User:Amanullah kathio
History Before development of combustion engine in late 19th century the main method for drilling rock was muscle power of man and animal. Rods were turned by hands using clamps attached to rods. The rope and drop methods were invented in china using a steel rods or piston raised and dropped vertically via a rope .mechanized version of this persisted until about 1970, using a can rapidly raise and drop by then was a steel cable.
Introduction Holes are drilled for a wide range of applications. for example exploration of all reserve or of an ore body , for exploration purpose ,blast hole generation, dewatering and for many other reasons virtually every mineral extraction project will require some form of drilling . As a consequence, drilling experience, drilling expertise is very important for a successful operation. Drilling knowledge has progressed through time from a fairy simple operation to a highly complex one. Involving a variety of different drilling techniques and methods each for a particular purposes or situation.
Drilling methods Drilling is basically a rock breakage process. Drilling has variety of devices designed to handle different methods, for drilling in earth crust there are two fundamental drilling techniques. (1) Percussive drilling. (2) Rotary drilling.
(1)Percussive drilling.
Also called churn or cable tool drilling. Percussion drilling involves repetitive application of mechanical energy i.e. a hammer like blow through wedge shape tool (bit) to rod. The repetitive blow of bit set up a compressive pulse into rock causing the bit to penetrate and fragment the rock. Top of wedge shape tool developed a crushed zone following by indentation (act of cutting into points) in rocks, which resulted in chip formation by initial crack or fracture?
Two types of percussive drill are commonly used, they are piston drill and hammer drill comprise of drill rod attached to piston. Drill steel and piston reciprocate and rotate piston drill today is rarely used. Hammer drill are generally the energy transmitting of hammer which classified into hydraulic or pneumatic drilling and top hammer or down hole (DTH) .
(2)Rotary drilling
Rotary drilling technique has long been practiced by civil, mining and petroleum industries. Rock drilling by rotary method comprises mainly of two distinct mechanism crushing\g and shearing. A vertical force is applied to bit as a result (i.e. shearing action) is applied to a bit as a result of applying torque to turn bit.
The result of these two forces generates small chips and then reaches to major chips as forces rise. Faster drilling of rotary bit made method most popular due to higher drilling rate.
Auger drilling
Auger drilling is done with a helical screw which is driving into the ground with rotation. The earth is lifted up the bore hole by the blade of the screw. Auger drilling is used for the well drilling, fence construction, soil engineering and geochemistry reconnaissance (preliminary or starting examination) work in exploration for mineral deposits. In some cases mine shafts are dug with auger drills. Small auger can be mounted on the back of the utility truck with large auger used for sinking (digging) piles for bridge foundation.
Auger drilling is restricted (limited or confined) to generally soft unconsolidated material or weak weathered rock. It is cheap and fast.
Air core drilling
Air core drilling and related methods used hardened steel or tungsten blades to bore a hole into rock. The drill bit has three blades arranged around the bit heads which cut the rock. The rods are hollow and contain an inner tube which sits inside the hollow outer rod barrel. The drill cuttings are removed by injections of compressed air into the hole via the hollow inner rod. The cuttings rods are blown back to the surface via the outer space inside the barrel where they are collected if needed or discarded. Drilling continues with the addition of rods to the top of the drill strings.
Air core drilling can occasionally produce small chinks of cored rock this method of drilling is used to drill the weathered regulate, as the drill rig and steel or tungsten blades cannot penetrate fresh rock where possible air core drilling is preferred over RAB drilling as it provide a more representative sample.
Air core drilling can achieve depth approaching 200 meters in good conditions as the cuttings are removed inside the rods and are less likely to clog. However this method is most costly and slower than RAB.
Diamond core drilling
Multi combination drilling rig(capable of both diamond and reverse circulation drilling). Rig is currently set up for diamond core drilling ( exploration diamond drilling ) utilizes an annular diamond impregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of solid rock. The diamonds are fine to micro fine industrial grade diamonds. They are set in metals of varying hardness from grass to high grade steel. Matrix hardness, diamond size and dozing can be varied according to the rock which must be cut. Holes within the bit allow waters to be delivered to the cutting face this provides three essential lubrication, cooling and removal of drill cutting from the hole.
Diamond drilling is much slower than reverse circulation (RC) drilling due to the hardness of the ground being drilled. Drilling of 1200to 1800 meters in common and at those depth ground is mainly hard rock. Diamond rig need to drill slowly to lengthen the life of drill bit and rods, which are very expensive.
Core samples are retrieved via the use of lifter tube; a hollow tube lowered inside the rod string by a winch cable until it stops the core barrel. As the core is drilled the core lifter slides over the core as it is cut. an over shot attached to the end of the winch cable is lowered inside the rod string and locks on to the backend , located on the top end of the lifter tube. The winch is retracted, pulling the lifter tube to the surface. The core does not drop out the inside of the lifter tube when lifted because a “core lifter spring” located at the bottom of the tube allows the core to move inside the tube but not fallout.
Once a rod is removed from the hole, the core sample is removed from the rod and catalogued the driller’s off.
The power system
It is another major component of the rig. Its primary function is to support all of other system by providing a source for the energy requirements of modern rotary drilling operations. The power system consists of two major sub-components:
(1)The primary power source. (2)The power transmission system.
(1)The primary power source.
For a rotary drilling complex is usually provided by large internal combustion engine called “the prime movers” the prime movers are required to drill a well vary in numbers depending on their size and the anticipated depth and construction of a well. They are usually assembled in a single location on the rig. Most drilling operation requires two or three engines. Deeper well have a greater power requirements and may need up to four engines, these engines ranges from 500 to 5000 horse powers. Diesel fueled (compression) engines are most commonly used on drilling rigs most of the power generated by engines is used by the draw works rotary and mud pumps.
(2)The power transmission system.
From the prime movers to the major working system is accomplished by one of the two methods. Depending on the type of engines being used the two methods are.
(a) Mechanical transmission. (b) Electrical transmission.
(a) Mechanical transmission.
Mechanical power transmission mean that power generated by the engines must be mechanically transmitted this is accomplished in the following manner.
Once power has been generated by the prime movers it must be linked together with other engines in order to deliver adequate amount of power. This is done with hydraulic coupling or torque converters which linked together. The power is transmitted through chain system which physically distributes needed power to the appropriate units to run the drilling operations.
(b) Electrical transmission.
Electrical power transmission is used mostly in modern drilling rigs. In this system the diesel engines drives an electrical generator which is mounted on the front of the block. The generators produce electricity that is sent through cables to a control cabinet. From the control cabinet the electrical power is transmitted through additional cables to electric motors that are directly connected to the equipments of different rig systems.
Independent variables The drilling performance is significantly affected by the independent variables. These variables are individually controlled by the operators, but as a result of the drilling process these parameters are inter related.
(a) Weight on bit (WOB). (b) Feed rate. (c) Rotation bit speed.sss (d) Drilling medium flow.
(e) Weight on bit (WOB).
The thurst or weight on bit (WAB) is one of the basic controllable veriables of certain types of rig there is a range of terms used to define this parametres eg thurst load on bit and axial load or axial pressure etc
The bit penetrates the rock when the pressure exerted on the bit indentors over come the strength of the rock and feeds it forward the weight on bit requires depend on size and geometery of bit and the resistance (strength) of rock being drilled . the rig must be capable of producing the required Wob with sufficient stability for driiling a given hole size with a selected bit size.
Low weight on bit can cause exessive bit wear because the bt sides over the surface of the formation without producing penetration . high WOB can result in changing of the bit and may cause drill rod failure or hole deviation. The gradual increase in bit weight untill it maintains a given penetratio9n rate , provide the availability of required torque to rotate the bit , will be the optimum . level of the weight on the bit. The optimum WOB also depends upon the other optimal . drilling conditions.
(b)FEED RATE The feed rate of the drill bit may be controlled by the function of the independent variables depending on the type of the rig . the rate of advance is expressed as distance drilled per unit time (m/min). penetration rate can be expected to increase with rotational speed and bit weight (WOB). For given condition s the relation ship between feed rate and those variables is basically linear whilst one variable is kept constant. It is a common believe that in any drilling process. The rate of penetration is expressed as a linear advanced.
(c)Rotationalbit speed The drilling performance of drag bit is significantly affectedd by the rotational speed. An increase in bit speed leads to reduction in bit weight to achieve the given penetration rate . the increase in bit speed may be restrict by thermal transients in the cutting insert of the bit , which may cause in bit , micro chipping or thermal cracking .
Rowlands 1971 states that increasing the bit rotational speed increase the time rate of wear since more work is done in given time.. however it may not increase the depth rate of wear. Since there may not be an optimum speed at which bit wear rate can be kept at minimum .
in order to increase a given rate of penetration the bit weight and rotational spead must be continuously maintained , provide adequqte rock cuttings removal from the hole . the eliminations to rotational bit speed may also be restricted b the stability of the rig and drill rod.
(d)Drilling medium flow
The affective drilling process can only be proceeded with when the bottom of the hole is kept clean . this can be achieved b an adequqte flushing medium which can be water air mud or foam.
The flushing medium flow serves the tow primery purposes , to cool and lubricate the bit and remove and transport cutting away from the bottom of the hole as they are made t edrilling medium flow should be a function of the volume of a material removed by the bit and the size of the maximum rock partical liberated .
An extra volume of the flushing mediam will be required to remove increased rock cuttings by inctreased penetration rate if the fluid flow is inadequate then the rock cutting may clog the bit advance and then regrinding may cause excessive abrasive wear on the bit.
(2) dependent variables
The drilling variables which can not be directly controlled by the operator are grouped as dependent variables
(A)TORQUE. (B)FLUSHING MEDIAM PRESSURE.
(A)TORQUE
Torque can be definedd as the turning force applied to drill rod , which cause the bit to rotate against the resistance . the torque is the result of the force resisting the cutting and shearing action generated at the bit rock cotact b the rotation of the bit . it is usually measured in Nm or lb-ft. Pane 1966 discovered a linear relationship b/w torque and penetration rate upto a limiting value of torque where the curve flattend , showing an upper value of toque. Attained roalands 1971 statess that in shallow veticalsholes , torque increase with penetration and thrust.
(B)FLUSHING MEDIAM PRESSURE. The flushing medium pressure directly controls the drilling medium flow. During the rotation of the drill rod also cause bouyancy of flushing water which carries the cuttings . the rotations also suspension of the chips.
Increases in penetration rate either b thrust or rotational speed will result in increased weight of suspended rock particald and hence differential fluid pressure is used to overcome the frictional resistance and weight imbalance of suspended rock particals. Rowlands 1971 describes that as the penetration increases the flow area at the bit face is reduced and flush mediam is increased .
Formation characteristic: The elastic limit and ultimate strength of the formation are most important formation property affecting penetration rate. The shear strength preddicted by the mohr failure criteria some is use to characterize the strengthof the formation. Mohr has reported that crater volume produced beneath single tooth is inversily proportional to both compressive strenght of the rock and the shear strenth of the rock. The threshold force or bit weight required to initiate drilling was obtained by plotting drilling rate as afunction of bit weight per bit diameter and then extrapolating back to azero drilling rate. The permeability of formation also has significant effect on the penetration rate . in permeable rocks the drilling fluid filterates can move into the rock a head of the bit and equalize the pressure differential acting on the chips formed beneath each 100th. It also can be argued that the nature of the fluid contained in the pores spaces of the rock also affect this mechanism since more filterate vo;lume could be equilize the pressure in arock containing gas than arock containing liquid .
The mineral composotion of the rock also has some affect
On penetration rate . rocks containing hard abrassive minerals can cause rapid dulling of the bit teeth . rocks containing gumm clay minerals can cause the bit to ballup and drill in avery ieffient manner.
LOGGING
Another valueable method of analyzing the sub surface is b examination of the various logging records which are available: (1)driller log (2)cutting log (3)mud log (4)electrical log (5)radioactive log (6)microlog (7)later log (8)induction log
(1)driller log This is probly the oldest logging method and consists of the driller’s interpretation of the formations he has encountered durin his tour of duty . the valur of driller’s logs varies from almost worthlessto very valueable , depending upon experience and methods used b the drillers in preparing the log. Agood drillers log is prepared after carefully observing drilling rate , cuttings and the action of the cutting tools with the experience most drillers have been acumulated , a valueable log can be compiled if it is basedd on overall consideration of the factors.
(2)cutting log
The cuttings which have been circullated to the surface or recovered by bailing in cable tool operation provide a continuous record of the formationsencountered . the analysis of the cuttings from a rotary drilled well requires considerable experience , as a great percentage of cuttings recoverd at a given depth may have been retained in the circulating system from other depths or may have fallen off the from some point up thehole.
This not un usual for as much as 98 percent of the given depth not to be representative of the formation actually being cut , there is a log between the time the formation is cut and time the cutting reach the surface . this time log depends on the depth of the well , velocity of the mud flow, and fficiency of the cuttings removal.
(3)mud log Mud logging is a technique where by adrilling fluid is continuously analyzed for a small traces of hydrocarbons.
(4)electical log The electrical log is the one of the most widely used logging tools . it provides a continuous record of the formations encountered in borehole , the log is obtained by lowering intowell bore on an insulated cable tool which measures the spontaneus potential and resistivity of a subsurface formation . under favourable circumstances ,indicates the lithologic and fluid ontent of the formations an electrical log can be obtained in a cased well bore hole . (5)radioactivity log Radio activity log can be run either in cased or open holes . these llogs can be obtainrd by lowering atool -------- into the well bore on an insulated cable . the measure radio action intensity and the presence of the hydrogen in the formationswill, in most instances , provides some indications of the nature of the formations and formation fluids .
(6)microlog The microlog is the variation of the onventional electrical loging system , where the area of the investigation is quite small, normally one or two inches . it is used primarily to indicates permeable formations and is normally obtainedd in aconjuctioin with conventional electrical log. (7) later log The later log is aso an electrical resistivity measuring intrument. It is run on an isulated cable . and provides a continuous records of the formations . it can be employed to good advantage where low resistivity drilling mud , such as salt mud , are used this beds can be easily identifid with the later log. (8)induction log The induction log is an otherrefinement of basic electrical logging principle . it measures formations conductivity instead of resistivity . the scope of investigtion is narrow and for this reason adjacent beds have very little influence on the log . it is particularly adaptable to logging in holes drilled with oils and oil based drilling fluids or in empty holes , where there is no drillin fluids .
Drillers log. The hole truth A useful approach to logging Using the log form Uses of the driller’s log
The engineers , geologist or client relies heavil on the information provided by driller. The drill log reveals the full history of the drilling operations and variations in the formations comes to the notice of the driller while the drilling is in progress, the driller should note in the log and pass the informationon to the geologist . the information logged b the driller therefore should be accurate and truly representative of the hole or site conditions. The driller job is to provide all the information possible this information include the following: The strata penetrated .
The actual occurencess: the driller may see, hear, feel,smell or even sense that some thing has occurred or in occuring. The drillers interpretation : the driller because of his knowledge and experience , can interpret what occurs and draw a conclusion from the event . no other person is in a position to make interpretations with the same occuracy and confident the drillers log must record this information and conclusions.
The events or occurences to be covered b the drillers log fallinto three catagories :
(1)machine behaviour:including penetration rate, thrudt and torque levels, smoothness/vibration. (2)borehole behaviour: hole deviation ,wall stability, chip condition, shap and size. (3) formation nfluid behaviour: water level or mud level changes , permeability fluid gains or loses gas kicks.
CASING functions and requirements and of casing:
Casing is an essential part of drilling and completion of an oil well . it consists of lengthes of steel pipe either screwed or welded together to form a continuous tube to mbe desired depth . one or more of the following strings of casing is required in every well.
(1)conductor casing . (2)surface casing (3)intermediate or protective casing (one or more strings) (4)oil string casing The total length of a continuous tube used in a well is called a string of casing , and may or may not have the same dimensions throughout its length conductor casing is the largest diameter casing used in a well , and is required only where the surface soil ,are so in competent that the washing and eroding action of the drilling mud would create a large cavity at the surface . the conductor casing controls this eroding action.th next smaller diameter casing which is required in the surface. Casing its principle function is to protect fresh water sands and provide an anchoe for blow out preventer equipment . the amount of surface casing that may be required will depend primarily on depth of the the fresh water. Sands in the area , which may vary from onl a few feet to several thousand feet.
following the setting of adequate surface casing , one or more string of casing, one or more strings og casings will still be required ,l the no depending on the depth of the well and the problems found in drilling . if the well is unusually deep or severe drilling problems are encountered , such as abnormal pressures formations , heaving formations or lost circulation zones it may be necessary to set an intermediate strings of casing to seal off the long open hole or the zones casing trouble . after all necessary intermediate strings of casing have been set . the final string of casing required is the long string or soil string .this is the string of casing which is set immediatly above or through , producing formation .
if the oil string is set on top of the producing formation , an open hole completion results . if the bottoim of the casing is set below the producing horizone , it become necessry to perforate the casing to permit communication between inside of the casing and the producing formation . this result in a perforated casing completion . a schemic diagram of a typical caserd well is shown in fig: the oil string of casing is required to prevent --------------- and maintain a clean hole , seal off or separate oil, gas or water bearing zones, and seal off thief zone into which the formation fluid might migrate .
the term casing is generally applied to strings of pipe which extends from the surface , downward to the bottom of the particular string which is reffered to as its seting depth in contrast. The term linear is generally applied to strings of pipe which do not extend s to the surface . linear may serves as the oil strings through several hundreds feet in the bottom part of the well .thry have been used where drilling problems required . that the intended oil string of casing be set before the total depth of the well was reached and it was deemed un necessary to extend to the surface the smaller pipe , which was lator set . and cemented in the better part of the well.
There are also linear for special purose , such as the slotted linear perforated liner and gravel perforated liners which are set opposite producing zones for thr purpose of preventing sand from entering the well, liners are some times cemented in place ofr set with packers at their top or bottom or both some times merelly set on the bottom of the well.
Cementing operations
The cementing of casing in oil well is almost a universal practice and is done for a no of reasons. Depending on the particular casing being cemented . where conductor casing is required it must be cemented in order to preventing the drilling fluid from circulating outside the casing and thus causing the surface erosion which the casing was designed to prevent.
Surface casing must be cemented in order to seal off and protect fresh water formations provides a anchor for for blow out preventer equipment and give support at the surface for the deeper . strings of casing, intermediate strings of casing are cemented in order to seal off pressure formations which could cause excessive sloughing unless supported by casing and cemented and shut off zones off lost circulation in order to allow drilling to progress further, oil strings are cemented in order to prevent migration of fluids of thief zone and sloughing of formations which could cause a reduction in well productivit . cement also effectivily protects the casing from corrosive environments notably corrossive fluids which may exists in the sub surface formations.
With a few notable exceptions , portland cement is the principle constituent of most cementing materials . it is the ordinary cement which has been used by constuction industry for many years . however, with the sdvent of its use for cementing cxasing in oil wells . the aditional requirement of pumping ability at increased temperature and pressure necessitated some revisions in specifications . aditive have been developedd which change the specifications of portland cement to adapt them for use in oilwell cementing .
In order for an oil well cement to perform satisfactorily the task alloted it. Certain requirements must be met .
(1)the cement slurry must be capble of bieng placed in ht e desired position by means of pumping equipment at the surface.