Standpipe Centrifugal

Standpipe Surge Relief

STRING A STEEL

1 TANKS

STRING A STEEL

1 TANKS

Fig. 1.23-Schematic of example rig circulating system tor liquid drilling fluids.

Drill Rig String Schematic

(a) Duplex design.

Fig. 1.23-Schematic of example rig circulating system tor liquid drilling fluids.

  • b) Triplex design.
  • a) Duplex design.
  • b) Triplex design.

Fig. 1.24-Example mud circulating pumps.

Discharge

Discharge

Discharge

Discharge

Inlet or Suction Suction

(a) Double-acting (duplex) design.

Inlet or Suction Suction

  • a) Double-acting (duplex) design.
  • Piston Rorfl

Discharge if 1

Suction

(b) Single-acting (triplex) design. Fig. 1.25-Schematic of valve operation of single- and double-acting pumps.10

multiplying the pump factor by N, the number of cycles per unit time. In common field usage, the terms cycle and stroke often are used interchangeably to refer to one complete pump revolution.

Pumps are rated for (1) hydraulic power, (2) maximum pressure, and (3) maximum flow rate. If the inlet pressure of the pump is essentially atmospheric pressure, the increase in fluid pressure moving through the pump is approximately equal to the discharge pressure. The hydraulic power output of the pump is equal to the discharge pressure times the flow rate. In field units of hp, psi, and gal/min, the hydraulic power developed by the pump is given by

3-in.-diameter passage for fluid circulation to the drillstring.

1714

For a given hydraulic power level, the maximum discharge pressure and flow rate can be varied by changing the stroke rate and liner size. A smaller liner will allow the operator to obtain a higher pressure, but at a lower rate. Due to equipment maintenance problems, pressures above about 3,500 psig seldom are used.

The flow conduits connecting the mud pumps to the drillstring include (1) a surge chamber, (2) a 4- or 6-in. heavy-walled pipe connecting the pump to a pump manifold located on the rig floor, (3) a standpipe and rotary hose, (4) a swivel, and (5) a kelly. The surge chamber (see Fig. 1.26) contains a gas in the upper portion, which is separated from the drilling fluid by a flexible diaphragm. The surge chamber greatly dampens the pressure surges developed by the positive-displacement pump. The discharge line also contains a pressure relief valve to prevent line rupture in the event the pump is started against a closed valve. The standpipe and rotary hose provide a flexible connection that permits vertical movement of the drillstring. The swivel contains roller bearings to support the rotating load of the drillstring and a rotating pressure seal that allows fluid circulation through the swivel. The kelly, which is a pipe rectangular or hexagonal in cross section, allows the drillstring to be rotated. It normally has a

Example 1.3. Compute the pump factor in units of barrels per stroke for a duplex pump having 6.5-in. liners, 2.5-in. rods, 18-in. strokes, and a volumetric efficiency of 90%.

Solution. The pump factor for a duplex pump can be determined using Eq. 1.10:

Fp = ~LsEv(2df-d})

  • 18)(0.9)[2(6.5)2-(2.5)2]
  • 1991.2 in. 3/stroke.

Recall that there are 231 in.3 in a U.S. gallon and 42 U.S. gallons in a U.S. barrel. Thus, converting to the desired field units yields

1991.2 in.3/stroke x gal/231 in.3 xbbl/42gal

= 0.2052 bbl/stroke.

Mud pits are required for holding an excess volume of drilling mud at the surface. This surface volume allows time for settling of the finer rock cuttings and for the release of entrained gas bubbles not mechanically separated. Also, in the event some drilling fluid is lost to underground formations, this fluid loss is replaced by mud from the surface pits. The settling and suction pits sometimes are dug in the earth with a bulldozer but more commonly are made of steel. A large earthen reserve pit is provided for contaminated or discarded drilling fluid and for the rock cuttings. This pit also is used to contain any formation fluids produced during drilling and well-testing operations.

Dry mud additives often are stored in sacks, which are added manually to the suction pit using a mud-

mixing hopper. However, on many modern rigs bulk storage is used and mud mixing is largely automated. Liquid mud additives can be added to the suction pit from a chemical tank. Mud jets or motor-driven agitators often are mounted on the pits for auxiliary mixing.

The contaminant-removing equipment includes mechanical devices for removing solids and gases from the mud. The coarse rock cuttings and cavings are removed by the shale shaker. The shale shaker is composed of one or more vibrating screens over which the mud passes as it returns from the hole. A shale shaker in operation is shown in Fig. 1.27. Additional separation of solids and gases from the mud occurs in the settling pit. When the amount of finely ground solids in the mud becomes too great, they can be removed by hydrocyclones and decanting centrifuges. A hydrocyclone (Fig. 1.28) is a cone-shaped housing that imparts a whirling fluid motion much like a tornado. The heavier solids in the mud are thrown to the housing of the hydrocyclone and fall through the apex at the bottom. Most of the liquid and lighter particles exit through the vortex finder at the top. The decanting centrifuge (Fig. 1.29) consists of a rotating cone-shaped drum which has a screw conveyor attached to its interior. Rotation of the cone creates a centrifugal force that throws the heavier particles to the outer housing. The screw conveyor moves the separated particles to the discharge.

When the amount of entrained formation gas leaving the settling pit becomes too great, it can be separated using a degasser. A vacuum chamber degasser is shown in Fig. 1.30. A vacuum pump mounted on top of the chamber removes the gas from the chamber. The mud flows across inclined flat surfaces in the chamber in thin layers, which allows the gas bubbles that have been enlarged by the reduced pressure to be separated from the mud more easily. Mud is drawn through the chamber at a reduced pressure of about 5 psia by a mud jet located in the discharge line.

A gaseous drilling fluid can be used when the formations encountered by the bit have a high strength and an extremely low permeability. The use of gas as a drilling fluid when drilling most sedimentary rocks results in a much higher penetration rate than is obtained using drilling mud. An order-of-magnitude difference in penetration rates may be obtained with gas as compared with drilling mud. However, when formations are encountered that are capable of producing a significant volume of water, the rock cuttings tend to stick together and no longer can be easily blown from the hole. This problem sometimes can be solved by injecting a mixture of surfactant and water into the gas to make a foam-type drilling fluid. Drilling rates with foam are generally less than with air but greater than with water or mud. As the rate of water production increases, the cost of maintaining the foam also increases and eventually offsets the drilling rate improvement.

A second procedure that often is used when a water-producing zone is encountered is to seal off the

VALVE GUARD

PRESSURE SAUCE

CHARGING VALVE

COVER PLATE

VALVE GUARD

PRESSURE SAUCE

CHARGING VALVE

COVER PLATE

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