Vertical, Directional and Horizontal Drilling

10.5.1 Vertical Drilling

Though it is desirable on most wells to drill as near to vertical as possible no hole is drilled exactly vertically from spud to TD. It is generally accepted that a straight or vertical well is one that:

• Stays within the boundary of a “cone”, as specified by the client (usually about 3 degrees).

• Does not change direction rapidly (no more than 3 degrees per 100 feet of hole) and form a “dogleg”.

In order for the driller to be sure that he is maintaining a vertical hole within the limits set out in the drilling contract, periodic measurements of the inclination of the hole must be taken. In straight-hole drilling, the measuring device is used to determine inclination or drift. The azimuth of the borehole is not necessary.

10.5.1.1 Preventing and Correcting Deviation

A stiff bottomhole assembly (a BHA with many stabilizers) is run when deviation is expected. This BHA will resist any change in direction.

The pendulum bottomhole assembly is run to straighten a crooked or deviated hole. Removing the stabilizers near the bit and retaining one upper stabilizer makes a pendulum BHA. Light weight-on-bit is used while drilling.

10.5.2 Directional Drilling

10.5.2.1 Definitions

Directional drilling is the process of directing the wellbore along a trajectory to a predetermined target. While deviation control is the process of keeping the hole contained within a prescribed limit relative to the hole angle.

10.5.2.2 Common Applications of Directional Drilling

The most common applications of directional drilling are:

• Multiple wells from artificial structures: Today’s most common application of directional techniques is drilling multiple wells from offshore platforms.

Figure 90 Multiple deviated wells drilled from a platform.

• Fault drilling: The hole is steered across or parallel to a fault for better production. This eliminates the danger posed by drilling a vertical well through a fault zone that could slip and shear the casing.

Figure 91 Drilling through a fault.

Figure 92 Drilling in unaccessible locations.

• Sidetracking and straightening: These are remedial operations. The well is either sidetracked usually to avoid an obstruction, like stuck drill pipe that has been cemented off (Figure 94) or the hole is brought back to the vertical by straightening the crooked hole (Figure 93).

Figure 93 Controlling a vertical well.

Figure 94 Sidetrack wells.

• Salt dome drilling: Directional drilling is used to reach zones lying beneath the overhanging cap of the salt dome.

Figure 95 Drilling underneath the overhang of a salt dome.

• Relief wells: This is the first application of directional drilling. A directionally drilled well is used to kill a blowout.

Figure 96 Drilling a relief well.

10.5.2.3 Deflection Tools

A prime requirement for directional drilling is suitable deflection tools, along with special bits and other auxiliary tools. A deflection tool is a mechanical device that is placed in the borehole to cause the drill bit to be deviated from the present course of the borehole. There are numerous deflection tools available. The selection depends upon several factors, but principally upon the type of formation where the deviation is to start. The most common tools used for deflection are:

• Downhole Hydraulic Motors (with a bent sub) • Jet Bits

• Whipstocks 10.5.2.3.1 Downhole Hydraulic Motors

The downhole motor with a bent sub is the most widely used deflection tool. Mud flowing through the motor produces downhole rotary power, thus eliminating the need for rotating the drill pipe. There are types of downhole hydraulic motors:

Downhole motors present many advantages over the whipstock. They permit a full- gauge hole at the “kick-off point”, thus eliminating costly follow-up trips to open the hole. Orientation is also more accurate since the motors penetrate along a smooth, gradual curve in build-up and drop-off sections. Corrections, if needed, can be made downhole without making a trip. Finally, downhole motors eliminate the need for clean-up trips due to bridges, doglegs, etc., since the tool can be circulated, rotated and drilled to bottom.

10.5.2.3.1.1 Turbine Type Motor

Mud is deflected to the rotor, which is locked to the drive shaft, by the stationary stator. This causes the shaft to turn and the bit to rotate. The number of stages determines the torque generated. A turbine type motor or “turbodrill” consists of the following:

• Multistage vane-type rotor and stator: a stator and a rotor assembly form a stage.

• Drive shaft

• Drive sub (bit rotating sub and bearing section)

10.5.2.3.1.2 Positive Displacement Motor

A “positive displacement motor” (PDM) consists of a two-stage helicoid motor, a dump valve, a connecting rod assembly, and a bearing and shaft assembly. The helicoid motor has a rubber lined spiral cavity with an elliptical cross-section, which houses a sinusoidal steel rotor. As the mud is pumped, it is forced downward between the rotor and spiral cavity. The rotor is thus displaced and turned by the pressure of the fluid column, which in turn powers the drive shaft and results in a rotational force that is used to turn the bit.

10.5.2.3.1.3 Bent Sub

The bent sub is used to impart a constant deflection to the tool. Its upper thread is cut concentric to the axis of the sub body, and its lower thread is cut with an axis inclined 1 to 3 degrees in relation to the axis of the upper thread. In addition, the “hydraulic bent sub” can be locked into position for straight drilling, or unlocked and reset for directional drilling.

10.5.2.3.2 Jet Bits

A jet bit has all but one of the jet nozzles are closed off or reduced in size. It can be used to deviate a hole in areas where subsurface formations are relatively soft. To deviate a well using a jet bit:

1. When the bit is on bottom, the open nozzle is oriented in the proper direction. 2. The pumps are started without rotating the drill string. The jetting action

literally washes the formation away.

3. After jetting has set a course, the drill string is rotated and weight is added. The bit and drill string will follow the set course because it is the path of least resistance. Extra weight is then applied to bow the collars, and the drilling continues until the correct hole angle is attained.

10.5.2.3.3 Whipstocks

The standard “removable” whipstock consists of a long inverted steel wedge that is concave on one side to hold and guide a whipstock drilling assembly. It also has a chisel point at the bottom to prevent the tool from turning, and a heavy collar at the top to help withdraw the tool from the hole. It is used to initiate the deflection and direction of a well, sidetrack cement plugs, or straighten crooked holes.

The “circulating” whipstock is run, set and drilled like a standard whipstock. In this case, the drilling mud flows through a passage to the bottom of the whipstock and circulates the cuttings out of the hole, ensuring a clean seat for the tool. It is most efficient for washing out bridges and bottom hole fills.

The “permanent” casing whipstock is designed to remain permanently in the well. It is mainly used to bypass collapsed casing, junk in the hole or to reenter and drill out old wells. After the bit has drilled below the whipstock, increased weight is applied until approximately 20 feet of hole has been drilled. The whipstock is then retrieved and the hole opened to full gauge with a bit and hole opener.

10.5.3 Horizontal Drilling

10.5.3.1 Introduction

There are many reasons for operators to drill horizontal wells, the most important being the ability to increase production by penetrating more of the reservoir using this technique. Horizontal drilling enhances many of the reservoir production parameters, for example the minimization of fluid coning problems.

Aside from greatly increasing the interval of the reservoir for analysis, horizontal wells provide the geologist, petrophysicist and engineers with an opportunity to evaluate the reservoir in more detail laterally.

10.5.3.2 Basic Horizontal Well Patterns

There are three basic borehole patterns:

• Short Radius Well: The targeted formations are “small” with vertical fractures and low energy production characteristics. Build rates are between 1.5 to 3 degrees per foot, and the horizontal section is rarely longer than 1000 feet.

• Medium Radius Well: The targeted formations are thin, low permeability reservoirs with a limited extent. Build rates are between 8 to 20 degrees per 100 feet and the horizontal intervals may extend up to 4000 feet.

• Long Radius Well: It is used for “extended reach” wells (e.g. Wytch Farm), and can be used to navigate around fault blocks. Build rates are between 2 and 6 degrees per 100 feet, and the horizontal intervals can exceed 5000 feet.