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Common Types of Directionally Drilled Wells

Many complex factors go into the design of a directionally drilled well; however, most deviated wells fall into one of three types. For years, the most common type was a simple ramp well (Fig. 3-9a), sometimes called an L-shaped hole. These wells are drilled vertically to a predetermined depth and then deviated to a certain angle, which is usually held constant to TD of the well. Many wells are drilled with an S-shaped design. For an S-shaped hole, the well begins as a vertical hole and then builds to a predetermined angle, maintains this angle to a designated depth, and then the angle is lowered again, often going back to vertical (Fig. 3-9b). Finally, horizontal wells are configured by continuously building the deviation angle until the desired near-horizontal orientation is reached (Fig. 3-9c). Under very rare circumstances, such as in the tar sands of Canada, wells may be spudded at an angle. This allows for larger step-out distances at very shallow depths.

General Terminology

The terms used to describe various aspects of a directionally drilled well are defined in Table 3-2 and illustrated in Figure 3-9.

Table 3-2 Aspects of directionally drilled wells

KOP

Kick-off point. Depth of initial deviation from vertical measured as measured depth (MD), true vertical depth (TVD), or subsea true vertical depth (SSTVD).

Build rate

Build angle. Rate at which the angle changes during deviation. It is usually expressed in degrees per 100 ft drilled. Example: 2 deg per 100 ft.

Ramp angle

Hole angle, drift angle, angle of deviation. Angle from the vertical that a well maintains from the end of the build through the ramp segment of the well.

BHL

Bottomhole location. Horizontal and vertical coordinates to the total depth point usually measured from the surface location.

Drop rate

Rate at which the ramp angle changes in degrees per 100 ft. Measured in S-shaped holes.

Vertical point

The depth where the well is back to vertical, measured as MD, TVD, or SSTVD.

Horizontal Wells

Horizontal wells are typically considered to be wells with the borehole drilled within about 3 deg of bed dip or wells drilled nearly horizontally. These include extended-reach wells with long horizontal displacements, as well as long- and medium-radius horizontal wells. Short-radius horizontal wells are often called drain hole wells.

Extended-reach wells can be similar to the S-shaped well but with very high ramp angles in the 80 deg range. Although they are nearly horizontal, they might not be considered true horizontal wells. These wells are generally drilled when the surface location is necessarily a great distance from the target. Long-radius horizontal wells have build rates in the 3 deg per 100-ft range, and generally the horizontal part of the borehole is several thousand feet in length. Medium-radius horizontal wells have build rates in the 30 deg per 100-ft range and are usually drilled for shallow objectives. Short-radius horizontal wells are borehole segments drilled from a vertical borehole that penetrated the objective interval, with the deviation from vertical to horizontal made within a vertical interval of about 20 ft. These wells usually have horizontal segments of only a few hundred feet, but several horizontal segments may be drilled from the same vertical wellbore. These drain hole wells are used in low permeability reservoirs and enhanced recovery projects.

The purpose of drilling most horizontal wells is to improve the economics of a project by increasing production rates. For example, horizontal wells can improve production rates from (1) reservoirs containing heavy oil, such as the Orinoco heavy oil belt (Venezuela); (2) reservoirs with mostly fracture porosity, such as the Austin Chalk (onshore Gulf of Mexico); and (3) reservoirs of low permeability. This latter feature is responsible for the rapid development of unconventional reservoirs in the United States and the rapid shift of well type from predominantly vertical in 1990 to dominantly horizontal in 2018 (Fig. 3-12).

FIGURE 3-12

Figure 3-12 Percentage of wells of various well types drilled in the United States. Most nonvertical wells drilled after 2007 are horizontal wells. By 2018, 88% of wells drilled in the United States were horizontal. Data from Baker Hughes website. (Published with permission of J. Brenneke.)

Horizontal wells can also be used to penetrate and produce from multiple reservoirs that are laterally discontinuous. For example, separate fluvial channel sands that are in the same stratigraphic interval but are laterally discontinuous can be penetrated by a single horizontal well to drain multiple sand bodies. Also, for multilobed reservoirs with attic reserves above the highest wells on the structure, a horizontal well can be designed to encounter all the lobes, whereas a vertical well might miss some of the lobes that truncate up-dip. Another application of horizontal wells is to drill for thin oil zones over water. These zones are subject to coning of water into vertical wells. A horizontal well can significantly reduce the problem of water coning. There are other applications of horizontal wells, but most well plans are based on increasing production rates and shortening well life to improve project economics.

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