From Naturally Fractured Reservoirs, Second Edition
Drilling directional or horizontal wells perpendicular to the preferential orientation of natural fractures increases the probability of success in these types of reservoirs. The rationale behind this idea is that many fractures below 3000 ft are nearly vertical or of high inclination. Consequently, a deviated or horizontal hole will have more chances of intercepting a vertical fracture than a vertical hole as shown on Figure 2 1. Although a vertical hole may intercept some fractures, experience indicates that directional holes represent a more sound approach for the development of naturally fractured reservoirs. This has been observed in practice in many places including the Mississippian lime of Oklahoma, certain offshore plays where directional holes are necessary, the Monterey shale of California, the Austin chalk of Texas, and the Palm Valley gas field of Central Australia as discussed in Chapter 1.
Figure 2 1: Vertical and deviated holes in a naturally fractured reservoir with vertical fractures. Well A (vertical) is dry; well B (deviated) is a discovery well.
DRILLING FLUIDS
Naturally fractured reservoirs are found in hard rocks which can usually be drilled with inexpensive muds. Soft rocks, on the other hand, must be drilled with muds of very precise properties which usually make the drilling muds very expensive.
In some cases it is possible to drill hard rock using air or gas as a drilling fluid with excellent results. However, high reservoir pressures and the presence of water in the subsurface put drastic limitations on the use of air and gas. It is...
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