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Thursday, June 30, 2011

3. Applications of Directional Drilling

What is Directional Drilling?
Directional drilling is the science of deviating a wellbore along a planned path to a target located at a given lateral distance and direction from vertical. This includes drilling as vertically as possible from a given TVD.
The figure below shows a vertical well and a deviated well.
As a starter one can consider that any well which gets deviated from the vertical axis to achieve the desired target (hydrocarbon reserve in our case) may be termed as deviated well (or Directional well). 
Vertical Versus Deviated (Directional) Wells
What are the applications of Directional Drilling?

1. SIDE TRACKING
Sidetracking is one of the primary uses for directional drilling. Sidetracking is an operation which deflects the borehole by starting a new hole at any point above the bottom of the old hole as in Figure below. 

The primary reason for sidetracking is to bypass a fish which has been lost in the hole; however, there are several other reasons for sidetracking. A sidetrack can be performed so the bottom of the hole can intersect a producing formation at a more favorable position such as up dip above the oil-water contact. A well can be sidetracked to alleviate problems associated with water or gas coning. A sidetrack can be performed in an old well to move the location of the bottom of the hole from a depleted portion of the reservoir to a portion that is productive, such as, across a fault or permeability barrier. 

Most often, a sidetrack is accomplished by setting a cement plug in the hole and dressing off the plug to a depth at which the sidetrack will commence. The sidetrack can be either "blind" or "oriented". In a blind sidetrack, the direction of the sidetrack is not specified and is not considered a directional well. In either case, a deflecting tool is used to drill out the old hole and start a new hole.
Sidetracking a stuck BHA


2. STRAIGHT HOLE DRILLING
Straight hole drilling is a special case of directional drilling where an attempt is made to keep the hole vertical. Some reasons for wanting to keep the hole vertical are:
a.To keep from crossing lease lines;
b.To stay within the specifications of a drilling contract;
c.To stay within the well spacing requirements in a developed field (Figure below).
Straight hole Drilling


3. CONTROLLED DIRECTIONAL DRILLING
Controlled directional drilling is used when drilling multiple wells from an artificial structure such as offshore platforms, drilling pads, or man made islands (Figure below). The economics of building one offshore platform for each well would be prohibitive in most cases. However, since wells can be directionally drilled, forty or more wells can be drilled from a single platform. Without controlled directional drilling, most offshore drilling would not be economical.

Multiple Wells from an Artificial Structure
4. DRILLING MULTIPLE SANDS WITH A SINGLE WELLBORE
There are special cases when multiple sands are drilled with a single wellbore. Where steeply dipping sand zones are sealed by an unconformity, fault, or salt dome overhang, a number of vertical wells would be required to produce each sand, which are separated by a permeability barrier. However, all the sand zones can be penetrated with one directionally drilled well thereby greatly reducing the cost of production (Figure below).

Drilling Multiple Sands from a Single wellbore

5. INACCESSIBLE LOCATIONS
There are times when oil deposits lie under inaccessible locations such as towns, rivers, shorelines, mountains, or even production facilities (Figure BELOW). When a location cannot be constructed directly above the producing formation, the wellbore can be horizontally displaced by directional drilling. This allows production of an otherwise inaccessible hydrocarbon deposit.

Inaccessible location

6. FAULT DRILLING
Directional drilling is also applicable in fault drilling (Figure BELOW). It is sometimes difficult to drill a vertical well in a steeply dipping, inclined fault plane. Often, the bit will deflect when passing through the fault plane, and sometimes the bit will follow the fault plane. To avoid the problem, the well can be drilled on the upthrown or downthrown side of the fault and deflected into the producing formation. The bit will cross the fault at enough of an angle where the direction of the bit cannot change to follow the fault.

Fault Drilling

7. DRILLING SALT DOME REGION
Many oil fields are associated with the intrusion of salt domes. Directional drilling has been used to tap some of the oil which has been trapped by the intrusion of the salt. Instead of drilling through the salt overhangs, the wells can be directionally drilled adjacent to the salt dome and into the underlying traps as shown in Figure BELOW. However, since the development of salt saturated and oil based muds, the amount of directional drilling has decreased. It is difficult to drill long intervals of salt with fresh water muds. Directionally drilling around the salt, alleviates a lot of the problems associated with drilling salt.
Salt Dome Drilling


8. RELIEF WELL
A highly specialized application for directional drilling is the relief well. If a well blows out and is no longer accessible from the surface, then a relief well is drilled to intersect the uncontrolled well near the bottom (Figure). Water or mud are then pumped through the relief well and into the uncontrolled well. Since it is sometimes required that the relief well intersect the uncontrolled well, the directional drilling has to be extremely precise and requires special tools. Survey data is not accurate enough to intersect a wellbore at depth. Proximity logging is required when drilling relief wells.
 
Relief Well Drilling

9. DRILLING HORIZONTAL WELLS
Horizontal drilling is another special application of directional drilling and is used to increase the productivity of various formations (Figure below). One of the first applications for horizontal drilling was in vertically fractured reservoirs. In fractured reservoirs, a significant quantity of the production comes from fractures. Unless a vertical well encounters a fracture system, production rates will be low.
Horizontal drilling is used to produce thin oil zones with water or gas coning problems. The horizontal well is optimally placed in the oil leg of the reservoir. The oil can then be produced at high rates with much less pressure drawdown because of the amount of formation exposed to the wellbore.
Horizontal wells are used to increase productivity from low permeability reservoirs by increasing the amount of formation exposed to the wellbore. Additionally, numerous hydraulic fractures can be placed along a single wellbore to increase production and reduce the number of vertical wells required to drain the reservoir.
Horizontal wells can be used to maximize production from reservoirs which are not being efficiently drained by vertical wells.

Horizontal Drilling

10. DRILLING MULTILATERAL WELLS
Directional drilling can also be used to drill multilateral wells. Multilaterals are additional wells drilled from a parent wellbore as illustrated in Figure 1-11. Multilaterals can be as simple as an open hole sidetrack or it can be more complicated with a junction that is cased and has pressure isolation and reentry capabilities. Multilaterals are used where production can be incrementally increased with less capital costs. Multilaterals can be used offshore where the number of slots are limited. It is also used to place additional horizontal wells in a reservoir.

Multilateral Wells Drilled From a platform

11. EXTENDED REACH DRILLING
Another application of directional drilling is what is commonly termed extended reach drilling. As illustrated in Figure below, extended reach drilling is where wells have high inclinations and large horizontal displacements for the true vertical depth drilled. Extended reach drilling is used to develop reservoirs with fewer platforms or smaller sections of a reservoir where an additional platform cannot be economically justified. Extended reach drilling will become more popular as the cost of platforms in deeper water and severe environments becomes more expensive.


Extended Reach Drilling
Advances in technology have allowed operators to drill extended reach wells with very high HD/TVD ratios (the ratio of the horizontal displacement to true vertical depth). Wells have been drilled with HD/TVD ratios in excess of 6/1. In these wells the horizontal departure was more than six times the true vertical depth with the total measured depth exceeding 32,800 feet (10,000 m).

Tuesday, June 28, 2011

2. Drilling Fundamentals (Part-2)

THIS BLOG IS DEDICATED TO MR. DP RAI, CHIEF ENGINEER (DRILLING), ONGC LTD., AHMEDABAD ASSET, INDIA
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Before answering the questions posted in the previous blog, I would like to share with you the objective behind writing this blog. While going through several books I found that the explanation given in the books are sometimes difficult to grasp or are of higher standard to the beginners. So my approach and the objective of writing this blog is to present the fundamentals of Directional Drilling in the simplest way possible. That's the only reason why I decided to start off with the fundamentals of conventional drilling.
So lets move ahead ..


Q1. What is Drilling?
In simple words, (In Oil & Gas industry) drilling can be defined as a process of making a hole below the earth surface using set of Drill Pipes (which are simply the steel pipes which are connected to one another to form a long string called Drill String) and Drilling bit. Here the function of drilling bit is to drill the formation so that we can move ahead in depth. The drilling operation is continued till we reach our target zone (which is the hydrocarbon reserve).
The drilling rig is a medium to support and hang the drill pipe and the mechanisms provided on the drilling rig helps to add another drillpipe to the present drill string, provide load on the bit so that we keep moving ahead in depth, helps rotating the drill string so that rotational force at bottom helps to drill the formation. There are several other functions which will be explained ahead.
The figure below presents a simplified view of the drilling operation.


 Q2. Will be answered at the end.
Q3. What is a Drilling Rig?
Q4. What are the various components of drilling rig?

A drilling rig is a structure used to create hole (usually called boreholes) in the ground.
They can be mobile equipment mounted on trucks, tracks or trailers, or more permanent land or marine-based structures. 

Classification of Drilling Rig:

A drilling rig has several components which together provides a driving force to drill ahead. Every component of the drilling rig has a specific function and equal importance.
A simplified figure of an onshore (land) drilling rig is shown below.


There are four major components of a drilling rig:
a. Hoisting System
b. Rotating Equipment
c. Circulating System
d. Power System

Now let us go through each of the above mentioned components in detail.

HOISTING SYSTEM
The hoisting system is used to raise and lower pipe in and out of the hole and to support the drill string to control the weight on the drill bit during drilling.
It consists of Crown Block, Derrick, Traveling Block, Drilling Line and Drawworks.  


The derrick is a steel tower that is used to support the traveling and crown blocks and the drill string. There may be no more identifiable symbol of the oil and gas industry than the derrick on a drilling rig. 


The crown and traveling blocks are a set of pulleys that raise and lower the drill string. The crown block is a stationary pulley located at the top of the derrick. The traveling block moves up and down and is used to raise and lower the drill string. These pulleys are connected via a large diameter steel cable called drilling line.


One end of the cable is connected to a winch or drawworks. The drawworks contains a large drum around which the drilling cable is wrapped. As the drum rotates one way or the other, the drilling cable spools on or off the drum and raises or lowers the drill string. 

Look at the figure below. It clearly shows how the steel cable passes through the Crown block and Traveling block and its one end goes to the drawwork.

Now let us have a look at traveling block closely .. ! The pulleys of the traveling block can be easily seen the the picture below.

  
This is how actually the traveling block looks like when the pulleys are covered using a box :

Figure below shows a Crown Block. It sits on the crown platform, which is a steel platform located along the upper portion of the rig. The crown block works in conjunction with a similar component, the traveling block, which is positioned just below the crown platform. Together, these two systems are known as the block and tackle.
Each crown block consists of a series of pulleys and steel cables, or sheaves. These cables and pulleys sit on a steel frame, which may be built into the structure of the derrick. The sheaves serve as drilling lines, and pass through the traveling block below to connect to the rig's hoisting drum.
While the crown block itself is fixed and used to stabilize the sheaves, the traveling block moves up and down with the cables. This traveling block contains internal pulleys, which are surrounded by steel to protect both the pulleys and the cables from corrosion. 

As said before, a key component of drilling rigs the drill line is made of multiple thread wounded wire ropes to be used for the lowering and raising of the drill string into and outside of the well bore. The drill line threads through the traveling and crown blocks.
If using a large size of line, it is possible to reach a million pounds of tension strength.


The function of a derrick is to provide the vertical clearance necessary to the raising and lowering of the drill string into and out of the hole during the drilling operations.
Derricks are rated according to their ability to withstand two types of loading:

1.      Compressive loads

2.     Wind loads
The allowable compressive load of a derrick is computed as the sum of the strengths of the four legs.  Derricks with load capacities from approximately 86,000 to 1,400,000 lb, depending on steel grade and leg size are available.

Allowable wind loads for API derricks are specified in two ways, with or without pipe setback.



With pipe setback, the wind may be blowing perpendicular to it, which is essentially a pipe wall.  This is the worst possible condition.



Wind loads are calculated by the formula:

        P = 0.004V2

where      P = wind load, lb/ft2

                V = wind velocity, mph
  

Calculation of Derrick Loads



The block and tackle arrangement for a rotary rig is shown in Figure 4.  Assuming that the system is frictionless, the following relationship are apparent:

where,    
Fd = total compressive load on the derick
n   = number of lines through the traveling block (those supporting W).
W = hook load 


The derrick load is always greater than the hook load by the the factor of (n+2)/2 due to the two additional lines (drawworks and anchor) exerting a downward pull.
 Thus, the derrick load is a function of 'n'. More the value of 'n', less is the derrick load.
                            
The drawworks or hoist is the key piece of equipment on a rotary rig.   
The functions of the drawworks are:
a.    It is the control center from which the driller operates the rig.   It contains the clutches, chains, sprockets, engine throttles and other controls which enable the rig power to be diverted to the particular operation at hand.
b.     It houses the drum which spools the drilling line during hoisting operations and allows feed-off during drilling.

where        
W = Hook load, lb

vh = hoisting velocity of travelling block, ft/min

33,000 = ft.lb/min per horsepower

e = Hook to drawworks efficiency


Hook to drawworks efficiencies are commonly between 80 to 90% depending on the  number of line in use.
 

The figure below summarizes the complete hoisting system.

ROTATING EQUIPMENTS
The rotating equipment turns the drilling bit. This equipment consists of the swivel, the kelly, the rotary table, the drill pipe, the drill collars, and the bit.

SWIVEL
The swivel allows the drill string to rotate while supporting the drill string's weight and providing a pressure-tight connection for the circulation of drilling fluid . The drilling fluid enters the swivel by way of the goose neck, a curved pipe connected to a high pressure hose. Connected to the swivel is a three-, four-, or six-sided 40 ft (12.2 m) length of hollow steel known as the kelly.

KELLY
The kelly is a square or hexagonal shaped section of pipe that is attached to the swivel. The kelly fits in a matching slot in the rotary table. As the rotary table turns the kelly is also turned. The movement of the kelly rotates the drill string and the drill bit. 

The figure below shows a hexagonal kelly placed in the kelly bushing.



ROTARY  TABLE
A rotary table is a mechanical device on a drilling rig that provides clockwise (as viewed from above) rotational force to the drill string to facilitate the process of drilling a borehole.
The kelly is inserted through the center of the rotary table and kelly bushings, and has free vertical (up & down) movement to allow downward force to be applied to the drill string, while the rotary table rotates it.

TO BE CONTINUED ...

                                                                

1. Drilling Fundamentals (Part -1)

1. What is Directional Drilling ?
Directional drilling is the science of deviating a wellbore along a planned path to a target located at a given lateral distance and direction from vertical. This includes drilling as vertically as possible from a given TVD.
The figure below shows a vertical well and a deviated well.
As a starter one can consider that any well which gets deviated from the vertical axis to achieve the desired target (hydrocarbon reserve in our case) may be termed as deviated well (or Directional well).




What are the concepts one should have a knowledge of before moving ahead to Directional Drilling study ?
As per my experience, to get a clear concept of Directional Drilling one should have a strong basic of conventional drilling methodology. Mentioned below are some of the basic question one should be able to answer before moving ahead to Directional Drilling Technology.
Q1. What is Drilling?
Q2. How do we drill a well to the desired target?
Q3. What is a drilling Rig?
Q4. What are the components of a drilling Rig?
Q5. What are drilling tubulars?
Q6. What is Bottom Hole Assembly?
Q7. What is a Target Zone?
Q8. What is Measered Depth (MD) and True vertical Depth (TVD) ?
Q9. What are Drilling Parameters?
Q10. What is Drilling Fluid?
Q11. What are the Drilling Fluid Parameters?
Q12. What is spud?
Q13.  What is tripping? Trip-in? Trip-out?
Q14. What is Short Trip and Wiper Trip?
Q15. What is Circulation and Mud Conditioning?
Q16. What is Casing?
Q17. What is Cementing?
Q18. What is wellhead and Wellhead fitting?
Q19. what is Cement Drill?
Q20. What is Leak-off Test?
Q21. What is Blow Out Preventor (BOP)?
Q22. What is Primary, Secondary and Tertiary Control?
Q23. What is Leak off Test?
Q24. What is Drilling Break?
Q25. What is kick?
Q26. What are downhole problems?
Q27. What is the difference between Casing and Liner?
Q28. What is drilling jar?
Q29. What is Geo Technical Order (GTO)?
 Q30. Terms used to write a Drilling Progress Report (DPR) and the other abbreviations used in Oil & Gas industry?

I guess if one is able to answer these basic questions or gains knowledge on these topics, he/she can switch over to Directional Drilling at ease.
In my next post I'll answer these question in the simplest possible way to make the learning more enjoyable !