011 2 697050 / 011 2 694835 hyd3@wrb.gov.lk
                   

Drilling Division

Our services are supported by experienced, qualified personnel and a well selected and maintained fleet of rigs, plant and equipment. The fleet of rigs that our water well drillers use has the flexibility to change between RC, Air and Mud Rotary methodologies on site. This provides timely outcomes to meet project schedules and without costly delays. Self-contained camps are available for remote drilling projects. With large capacity mud pumps, high capacity compressors, and self-contained mud tanks Direct Drilling is able to provide both air and mud drilling with the one rig. Our Dual purpose rigs have the ability to change seamlessly from RC to Rotary Air or Mud on site.

Auxillary equipment supports the rigs, including diesel tankers, water tanks, backhoes, compressors, generators, All equipment is mine site compliant.

Water Well Drilling Methods

There are several different types of drilling methods used in world. This section covers the most common methods used in Water Resources Board.

Rotary Drilling

Rotary drilling is the most popular well drilling method in Sri Lanka. Mud rotary is widely used in the Puttalam and similar areas of Sri Lanka where substantial overburden exists. The principle of rotary drilling is based upon a rotating drill stem made of lengths of drill pipe about 3 meter long. A bit is attached to a heavy stabilizer or drill collar at the end of the column of drill pipe. The extra weight and larger outside diameter of the stabilizer just above the bit helps to maintain a straight drill hole. The drill stem is hollow and has a drilling fluid of either mud or air circulating down the drill stem out through the nozzles in the bit and up along the outside of the drill stem. The rotating action of the bit breaks up the material and the drilling fluid carries the cuttings to the surface where they settle out in a mud tank.

Several types of bits are available to the rotary driller. The bit most generally used in Water Resources Board is the tri-cone roller bit. The type and number of cutting teeth on the bit cones vary depending upon the type of formations to be penetrated.

The upper end of the drill stem is attached to a kelly on a table drive rig and swivel which hare mounted on a large mast. Hydraulic controls lower or raise the drill stem and operate the rotary motion. When a hole has been drilled the full length of the kelly, the drill stem is raised, the joint between the kelly and drill pipe is broken, and an additional length of drill pipe is added. The drive mechanism for the drilling operation is provided either at the rotary table (table drive) or at the swivel (top head drive). The rig also contains a cable called a casing line which is used to raise and lower sections of drill pipe and casing.

In rotary drilling, the borehole size is larger than the casing size. In drift formation, the entire hole is completed before casing is installed. In rock wells, the length of hole to be cased is drilled, the casing is installed, then the bit size is reduced and the rock portion of the well is completed.

Mud rotary utilizes a drilling fluid of bentonite clay and water. The mud serves several purposes:

  1. remove cuttings from the drillhole,
  2. prevent collapse of the drillhole and reduce water loss to the formations by forming a filter cake on the borehole wall,
  3. suspend cuttings when drilling is stopped, cool and clean the drill stem and bit, and
  4. lubricate bit bearings and mud pump parts. After the cuttings are allowed to settle in the mud tank, the mud is recirculated via a mud pump to the swivel at the top of the kelly, then down through the drill stem. The mud tank is usually rectangular in shape with a mud volume of 200-800 gallons and may contain several baffles to aid in separation of cuttings from the drilling mud before it enters the pump intake for recirculation. A device known as a sand separator may be used to further remove sands and other “parasites” from the drilling mud. Samples of cuttings may be obtained directly from the borehole before the fluid and cuttings spill into the mud tank.

Most larger rotary rigs have an air compressor to enable the contractor to also use air as the drilling fluid. The high velocity of the air as it exits the bit is sufficient to blow the cuttings away from the bit and carry them up to the surface where they settle out around the borehole. Air rotary is used primarily for drilling in consolidated (rock) formations. In rock wells with substantial overburden, mud will be used for drilling through the drift, and after the casing is set the drilling operation will be converted to air rotary for completion of the rock portion of the well. Clean water is often used for drilling the rock portion of the hole after setting the casing.

Air Hammer Drilling

Air hammer drilling, sometimes referred to as down hole drilling, is used extensively in Sri Lanka’s hard rock areas. The bit used in this drilling method is essentially a neumatic hammer operated at the end of the drill stem. Compressed air operates a piston which strikes the top of the bit at a very rapid rate. The cutting tips on the bit are made of tungsten-carbide which are extremely resistant to abrasion. The combined hammering and rotation of the bit results in penetration of hard rock at a rate faster than any other drilling method.

Water Well Components

This section covers the major water well components used when installing a water well.

  • Borehole

Borehole is a vertical boring to reach aquifer (water bearing geologic material). In a well terminating into rock, an open borehole will extend beyond the bottom of the well casing.

  • Well Seal

Well seal is a mechanical device to prevent contamination from entering well casing that is installed after well completion. All well caps and seals shall be weathertight, tightly secured, and vermin proof.

  • Casing

Well casing is steel or plastic pipe installed to keep borehole wall from collapsing and houses the submersible pump and drop pipe. Comparison of PVC plastic casing and steel casing:

PVC vs. Steel

Noncorroding :Corrodes

Lower strength :Higher strength

Fewer water quality complaints Rusty water

Rotary construction only Suitable for any drilling method

  • Grout

Grout is impermeable cement or clay placed in annular space between borehole and casing to prevent well contamination, maintain separation of aquifers, and preserve artesian aquifers.

  • Screen

A well screen is a filtering device that serves as the intake portion of wells constructed in unconsolidated or semi consolidated aquifers. A screen permits water to enter the well from the saturated aquifer, prevents sediment form entering the well, and serves structurally to support the unconsolidated aquifer material.

Slot openings have been designated by numbers that correspond to the width of the openings in thousandths of an inch. A No.10 slot, for example, is an opening of O.010 inch. Slot size may also be expressed in metric units; for example, 0.010 inch equals 0.25 millimeter (mm). For small-diameter screens covered with wire mesh, the number of openings in the mesh per inch are designated by gauze numbers.

Geological Material Slot Size Opening (inches) Opening (mm) Gauze Size
Clay & Silt
--
0.003
0.10 Fine
--
Fine sand
6
7
8
10
0.006
0.007
0.008
0.010
0.15
0.18
0.20
0.25
90
80
70
60
Medium sand
12
15
18
20
0.012
0.015
0.018
0.020
0.30
0.40
0.45
0.50
50
--
40
--
Coarse sand
25
35
0.025
0.035
0.65
0.90
30
20
Very coarse sand
50
0.050
1.27
--
Very fine gravel
90
0.090
1.29
--
Fine gravel
150
250
375
0.150
0.250
0.375
3.81
6.35
9.52
--
--
--

When selecting the proper screen to install, the following selection criteria need to be

considered:

  • Maximize the percent of open area.
  • Non clogging openings.
  • Corrosive resistance.
  • Column and collapse strength.
  • Screen opening (slot size) based on aquifer material.
  • Screen diameter that provides a water entrance velocity of less than .1 foot/second.

Fact or Fiction? – “Doubling the well diameter appreciably increases well yield.”

Answer

Doubling the well diameter increases the well yield only 10 percent.

Doubling the screen length increases the well yield 100 percent.

Services

  • Construction of Deep Tube Wells.

  • Cleaning and Flushing of Tube Wells

  • Installation of Hand Pumps

  • Rehabilitation and Maintenance of Tube Wells

  • Construction of Boreholes for Hydrogeological Research / Studies