Jetting

The jet bit method of deflecting a well was, at one time, the most common method used in soft formations. Jetting has been successfully used to depths of 8,000 feet (2,400m); however the economics of this method and the availability of other directional drilling tools limit its use.

Geology is the most important influence on where jetting can be used; next in importance is the amount of hydraulic energy available for jetting. Sandstones and oolitic limestones that are weakly cemented are the best candidates for jetting. Unconsolidated sandstones and some other types of very soft rocks can be jetted with some degree of success. Very soft rocks erode too much, making it difficult to jet in the desired direction; when rotation begins, the stabilizers cut away the curved, jetted section and return to a nearly vertical well path.

Even though shales may be soft, they are not good candidates for jetting. Most medium-strength rock is too well cemented to jet with conventional drilling rig pumps, so it limits the depth to which jetting can be applied. Higher pressures and more hydraulic energy can extend the depth to which jetting is practical.

A major drawback to jetting is that the formation must be favorable at a shallow depth or in the desired kickoff interval; otherwise, the technique is no better than the use of a mud motor with a deflecting device. Another problem is that if jetting is continued too long without conventional drilling being resumed, large doglegs can be created.

The drill string is set on bottom and if the formation is sufficiently soft, the WOB drills off. A pocket is washed into the formation opposite the large nozzle. The bit and near-bit stabilizer work their way into the pocket (path of least resistance). Enough hole should be jetted to "bury" the near-bit stabilizer. If required, the bit can be pulled off bottom and the pocket spudded. The technique is to lift the string about 1.5m (5') off bottom and then let it fall, catching it with the brake so that the stretch of the string (rather than the full weight of the string) causes it to spud on bottom. Spudding can be severe on a drillstring, drilling line and derrick and should be kept to a minimum. Another technique that may help is to 'rock' the rotary table a little (15o) right and left of the orientation mark while jetting.

Jet Bits Nozzle

Jetting Bit

There are special bits made for jetting including those with two cones and an elongated jet nozzle replacing the third cone. The elongated nozzle provides the means to jet the formation while the two cones provide the mechanism for drilling. Other tri-cone deflection bits are available with an enlarged fluid entrance to one of the jets. This allows a greater amount of fluid to be pumped through one of the jets during jetting operations.

To deflect a well using the jet method, the assembly is run to the bottom of the hole, and the large jet is oriented in the desired direction. The kelly should be high to allow rotary drilling after the deflection is started. The center of the large nozzle represents the toolface and is oriented in the desired direction. Maximum circulation rate is used while jetting. Jet velocity for jetting should be 150 m/sec (500 ft/sec).

Jetting Bit

Moving The

Orienting Larje Jertin« A Bit Into Tut Normal

Nozzle Pocket Port« (Still Jetting) Ftoory Drilling

Moving The

Orienting Larje Jertin« A Bit Into Tut Normal

Nozzle Pocket Port« (Still Jetting) Ftoory Drilling

Dog Leg Severity Drilling

After a few feet have been jetted, the pumps are cut back to about 50% of that used for jetting and the drill string is rotated. It may be necessary to pull off bottom momentarily due to high torque (near-bit stabilizer wedged in the pocket). High WOB and low RPM are used to try to bend the collars above the near-bit stabilizer and force the BHA to follow through the trend established while jetting. The remaining length on the kelly is drilled down. Deflection is produced in the direction of the pocket i.e. the direction in which the large jet nozzle was originally oriented.

To clean the hole prior to connection/survey, the jet should be oriented in the direction of deviation. After surveying, this orientation setting (toolface setting) is adjusted as required, depending on the results achieved with the previous setting. Dogleg severity has to be watched carefully and reaming performed as required.

The operation is repeated as often as is necessary until sufficient inclination has been achieved and the well is heading in the desired direction. The hole inclination can then be built up to maximum angle using 100% rotary drilling and an appropriate angle build assemble.

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  • lalli pirhonen
    How to rotate shale gas drilling bits?
    8 years ago
  • ernest
    What is jetting in petroleum engineering?
    2 years ago
  • matthias
    What is jetting in drilling?
    10 months ago
  • Salla
    What happens to the mud during jetting operations?
    6 months ago

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