Pipe inserted into a borehole

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Casing is a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole. Similar to the bones of a spine protecting the spinal cord, casing is set inside the drilled borehole to protect and support the wellstream. The lower portion (and sometimes the entirety) is typically held in place with cement.[1] Deeper strings usually are not cemented all the way to the surface, so the weight of the pipe must be partially supported by a casing hanger in the wellhead.

Casing that is cemented in place aids the drilling process in several ways:[2]

• Prevents contamination of fresh water well zones.
• Prevents unstable upper formations from caving in and sticking the drill string or forming large caverns.
• Provides a strong upper foundation to allow use of high-density drilling fluid to continue drilling deeper.
• Isolates various zones, which may have different pressures or fluids, in the drilled formations from one another.
• Seals off high pressure zones from the surface, minimizing potential for a blowout.
• Prevents fluid loss into or contamination of production zones.
• Provides a smooth internal bore for installing production equipment.

Optimum design of the casing program decreases the well construction costs, enhances the efficiency of operations and also diminishes the environmental impacts.[3]

A slightly different metal string, called production tubing, is often used without cement inside the final casing string of a well to contain production fluids and convey them to the surface from an underground reservoir.

Design

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In the planning stages of a well, a drilling engineer, usually with input from geologists and others, will pick strategic depths at which the hole will need to be cased in order for drilling to reach the desired total depth. This decision is often based on subsurface data such as formation pressures and strengths, well integrity,[4] and is balanced against the cost objectives and desired drilling strategy.[2]

With the casing set depths determined, hole sizes and casing sizes must follow. The hole drilled for each casing string must be large enough to accommodate the casing to be placed inside it, allowing room for cement between the outside of that casing and the hole. Also, subsequent bits that will continue drilling obviously must pass through existing casing strings. Thus, each casing string will have a subsequently smaller diameter. The inside diameter of the final casing string (or penultimate one in some instances of a liner completion) must accommodate the production tubing and associated hardware such as packers, gas lift mandrels and subsurface safety valves.

Casing design for each size of designed pipes is done by calculating the worst conditions that may be faced during drilling and over the producing life of the well. Mechanical properties such as longitudinal tensile strength, and burst and collapse resistance (calculated considering biaxial effects of axial and hoop stresses), must be sufficient at various depths. Pipe of differing strengths often comprises a long casing string, which typically will have the greatest axial tension and perhaps highest internal burst pressure differentials in the upper parts, and the greatest collapsing loads deeper in the well from external pressure vs lowered internal pressure.

Casing strings are supported by casing hangers that are set in the wellhead, which later will be topped with the Christmas tree. The lower members of the wellhead usually are installed on top of the first casing string after it has been cemented in place.

Intervals

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Typically, a well contains multiple intervals of casing successively placed within the previous casing run.[2] The following casing intervals are typically used in an oil or gas well:

• Conductor casing
• Surface casing
• Intermediate casing (optional)
• Production casing
• Production liner

The conductor casing serves as a support during drilling operations, to flowback returns during drilling and cementing of the surface casing, and to prevent collapse of the loose soil near the surface. It can normally vary from sizes such as 18 to 30 in (460 to 760 mm).[5]

The purpose of surface casing is to isolate freshwater zones so that they are not contaminated during drilling and completion. Surface casing is the most strictly regulated due to these environmental concerns, which can include regulation of casing depth and cement quality. A typical size of surface casing is 13+3&#;8 inches (340 mm).[5]

Intermediate casing may be necessary on longer drilling intervals where necessary drilling mud weight to prevent blowouts may cause a hydrostatic pressure that can fracture shallower or deeper formations. Casing placement is selected so that the hydrostatic pressure of the drilling fluid remains at a pressure level that is between formation pore pressures and fracture pressures.[6][5]

In order to reduce cost, a liner may be used which extends just above the shoe (bottom) of the previous casing interval and hung off downhole rather than at the surface. It may typically be 7", although many liners match the diameter of the production tubing.[5]

Few wells actually produce through casing, since producing fluids can corrode steel or form deposits such as asphaltenes or paraffin waxes and the larger diameter can make flow unstable. Production tubing is therefore installed inside the last casing string and the tubing annulus is usually sealed at the bottom of the tubing by a packer. Tubing is easier to remove for maintenance, replacement, or for various types of workover operations. It is significantly lighter than casing and does not require a drilling rig to run in and out of hole; smaller "service rigs" are used for this purpose.

Cementing

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Cementing is performed by circulating a cement slurry through the inside of the casing and out into the annulus through the casing shoe at the bottom of the casing string. In order to precisely place the cement slurry at a required interval on the outside of the casing, a plug is pumped with a displacement fluid behind the cement slurry column, which "bumps" in the casing shoe and prevents further flow of fluid through the shoe. This bump can be seen at surface as a pressure spike at the cement pump. To prevent the cement from flowing back into the inside of the casing, a float collar above the casing shoe acts as a check valve and prevents fluid from flowing up through the shoe from the annulus.

Casing Wear

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A prolonged, recurrent axial and rotational movement within casing would cause wear to the casing interior, with the probability of blowouts, production loss, and other hazardous and costly complications.

The following conditions contribute to casing wear:

• Drill pipe weight
• Mud and additives
• RPM and ROP
• Tool joint coating
• Well path and dogleg

The following are recommendations for preventative measures to minimize casing wear:

• Minimization of dogleg severity and expect real dogleg at least 1.5 times higher than the planned value.
• Usage of casing friendly tool joint materials.
• Minimize rotor speed and use downhole motor.
• Increase ROP.
• Select proper mud type and add lubricants to minimize wear and friction.
• Usage of drill pipe protectors.
• Usage of thick wall casing in the anticipated wear section area.
• Usage of software to reduce risks.

References

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To Drill a Water Well, Powered Equipment is Required. This Increases the Speed and Depth That Can Be Reached.

To drill a water well, powered equipment is required. This increases the speed and depth that can be reached.

Note: The content on this page has been adapted from publications of Lifewater International, written by Fred Proby.

The objective in choosing a method to drill a water well is to use the least expensive method that can be successful given the type of material that must be drilled through and the depth that must be drilled to reach an acceptable source of groundwater.

Often, there are no options, and the choices we have are limited, maybe even non-existent. But the method used to drill a water well must match the geology.

Powered Methods Used to Drill a Water Well

Most manual well drilling methods have been adapted to use machine power instead of human power. Also, powered methods have been developed that can drill larger diameter boreholes much deeper and faster than any manual method. Machines used to drill a water well are typically called a "drill rig" or just a "rig".

Jetting

This method employs a pump to force a flow of water down a drill pipe and out a narrow nozzle to make a ''jet'' of water that loosens the sediment. The return flow of water outside the drill pipe carries cuttings up to the surface and into a settling pit. The pump then returns the water back down the pipe. The drill pipe is suspended from a tripod and rotated by hand to keep the borehole straight.

Advantages:

This method only requires lengths of pipe and a water pump that can generate sufficient pressure. The pipe is often left in the ground to serve as the well casing.

For more Water Well Drill Pipeinformation, please contact us. We will provide professional answers.

In fine sand, a 5 cm diameter PVC pipe can be rapidly advanced to more than 60 meters. It only takes two people to jet a well.

Disadvantages:

Jetting is only suitable for soft, fine-grained sediments. Gravel is too heavy for the return water to bring it to the surface.

The diameter of the borehole is only slightly larger than the drill pipe/casing. Therefore, it is difficult to install an adequate sanitary seal to protect the well from surface water contamination.

Cable Tool

This is a mechanized version of manual percussion drilling. The heavy drill bit and related parts are called the ''tools'' and they are raised and dropped on a steel cable.

Cuttings are removed with a bailer. Several meters of water must be maintained in the borehole to keep the cuttings suspended. The machinery ranges from a very simple skid-mounted powered winch with a tripod to a complex set of pulleys and drums with a large mast.

The larger cable tool rigs are mounted on a trailer or the bed of a truck and use hydraulic motors to raise and lower the mast and rotate the drums of cable.

Advantages:

A cable tool rig can drill through anything. The larger versions can drill a water well hundreds of meters deep. Compared with other powered drill rigs, the machinery is simple and has a relatively low rate of fuel consumption

Disadvantages:

Compared to other drill rigs of a similar size, a cable tool rig will drill a water wellvery slowly. When drilling in loose sediments, it is necessary to drive steel pipe behind the drill bit to keep the borehole from collapsing.

The sections of this drive casing must be welded together going in and cut apart coming out. So an arc welder and a cutting torch are needed on all but the smallest cable tool rigs.

The tools on a medium to large sized rig are very heavy and require a cable and winch to move around. There are many ways to get injured on a cable tool rig.

Mud Rotary

This method used to drill a water well starts with the basic concept of well jetting described above. Add a larger cutting bit, lengths of steel drill pipe with threaded joints, a motor to turn and lift the drill pipe, and a sturdy mast to support the pipe and you have the elements of a mud rotary drill rig. A further refinement is mixing bentonite clay or other materials in the water to improve its ability to lift cuttings out of the hole; this fluid is called ''drilling mud'' or just ''mud.''

There are many kinds of mud rotary drill rigs used to drill a water well. They fall in two basic categories; table drive, where the drill pipe is turned by a rotating mechanism near the base of the rig, and top-head drive, where the drill pipe is turned by a motor attached to the upper end of the pipe.

In both types, the upper end of the drill pipe is attached to a lifting mechanism that raises and lowers it along the mast. Both types of mud rotary rigs also have a swivel attached to the upper end of the drill pipe that allows drilling mud to be pumped down the drill pipe while the pipe is rotating.

The larger the rig, the faster and deeper it can drill. The LS100 and LS200 drill rigs are mud rotary rigs at the small end of the range of drill rig sizes.

Advantages:

Because the borehole is kept open by the pressure of the drilling mud, it is not necessary to use a drive casing as with cable tool drilling. 

Mud rotary drilling is also much faster than cable tool. A large mud rotary rig can drill a borehole 60 cm in diameter to 1,000 meters or more. Even a small rig like the LS200 can drill a 20 cm porthole to a depth of 60 meters.

Disadvantages:

Drilling through rock requires a great amount of weight on the drill bit so only the larger mud rotary rigs can effectively drill in rock.

Most mud rotary rigs have a motor to rotate and lift the drill pipe and a motor to operate the mud pump.

As a result, mud rotary rigs use more fuel per hour than a comparable cable tool rig. Most drilling operations that use a large mud rotary rig also require support vehicles to haul water and drill pipe.

Air Rotary

The mechanical elements of an air rotary drill rig are similar to a mud rotary rig; table drive and top-head drive are the two basic options for rotating the drill pipe. The principal difference is an air rotary rig uses compressed air to remove cuttings rather than drilling mud.

A type of ''foam'' can be added to the air stream to improve cuttings removal and provide some borehole stability. An air rotary rig can use the same type of drill bits as a mud rig, but it can also drill with a down-the-hole hammer.

This type of bit uses compressed air to break up rock and it can drill very fast. A large air rotary rig can drill a borehole 60 cm in diameter to 500 meters or more.

Advantages:

Because there is no drilling mud to mix or settling pits to dig, an air rotary rig can be set up very quickly. An air rotary rig also drills much faster than any other rig of a comparable size.

Disadvantages:

An air rotary drill rig requires a very large air compressor, especially if a down-hole hammer is used. This adds significantly to the cost of the rig, its maintenance needs, and its fuel use.

A large air rotary rig will consume 40-60 liters per hour of fuel, making it one of the most expensive types of drill rig to operate. Large air rotary rigs also require support vehicles.

Swiss Centre for Development Cooperation in Technology and Management (SKAT) has published a manual entitled "Drilled Wells" which covers additional topics and information on the subject of how to drill a well. The PDF of that manual can be downloaded here.

SKAT Well Drilling Manual 

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