Directional Drilling

Wells are drilled for the purpose of penetrating the rock at some definite subsurface location.In most instances this is accomplished by drilling a reasonably straight & reasonably vertical hole.

Three concepts for directional progress of a hole.A vertical hole is one that is maintained within few degrees, usually 30 to 50, of the vertical,and usually no attention is paid to the directions of the deviation. It may contain a number of sharp bends, which would cause problem in the subsequent drilling & production of the well.A straight hole is one that contains no sharp curvature, it is restricted to either vertical holes or holes which maintain a constant deviation from the vertical.A directionally drilled well is one which is deviated from the vertical & whose direction is controlled so that it follows a predetermined course.

DIRECTIONAL DRILLINGDirectional drilling is defined as an art and science involving deflection of a well bore in a specified direction in order to reach a predetermined object below the surface of the earth.

APPLICATIONS OF DIRECTIONAL DRILLING

1. Multiple wells from single location.2. Inaccessible locations.3. Drilling to avoid geological problems. Fault drilling. Salt dome drilling.4. Side tracking and straightening.5. Relief well drilling.6. ERD well.7. Horizontal well.8. Multilateral drilling.

NON PETROLEUM APPLICATIONS Mining industry.

Construction industry.

Geo thermal engineering.

MULTIPLE WELLS FROM SINGLE LOCATIONOptimum number of wells can be drilled from a single platform or artificial island. This greatly simplifies gathering systems and production techniques .

INACCESSIBLE LOCATIONSIf reservoir located under river beds, mountains, cities etc, this technique of directional drilling is used .

DRILLING TO AVOID GEOLOGICAL PROBLEMSA. FAULT DRILLING: This eliminates the hazard of drilling a vertical well through steeply inclined fault plane which could slip and shear the casing.

B. SALT DOME DRILLINGTo reach the producing formation which often lie underneath the over hanging cap of the dome, the well is first drilled at one side of the dome and is then deviated to producing zone to avoid drilling problems such as large washouts, lost circulation and corrosion .SOURCE: T.A.INGLIS

SIDE TRACKING AND STRAIGHTENINGIt is used as remedial operation either to side track obstruction by deviating the well bore away from obstruction by deviating the well bore back to vertical by straightening out crooked holes.

RELIEF WELL DRILLINGThe technique is applied to the drilling of relief wells so that mud may be pumped into the reservoir of the uncontrolled well.

HORIZONTAL WELLHorizontal well may be defined as an extension of highly deviated wells, drilled in order to situate a drain of considerable length at a precise depth in a reservoir with an angle of inclination approaching 90o from vertical. Advantages:Increasing the drainage area.Prevention of gas coning or water coning problems.Increased penetration of the producing formation.Increasing the efficiency of enhanced oil recovery ( EOR ).Improving productivity in fractured reservoirs by intersecting a numbers of vertical fractures.

ERD WELLIn these well departure of the well is twice or more of TVD. This separates it from conventional directional wells.Advantages:Increased horizontal displacement from central platform.Increased penetration length of reservoir.Require less number of wells to develop a field. Require less number of platforms to develop a field in offshore.

Multilateral Drilling Multilateral well has been defined as a well that has more than one horizontal or near horizontal laterals drilled from single site and connected back to a single well bore.Applications: .Greater reservoir exposure. Drain more than one reservoir. Exploit irregular reservoirs efficiently. Speed up reservoir drainage. Reduction in drilling cost per unit length of the well bore contacting, the reservoir rock. Ability to obtain a given length of horizontal well bore in reservoir where drag would perhaps limit the length of single horizontal well bore. Reduction in number of slots and thus the number of production platforms.

NON PETROLEUM APPLICATIONSA. MINING INDUSTRY Directional wells are used to produce methane gas that is contained in coal seams. - Methane presents a safety hazard and must be drained off before mining operations can begin. In deep coal seams that are beyond the reach of conventional mining techniques, directional wells can be drilled for in situ gasification projects.

B. CONSTRUCTION INDUSTRYA small diameter pilot hole is drilled in a smooth arc beneath the river until it immerges on the other side. This acts as a guide for the large diameter pipe forming the conduit.The hole is drilled through soft sediments about 40 below the river bed. This techniques has been used to cross rivers up to 200 wide.

GEO THERMAL ENGINEERINGHigh geothermal gradient found in some rocks( e.g. granite) can be harnessed to provide energy.Extracting the heat from this rocks requires the drilling of injection and production wells.

TYPES OF WELL PROFILES L- TYPES- TYPEJ- TYPE

TYPE I (BUILD AND HOLD OR L TYPE)

This is the most common and simplest profile for a directional well.The well is drilled down vertically to KOP, where the well is deviated to required inclination and further maintained to target.Shallow KOP is selected to reduce the inclination. This profile can be applied where large displacements are required at relatively shallow target depths.Under the normal condition inclination should be 15 to 55.

TYPE II (BUILD HOLD AND DROP OR S TYPE)

This profile is similar to type-I up to tangential section. After that profile enters in a drop of section where inclination is reduced and in some cases becomes vertical as it reaches the target. More torque and drag can be expected due to the additional bend.Used where target is deep but horizontal displacement is relatively small.It has also application when completing a well that intersect multiple producing zones.Drilling of relief well where it is necessary to run parallel to wild well.Lease or target limitations.

TYPE III (DEEP KICK OFF AND BUILD J TYPE)

Initial deflection is started well below the surface and angle is built up to bottom. It is used in particular situations like salt dome drilling, fault drilling and side- tracking or repositioning of target. Disadvantages: Formation may be harder & less responsive to deflection.More tripping time to change BHA while deflecting. BUR is more difficult to control.

HIGHLY DEVIATED AND HORIZONTAL WELLS

HIGHLY DEVIATED WELLSHIGHLY DEVIATED WELLS MAY BE DEFINED AS THOSE WELLS FOR WHICH ANGLE OF INCLINATION EXCEEDS 600 FOR MOST OF THEIR LENGTH.

EXTENDED REACH DRILLINGERD WELLS ARE THOSE HIGHLY DEVIATED WELLS FOR WHICH:MD / TVD2HD / TVD1 IN THE PRESENT WORLD WIDE SCENARIO WHEN HORIZONTAL DEPARTURE HAS CROSSED THE BARRIER OF 10 KMS. (10,728 M), MD 11,278 M AT TVD OF 1,637 M ONLY THE DEFINITIONS ARE REDEFINED AS BELOW:

REACH / TVD< 2CONVENTIONAL DIRECTIONAL DRILLING (NON ERD)REACH / TVD= 2 3EXTENDED REACH DRILLINGREACH / TVD> 3SEVERE EXTENDED REACH DRILLING

ADVANTAGES OF ERD WELLSINCREASED HORIZONTAL REACH FROM CENTRAL PLATFORM (LESS NO. OF PLATFORMS )INCREASED LENGTH OF THE COMPLETION ZONE THROUGH THE RESERVOIR. IN SOME RESERVOIRS SMALLER NO. OF ERD WELLS CAN DRAIN THE RESERVOIR MORE EFFICIENTLY THAN A GREATER NO. OF CONVENTIONAL WELLS.

LENGTH OF PENETRATION vs. HOLE ANGLE 80o 70o 60o

DRAINAGE AREA vs HOLE ANGLE200