07 horizontal drilling technology

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Horizontal drilling technology

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Horizontal wells are of great interest to the petroleum industry today because they

provide an attractive means for improving both production rate and recovery

efficiency. These are due to that horizontal wells provide a large-area of contact with

the reservoir than do vertical; wells and in addition, they provide a means for the

lateral transportation of fluid. Thus, the horizontal wells can be drilled as new wells or

horizontal sidetracks, drilled to revitalize the performance of exiting vertical wells that

are called drain holes The past few years have seen great improvement in drilling

technology. Development such as the use of bent, down hole drilling-motors, top-drive rigs and MWD (measurement while drilling) or advanced MWD called

geosteering technique together with steerable drill systems have greatly reduced cost.

Recent horizontal wells have cost no more per meter of well drilled than comparable

conventional wells. Thus, great advances have also been in methods for-drilling short-

radius drain holes from existing vertical wells. The construction and placing of

horizontal wells has become routine Usually it is no longer speculative as to whetherhorizontal wells can be drilled .In most cases now, the choice is not whether one can

drill horizontally but whether on should.

Great advances in the technology of drilling and locating horizontal wells continue to

be made. Today much attention is being paid to the problems of re-entering existing

vertical wells using smaller diameter, medium-radius and short-radius equipment.

These improvements will allow a much larger proportion of existing conventionalwells to have their live extended by re-completion with long, horizontal drain holes.

The provision of MWD tools that will operate in smaller diameter holes is a

particularly active area. There are developments, too, in logging tools. Tools are now

available that can be operated while drilling (LWD) to provide information about the

reservoir being encountered. Locating the logging sensors closer to the drill bit to

allow a more timely evaluation of the bit position and of the rock being penetrated is


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another area of active development.

Application of horizontal wells

Thin formationsWhen considering thin formation for hemogenous formation, it should avoid drilling a

horizontal well into this formation thicker than 22 ft maximum pay zone thickness

does not hold true for formation heterogeneous formation or formation with vertical

naturally fractured. Thus, the productivity index (PI) for a horizontal well reflects the

increases area of contact of the well with the reservoir. Typically, the PI for a horizontal

well may be increased by a factor of 4 compared to a vertical well penetrating the same

reservoir, although enhancement by a factor of 02 or more may be achievable in

certain circumstance.

Vertical naturally fractured formations

A horizontal well provide a means of communicating with natural vertical fractures,

e.g., and a high fractured limestone. If the well is oriented to interior these fractures

the productivity index can be substantially increased even when the fracture density is

low. Unfortunately, this can also accelerate the movement of gas or water to reach the

well bore.

Low permeability formations

One question is often asked by the operator that is how low should the permeability be

in a homogenous formation before considering drilling a horizontal well? Gieer

determined the amount of oil that could be produced under the same following

conditions for horizontal well and vertical well using the same homogeneous reservoir

for various reservoir permeability’s. That is the homogeneous formation with reservoir

permeability’s greater than 02 md should not be considered for a horizontal well

completion. For 022 md the same oil volume would be produced after about 22 days

from cither types of well? Even during the first 22 days, there is little difference in the

amount produced from a horizontal well versus a vertical well because formation is not


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homogeneous. However, the productivity, obtained by drilling a horizontal

well partially depends on the magnitude of the vertical permeability and

the length of the drain hole. Where the ratio of vertical permeability to horizontal

permeable is high a horizontal well may produce more cost effectively than a vertical.

Heterogeneous reservoir formations

A horizontal well may provide a number of advantages when reservoir heterogeneity

exists in the horizontal plane. A horizontal well bore reservoir provides potential for far

more information about the reservoir than would normally be available. As logging and

completion techniques become more sophisticated, this aspect of horizontal wells is

likely to be used advantage.

From a production viewpoint, a horizontal well in an irregular reservoir may provide a

means of accessing isolated productive zones, which might otherwise be missed.

Furthermore, in heterogeneous reservoirs, the influence of the heterogeneous along

the well bore is reduced by the composite flow geometry, so that production rates are

generally enhanced in these circumstances

Application in reservoirs with bottom water or with a gas cap

In many cases, the most important factor limiting the production of oil from a reservoir

is the tendency for water from an underlying aquifer, or gas from gas cap, to be drawn

vertically to the production well. Horizontal wells can have substantial advantages in

such reservoirs. The conventional way of reducing the effect of coning is to complete

the vertical well over a limited vertical distance to maximize the stand off from the

water or gas cap as the case may be. Because of its extended contact with the

reservoir, a horizontal well usually has less pressure draw down for a given production

rates than does a vertical well. This reduced draw down lessens the tendency for the

coning of water or gas with the produced oil. Thus, for example, horizontal wells may

be operated at the same rates as conventional wells but with less sometimes much

less-coning, i.e., with better water-oil ratios or gas-oil ratios or both in some case,

production without coning may be economic using horizontal wells.


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Where it would be prohibitively slow with conventional wells. In situations

where the initial rate for production without free gas coning would be

impractical with vertical wells, it may be possible with horizontal wells to achieve

economic production by gravity drainage with only a small rate of gas injection to

maintain gas cap pressure. Even if operation below the critical rate for coning is

impractical because of economics, there can still be a large advantage for horizontal

wells. This situation is common when viscous, conventional heavy oils are produced

from above a water layer. When the high oil viscosity and the low difference in density

between the oil and water make coning, or more correctly fingering, occur even at very

low production rates. In these cases, the volume of oil that is produced is

approximately proportional to the volume swept by the water finger. Horizontal wells

have an advantage over vertical wells here because they (really a crest shaped liked

the roof of a house along the length of the horizontal well) have a much larger volume

and this larger crest displaces a much larger volume of oil.

Advantages of horizontal wells in offshore application

Many horizontal wells have been drilled from offshore platform. Such well offer savings

in platform costs in addition to the advantages found onshore, For example, one

operator states that the cost of his North Sea platforms approximately $ 6 million per

well slot. Using horizontal wells, the same number of well slots on a platform can

produce since each horizontal well more productive than each conventional well.

Furthermore, since offshore wells are normally highly deviated in any case the extra

cost for horizontal drilling can be relatively small. Commercial offshore horizontal well

projects in various areas including the Adriatic, the North Sea and the Java Seas arc

described in the literature.

Heavy oil application

Probably the most prospective area for using horizontal wells lies in the field of heavy

oil recovery, particularly thermal recovery using stream- For example, the bitumen


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deposits is Canada, which are impossible to recover economically by

conventional methods, have a volume of oil in place approximately equal to

that of all the known conventional crude oil in the world. One approach used to recover

these resources is open pit mining. However, this is limited to the small fraction of the

Tabasco reservoir that is close to the surface and the approach involves handling vast

quantities of material. In situ thermal recovery is more generally applicable, cheaper

and less damaging environmentally.

Thermal recovery normally requires close well spacing. Typical projects have a spacing

of . to acres per well and, in many cases, these are latter in filled to improve

recovery. Stream flood projects in California with spacing are as low as /5-acre arc

being operated. In such circumstances, a single horizontal wells. This may become one

of the most important applications of horizontal wells. The successful operations of

field pilot in Cold Lake, the Lloyd Minster area and in Athabasca using horizontal wells

and streams-assisted gravity drainage (SAGD) are discussed in the literature. A

particularly important feature of the use of horizontal wells for stream recovery is that

it is possible to operate and obtain high recoveries with little stream production, I, e.

with little streams bypassing by cresting. With horizontal wells, it is possible to produce

economically below the critical rate for stream by pass; with vertical wells, stream

flooding is impractical without the bypass of stream except on very lose spacing.

Sand production

At higher draw down-pressure, sand production is a common problem especially the

production from unconsolidated and fire grained sand. Sand erodes and plugs the

equipment and restricting the flow rates. Screens and gravel placing limit sand entry

into the well bore and in some cases reduce productions rates, less pressure draw

down eliminates the need for screen and gravel placing and allows higher production

rates from drainhole or horizontal well.


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Types of horizontal wells and different drilling technique used :

The choice of drilling method depends upon drilling cost, well spacing and the

mechanical condition of a vertical well bore is existing. Also, the reservoir

considerations are also important in selecting the drilling method. During the last

decade, the incremental cost of drilling horizontal wells and drainhole. Over a vertical

well cost, has come down considerably. But today oil industry grains more experience

and use newer drilling technologies; in turn the cost of drilling horizontal wells may be

further reduced. Accordingly the practical horizontal drilling methods can be classified

into four categories depending upon the turning radius required to turn from a vertical

to a horizontal direction. Also, the use of a top-drive drilling system (TDS) is essential to

the successful implementation of a horizontal drilling program for: deeper wells; larger

well bores; outer-row wells (offshore platforms); Gumbo or tendonitis hole sections

when drilled with water base mud; where simultaneous high torque and tension is

required to be applied drill string. A discussion of various drilling methods is given

below for each type of horizontal well and drainhole.

Ultra – Short turning radius

Ultra-short turn-radius horizontal holes, sometimes called drainhole. In this method, it

utilized water jet to drill 022-22 ft long drainhole with a turning radius of 0 to ft.

these arc drilled in previously cased often multiple horizontal laterals are drilled from

the same well bore, It is reported that, sidetracking may be done with a whipstock

deviating tool with a curved guide. Also, a long slender steel tube fits inside the dill pipe

into the top of the whipstock guide. The upper end of the tube has a pressure seal to

contain pressure and divert drilling fluid through the tube. The jet nozzle Fits on the

lower end of the tube. Thus, drilling rate is controlled with a retaining cable connected

to the top of the tube.


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Short – Turning radius

Short-turn horizontal patterns have a turn radius of about 2 – 62 ft, for a

drilling from cased holes. The process begins by sidetracking, building angle, and drilling

the curved section with a special angle building assembly. Thus, for a re-entry drilling

system to be technically successful, it must be capable of drilling a consistent radius of

curvature and of drilling curve in the desired direction.

These requirement arise from the needs

0.To position the end of the curve within a precise depth interval so that the lateral can

traverse the pay zone as desired. To place the lateral in a direction dictated by well spacing, desired sweep patterns,

or other geological considerations.

. To establish a smooth curve to facilitate drilling the lateral and completing the well.

Several types of short-radius curve-drilling systems arc commercially available. The

most common types uses a mud motor to rotate a drill bit that is titled bit drills a

curved path, and the rotational orientation of the motor housing in the boreholedetermines the direction of the curve. Either a steering tool or a measurement-while

drilling (MWD) tool is required to keep the motor housing oriented during drilling. The

system may be used with conventional or work over rigs or with coiled-tubing units.

This is the most popular method of drilling a curved borehole, but it is often too

expensive to be economical for re-enters in mature fields.

Constrained-rotary systems arc second category of commercially available tools. They

have a flexible drive shaft inside an articulated non-rotating housing. Since originated

by Zublin in 0, this approach has been greatly refined. A resilient curve guide acts as

a spring that applies a side force to the bit and force the bit to drill a curved path. The

curve guide initially is oriented in the desired direction and then relics on well bore

friction to maintain orientation as it advances along the curve. Because of the

considerable hardware required and the associated operating procedures, use of

constrained-rotating systems has declined in favor of the more reliable mud-motor


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systems.

Medium – Turning radius

Although the long-radius approach to drilling horizontal wells is highly developed and

very successful, the radius required restricts the applications possible. Medium-radius

drilling extends the techniques so that build sections with a radius down to about 22

ft (2m) can be drilled. Holes drilled by medium-radius techniques have several

advantages compared to long-radius wells and almost no disadvantages. They can be

drilled with conventional drilling rigs, although they require some special, but now well-

developed, equipment. Medium-radius techniques use non-articulated drill strings and

bend mud motors. These are three principal requirements:

A-The bottom-hole assembly must be able to drill along a trajectory with the required

radius.

B-The drill string must be sufficiently flexible to follow the drill without mechanical

failure.

C-Tools used in the hole must be able to be moved around the curved parts of the hole.

It is important to note that the third requirement listed above limits the tools that can

be used in the horizontal part of the hole even through the curvature there is

negligible. These requirements place limitations on what can be achieved by medium-

radius drilling. The requirement for drill string flexibility means drill strings must be

smaller in diameter for higher curvatures. At the limit, it is necessary, e.g., articulated

strings, coiled tubing type strings stressed beyond the elastic limit, and strings made of

exotic higher-strength the materials such as titanium, carbon fiber or steel-reinforced

hoses. Charges of this sort move the technique beyond the boundary of normal

medium-radius drilling.

The trend to using smaller diameter drill pipe goes along with the drilling of smaller

diameter holes. Another driving force moving the technology towards small diameters

is the incentive to drill horizontal drains starting with existing vertical wells. To do this,


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medium-radius equipment that can be inserted down the existing vertical

well casing is needed.

Drill pipe for Medium Radius Drilling:

Dech, Hearn, Schuh and Ienhart described the first medium-radius drilling system in

056. It allowed holes 06 in. in diameter to be drilled to horizontal distance of 0222 ft

with a build rate of about 2: 2 m (022ft). The system used a narrow-diameter, special

compressive service drill pipe (CSDP) carrying larger-diameter wear knots. In holes with

a lower build rate than 0/2 m, heavy-wall drill pipe is used. This pipe has a wall

thickness, which makes it about two and a half times as heavy as standard drill pipe (e.

g. 6. Kg/m for 4. in. pipe compared to . Kg/m for standard pipe). In addition,

hevi -wate pipe has, in each 2 ft length; a central upset section, which behaves

similarly to the wear knots in CSDP. The wear knots keep the drill pipe away from the

wall of the hole in the curved section. This reduces both rotating and longitudinal

friction, resulting in less striking. It is also through the wear knots help keep the cuttings

in suspension in the drilling fluid.

Long-Radius Horizontal Well :

Long-turn radius horizontal well classifications are drilled mainly by deviation in open

holes. Wells in this classification are characterized by larger hole sizes and are very

susceptible to high drag and torque because of long open hole section. Hole size range

up 0. in diameter although smaller diameter holes are more common.

Wells with a radius of curvature of about 22 m (0222 ft) in the deviated sections are

commonly drilled using the techniques a steerable bent mud motor and MWD to locate

the hole as drilling proceeds. Also, there are significant advantages in using top-drive

rather than rotary table drilling rigs, and top-drive rigs are commonly used in offshore

applications. Few land rigs have had top-drive because of the cost installation,

however, new top-drive designed specifically for land operations are available.


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Over View


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Horizontal Well Profiles


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Design of a horizontal trajectory

Design by using simple method:

In accordance with the horizontal well drilling, there are three sections namely:

1 ) Vertical section:

It is drilled from seabed (mud line) until kick-off point (KOP).

2 ) Turning or curved or angle build section:

It is drilled from kick-off point (KOP) to the end-of-curve (EOC). This section includes

the first build arc, the straight tangent , and the second build arc.

3 ) Horizontal section:

It is drilled from the end of second build arc (EOC) to the end of proposed

distance to be drilled horizontally in the pay zone, in accordance with the type of

horizontal well to be drilled.


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07 horizontal drilling technology

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