Text 3 Mud System

The mud system is used on a vast majority of wells. This is a cycling process which starts at the slush pump. The pump picks up the fluid from the suction tank and delivers it through the stand-pipe and mud hose to the swivel. Then it passes through the kelly, drill pipe, drill collars and bit. The fluid passes upward around the drill string, over a shale shaker where the cuttings are removed. It drops into a settling tank and then goes to the suction tank to start a new cycle.

The term «fluid» means anything which flows. In the drilling fluids, this could mean either liquid., or air, or gas. The drilling fluid is now recognized as one of the major factors of the drilling operation. There are the following drilling fluid types:

1. Clay water-base muds and hydrocarbon-base muds.

2. Aerated muds.

3. Oil-base muds.

4. Emulsion muds.

5. Natural muds (carbonate, sulphate).

6. Water - fresh or salt.

7. Air and gas.

The specific gravity of the fluid may range from 0.8 to 2.3 gr per cm³. To maintain the desirable quality chemical agents are added to the circulating fluid. The follow ing parameters determine the quality of the fluid: the specific gravity, the viscosity, the w^rater loss, the gel strength, the sand content, the alkali ne percentage.

Mechanical and hydraulic mud mixers as well as chemical agents are used for the preparation of the circulating fluid at the surface. Cuttings, different salts, formation water, gas and oil enter the drilling fluid in the process of drilling and as a result the parameters of the fluid may alter.

Special devices and machinery - grooves, sediment boxes, shakers, hydrocyclones and etc. are used for cleaning the fluid at the surface.

The hydrocone provides a simple and efficient method of removing sand or solid from a light density drilling fluid. The fluid is pressured by a pump and introduced into the cone at a tangent to the cone wall. This tangential introduction causes the fluid to follow a swirling or spiraling path down the cone wall. The change in direction caused by the conical shape of the cone creates a centrifugal force. This force settles the sand,or light solid toward the wall where they collect and spiral down to-a bottom apex of disc harge valve.

Meanwhile, the spiral effect has created i n the; cone a voitex or vacuum analogous to the eye of a cyclone. The lighter fluid moves toward the center and is picked up in the updraft, carried out and discharged free from sand solids. These cones operate with any feed pressure from 1 to 40 PS I and are easily adjusted to handle any amount of sand.

The composition of drilling mud will depend upon the require­ments of the particular drilling operation. For some drilling operations dirty water may be the ideal drilling fluid. Holes must be drilled through a large number of different types of formations so it is natural to expect that various improvements in the drilling fluid will be necessary to meet the various conditions encountered as the hole is drilled into the earth. In some areas drilling can be started with water and, as drilling progresses, the clays and shales picked up from the formations will be dispersed in the water, resulting in a good drilling mud. In other areas formations such as limestones, sand, or gravel may be encountered which do not form mud. Under such conditions it will be necessary to add clays.

The drilling fluid must perform six primary functions:

J Bring the cuttings to the surface.

2. Cool and lubricate the bit.

3. Wall up the hole.

4. Keep the wall under control, that is create a counter pressure on the formation.

5. Reduce friction on the drill string.

6. Drive the turbodrill hydraulically.

The choice of mud type for a specific instance is governed by those functions which are the most critical to the well in question.

Ability to carry cuttings up the hole and into a settling pit de­pends on the velocity of the fluid stream in the annulus between the drill pipe and the wall of the; hole.

Text 4

Cooling and Lubrication of Bit

The heat generated by friction at the bit and other contact points between the drill string and the hole wall is absorbed by the drilling fluid. Improper lubrication of the bit and drill string will cause excessive torque and reduce bit bearing life.

Lubricants coat metal surfaces such as bit bearings with a high strength lubricating f)lm, thereby reducing metal-to-metal friction and reducing wjear. .

The addition of oil such as in oil emulsion rnuds, also improves down hole lubrication and bit wear.

Text 5

Use of Compressed Air or Gas as the Circulating Medium m

Rotary Drilling

It is possible to use compressed air or gas as the circulating me­dium for removing drill cuttings instead of the usual tnud fluid. With sufficient gas pressure and volume, the drill cuttings are readily swept out of the drilling zone and lifted to the surface through the annulus between the drill pipe and the wall of the well while suspended in the stream of flowing gas. Definite advantages are possible by this method, including greater drilling speed and longer bit life. However, there are certain disadvantages in the use of compressed air or gas: higher abrasive wear of pipes, the necessity of additional surface equipment for well head hennetizatioiii, higher capacity compressors as well as the increase of the rig load capacity.

No provision can be made for consolidating and sealing the walls of the well in soft formations, and it is difficult to control high formational fluid pressures when they are encountered.

If the strata penetrated yield water to the well, the drill cuttings become wet and sticky and are likely to cling to the walls and drill pipe.

If natural gas is used as the circulating medium, a considerable fire hazard is presented about the well head and in surface facilities.