CIRCULATING SYSTEM When drilling is in progress, the components of the hoisting system, mud pumps and prime movers are used to circulate drilling fluid from the mud pits through the drillstring and out the bit. Cuttings are flushed from the bottom of the borehole up to the surface, thus cleaning the bottom of the hole and providing the logging geologist with samples at the surface.
Mud Pumps
y
A
drilling rig usually has two mud pumps, and these are the heart of a fluid-circulating
system. y
Their
function is to circulate the drilling fluid under pressure (up to 7500 psi) from a
surface pit, through the drillstem, to the bit, return it up the annulus, and back to the pit. y
There
are two types of pumps used in the oil industry.
1. Duplex (2 pistons). 2.
Triplex (3 pistons).
y
A
recent development is Hex pump which contains 6 pistons.
y
A
basic pump consists of a piston (the liner) which moves back and forth (reciprocates)
inside a cylinder. y
The
normal mud pump consists of two main assemblies.
1.
The Fluid End: It produces the pumping process with valves, pistons & liners. To reduce severe vibration caused by the pumping process, these pumps incorporate both suction & discharge pulsation dampener.
2. The Power End: It converts the rotation of the drive shaft to the reciprocating motion of the pistons. Single-Acting (Triplex Pump) pump is said to be single-acting or triplex if it pumps fluid on the forward strokes. It contains 3 pistons. A
y
Triplex
single-acting pumps put pressure on only one side of the piston.
y y
y
y
These
pumps have three pistons and are much lighter than duplex pumps. More power can be obtained from a relatively small triplex pump because it operates at higher speeds, 120 to 160 spm. As triplex pumps operate at higher speeds, they usually have a centrifugal pump to charge the suction. With a properly charged suction, triplex pumps can operate at nearly 100% volumetric efficiency. When efficiency problems are discovered, it is usually because of problems with the suction pump.
Double-Acting ( Duplex)
pump is said to be double-acting or duplex if it pumps fluid on both forward & backward strokes. It contains 2 pistons. A
y
y
y y y y
In
duplex pumps, each of the two cylinders is filled on one side of the piston & at the same time fluid is being discharged on the other side of the piston. Each complete cycle of the piston results in the discharge of a fluid volume that is twice the volume of cylinder, minus the volume of the rod. Double acting pumps stroke f our times during each cycle. Strokes per minute (spm), on a duplex pump actually means cycles per minute. Duplex pump operates at relatively low speed i.e. 60 to 70 spm. The volume of fluid pumped per minute is determined by multiplying the volume pe r complete cycle by the number of strokes/cycles per minute.
Volume of f luid pumped (per min) = Volume (per complete cycle) × Strokes/Cycles (per min) Vol of f luid pumped/min = Vol/cycle × Spm
Centrif ugal Pumps This
type of pump uses an impeller (Rotor inside a tube or conduit used to increase the pressure & flow of fluid) for the movement of fluid rather a piston reciprocating inside a cylinder. Centrifugal pumps are commonly used to move liquids through a piping system. They are used to surcharge mud pumps & providing fluid to solids control equipment & mud mixing equipment. A centrifugal pump works by converting kinetic energy into potential energy, measurable as static fluid pressure. Pump Liners Pump liners fit inside the pump cavity. They affect the pressure rating & flow rate from the pump. For a given pump, a liner has the same OD (Outer Diameter) but different ID (Internal Diameter). The smaller liner (small ID) is used in the deeper part of the well where low flow rate is required but much higher operating pressure. Power Requirement The
horsepower requirement of the pump depends on flow rate & the pressure.
Operating Pressure The
operating pressure depends on flow rate, depth & size of the hole, size of the drill pipe & drill collars, mud properties & size of nozzle used. Size Determination The
size of the pump is determined by the length of its stroke & the size of the liner.
Volumetric & Mechanical E ff iciency Drilling mud usually contains little air & is slightly compressible. Hence the piston moves through a shorter stroke than theoretically possible before reaching discharge pressure. As a
result, the volumetric efficiency is always less than one; typically duplex. In
95%
for triplex & 90% for
addition, due to power losses in drives, the mechanical efficiency of most pumps is about
85%.
Terminology Mud The liquid circulated through the wellbore during rotary drilling and work over operations. Mud Acid A mixture of hydrochloric and/or hydrofluoric acids and surfactants used to remove wall cake from the wellbore. Mud Cake The sheath of mud solids that forms on the wall of the hole when liquid from mud filters into the formation. Also called filter cake or wall cake. Mud Centrif uge A device that uses centrifugal force to separate small solid components from liquid drilling fluid. Mud Cleaner A cone-shaped device, a hydro cyclone, designed to remove very fine solid particles from the drilling mud. Mud Engineer An employee of a drilling fluid supply company whose duty it is to test and maintain the drilling mud properties that are specified by the operator. Mud-Gas Separator A device that removes gas from the mud coming out of a well when a kick is being circulated out. Mud Hopper A large funnel- or cone-shaped device into which dry components (such as powdered clay or cement) can be poured to later mix with water or other liquids. The dry component is inducted through a nozzle at the bottom of the hopper. Mud Hose The hose on a rotary drilling rig that conducts the drilling fluid from the mud pump and standpipe to the swivel and Kelly; also called the mud hose or the Kelly hose. It is a steel reinforced, flexible hose that is installed between the standpipe and the swivel or top drive. It is also called Rotary Hose. Mud Line
A
mud return line.
Mud Logging The recording of information derived from examination and analysis of formation cuttings made by the bit and of mud circulated out of the hole. A portion of the mud is diverted through a gas-detecting device. Cuttings brought up by the mud are examined under ultraviolet light to detect the presence of oil or gas. Mud logging is often carried out in a portable laboratory set up at the well site. Mud Motor A drilling tool made up in the drill string directly above the bit. It causes the bit to turn while the drill string remains fixed. It is used most often as a deflection tool in directional
drilling, where it is made up between the bit and a bent sub (or, sometimes, the housing of the motor itself is bent). Two principal types of downhole motor are the positive-displacement motor and the downhole turbine motor. It is also called downhole motor. Mud Pit Originally, an open pit dug in the ground to hold drilling fluid or waste materials discarded after the treatment of drilling mud. For some drilling operations, mud pits are used for suction to the mud pumps, settling of mud sediments, and storage of reserve mud. Steel tanks are much more commonly used for these purposes now, but they are still usually referred to as pits. Mud Pump A large, high-pressure reciprocating pump used to circulate the mud on a drilling rig. A typical mud pump is a two or three-cylinder piston pump whose replaceable pistons travel in replaceable liners and are driven by a crankshaft actuated by an engine or a motor. Mud Return Line A trough or pipe that is placed between the surface connections at the wellbore and the shale shaker. Mud Tank One of a series of open tanks, usually made of steel plate, through which the drilling mud is cycled to remove sand and fine sediments. Mud Weight A measure of the density of a drilling fluid expressed as pounds per gallon, pounds per cubic foot, or kilograms per cubic meter. Mud weight is directly related to the amount of pressure the column of drilling mud exerts at the bottom of the hole.
