THE DRILLSTRING

Starting at the bottom, a basic drillstring for rotary drilling consists of the (1) bit, (2) drill collars and Bottom-Hole Assemblies (BHAs), and (3) drill pipe (see Figure 5).

The BHA is located just above the bit and consists of drill collars combined with one or more bladed stabilizers (to keep the BHA and bit concentric), possibly a reamer (to keep the hole from becoming tapered as the bit diameter wears down) and other tools. MWD tools and mud motors are generally located low in the BHA, usually just above the bit. Sometimes, a set of “jars” is located near the top of the BHA. Jars can free stuck pipe by giving a hammering action when they are set-off by pulling hard.

Drill collars are thick-walled, heavy joints of pipe used in the BHA to provide weight to the bit. Usually, one of the collars is made of non-magnetic metal so that a magnetic compass tool (survey tool) can be used to determine the inclination of the lower BHA and bit without interference from magnetic metals.

Each joint of drill pipe is approximately 30 ft long, and has a box (female connection) welded onto one end and a pin (male connection) welded to the other. These threaded couplings (tool joints) must be strong, reliable, rugged and safe to use. They must be easy to make up (connect) and break out (disconnect). Outer diameters for drill pipe range from 23⁄8 to 6 5⁄8 in.

The hollow drill string provides a means for continuous circulation and for pumping drilling mud under high pressure through the bit nozzles as a jet of fluid. The blast of mud knocks rock cuttings from under the bit, gives a new rock surface for the cutters to attack and starts the drill cuttings on their trip to the surface. This transmission of hydraulic horsepower from the mud pumps to the bit is a very important function of the mud.

Coiled-tubing drilling. This method employs a continuous string of coiled tubing and a specialized, coiled-tubing drilling rig. Rather than drilling with separate joints of the traditional, largediameter, rigid drill Rather pipe, the drillstring is smaller-diameter, flexible tubing. Unlike drill pipe which is screwed together to form the drillstring, and which must be disconnected into stands that are racked in the derrick during trips, the tubing comes rolled on a reel that unwinds as drilling progresses and is subsequently rewound onto its spool during trips. The coiled-tubing method greatly facilitates lowering and retrieving the drilling assembly.

Traditionally, coiled-tubing rigs have been used for workover and completion operations where mobility and compact size were important. With the development of downhole mud motors which do not require the use of a rotating drillstring to turn the bit, coiledtubing units are now functioning as true drilling rigs.

Date: 2015-02-28; view: 3342