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Information about Drilling

Published on April 10, 2013

Author: kaissfrikha

Source: slideshare.net

DRILLING FLUIDSThe key to making the rotary drilling system work isthe ability to circulate a fluid continuously downthrough the drill pipe, out through the bit nozzles andback to the surface.The drilling fluid can be air, foam (a combination of airand liquid or a liquid.Liquid drilling fluids are commonly called drilling mud.All drilling fluids, especially drilling mud, can have awide range of chemical and physical properties. Theseproperties are specifically designed for drillingconditions and the special problems that must behandled in drilling a well. 1

Purpose of Drilling Fluids1. Cooling and lubrication. As the bit drills into the rock formation, the friction caused by the rotating bit against the rock generate heat. The heat is dissipated by the circulating drilling fluid. The fluid also lubricates the bit.2. Cuttings removal. An important function of the drilling fluid is to carry rock cuttings removed by the bit to the surface. The drilling flows through treating equipment where the cuttings are removed and the clean fluid is again pumped down through the drill pipe string.3. Suspend cuttings. There are times when circulation has to be stopped. The drilling fluid must have that gelling characteristics that will prevent drill cuttings from settling down at the bit. This may caused the drill pipe to be stuck. 2

4. Pressure control. The drilling mud can be the first line of defense against a blowout or loss of well control caused by formation pressures. The hydrostatic head produced by the mud in psi is = 0.052 x G x H where G = density of mud in ppg H = depth of the hole in feet. This hydrostatic head will counter the formation pressure in order to avoid a blowout while drilling. For example, Lets say a well is being drilled in a salt-water basin (pressure gradient of 0.465 psi/ft), the pressure in the formation at 10,000 feet would be expected to be: 10,000 x 0.465 = 4,650 psi The weight of mud required to counter this pressure is calculated as follows. P = 0.052GH 4,650 = 0.052 x G x 10,000 G = 8.94 ppg 3

5. Data source. The cuttings that the drilling mud brings to the surface can tell the geologist the type of formation being drilled.6. To wall the hole with impermeable filter cake. This will give a temporary support to the wall of the borehole from collapsing during drilling. Drilling fluid can solve problemsMany drilling problems are due to conditions orsituations that occur after drilling begins and forwhich the drilling fluid was not designed.Some of these problems can be solved by addingmaterials to the drilling fluid to adjust itsproperties.Other cases, it may be necessary to replace thedrilling fluid being used with another fluid system. 4

The most common changes is the mud weight ordensity. Weighting material is added when high-pressure formations are expected.Some of the problems are:1. Lost circulationLost circulation can occur in several types offormations, including high permeable formations,fractured formations and cavernous zones.Lost circulation materials can be added to the mudto bridge or deposit a mat where the drilling fluidbeing lost to the formation. These materials includecane and wood fibres, cellophane flakes and evenpadi husks were used in oil drilling in Sumatra. 5

2. Stuck pipeStuck pipe can occur after drilling has been haltedfor a rig breakdown, while running a directionalsurvey or when conducting other nondrillingoperation.The drill pipe may stick to the wall of the hole dueto the formation of filter cake or a layer of wet mudsolids on the wall of the hole in the formation.3. Heaving or sloughing holeThis occurs when shales enter the well bore afterthe section has been penetrated by the bit. To solvethis problem, drilling is suspended the hole isconditioned (by letting the mud in circulation for aperiod of time) 6

Types of drilling fluids1. Water-base mudThis fluid is the mud in which water is the continuousphase. This is the most common drilling mud used inoil drilling.2. Oil-based mudThis drilling mud is made up of oil as the continuousphase. Diesel oil is widely used to provide the oilphase. This type of mud is commonly used in swellingshale formation.With water-based mud the shale will absorb thewater and it swells that may cause stuck pipe.3. Air and foamThere are drilling conditions under which a liquiddrilling fluid is not eh most desirable circulatingmedium. Air or foam is used in drilling some wellswhen these special conditions exist. 7

Mud Properties1. Mud density or mud weightMud weight is measured by means of a mud balance.The weight of water is 8.33 ppg. The mud weight canbe increased by adding barite (barium sulphate).Barite has a specific gravity of between 4.2 – 4.3.Other materials can be used to increase mud weightsuch as ilmenite (S.G of 4.58)2. Mud viscosityMud viscosity is difficult to measure but in the fieldthe Marsh funnel and the Fann V-G meter iscommonly used.The Marsh Funnel is filled with mud, the operatorthen notes the time, removes his finger from thedischarge and measures the time for one quart (946 8

cm3) to flow out. Marsh funnels are manufactured toprecise dimensional standards and may be calibratedwith water which has a funnel viscosity of 26 ± 0.5sec.In using Fann V-G (Viscosity-gel) meter, readings aretaken at 600 rpm and 300 rpm.The viscosities are defined as follows: µp = φ600 - φ300 µaF = ½ φ600 Yb = φ300 - µp Where µp = plastic viscosity, cp µaF = apparent viscosity, cp Yb = Bingham yield point, lb/100 ft2 φ = Torque readings from instrument dial at 600 and 300 rpm. 9

From these relationships: Yb = 2(µaF - µp) µaF = µp + ½ YbTrue yield point: Yt = ¾ YbYield point is influenced by the concentration of solids,their electrical charge, and other factors. If not atthe proper value, it can also reduce drilling efficiencyby cutting penetration rate, increasing circulatingpressure, and posing the danger of lost circulation.3. Gel strengthThe gel strength of a mud is a measure of the shearingstress necessary to initiate a finite rate of shear.With proper gel strength can help suspend solids in thehole and allow them to settle out on the surface,excessive gel strength can cause a number drillingproblems. 10

4. FiltrationThe filtration, water loss or wall building test isconducted with a filter press.The rate at which filtrate will invade permeable zoneand the thickness of the filter cake that will bedeposited on the wall of the hole as filtration takesplace are important keys to trouble-free drilling Drilling Fluid treating and monitoring equipmentIn addition to the main mud pumps, several items ofmud treating equipment are found on most rigs. Muchof this equipment is aimed at solids removal, includingshale shakers, desanders, desilters and centrifuges.Shale shakers remove larger particles from the mudstream as it returns from the bottom of the hole.Shakers are equipped with screens of various sizes,depending on the type of solids to be removed. 11

Finer particles in the mud stream are removed withdesanders, desilters and centrifuges. Each of theseitems of solids-control equipment is applicable onlyover a certain range of particle sizes.In addition to removing solids, mud handling equipmentmay also include a mud degasser to remove entrainedgas from the mud stream. Degassing the drilling fluidis sometimes necessary when small volumes of gas flowinto the well bore during drilling.Additional equipment include mixers to agitate mud inthe tanks, smaller pumps to various duties andequipment for adding chemicals and solid materials tothe mud system. 12

Drilling hazardsThe following are some of the most common hazards indrilling and can be overcome by proper control of themud properties.1. Salt section hole enlargement Salt section can be eroded by the drilling fluid and causes hole enlargement. These enlargement will require larger mud volume to fill the system and in case of casing the hole, larger cement volume is required. To avoid these problems a salt saturated mud system is prepared prior to drilling the salt bed.2. Heaving shale problems Areas with shale sections containing bentonite or other hydratable clays will continually absorb water, swell and slough into the hole. 13

Such beds are referred to as heaving shales andconstitute a severe drilling hazard whenencountered.Pipe sticking, excessive solid buildup in the mudand hole bridging are typical problems.Various treatments of the mud are sometimessuccessful, such as • Changing mud system to high calcium content by adding lime, gypsum etc which reduces the tendency of the mud to hydrate water sensitive clays. • Increasing circulation rate for more rapid removal of particles. • Increasing mud density for greater wall support • Decreasing water loss mud • Changing to oil emulsion mud • Changing to oil-based mud. 14

3. Blowouts Blowout is the most spectacular, expensive and highly feared hazard of drilling. This occurs when encountered formation pressure exceed the mud column pressure which allows the formation fluids to blow out of the hole. Mud density or the mud weight is the principal factor in controlling this hazard. In drilling a blow out preventer (BOP) stack is always attached at the top of the conductor pipe. In case of a gas kick (a sign that may lead to a blow out) the BOP stack can close the annular space between the drilling pipe and the conductor pipe or casing or shut the whole hole (with a blind ram of the BOP). 15

4. Lost Circulation Lost circulation means the loss of substantial amount of drilling mud to an encountered formation. Lost circulation materials are commonly circulated in the mud system both as a cure and a continuous preventive. These materials are the fibrous materials such as the hay, sawdust or padi husk and lamellated (flat and platy) materials such as mica, cellophane. Drilling Mud CalculationsThe most common mud engineering calculations arethose concerned with the changes of mud volume anddensity caused by the addition of various solids orliquids to the system. 16

The first step is to compute the system volume, whichis the sum of the mud in the hole and surface pits.Consider then the volume and density change of a mud(or water) resulting from the addition of solids.Two basic assumptions must be made: 1. The volumes of each material are additive. 2. The weights of each material are additive.Expressions for these assumptions: Vs + Vm1 = Vm2 ρsVs + ρm1Vm1 = ρm2Vm2 where Vs = volume of solid Vm1 = volume of initial mud Vm2 = final volume of mixture ρs = density of solid ρm1 = density of initial mud ρm2 = density of final mudSolving for Vs : ρsVs + ρm1Vm1 = ρm2Vm2 ρsVs = ρm2Vm2 − ρm1Vm1 17

= ρm2Vm2 − ρm1(Vm2 − Vs) ρsVs − ρm1Vs = ρm2Vm2 − ρm1Vm2 Vs(ρs − ρm1) = Vm2(ρm2 − ρm1) Vm2 (ρm2 − ρm1 ) Vs = ρs − ρm1As to units, the densities may be in any consistent set.The corresponding weight to add is ρsVm2 (ρm2 − ρm1 ) ρsVs = ρs − ρm1Example:A 9.5 lb/gal mud contains clay (S.G.=2.5) and freshwater. Compute (a) the volume % and (b) the weight %clay in this.Solution:(a) From the equation V volume of solid = s Vm2 (ρm2 − ρm1 ) × 100 Vs Vm2 = ρs − ρm1 ρ V ρ (ρ − ρm1 ) (b) Weight % solids = s s × 100 = s m2 × 100 (ρm2 − ρm1 ) ρm2Vm2 ρm2 (ρs − ρm1 ) = × 100 ρs − ρm1 18 20.8(.9.5 − .8.33) 9 5 − 8 33 = × 100 = = 9..6% × 100 20 4% (25()(8.8 −)8.3333 9..5 20 33 − 8. )


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