LA HISTORIA DE VIDA DE LUIS MORALES

An essential operation when drilling an oil or gas well is to periodically line the hole with steel pipe, or casing.

Successively smaller-diameter lengths of steel pipe are either screwed or welded (in the case of large conductor pipe) together to form a continuous tube to the desired depth. When installed, this casing is cemented in place to provide additional support and a pressure-tight seal.

Casing in a well has a number of functions:

 Prevents formations from caving into the hole.

 Isolate unstable or problem formations such as high-pressure zones, aquifers, gas zones, weak zones.

 Protects productive formations.

 Provides greater kick tolerance. The deeper the casing, the greater the fracture pressure of the formation the casing is set in, meaning higher formation pressures can be controlled as the well is deepened.

 Allows for production testing.

 Serve as an attachment for surface equipment and artificial lift equipment.

5.5.2 Types of Casing

One or more of the following types of casing is required in every well:

 Conductor pipe—is a short string installed to protect surface sediments from erosion by drilling fluids. It raises the drilling fluid high enough to be returned to the mud pits and prevents washing out around the base of the rig. When shallow gas sands are anticipated, it can serve as the attachment for a BOP.

 Surface casing—is set to protect fresh-water formations and prevent loose formations from caving into the hole. It also serves as an anchor for the BOP to forestall problems with abnormal pressure

zones. The casing must be strong enough to support a BOP, and to withstand gas or fluid pressures that might be encountered as drilling proceeds below this casing.

Surface casing should be set deep enough in a strong, consolidated formation with a fracture gradient high enough to support the maximum mud weight that is needed to drill to the next casing setting point.

 Intermediate casing—is primarily used to protect the hole against lost circulation. It is run to seal off weak zones that may break down as a heavier mud weight is needed to control higher formation pressures as the well is drilled deeper. It may also be set below high pressure formations so that lighter mud weights can be used when drilling proceeds.

 Liner string—is run in a deep hole to prevent lost circulation in weak upper zones while drilling with normal weighted mud to control normal-pressure formations at deeper intervals.

Liners protect against downhole blowouts into normal-pressure formations when drilling abnormal pressure zones. Unlike casing which is run from the surface to a given depth and overlaps the previous casing, liner is suspended from the bottom of the previous casing by a hanger and run to the bottom of the hole.

Liner string offers a cost advantage because of its shorter length; however, a tieback string is sometimes run after the hole is drilled to total depth to connect the liner to the surface.

 Production casing—is the last casing string in a well, usually set immediately above or through the producing formation. It isolates the oil or gas from undesirable fluid in the producing formation and from other formations penetrated by the wellbore. It serves as the protective housing for the tubing and other equipment used in a well.

5.5.3 Surface Equipment

Elevators, casing tongs and casing spinners are designed for specific casing diameters, to lift the casing joints and connect to each other at the correct torque.

The most common type of cement mixing system is the jet type. Water is forced through a reduced section of line at high velocity and cement is added from a hopper above. Cement pumps are used to control the pressure and the rate of displacement during mixing. When the cement is pumped down into the casing, rig pumps are used to pump mud and displace the cement from inside the casing to the annulus.

A cementing head, or retainer head, is an accessory attached to the top of the casing to facilitate cementing the casing. It has passages for cement slurry, and retainer chambers for cementing wiper plugs so that mud, slurry and plugs can all be pumped consecutively in one continual operation.

5.5.4 Subsurface Equipment

A guide shoe (or shoe collar) is a short, concrete-filled, cylindrical section of steel placed at the bottom end of the casing string. This guides the casing into the hole, past any obstructions therefore minimizing the risk of the casing from becoming caught up on irregularities in the borehole as it is lowered.

A float collar is usually installed between the first and second joint of casing. It is equipped with a check valve assembly, which allows downward movement of fluid but prevents upward movement, thus preventing mud from entering the casing as it is being lowered into the hole. This enables floating the casing into the wellbore and decreasing the load on the blocks and derrick. It also prevents cement from backing up into the casing during the cementing operation and after it is displaced.

Variations may include float collars that allow partial filling of the casing with mud as it is lowered into the wellbore, and collars that combine both the guiding and floating apparatus.

Wiper plugs are rubber plugs that are used to separate cement and drilling fluid as they are pumped down the casing during cementing. The bottom plug, which is pumped ahead of the cement, wipes residual drilling mud from the inside casing walls and prevents the drilling mud below it from contaminating the cement.

The top plug, which is released after the calculated volume of cement is pumped, wipes residue cement from the inside casing walls and prevents the drilling mud above it from contaminating the cement.

Centralizers are secured around the casing at regular intervals to hold the casing away from the wellbore walls (Figure 43). Centering the casing in the hole allows for a more uniform cement sheath to form around the pipe.

Figure 43: Centralizer and Wall Cake Scratcher

A scratcher is a stiff-wired device fastened to the outside of the casing that is used to condition the hole for cementing. By rotating or moving the casing string up and down as it run into the hole, the scratcher removes mud cake from the wellbore walls so that the cement can bond solidly to the formation.

A liner hanger is a circular, frictional-gripping assembly of slips and packing rings used to suspend liner string from the bottom of the previous casing. Using a liner hanger saves on the expense of running casing all the way back to the surface.

5.5.5 Preparing to Run Casing

Before running casing, drilling mud is circulated to remove cuttings and excess filter cake from the hole;

this conditions the hole and conditions the mud to ensure uniform properties. Failure to condition the hole thoroughly and treat the mud properly can lead to stuck pipe, poor cementing, extra well costs for cement squeeze work, and even re-drilling the hole.

When conditioning the hole, drilling mud should be pumped around at least twice while weight, viscosity, and fluid loss properties are recorded. If mud treatment appears necessary, circulation while slowly rotating and working the pipe must be maintained until the mud fluid is in suitable condition for running casing.

Before casing is run into the hole, an electric log is run to confirm the bottomhole formation for setting the casing shoe, and to confirm hole depth so that the exact length of casing can be run.

A caliper log is also run to determine hole diameter and the volume of cement required. Cement is pumped to fill the annulus, and into the previous casing. Typically, an extra 25% volume may be pumped to allow for error and losses to the formation.

5.5.6 Running Casing

As casing is run, it is periodically filled with drilling mud unless automatic fill-up float equipment is used. If not filled while running in, the hydrostatic pressure of the mud column acting on the outside of the casing would cause it to collapse.

Using a lightweight filling line with a quick-opening valve, each joint is filled from drill floor while the next length is picked up and prepared for stabbing. Because it is usually not possible to fill a joint completely, it is common practice to stop running casing every five to ten joints and fill completely.

It is crucial that the mud displacements are accurately monitored for the whole duration of the casing run.

Due to the fact that the casing tubular is effectively close-ended, together with very small annular clearance, surge pressures while running casing are large. To minimize this, the casing joints are run in at a very slow speed; surge pressure can cause weaker formations to fracture resulting in loss of drilling mud to the formation. Not only might fracturing the formation result in a poor cement job, it might also result in

a blowout if sufficient mud is lost from the annulus to reduce the mud hydrostatic below the formation pressure of a permeable formation elsewhere in the wellbore.

Therefore, mud returns and displacements are closely monitored for any indication of losses to the formation (Figure 44).

Figure 44: Monitoring Mud Returns and Displacements

The volume of fluid displaced from the hole, as each joint of casing is added to the string, should be equal to the closed displacement of the casing. Final volume gains in the suction pit should be equal to the volume of steel only (open displacement) that is run into the well, if there is no fluid loss.

Provided proper mud returns are obtained, it is usually possible to run the casing all the way into the hole before attempting circulation. When establishing circulation, care must be taken not to run the pump too fast as the casing is lowered, so as to minimize pressure surges. If there is any indication of lost returns, the pumping rate should be reduced immediately.

Circulating drilling mud through the casing after reaching bottom serves two important functions. One is to test the surface piping system. Another is to condition the mud in the hole, and to flush out cuttings and filter cake before cementing.

Circulation time extends for as long as required to condition the mud; the casing is reciprocated (moved up and down) and/or rotated, with or without scratchers, throughout circulation. Minimum adequate circulation before cementing distributes a volume of fluid equal to the volume inside the casing and annulus.

5.5.7 Cementing Operation

Cementing is a process of mixing and displacing a cement slurry consisting of dry cement mixed with water, into the annulus, i.e. the space between the casing and the open hole. By bonding the casing to the formation, cementing serves several valuable purposes:

 Protects the productive formation.

 Helps control blowouts from high-pressure zones.

 Seals off lost-circulation or other troublesome formations before drilling deepe.r

 Helps support the casing.

 Prevents casing corrosion.

Generally, 10 – 15 barrels of water are pumped into the hole before pumping cement slurry. The water acts as a flushing agent and provides a spacer between the drilling mud and the slurry. The water also helps to remove any remaining filter cake and flushes mud ahead of the cement, thereby lessening contamination.

To prepare for cementing, the cementing head is installed on the top casing joint. A discharge line from the cementing pump is attached to the cementing head so that the slurry can be circulated. A bottom wiper plug is placed in the cementing head, followed a top wiper plug.

As the cement slurry from the pump discharge reaches the cementing head, the bottom plug travels down the casing ahead of the slurry. When the calculated volume of cement is pumped, a retainer pin is pulled to release the top wiper plug from the cementing head (A). Refer to Figure 45.

The plugs and cement are pumped to the bottom of the casing by the mud / rig pumps. The bottom plug seats in the float collar (B). Refer to Figure 45. Mud continues to be pumped to displace the cement, which passes through the open valve in the float collar, out the guide shoe and into the annulus.

Meanwhile, the casing is reciprocated and/or rotated to help displace the mud.

Again, it is important to monitor pit levels through this operation to ensure the much denser cement slurry is not being lost to the formation.

Figure 45: Cementing Operation

When all of the cement is displaced from inside the casing, the top plug seats (bumps) on top of the bottom plug held in the float collar (C). Refer to Figure 45. At this point, the pump pressure increases immediately because no mud can get past the solid top plug (that is, mud is being pumped into a closed space). The pump is then immediately shut down and the pressure is bled off. With the pressure released from the casing, the valve in the float collar closes to keep cement from backing up in the casing.

It is important to release the pressure on the casing before the cement sets as the pressure causes the casing to bulge. If the cement is allowed to set, then the casing will pull away from the hardened cement when the pressure is released, thereby loosening the bond.

Cement should be displaced quickly to create turbulence in the annulus and to remove the maximum amount of mud cake as possible. However, excessive pressure on the casing and surface connection can cause a rupture, excessive flow (or pressure) in the annulus can lead to formation breakdown and result in lost circulation, and excessive flow in the annulus can cause mud waste through overflow.

5.5.8 Other Applications

 Secondary cementing operations are performed as part of well servicing and workover. Secondary cementing can also be performed to plug-back a well to another producing formation (and change testing zones), to plug a dry hole (and abandon the well) and to plug-back a hole to sidetrack.

 During cementing, the cement can fail to rise uniformly between the casing and the borehole wall, leaving spaces devoid of cement; this is called cement channeling. This channeling can be rectified by performing a secondary cementing operation called a cement squeeze. In this case, cement is pumped behind the casing under high pressure to re-cement channeled areas or to block off an uncemented formation. It can be performed to isolate a producing formation, seal off water or repair

casing leaks.