However; sensors in the drill string are capable of detecting any deviation from the planned path, enabling operators to make any required corrections before damage takes place.
There are basically two types of deviations that happen in a drilling operation, namely, lineal and angular deviation. They are broadly known as technical hole deviations (THD).
The 1st order lineal deviation includes vertical deviation (msVD) and horizontal deviation (msHD). It is a measure of the difference between the planned and actual bore path in the horizontal or vertical direction during the drilling process. HD will take place either in the east or west direction of the bore path while VD will take place either to the north or south of the bore path. VD and HD can be easily visualized to determine the location of the wellbore in space.
The 2nd order lineal deviations are – relative change in vertical deviation (RCVD) and relative change in horizontal deviation (RCHD). These parameters are able to capture important data to predict VD or HD. The insight provided by RCVD and RCHD help to correct the difference by adjusting tool settings before the planned bore path is reached.
The 1st order angular deviation includes azimuthal deviation (msAD) and inclinational deviation (msID). It represents the difference between the tangent angles of the planned and actual bore path. Compared to VD and HD, AD and ID are difficult to visualize but easier to control.
The 2nd order angular deviations are – relative change in azimuthal deviation (RCAD) and relative change in inclinational deviation (RCID). These parameters also help to predict AD and ID as the drill progresses. It has been observed that when VD and RCVD are nil, and ID is either greater than or less than nil, VD will not continue to be nil for long, unless ID is also nil.
Causes of Deviation
There are many reasons why a hole may deviate right from the set up process. Slight setup error like instability of the drill rig or drill rod can lead to substantially deviated boreholes. Over a longer distance, the deviation may become unacceptable. Other causes of deviation include:
Layered rocks with alternating soft and hard strata
Fissile and folded rocks
Worn and loose drill strings
Weak core barrels
Incorrect bit design or excess force on bits leading to buckling of the bit
Improperly placed stabilizers
Too slow or too fast advance rate of the bore hole
Operator error etc.
The major factor affecting borehole deviation is the rock or soil mass that the borehole traverses. In softer rocks, the deviation is more probable than in hard rocks because in hard rock, the bit may wander from a slight to a strong left if the threshold strength of the rock is not crossed. In ground conditions with hard and soft layers, the bit will have the tendency to turn into the soft layer, and in fissured ground, the drill will tend to follow the fissures that will provide the path of least resistance.
Dealing with Deviation
Drill operators use predictive methods like linear projection and path projection to adjust and control the path of the drill. Often, quick mental calculations of numerical information and graphs are also helpful. However, absolute accuracy in predicting deviation is not possible, as it is difficult to know what lies just ahead of the drill bit. A THD log with data relating to VD, HD, AD and ID, and their 2nd order deviation provides the basis to form opinions regarding the course of action for directing the drill bit.
Geotechnical investigation of the planned bores site also helps prevent deviation to a certain extent. Knowing the topography of the area including taking core samples at regular intervals to understand the kind of rock and soil the drill bit will encounter can help the contractor to prepare for appropriate drill bits and other accessories required to prevent deviation. During the drilling operation, sensors fitted in the drill string indicate parameters like the angle and direction of the bore, and the tool face orientation to the operator who can then adjust the drill to maintain the planned drill path.
Practices to Reduce Deviation
A successful drilling operation can be attributed largely to the drill operator and to an extent to the design. The operator should be well equipped with information about ground conditions and provided with the correct tools to conduct the operation, including a detailed geotechnical report that provides information regard the soil conditions below.
The collaring of the hole should be set up accurately to prevent the bit from skating over the surface causing the bit to tilt off-center. Stabilizers can also be used to steady the bit, and the drill string should be checked to ensure that it is in good condition with proper stiffness because a very flexible drill string will not be able to resist side forces. The drill bit should be designed for the particular soil or rock strata that the bore path will encounter, and the bits and rods should be examined and changed regularly. (Learn more in "The Right Drill Bit for Soft, Medium & Hard Ground Conditions.")
The rate at which the borehole progresses should be balanced because if the bit progress too fast or too slow, deviation can occur. The feed pressure should be reduced when drilling through geological anomalies.
Deviation is a problem that is commonly encountered in all horizontal drilling operations. Due to gravity, the string sags in the center, forcing the bit to point upward and rotate off-center. The weight of the string can also cause the bit to pivot. While it is difficult to eliminate the problem of deviation altogether, it can be controlled. In addition, information about what lies underneath with regards to pipes and cables should also be thoroughly studied to avoid cross boring into sensitive pipelines like those carrying sewer, gas and chemicals. During the drilling operation, the THD log is a very handy tool to predict deviation and control it before the hole deviates out of control.