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The vertical distance from a point in the well usually the current or final depth to a point at the surface, usually the elevation of the rotary kelly bushing RKB. This is one of two primary depth measurements used by the drillers, the other being measured depth. TVD is important in determining bottomhole pressures, which are caused in part by the hydrostatic head of fluid in the wellbore.
For this calculation, measured depth is irrelevant and TVD must be used. For most other operations, the driller is interested in the length of the hole or how much pipe will fit into the hole.
For those measurements, measured depth, not TVD, is used. The target, or objective, is the theoretical, subsurface point or points at which the wellbore is aimed. In the majority of cases it will be defined by someone other than the directional driller.
Usually this will be a geologist, a reservoir engineer or a production engineer. They will often define the target in terms of a physical limitation - i. If multiple zones are to be penetrated, the multiple targets should be selected so that the planned pattern is reasonable and can be achieved without excessive drilling problems. Some care should be taken with target definition. Any target can be reached - given enough time, money and effort but the economics of drilling dictate the use of as large a target as possible.
A circular, square or rectangular shape with at least one side fixed by a physical constraint e. Targets may be defined as an angle limitation at depth - e. When targets are defined the directional driller must also know the true vertical depth at which the target applies. In some cases this depth may not be available within several hundred meters and could be specified as the wellbore intercept of a given formation top.
This top of target would almost certainly preclude the use of Build and Hold wells and require use of "S" shaped wellbores.
Target displacement is defined as the horizontal distance from the surface location to centre of the target in a straight line. This is also the directional summation of the departure the due East or West displacement and the latitude the due North or South displacement. The target bearings are a measure of the direction in degrees, minutes and seconds or decimals and typically expressed with reference to well centre.
True Vertical Depth TVD is the depth of the wellbore at any point measured in a vertical plane and normally referenced from the horizontal plane of the kelly bushing of the drilling rig.
This is the point at which the first deflection tool is utilized and the increase in angle starts. The selection of both the kick off point and build up rate depend on many factors including the formation s , wellbore trajectory, the casing program , the mud program, the required horizontal displacement, maximum allowable dogleg and inclination. Fewer problems are faced when the angle of the hole is between 30o and 55o.
The deeper the KOP is, the more angle it will be necessary to build, possibly at a more aggressive rate of build.
The KOP should be at such a depth where the maximum angle to build up would be around 40o; the preferred minimum is 15o. In practice the well trajectory may be calculated for several choices of KOP and build up rates and the results compared. The optimum choice is that which gives a safe clearance from all existing wells, keeps the maximum inclination within the desired limits, avoids unnecessarily high dogleg severity 's and is the best design from a cost point of view.
The build rate is achieved through the use of a deflection tool positive displacement motor with a built in adjustable housing or purposefully designed stabilized bottomhole assembly. This is the part of the hole where the vertical angle is increased at a certain rate, depending on the formations and drilling assembly used.
During the Build Up the drift angle and direction are constantly checked in order to see whether a course correction or change in build rate is required. This part of the hole is the most critical to assure the desired wellpath is maintained and the final target is reached. This section, also called the Hold Section, is a straight portion of the hole drilled with the maximum angle required to reach the target.
Subtle course changes may be made in this section. Many extended reach drilling projects have been successfully completed at inclinations up to 80o, exposing much more reservoir surface area and reaching multiple targets. However, inclination angles over 65o may result in excessive torque and drag on the drill string and present hole cleaning, logging, casing, cementing and production problems. These problems can all be overcome with today's technology, but should be avoided whenever there is an economic alternative.
Experience over the years has been that directional control problems are aggravated when the tangent inclination is less than 15o. This is because there is more of a tendency for bit walk to occur, i. Example A well is drilled to feet. It has been determined that the well was inclined 10 degrees from vertical.
What is the TVD? The depth was measured along the length of the inclined hole. That length becomes the hypotenuse of a right triangle. Using the trigonometric cosine function;.
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