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Directional Well Plots

Directional survey data are used to determine the depth and lateral position of the borehole along its entire length. These data are normally plotted on a base map in one of two ways: (1) as straight lines from the surface to BHL or (2) as detailed directional plots.

The simplified straight-line method of plotting a directional well is shown in Figure 3-23a. The only data required and plotted on the base map are the surface location and BHL, which may be the only data available. The MD to TD may be recorded next to the BHL. A straight dashed line is usually drawn between the surface location and BHL. This directional well plot provides absolutely no information about the position or depth of the wellbore in the subsurface between the surface and bottomhole locations. Such a plot is not helpful in the interpretation, construction, and evaluation of fault, structure, or isochore maps. Some computer databases use this type of map to show what wells have been drilled in a given area. Data in the database is then tied to the wells on the screen and is available with the click of a mouse.

FIGURE 3-23

Figure 3-23 (a) Straight-line method of plotting directional wells in map view. (b) Detailed plot of directional survey data indicating the location and subsea depth of the wellbores along their entire length. Compare this plot to that in Figure 3-13a.

When directional survey data are actually plotted to provide detail about the lateral position and subsea depth of the wellbore, as shown in Figure 3-23b, the plot has real value. Such a plot provides a visual guide (in map view) to the location and subsea depth of the wellbore anywhere along its path. It saves time in preparing subsurface maps and is extremely helpful in the interpretation, construction, and evaluation of fault, structure, and isochore maps. Later chapters examine several important benefits to fault surface and structure mapping derived from plotting on a base map the actual location and subsea depth, at fixed increments (usually 500 ft or 1000 ft), of all directional wells. This type of basemap is seldom used these days, as computer workstations plot the location of formation tops and fault cuts on directional well paths.

Figure 3-24 illustrates the cross-sectional view of a directionally drilled well and the detailed map-view plot of the directional data in 500-ft increments of subsea TVD along the actual well path. If subsea data are to be used in mapping, the directional well data are corrected to subsea before being plotted on the base map.

FIGURE 3-24

Figure 3-24 Cross-sectional view of a directional well and its detailed map-view plot in increments of 500 ft.

Figure 3-25 shows the map-view well plot for the Cognac “A” Platform in the Mississippi Canyon Block 194 in the U.S. Gulf of Mexico. The platform is the largest multiple-well platform in the world. The platform is located in 1000 ft of water and has a total height of 1260 ft. Wells were deviated with high angles, up to 75 deg. Horizontal displacements up to 11,500 ft result in a well pattern that covers an area with a diameter of more than 4 mi. Total cost for the project was over $1 billion.

FIGURE 3-25

Figure 3-25 Spider directional well plot for the Cognac “A” Platform, Offshore Gulf of Mexico. (Published by permission of Gardes Directional Drilling.)

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