Changes

==Conventional wells==

==Conventional wells==

In the early days of the oil industry, drilling wells was a simple operation. A well location was picked at top reservoir, and the well was drilled directly down to the target as a vertical well. Then drilling became more sophisticated when the art of deviating wells was perfected. Here, the drill bit is deflected at an angle from the vertical toward a specific target. Deviated wells are commonly drilled from fixed drilling locations such as an offshore platform.<ref name=Cheatham_1992>Cheatham, C., 1992, [http://archives.datapages.com/data/specpubs/methodo1/data/a095/a095/0001/0050/0071.htm Wellbore trajectory], ''in'' D. Morton-Thompson and A. M. Woods, eds.,  Development geology reference manual: [http://store.aapg.org/detail.aspx?id=612 AAPG Methods in Exploration Series 10], p. 71-75.</ref> One method of directional drilling uses an assembly with a mud turbine and a bit. The flow of mud through the turbine causes the attached bit to rotate while the drill string remains stationary. Drillers refer to this type of drilling as being in sliding mode because the drill pipe slides along the hole behind the turbine. To deflect the bit in the appropriate direction, a bent sub is used; this is a piece of drill pipe bent to about 1–2&deg; angle, which is inserted behind the mud turbine and oriented from the surface along the planned direction for the well.<ref name=Inglis_1987>Inglis, T. A., 1987, Directional drilling: London, Graham & Trotman, 260 p.</ref>

In the early days of the oil industry, drilling wells was a simple operation. A well location was picked at top [[reservoir]], and the well was drilled directly down to the target as a vertical well. Then drilling became more sophisticated when the art of deviating wells was perfected. Here, the drill bit is deflected at an angle from the vertical toward a specific target. Deviated wells are commonly drilled from fixed drilling locations such as an offshore platform.<ref name=Cheatham_1992>Cheatham, C., 1992, [http://archives.datapages.com/data/specpubs/methodo1/data/a095/a095/0001/0050/0071.htm Wellbore trajectory], ''in'' D. Morton-Thompson and A. M. Woods, eds.,  Development geology reference manual: [http://store.aapg.org/detail.aspx?id=612 AAPG Methods in Exploration Series 10], p. 71-75.</ref> One method of directional drilling uses an assembly with a mud turbine and a bit. The flow of mud through the turbine causes the attached bit to rotate while the drill string remains stationary. Drillers refer to this type of drilling as being in sliding mode because the drill pipe slides along the hole behind the turbine. To deflect the bit in the appropriate direction, a bent sub is used; this is a piece of drill pipe bent to about 1–2&deg; angle, which is inserted behind the mud turbine and oriented from the surface along the planned direction for the well.<ref name=Inglis_1987>Inglis, T. A., 1987, Directional drilling: London, Graham & Trotman, 260 p.</ref>

A more recent technique for deviating a well involves using a rotary steerable assembly. Signals from the surface can be sent to the tool to deflect the bit in the appropriate direction while it is still drilling ahead in rotary mode. Drilling can be more efficient this way because there is less risk that the drill pipe will get stuck, it turns instead of slides, and the rate of penetration is faster.<ref name=Downtonetal_2000>Downton, G., A. Hendricks, T. S. Klausen, and D. Pafitis, 2000, [https://www.slb.com/~/media/Files/resources/oilfield_review/ors00/spr00/p18_29.pdf New directions in rotary steerable drilling]: Oil-field Review, Spring 2000, v. 12, no. 1, p. 18-29.</ref>

A more recent technique for deviating a well involves using a rotary steerable assembly. Signals from the surface can be sent to the tool to deflect the bit in the appropriate direction while it is still drilling ahead in rotary mode. Drilling can be more efficient this way because there is less risk that the drill pipe will get stuck, it turns instead of slides, and the rate of penetration is faster.<ref name=Downtonetal_2000>Downton, G., A. Hendricks, T. S. Klausen, and D. Pafitis, 2000, [https://www.slb.com/~/media/Files/resources/oilfield_review/ors00/spr00/p18_29.pdf New directions in rotary steerable drilling]: Oil-field Review, Spring 2000, v. 12, no. 1, p. 18-29.</ref>