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Chapter 8 – DIRECTIONAL AND HORIZONTAL DRILLING                  175






                        ▪ Optimum orientation in the reservoir. Reservoirs are not nice
                        and even, the same in all directions. Permeability will be better
                        in one direction than another. In a fractured limestone reservoir,
                        the well should intercept as many fractures as possible to get the
                        best production. These factors will determine which direction the
                        well should be drilled in the reservoir. Some wells follow very
                        complex well paths, turning corners, to get in the best position in
                        the reservoir.
                     ▪ Remedial work (sidetracks). A well might be cemented back
                        to a shallower depth, and a new wellbore drilled away from the
                        original bore, for a number of reasons. One reason might be due
                        to drilling problems, such as stuck pipe that cannot be freed and
                        must be cut. Or an old producing well might be sidetracked to
                        another location.

                        ▪ Relief wells. The worst reason to have to drill a well is to create a
                        relief well. For example, a well has suffered a blowout and is still
                        blowing hydrocarbons into the environment. For some reason, the
                        well cannot be killed at the surface. Another well, the relief well,
                        is drilled to intercept the blowing well. Dense mud is circulated
                        down the relief well and into the blowing well, killing it from the
                        bottom. The Deepwater Horizon disaster in the Gulf of Mexico
                        in April 2010 was one of these situations. The BOPs on the
                        seabed would not operate to shut in the well at the seabed. Two
                        rigs moved in to drill relief wells to depths below 18,000 ft to shut
                        down the blowout.


                                Directional Well Planning


                    It is the job of the geologists and reservoir engineers to decide where
                 the wellbore should be placed. This could be as simple as determining

                 one single target, often defined as a tolerance of say 100 m around a target
                 point (so that would be a round target). It does not matter at what angle the
                 well enters the target. At the other extreme, the well may have to penetrate

                 multiple targets, and the final target could be defined as a cylinder that the

                 well must stay within, or perhaps the well must be drilled to follow the
                 “roof” of the reservoir and stay within a certain distance of the cap rock
                 above. This is called geosteering and will be covered later in this chapter.







        _Devereux_Book.indb   175                                                 1/16/12   2:10 PM
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