Drilling for Petroleum

DRILLING METHODS

When it has been established that a petroleum reservoir probably exists, the only way to verify this is to drill. Drilling for natural resources is not a new idea. As early as 1100 A.D., brine wells as deep as 3,500 ft were drilled in China, using methods similar to cable tool drilling.

Cable tool drilling. This was the method used by pioneer wildcatters in the nineteenth and early twentieth centuries and is still used today for some shallow wells. The method employs a heavy steel drill stem with a bit at the bottom, suspended from a cable. The tool is lifted and dropped repeatedly. The falling steel mass above the bit provides energy to break up the rock, pounding a hole through it. The hole is kept empty, except for some water at the bottom. After drilling a few feet, the drill stem (with its bit) is pulled out and the cuttings are removed with a bailer. The cable tool method is simple, but it is effective only for shallow wells. Progress is slow because of the inefficiency of the bit and the need to pull the tools frequently to bail out cuttings.

Rotary drilling. Rotary rigs are used for a variety of purposes — drilling oil, gas, water, geothermal and petroleumstorage wells; mineral assay coring; and mining and construction projects. The most significant application, however, is oil and gas drilling. In the rotary method (introduced to oil and gas drilling in about 1900), the drill bit is suspended on the end of a tubular drillstring (drill stem) which is supported on a cable/pulley system held up by a derrick (see Figure 3). Drilling takes place when the drillstring and bit are rotated while the weight of the drill collars and bit bears down on the rock.

To keep the bit cool and lubricated, and to remove the rock cuttings from the hole, drilling fluid (mud) is pumped down the inside of the drillstring. When it reaches the bit, it passes through nozzles in the bit, impacts the bottom of the hole and then moves upward in the annulus (the space between the drillstring and the wellbore wall) with the cuttings suspended in it. At the surface, the mud is filtered through screens and other devices that remove the cuttings, and is then pumped back into the hole. Drilling mud circulation brought efficiency to rotary drilling that was missing from cable tool drilling — the ability to remove cuttings from the hole without making a trip to the surface.

Equipment for rotary drilling is illustrated in Figure 3.

Date: 2015-02-28; view: 1033