Directional drilling

Directional drilling

      The most common applications of directional drilling are illustrated in and discussed briefly below

    - Multiple wells from artificial structures. Today's most common application of directional techniques is an offshore drilling where an optimum number of wells can be drilled from a single platform. This operation greatly simplifies production techniques and gathering systems, a governing factor in the economic feasibility of the offshore industry

   - Fault drilling. Another application is in fault control where the wellbore deflected across or parallel to the fault for better production. This eliminates the hazard of drilling a vertical well through a steeply inclined fault plane, which could slip and shear the casing

   - Inaccessible locations. The same basic techniques are applied when an inaccessible location in a producing zone dictales remote rig location, as in production located under riverbeds, mountains, cities, etc

   - Sidetracking and straightening. This is used as a remedial operation, either to sidetrack an obstruction by decimating the wellbore around and away from the obstruction, or to bring the wellbore back to vertical by straightening out cooked holes

  - Salt dome drilling. Directional drilling programs are also used to overcome the problems of salt dome drilling, to reach the producing formations, which often lie underneath the overhanging cap of the dome

   - Relief wells. Directional drilling, was first applied to this type of well so that mud and water could be pumped in to kill a wild and cratered well

Basic hole patterns

A carefully conceived directional drilling program on geological information, knowledge of mud and casing program, target etc., is used to select a hole Pattern suitable for the operation

   - Type I,  is planned so that the initial deflection is obtained at a shallow depth (approximately 1000 ft), and the angle is maintained as a "locked in," straight approach to the target. This pattern is mainly used for moderate drilling in areas where the producing formation is located in a single zone location and where no intermediate casing is required. It is also used to drill deeper wells requiring a larger internal displacement

   - Type II, called the "S" curve pattern, is also deflected neat the surface. The drift is maintained, as with type I, until most of the desired lateral displacement is obtained. The hole angle is then reduced and/or returned to vertical in order to reach the target

   - Type III, is planned such that the initial deflection is started well below the surface and the hole angle is maintained to buttonhole target. This pattern is particularly suited to special situations, such as fault or salt dome drilling, or to any situation requiring redrilling or repositioning of the bottom part of the hole

Deflection tools

      A prime requirement for directional drilling is suitable deflection tools, along with special bits and other auxiliary tools. A deflection tool is a mechanical device that is placed in the hole to ensure a drilling bit to be deviated from the present course of the hole. There are numerous deflection tools available for deflecting a hole or correcting direction. The selection of a deflection tool depends upon several factors, but principally upon the type of formation at the point where the hole deviation is to start. The most common tools used for deflection are

Jet biT

Down hole hydraulic motor with a "bent sub

Whip stocks

 Down hole hydraulic motors

          The down hole hydraulic motor with a bent sub is the most widely used deflection tool. It is driven by drilling mud flowing down the drill string to produce rotary power down hole, thus eliminating the need for rotating the drill pipe

         The first variation of the down hole motor is the "turbine" type motor, or "turbo drill." It consists of a multistage vane-type rotor and stator, a bearing section, a drive shaft and a bit rotating sub. The first stage is comprised of a rotor and a stator of identical profile. The stator is stationary and deflects the flow of drilling fluid to the rotor, which is locked to the drive shaft and thus transmits the rotary motion to turn the bit

           The second variation of a down hole motor is the "positive displacement" or "helicoids hydraulic" motor. It consists of two-stage helicoids motor, a dump valve, a connecting rod assembly, and bearing and shaft assembly. The helicoids motor has a rubber-lined spiral steel rotor. As the mud is pumped under pressure from above, it is forced downward between the rotor and the spiral cavity. The rotor is thus displaced and turned by the pressure of the fluid column, powering the drive shaft and resulting in a rotational force that is used to turn the bit

        The bent sub is used to import a constant deflection to the tool. Its upper thread is cut concentric to the axis of the sub body, and its lower thread is cut with an axis inclined 1 to 3 degrees in relation to the axis of the upper thread. In addition, the "hydraulic bent sub" can be locked into position for straight drilling, or unlocked and reset for directional drilling

         Both down hole motors can be used with the following assembly which consists of a full-range bit, the down hole motor, a bent sub or hydraulic bent sub, a non magnetic drill collar, and the normal drill string

Jet bits

         

Where subsurface formations are relatively soft, the hole can be deviated by using a jet bit. In this method, all but one of the jet openings in a conventional bit are closed off or substantially reduced in size. The jet left open has a very large nozzle. This nozzle is oriented in the proper direction on bottom and the pumps are started, but the drill string is not rotated. Instead, it is usually worked up and down slowly, approximately 10 feet off bottom. Then weight is applied on the drill string and bit while jetting, and the jetting action literally washes the formation out from under the jet. After the jetting has set a proper course, the drill string is rotated. Since the washed-out section is the path of least resistance, the bit will follows it. Extra weight is then applied to bow the collar, and the drilling continuous until the correct hole angle is attained

 Whipstocks

        The standard "removable" whipstock is used to initiate the deflection and direction of the well, sidetrack cement plugs, or straighten crooked holes. It consists of a long inverted steel wedge that is concave on one side to hold and guide a whipstock drilling assembly. It also has a chisel point at the bottom to prevent the tool from turning, and a heavy collar at the top to help withdraw the tool from the hole

          The "circulating" whipstock is run, set and drilled like the standard whipstock

However, in case, the drilling fluids flow through a passage to the bottom of the whipstock and circulate the cutting out of the hole, ensuring a clean seat for the tool. It is most efficient for washing out bridges and buttonhole fills

          The "permanent easing" whipstock is designed to remain permanently in the well

It is mainly used to bypass collapsed casting or junk in the hole, or to reenter and drill out old wells. After the bit has been drilled below the whipstock, increased weight is applied until approximately 20 feet of pilot hole has been drilled. The whipstock is then retrieved and the pilot hole opened to full range with a pilot bit and hole opener

 Knuckle joint

         Is one of deviation control methods, which employs the principle of universal joint. The owed half of the tool can be rotated at an angle to the centerline of the body. It is made up on the lower end of drill pipe

Methods of drilling wells

Methods of drilling wells

Drilling is the process of making holes in the earth crust. Many methods can be used for
drilling wells.
Drilling methods can be classified in accordance with various principles. Any methods of drilling
involves formation disintegration machine which can be used for drilling, disintegrate and
excavate rock by four basic mechanism:-
A - By mechanically induced stresses.
B - By thermally induced stresses.
C - By fusion and vaporization.
D - By chemical reactions.
From all the principles mentioned only mechanical drillings widely used for drilling oil and gas
wells. Drilling methods based on other mechanism of formation disintegration were tested in
laboratories and in the field but not used in the industry.
Industrial methods of mechanical drilling can be further subdivided according to the character
or rock design tools motion.
I. Drilling methods with reciprocating motion of the tool.
II. Drilling methods with rotary motions of the tool.
Mechanical methods of drilling with rotary motion of the working tool are the most
widely used methods in the oil and gas industry.
These methods can be classified in accordance with the position of a mover that drives the
tool:
1- Drilling methods with the mover on the earth surface.
2- Drilling methods with the mover situated near the bottom of the hole.
One of many possible classification of drilling methods is given below:
Now, two drilling methods are widely used:
1- Rotary drilling (about 85 % of total footage in the USA and other countries except the
USSR).
2- Turbo-drilling (about 85 % of the total footage in the USSR).
Cable-tool drilling still being used in the USA and about 10 % of the total footage is drilled
with this method. Drilling with electro drill is under wide field experiment since more than 10
years in the USSR.
Hence rotary drilling is the most important method of making bore holes in petroleum
industry and it is this method that should be mainly Considered in the course of Oil Well
Drilling Technology.

Drilling engineering history.

 Drilling engineering history.

Oil well drilling is one of the most important branches of petroleum industry. Drilling is
widely used in exploration for oil and natural gas in the early stage of a search for oil. When a
seismograph surveying method is used to discover geological structure favorable for oil and gas accumulation, it is necessary to drill shallow to make explorations.
There are many indirect methods of prospecting for oil and natural gas, but such methods
indicate that certain possibilities exist for oil gas accumulation. These methods cannot prove
presence of oil in favorable area. They give no possibility for estimation of deposit that is
supposed to be discovered.
Drilling a well is the only method to find oil or gas and to prove communicability
(profitability) of the deposit discovered.
After discovering oil of gas, it is necessary to extract them from the deep underground to
make them available for further processing and consumption. As oil is usually accumulated
rather deep strata the most economical method of extracting it is to drill bore holes that can
serve as conduits for oil from the oil trap to the surface.
Wells are drilled for not only extracting oil but also for the purpose of injecting water, gas ,
steam, into the oil bearing strata to maintain formation pressure ,to apply secondary recovery
methods etc..
Drilling boreholes is used not only in the petroleum industry. Wells are drilled for many other
purposes like water supply, ores and coals exploration etc. However, oil and gas well drilling is the most advanced out of all the drilling industries because it deals with the deepest wells and the most save underground conditions.
The “Colonel Drake well” drilled in the united states by Colonel Edwin L. Drake in 1859, is
considered by many to be the first commercial well drilled and completed.
Through 1956, the cumulative world crude oil production was 95 billion bbl, of which 55
billion had been produced in the USA.
In the late 1990, the USA is still the world’s largest oil consumer both terms of shear
volume (18.2 MMbbl/d).
The USA is also the largest petroleum importer (9.5 MM bbl/d, representing over
50% of consumption). Worldwide production is about 62 million bbl/d. the bulk of
petroleum reserves is clearly outside the industrialized world of north America and
western Europe (combined 57 billion bbl vs. 1.1 trillion bbl worldwide).
The majority of petroleum is found in the Middle East, where 600 billion bbl are
produced, 260 billion of which are from Saudi Arabia alone.
In the USA and Canada approximately 34,000 wells were drilled during 1995 and
1996, representing almost 60% of all wells drilled worldwide (about 58,000). Yet the
United States and Canada, combined, account for only 13% of the world’s petroleum
production

Origins of Gas Shows

Origins of Gas Shows
 

A gas show can be defined as a significant occurrence of hydrocarbons gases
detected from the mud stream and identifiable as being the result of the drilling of
specific interval of formation. This definition is apparently very simple and
readily understood. It is in fact ambiguous and if taken literally, can lead to
confusion.
The magnitude of a gas reading seen at the surface is not a true mark of its
significance. Nor is the fact that a gas reading can be identified as coming from a
volume of formation quantitative evidence of the volume of gas in place in that
formation or even liberated from it while drilling.
The object of good mudlogging is to plot those gas readings produced by gases
liberated from drilled formation in conjunction with the data relevant to their
interpretation. The object of mud log analysis is to reconstruct from these the
composition and mobility of reservoir hydrocarbons. In performing these tasks it
is necessary to appreciate the physicochemical processes active in the formation,
the mud circulation system, the gas sample extraction and analysis equipment.
The amount of gas, i.e., the number of “gas units” or “percentage of gas in air,”
detected by the gas detection equipment from a given formation is the result of a
complex of interacting variables. These factors begin to act before the formation
has been drilled and continue their effect until the gas sample enters the gas
detectors.
In order to reconstruct a picture of the fluids in place in a formation and the type
of fluid the formation may produce, it is necessary to study gas magnitude and
composition in the mud stream and cuttings, oil and water themselves, and
changes in the drilling process and circulation system which may affect or be 

affected by formation fluid behavior

Definitions
 

Prior to considering these complex factors, it is necessary to define the usage of
the term “gas show” as it is to be used in this manual. To do this it will be
necessary to define also the terms “background gas” and “circulation gas.”
Although apparently self-explanatory, these terms are commonly given multiple
uses in the industry.
 

Circulation Gas
 

This is the value of gas seen by a gas detector when circulating under normal
conditions, meaning a clean, balanced borehole with drill pipe in the hole and
rotating but with the bit off bottom and with no vertical movement. Under such
conditions some gas will be present in the sample drawn from the mud stream, but
it will represent only contamination or recycled hydrocarbons in the mud
 

Background Gas
 

When drilling through a consistent lithology, it is common for a consistent gas
value to be recorded. Certain lithologies (for example, overpressured shales) may
show considerable variation in the background gas. Background gas may be
observed to vary with drilling, mud or surface conditions without any change in
lithology or formation hydrocarbons. This should always be given consideration
in both formation and hydrocarbon evaluation
 

Gas Show
 

This any deviation in gas, amount or composition, from the established
background. This may or may not accompany a change in lithology, may or may
not indicate a significant or economic hydrocarbon accumulation. In other words,
“Gas Show” is a term describing an observed response on the gas detector, having
no causal or interpretive significance.
It is common for the surface data logger to be asked, “What is a good gas show?”
The answer to this is complex and relates to many factors beyond the simple
number of gas units seen. To decide whether a gas show is “good” or “poor”,
i.e., whether or not a significant hydrocarbon accumulation is indicated, requires a
total evaluation of all mudlogging parameters plus consideration of the many
variable system conditions.
The manner and extent to which shows will manifest themselves varies so greatly
among the many different regions that it is impossible to set even a general set of
requirements which must be met to qualify a show for additional evaluation or to

determine to what that evaluation should be. To a large extent, evaluation of
interesting zones becomes a matter of comparison of likes and differences.
Parameters of the log through the sections in question are compared with the same
parameters in adjacent known barren zones for perception of the degree to which
they changed or did not change. Generally speaking, the key to interpretation lies
not in the magnitude of the reading reached, but in the extent to which it did
change

  

Sources of Gas in Mud
 

Gas detected in the mud stream may originate from the formation via a number of
mechanisms. It is necessary for the surface data logger to isolate and attribute
these causes in order to draw the appropriate conclusions. Gas originating from
other sources or only indirectly from the formation will also be seen in the mud

 stream. This must, if possible, be recognized and removed from consideration

Gas from Drilling
 

This is often referred to as “liberated” gas since it is liberated from the crushed
formation produced by the drilling process. This should not be taken to imply that
the total volume of gas in place in the formation is liberated to the mud stream and
detected at the flowline. As discussed below, not all of the gas seen at the surface
will be present in the formation as free gas. Conversely, not all of the free gas in
place in the formation will be liberated at the surface. Some free gas will remain
trapped in the drill cuttings due to lack of permeability. Some gas, especially the
lighter hydrocarbons, will remain dissolved in the drilling fluid and not be
released at the surface. Thus, though it can be said that the gas liberated by drilling
reflects the composition and saturation of gas in place, no direct relationship can
be drawn
 

Free Gas and Liquefied Gas
 

Methane and ethane, the two lightest paraffins, have critical temperatures of
-82.5oC and 32.3oC, respectively. Since most petroleum reservoirs will have
temperatures in excess of this, the two will always exist in the formation only as
gases. All other hydrocarbons and water may exist in both liquid and gaseous
forms in equilibrium, dependent upon the temperature and pressure of the
formation.
Since the vapor pressures and critical temperatures of all hydrocarbons are
different, the composition of the free gas in the reservoir will differ from that of
the oil with which it is associated. Similarly, the composition and volume of the
gas detected at surface temperature and pressure will be different to that in place.

  

Dissolved Gas
 

Much of the gas detected at the surface is present in place in solution in oil and
water. This solubility depends upon the temperature and pressure of the reservoir.
Only when the oil and water are fully saturated with dissolved gas will a free gas
phase be present in the reservoir. Some gas will be liberated and detected from the
solution when carried to the surface. Other gases will remain in solution in oil and
water and be retained in the mud system.
The presence of gas as a liquefied, dissolved or free gas phase in the reservoir has
major influence on the relative permeability of the reservoir and its material
balance, i.e., the type of production and productivity. Similarly, due to the
differing solubilities of various hydrocarbons, the solution of gas in recovered oil
and water and mud filtrate will have an effect in reducing the amount of the gas
detected at the surface and in changing its composition

Gas Determination from the Drilling Mud

 Gas Determination from the Drilling Mud

As the drill bit breaks loose the formation, cuttings and gas in the formation are
transferred to and entrained in the drilling mud and transported to the surface.
With this in mind, the surface data logger hypothesizes the existence of a direct
relationship between the kind and amount of gas and/or oil in the drilling mud
arriving at the surface, and the gas and/or oil that was in place in the formation
being drilled at the time that particular mud was passing by the bit at the bottom of
the hole. It may be an over simplification at this point, but this represents the
situation in general and the description of the gas parameter.
The gases, if present, are assumed to be released by the formation and cuttings
into the mud stream to be entrained in solution in the mud. The concentrations or
amount of this entrainment normally encountered is on the low order of less
than 5%. This entrainment is also influenced by pressure, temperature, etc.

All that remains now is to convert this parameter to a meaningful representation of
the character of the formation before being disturbed by the bit. This conversion is
accomplished by three parts of the gas detector and related equipment. These parts
are:
1. The gas trap, which is the device for removing gases from the drilling mud.
2. The transporting equipment, consisting of a sample pump, pumping the
gas-air mixture to the gas detector, the hoses, the plumbing, and flow
regulation equipment.
3. The gas detectors proper (Total Hydrocarbon Analyzer and Gas
Chromatograph). These detectors are the Flame Ionization Detectors, see the
Sperry-Sun Drilling Services Gas Systems Manual for detailed information
on these detectors and flow conditioners

Aeration Gas Trap
The gas readings from the drilling mud as related to fluids and gases in-place in
the formation must be interpreted with the following consideration in mind:
The extraction of this gas from the drilling mud must be done in a manner that is
independent of variables such as density, viscosity and gel strength of the mud; in
a manner independent of the flow rate of the mud through the whole mud system;
in a manner so that all the gases as completely possible may be extracted even
from a high gel strength mud, and in a manner which would be considered reliable
around drilling rig conditions which tend to be destructive of sensitive equipment.
Sperry-Sun currently uses two types of gas traps. These are air powered and
electrically powered.
In operation, the bottom of the trap lies submerged about two inches under the
surface of the returning mud stream. The mud, tending to seek its own level, flows
in the inlet in the bottom of the trap canister. Rotation of the motor-driven impeller
blade causes this mud to whirled around rapidly. The centrifugal force of this
whirling action causes the level of the mud to be raised around its periphery inside
the canister until it flows out the discharge on the side of the trap.
The depth to which the trap is lowered into the mud should be adjusted to give a
continuous sample of 3 gallons per minute (1 quart in 5 seconds) of mud flowing
through the trap

Simultaneously, as the sample of mud is being pumped through the trap, the
whirling action of the impeller whips air from the atmosphere inside the canister
into the mud. These bubbles of air tend to become united with the tiny molecules
of gas entrained in the mud and being much larger, develop a size having a surface
tension sufficiently low to be released from bondage by the drilling mud. These
bubbles of air thus serve as a carrier by going into the mud, uniting with the gas 

 and carrying the gas out of the mud into the atmosphere of the canister

Hints and Precautions:
1. The trap should be located as near as possible to the discharge of the flowline,
or at least in a way so that it will have immediate access to the mud returning
to the surface.
2. The trap should be located in an open atmosphere. The trap discharge must
have immediate access to fresh air. This may not always be possible on some
rigs.
3. If difficulty is experienced with lost circulation material, removing the flange
on the trap bottom sometimes helps maintain a steady flow of mud.
4. Keep the locking screw on the adjustment jack tight after adjustments are
made.
5. Mud should not be discharging from the trap in an intermittent manner but
should exit in a continuous flowing manner

Transportation Equipment
After the mud has been sampled and the gases removed from it, these gases must
be transported to the gas detectors in the logging unit. This is accomplished by a
small motor-driven compressor which is connected to the trap by a length of
rubber hose. The compressor pulls a continuous stream of fresh air in through the
discharge of the trap. As the gases, if present, are being continuously extracted
from the mud in the trap, they will be continuously mixed with this stream of air
and carried with the air into the logging unit through the connecting hose. There,
the flow of air, or air-gas mixture, passes through additional flow regulation
equipment, plumbing and instruments and finally arrives at the detector element
for continuous detection and monitoring

In the logging unit, the flow is kept constant by the Total Hydrocarbon
Conditioner, and the flow rate is read on the flowmeter. The flow is split either
two or three ways, and the majority of the flow is discharged back to the
atmosphere.
The optimum flow of air from the trap is 6 to 8 cubic feet per hour (cfh). The total
flow meter is a small flowrater, calibrated to read directly in cfh of 0 to 10 cfh.
The flow is adjusted by an adjustment knob in the flowrater and should be kept at
6 to 8 cfh which will be maintained at this constant by the regulator. More volume
of the air-gas mixture than is necessary for detection is drawn from the trap in
order to minimize the lag time between the arrival of a show at the surface and its
detection.
It now becomes apparent that a reliable gas detecting technique demands that the
number of influencing variables be kept in control. Ideally, only the amount of gas
in the mud and its corresponding reading should be variable. For this reason, it is
important that the trap consistently pump a constant volume of mud, that the
amount of air drawn through the trap be maintained at a constant, that there be no
leaks or restrictions in the flow system

Gas Detection Equipment
See the Sperry-Sun Drilling Services, Surface Logging Systems, Gas Systems
Manual for the detailed operation of the Total Hydrocarbon Conditioner, Total
Hydrocarbon Analyzer, and Gas Chromatograph and the peripheral equipment
related to these instruments