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Wells Without Directional Surveys

Many wells that are planned to be vertical are drilled without running a directional survey. Wells that are reported to be vertical but do not have a directional survey are frequently not vertical. Anderson (1929) surveyed many “vertical” wells in California and showed that the wells frequently deviated significantly from the vertical. This situation is still true (Fig. 3-26) (Stigant 2012). If tops do not seem to match surrounding wells, it is frequently desirable to run a new directional survey if the wellbore is still available. New directional surveys should always be run in wells that are being sidetracked, and these directional surveys should be run as deep as possible.


Figure 3-26 Cross sectional profile of a number of wells in West Texas that were thought to be vertical before directional surveys were run in them. (Modified after Stigant 2012. Used with permission of American Association of Petroleum Geologists [AAPG].)

Reservoir Pressures

In onshore or shallow water fields, where well costs are relatively low, there are frequently multiple wells that penetrate the hydrocarbon/water contact in a reservoir compartment. Figure 3-22 shows how directional survey uncertainties can result in different contact depths. In deep water and frontier fields, where well costs are much higher, there may be few if any penetrations of the hydrocarbon/water contact. In these areas, wells may rely on reservoir pressure data from repeat formation testers to identify hydrocarbon contacts and recognize separate reservoir compartments (e.g., Brown 2003; Chen 2014). Differences in depth measurement between LWD and wireline logs can result in pressure data that suggests reservoir compartmentalization where none may exist. Carefully corrected depth data can identify where the risk of compartmentalization may have been overestimated (Fig. 3-27).


Figure 3-27 Pressure data from three wells in a discovery. (a) One well’s pressure data lies 10 psi off the pressure trend from the other two wells, suggesting that the third well lies in a different fault block than the first two wells. (b) Post-job depth correction of the pressure data from the third well shifted the data down 42 ft, resulting in the pressure data being within 1 psi of the pressure trend from the other two wells, suggesting that the risk of the wells being in separate faults blocks is less. (From Cribbs 2020.)

Just as water contacts that differ due to directional survey uncertainties can be corrected, resulting in corrected formation tops, pressure data trends that differ due to directional survey issues or along hole depth measurement issues can be shifted to align, shifting formation tops as well. Before shifting the depth of tops, it is important to have convincing evidence that wells are in the same pressure compartment.

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