Guide shoes: As the name suggests, a guide shoe is attached to the first joint of casing to be lowered into the hole. It is aluminum with a hole in the center and rounded, to guide the casing into the borehole, around obstructions.
Float collars: These devices permit the casing to literally float into the borehole, by virtue of being partially empty. It is a back pressure valve, which is closed by the outside fluid column, thereby preventing entry of the fluid as the casing is lowered into the hole. It also serves as a check-valve in the casing string, to prevent back flow of cement after being pumped outside the string. This is important because the density of the cement slurry is always greater than drilling mud. This back pressure valve serves to prevent a blowout through the casing, if a kick should occur during casing operations.
A float collar also serves as a “stop” for the two plugs when cement is displaced. This action allows a quantity of slurry to stay inside the casing at the casing shoe, so that the operator has reasonable assurance of there being adequate cement outside the casing at that point.
Centralizers and wall scratchers: These are attached to the casing for two main reasons: 1. To ensure a reasonably uniform distribution of cement around the pipe.
2. To obtain a competent seal all the way around the casing and with the adjoining formation.
Centralizers hold the casing away from the borehole wall and therefore serve to prevent differential sticking. Wall scratchers are mechanical wall cleaning devices, attached to the casing, that abrade the hole when worked by reciprocating or rotating. This helps to provide a more suitable surface for the cement to bond to.
Centralizer Scratcher
Wiper plugs: Wiper plugs are made of molded rubber and cast aluminum or plastic. They are designed for the following reasons:
• Wipe the casing free of mud.
• Separate mud from cement inside the casing.
The top, or follow, plug follows cement slurry, or
other fluids, down the CT string and serves as a wiper and means to separate the cement and the displacement fluid pumped behind the slurry. The top plug will seat on the top of the bottom plug when completely displaced.
The bottom, or lead, plug is a device to lead the
cement slurry, or other fluids, down the CT string. It also separates the fluid inside the coiled tubing and the cement slurry. Upon seating, the pins in the plug will shear at the selected pressure and allow the cement slurry to pass through the plug. The lead plug provides the seal area for the follow plug.
Cementing head: This provides the union for connecting the cementing lines from the cementing pump to the casing. This type of head makes it possible to circulate the mud in a normal manner, release the bottom plug, mix and pump the cement and pump it down, release the top plug, and displaces the cement without making or breaking any connections. Cementing heads are available in sizes from “4 1/2” to “20”, for working pressure of 2,000 to 10,000 psi. Cementing head sizes & working pressures are inversely related. Generally for smaller sizes mare working pressure and far larger sizes less working pressure are the requirements. Cementing heads could be of single plug or double plug types. Caps are “Acme” threaded or quick lack type and fitted with swivel chain assembly.
5. Cementing:
Oil well cementing is the process of mixing and displacing cement slurry down the casing and up the annular space behind the casing where it is allowed to set, thus bonding the pipe to the formation. Cementing procedures are classified as primary or secondary. Primary cementing is performed immediately after the casing is run into the borehole. Its objective is to obtain an effective zonal separation and help protect the casing. Cementing also helps in the following ways:
• Bonds the casing to the formation. • Protects the producing formations.
• Helps in the control of blowouts from high-pressure zones. • Seals off troublesome zones (i.e. lost circulation zones). • Provides support for the casing.
• Forms a seal in the event of a kick during drilling.
Primary cementing:
Most primary cement jobs are performed by pumping the slurry down the casing and up the annulus. There are modified techniques for special situations, such as:
• Single-Stage cementing through casing (normal displacement).
• Multi-Stage cementing (for wells having critical fracture gradients or requiring good cement jobs on long strings).
• Inner string cementing through drill pipe (for large diameter pipe).
• Outside or annulus cementing through tubing (surface pipe or large casing). • Multiple string cement (for small diameter pipe).
• Reverse circulation (critical formations).
Single-stage: (normal displacement technique) the general practice, once casing is set and
circulation has been assured, is to pump a 10 to 15 barrel “spacer” ahead of the bottom (red) plug, which is immediately followed by the cement. The spacer serves as a flushing agent and provides a spacer between the mud and cement. It also assists in the removal of wall cake and flushes the mud ahead of the cement, thereby lessening contamination.
Cement plugs consist of an aluminum body encased in molded rubber. Two plugs are usually contained in the cementing head to facilitate the operations. When the bottom plug reaches the float collar, the diaphragm in the plug ruptures to permit the cement to proceed down the casing and up the annulus. The top (black) plug, which is solidly constructed, is released when all the cement has been pumped. It is dropped on top of the cement, followed by drilling mud, to displace the cement from the casing.
This plug causes a complete shut-off when it reaches the float collar. Pumping is stopped as soon as there is a positive indication (pressure increase) that the top plug has reached the float collar. To ensure good cement circulation and drilling mud displacement, movement of the casing, either by reciprocation or rotation, may be continued throughout the pumping and displacement operations.
Multi-stage: Devices are used for cementing two or more separate sections behind the
casing string, usually for a long column that might cause formation breakdown if the cement were displaced from the bottom of the string. The essential tool consists of a ported coupling placed at the proper point in the string. Cementation of the lower section of casing is done first, in the usual manner, using plugs that will pass through the stage collar without opening the ports. The multi-stage tool is then opened hydraulically by special plugs, and fluid circulated through the tool to the surface. Placement of cement for the upper section occurs through the ports, which are subsequently closed by the final plug pumped behind the cement.
Single stage cementing
Secondary cementing:
Secondary cement work is done after primary cementing, and includes:
• Plugging to another producing zone. • Plugging a dry hole.
• Formation “squeeze” cementing.
The most important use of squeeze cementing is to segregate hydrocarbon producing zones from those formations producing other fluids. Squeeze cementing is also used to: • Supplement or repair a faulty primary cement job.
• Repair defective casing or improperly placed perforations. • Minimize the danger of lost circulation zones.
• Abandon permanently a non-producing or depleted zone. • Isolate a zone prior to perforating or fracturing.
Injection of the slurry is done under pressure through perforations. The pumping rate is slow enough to allow for dehydration and initial setting, or both. Pumping is continued until the desired “squeeze” pressure is reached.
Cement preparation:
Bulk cement storage and handling equipment is moved out to the rig. Making it possible to mix large quantities of cement on site. The cementing crew mixes the dry cement with water, using a device called a job mixing Hopper. The dry cement is gradually added to the hopper and a jet of water thoroughly mixes with the cement to make slurry (very thin, watery cement). Weighted slurries are often used to insure a control of the formation pressure.
6. Leak off test:
If drilling must continue after a casing is set, it must be determined how much is the maximum pressure that can fracture the formation at the casing shoe since it is the shallowest, unprotected formation than the lowest fracture pressure. By this measurement, drilling can continue until the mud weight (equivalent mud weight and fracture pressure equivalent mud weight) can be calculated.
As the depth of drilling increases, mud weight must be increased. The maximum mud weight that can be used but not fracture the formation at the shoe is measured as follows:
1. After cement in the casing is drilled out, casing shoe is drilled out. 2. About 10 feet of new formation below the casing shoe is drilled.
3. Hole will be circulated until all of the cuttings from the new formation is out of the hole and the hole and inside casing is very clean and free from shale, sand, and cuttings. 4. Special wellhead plugs, packers at the rig floor are set.
5. A special mud with known and pre-determined weight is pumped down the hole through the drill pipe.
6. The amount of pressure that is applied is monitored very carefully and accurately.
7. Pressure is increased slowly and steadily until the formation at the shoe or below the shoe is fractured or propped open and fluid can flow into the formation.
8. At the moment of formation fracture, the pressure is measured accurately.
9. Leak-off test pressure is calculated using this pressure. Fracture pressure or fracture pressure equivalent mud weight is the combination of pressure equivalent mud weight applied and the mud weight equivalent of the hydrostatic head.
7. Fishing:
When great stress is placed on downhole equipment, the probability exists that sooner or later, there will be a mechanical failure and some part of the equipment will be left in the borehole. Another common source of trouble is the drill string and associated equipment becoming “stuck” in the borehole. The technique of removing pieces/section of equipment is called “fishing”.
A “fish” is a piece of equipment, a tool, a part of the drill string that is lost or stuck in the hole. Small pieces, such as a bit cone, or any other relatively small non-drillable items, are called junk or “fish” in the hole. These must be removed or fished out so that drilling operations can continue.