Drilling Problems: Pipe Sticking

During drilling operations, a pipe is considered stuck if it cannot be freed and pulled out of the hole without damaging the pipe and without exceeding the drilling rig’s maximum allowed hook load. Differential pressure pipe sticking and mechanical pipe sticking are addressed in this section.

Differential-Pressure Pipe Sticking
Differential-pressure pipe sticking occurs when a portion of the drillstring becomes embedded in a mudcake (an impermeable film of fine solids) that forms on the wall of a permeable formation during drilling. If the mud pressure, pm , which acts on the outside wall of the pipe, is greater than the formation-fluid pressure, pff , which generally is the case (with the exception of underbalanced drilling), then the pipe is said to be differentially stuck (see Fig. below). The differential pressure acting on the portion of the drillpipe that is embedded in the mudcake can be expressed as

....................(10.1)

The pull force, Fp, required to free the stuck pipe is a function of the differential pressure, Δp; the coefficient of friction, f; and the area of contact, Ac, between the pipe and mudcake surfaces.

....................(10.2)

From Bourgoyne[1],

....................(10.3)
where

....................(10.4)

In this formula, Lep is the length of the permeable zone, Dop is the outside diameter of the pipe, Dh is the diameter of the hole, and hmcis the mudcake thickness. The dimensionless coefficient of friction, f, can vary from less than 0.04 for oil-based mud to as much as 0.35 for weighted water-based mud with no added lubricants.

Eqs. 10.2 and 10.3 show controllable parameters that will cause higher pipe-sticking force and the potential inability of freeing the stuck pipe. These parameters are unnecessarily high differential pressure, thick mudcake (high continuous fluid loss to formation), low-lubricity mudcake (high coefficient of friction), and excessive embedded pipe length in mudcake (delay of time in freeing operations).

Although hole and pipe diameters and hole angle play a role in the pipe-sticking force, they are uncontrollable variables once they are selected to meet well design objectives. However, the shape of drill collars, such as square, or the use of drill collars with spiral grooves and external-upset tool joints can minimize the sticking force.

Some of the indicators of differential-pressure-stuck pipe while drilling permeable zones or known depleted-pressure zones are an increase in torque and drag; an inability to reciprocate the drillstring and, in some cases, to rotate it; and uninterrupted drilling-fluid circulation. Differential-pressure pipe sticking can be prevented or its occurrence mitigated if some or all of the following precautions are taken:


-Maintain the lowest continuous fluid loss adhering to the project economic objectives.
-Maintain the lowest level of drilled solids in the mud system, or, if economical, remove all drilled solids.
-Use the lowest differential pressure with allowance for swab and surge pressures during tripping operations.
-Select a mud system that will yield smooth mudcake (low coefficient of friction).
-Maintain drillstring rotation at all times, if possible.

Differential-pressure-pipe-sticking problems may not be totally prevented. If sticking does occur, common field practices for freeing the stuck pipe include mud-hydrostatic-pressure reduction in the annulus, oil spotting around the stuck portion of the drillstring, and washing over the stuck pipe. Some of the methods used to reduce the hydrostatic pressure in the annulus include reducing mud weight by dilution, reducing mud weight by gasifying with nitrogen, and placing a packer in the hole above the stuck point.

Mechanical Pipe Sticking
The causes of mechanical pipe sticking are inadequate removal of drilled cuttings from the annulus; borehole instabilities, such as hole caving, sloughing, or collapse; plastic shale or salt sections squeezing (creeping); and key seating.

Drilled Cuttings. Excessive drilled-cuttings accumulation in the annular space caused by improper cleaning of the hole can cause mechanical pipe sticking, particularly in directional-well drilling. The settling of a large amount of suspended cuttings to the bottom when the pump is shut down or the downward sliding of a stationary-formed cuttings bed on the low side of a directional well can pack a bottomhole assembly (BHA), which causes pipe sticking. In directional-well drilling, a stationary cuttings bed may form on the low side of the borehole (see Fig. below). If this condition exists while tripping out, it is very likely that pipe sticking will occur. This is why it is a common field practice to circulate bottom up several times with the drill bit off bottom to flush out any cuttings bed that may be present before making a trip. Increases in torque/drag and sometimes in circulating drillpipe pressure are indications of large accumulations of cuttings in the annulus and of potential pipe-sticking problems.

Borehole Instability: This topic is addressed in Sec. 10.6; however, it is important to mention briefly the pipe-sticking issues associated with the borehole-instability problems. The most troublesome issue is that of drilling shale. Depending on mud composition and mud weight, shale can slough in or plastically flow inward, which causes mechanical pipe sticking. In all formation types, the use of a mud that is too low in weight can lead to the collapse of the hole, which can cause mechanical pipe sticking. Also, when drilling through salt that exhibits plastic behavior under overburden pressure, if mud weight is not high enough, the salt has the tendency of flowing inward, which causes mechanical pipe sticking. Indications of a potential pipe-sticking problem caused by borehole instability are a rise in circulating drillpipe pressure, an increase in torque, and, in some cases, no fluid return to surface. (Fig. below) illustrates pipe sticking caused by wellbore instability.

Key Seating: Key seating is a major cause of mechanical pipe sticking. The mechanics of key seating involve wearing a small hole (groove) into the side of a full-gauge hole. This groove is caused by the drillstring rotation with side force acting on it. (Fig. Below)illustrates pipe sticking caused by key seating. This condition is created either in doglegs or in undetected ledges near washouts. The lateral force that tends to push the pipe against the wall, which causes mechanical erosion and thus creates a key seat, is given by

....................(10.5)

where Fl is the lateral force, T is the tension in the drillstring just above the key-seat area, and ϴdl is the abrupt change in hole angle (commonly referred to as dogleg angle).

Generally, long bit runs can cause key seats; therefore, it is common practice to make wiper trips. Also, the use of stiffer BHAs tends to minimize severe dogleg occurrences. During tripping out of hole, a key-seat pipe-sticking problem is indicated when several stands of pipe have been pulled out, and then, all of a sudden, the pipe is stuck.

Freeing mechanically stuck pipe can be undertaken in a number of ways, depending on what caused the sticking. For example, if cuttings accumulation or hole sloughing is the suspected cause, then rotating and reciprocating the drillstring and increasing flow rate without exceeding the maximum allowed equivalent circulating density (ECD) is a possible remedy for freeing the pipe. If hole narrowing as a result of plastic shale is the cause, then an increase in mud weight may free the pipe. If hole narrowing as a result of salt is the cause, then circulating fresh water can free the pipe. If the pipe is stuck in a key-seat area, the most likely successful solution is backing off below the key seat and going back into the hole with an opener to drill out the key section. This will lead to a fishing operation to retrieve the fish. The decision on how long to continue attempting to free stuck pipe vs. back off, plug back, and then sidetrack is an economic issue that generally is addressed by the operating company.

To Be Continued in the second Part. Leave your Comments
Read More HERE

Read More

The Chemistry and Technology of Petroleum FOURTH EDITION

The Chemistry and Technology of Petroleum offers a 21st century perspective on the development of petroleum refining technologies.
The Chemistry and Technology of Petroleum traces the science of petroleum from its subterranean formation to the physicochemical properties and the production of numerous products and petrochemical intermediates.

Written by: James G. Speight

No. of Pages: 217

Presenting nearly 50 percent new material, The Chemistry and Technology of Petroleum emphasizes novel refining approaches that optimize efficiency and throughput.

Download link is below post in First comment

Don't forget to leave your comment

Read More

Perforations Part 1

Overview
Perforating is a critical part of any well completion process. The perforating process generates holes -perforation tunnels- in steel casing surrounding cement and the formation.

In the past, perforation was regarded simply as holes in steel casing made .By different methods. But perforation is not just a simple hole drilling process.Perforated completions play a crucial role in economic oil and gas production. Long term well productivity and efficient hydrocarbon recovery.

2.2. History of Perforation in Brief 

1. Prior to the early 1930's, casing could be perforated in place by mechanical perforators. These tools consisted of either a single blade or wheel-type knife which could be opened at the desired
level to cut vertical slots in the casing.
2. Bullet perforating equipment was developed in the early 1930's and has been in continuous and widespread use since that time.
-The major drawbacks with this method were that the bullet remained in the perforation tunnel, penetration was not very good, and some casings could not be perforated effectively.
3. After World War II the Monroe, or shaped – charge, principle was adapted to oil well work, and the resulting practice is now commonly referred to as jet perforating.
-The principle of the shaped charge was developed during World War II fo armor piercing shells used in bazookas to destroy tanks. This new technology allowed the oil producers to have some control over the perforating design (penetration and entry hole size) to optimize productivity. 2.3. Gun systems
2.3.1. Overview

In order to allow oil and gas to flow into the well, conduits need to be made into the formation. To do this, a gun is positioned across the producing formation and is detonated to create perforations through the casing and cement.
The guns used for this purpose are known as perforating guns.
2.3.2. Perforating guns are divided into two primary categories:
- Capsule guns
- Carrier guns
2.3.3. The perforating gun performance is affected by the
- Gun size
- Clearance
- Entrance hole diameter
- Shot density
- Gun phasing
- Perforating length
- Temperature rating

After firing the gun and while retrieving, unwanted solids enter into the wellbore or formation through perforating tunnels. These are called the perforating debris. Perforating debris can create problems in highly deviated or horizontal wellbores and can also create problems with the completion hardware.
Sources of debris are not only gun system, but also from the casing, cement and formation.
Gun hardware contributing to debris are:
- Gun body
- Shaped charge liner slug and jet
- Shaped charge cas
- Shaped charge retaining system (that holds the charge inside the gun).
2.3.4.1. Shaped charge liner
Perforating debris sources can be controlled if properly engineered.

Shape charge liner used in deep penetrating charges is made of powder metal, which eliminates the carrot and slug associated with liner penetration into the formation during charge detonation. Big hole charges us solid liners in order to produce large hole into the casing. However pf4621 power flow liners, produce big holes and yet leave no slugs into perforating tunnels, this new technology charge can replace the ultrapack charges. Attempts are made to contain the debris in the gun,
collect it after perforating or minimize the quantity expelled.
To address this problem of controlling the debris,
two methods are used. These are:
- Zinc casing method
- Patented packing method
Additional techniques that contribute to reduced perforating debris include powder metal liners and non-plastic charge retention systems. These recent innovations help in limiting problems arising from perforation debris.
2.3.4.1.1. SHAPED CHARGE THEORY
The ultimate goal of perforating is to provide adequate productivity. Test laboratories evolved over the years to provide means of predicting and improving well performance. Today, the performance of the charges is determined according to the procedures outlined in the API RP 43 (standard procedure for evaluation of well perforators) fifth edition, published in 1991. From Figure B1 it can be seen that the penetrating power of a cylinder of explosive is greatly increased by a cavity at
the end opposite to the detonator. Furthermore, placing a thin metallic liner in the cavity increases penetration. A typical shaped charge consists of four main components: a case, a high order explosive powder, primer and a liner, as shown in Figure shown

The case simply holds all the components together.

- The explosive (RDX, HMX and HNS) is a complex mixture designed to allow packing and shipping in the case.
- The primer is a purer mixture of explosive which is more sensitive to the detonation of the detonating cord.
- The liner is used to form a jet which physically does the perforating.
- The detonating cord, which is initiated by a blasting cap, detonates each charge.
The selection of explosive material is based on the well temperature and anticipated exposure time at that temperature (Figure B3). RDX, HMX and HNS are all explosives used in oil well shaped charge manufacture. For deep penetrating charges, the liner is made from a mixture of powdered metals pressed into the shape of a cone. High precision inthe pressing operation is required and it must be done in an extremely uniform and predictable manner. For Big Hole charges, the liner is
drawn from a solid sheet of metal into hemispherical, parabolic, or more complex shapes.
For each of the two types of charges, there is a trade-off between entrance hole size and penetration. The sequence of events in firing is illustrated in Figure B4 from top to bottom.
The detonator initiates the cord which detonates at a rate of approximately 7000 m/s (23,000 ft/sec.) The pressure impulse from this detonation initiates the primer in the charge and the
explosive begins to detonate along the length of the charge.
The high pressure wave 30x106 kPa, 4,500,000 psi) strikes the liner and propels it inward. The liner collapses from apex to skirt imparting momentum with a velocity approaching
2500 m/s (8000 ft/sec). At the point of impact on the axis the pressure increases to approximately 50x106 kPa (7,000,000 psi) and from this high pressure region, a small amount of material is
propelled out at velocities in excess of 7000 m/sec (23,000 ft/sec). As the liner collapses further down the cone, more and more material must be propelled by less and less explosive such that the
impact pressure is substantially less. Thus the tip of this so-called jet is travelling 20 times faster than the rear portion and gives the elongated shape to the jet. The penetration depth depends on this stretching action. As the liner walls collapse inward, the resultant collision along the axis divides the flow into two parts, as in Figure B5. The inner surface of the liner material forms
the penetrating jet which is squirted out along the charge. The outer surface of the liner, which was in contact with the explosive, forms a rear jet or slug which moves forward slower than the forward jet. In the zone of collision, where division of the material forming the jet and slug takes place, there is a neutral point which moves along the axis as the liner collapse process continues. The very fast jet impacting a casing generates a pressure of approximately 70x106 kPa (10,000,000 psi). At this pressure the steel casing flows plastically and the entrance hole is formed. A similar behavior occurs with formation material as the jet penetrates. In addition, crushing and compacting of the formation material around the perforation may also occur. The entire process from detonation to perforation completion takes from 100 to 300 microseconds. The jet material arriving last at the target, making the end of the perforation, comes from the skirt or base. As discrete portions of the jet strike at this end of the hole, they penetrate, expending their energy in the process.
Portions of the jet continue the penetration process, until the entire jet is expended.
The perforation occurs so fast that, essentially, no heat is transferred. Indeed, it has been demonstrated that a stack of telephone directories can be penetrated without singeing a single page. It follows that no fusing of formation material occurs during penetration. However, crushing and
compacting of formation material is to be expected, and will be reviewed later.

2.3.4.1.2. SHAPED CHARGE DESIGN

Liner aspects, such as geometry, angle, material, and distance from base to apex, as well as stand off, and explosive density are more important than the amount of explosive (Figure B7a). Only about 20% of the available explosive energy goes into the useful jet. It has been proven that properly designed charges can out perform poorly designed charges that have twice the explosive load.

This is important in situations where a higher explosive load causes casing damage. Once a charge is designed for entrance hole and penetration efficiency, manufacturing quality control and
consistency become significant in shaped charge performance. Perforation efficiency is accomplished with maximum penetration, uniform crushed zone, and minimal plugging due to slug debris. This is achieved by designing a liner that will provide a uniform jet diameter and velocity with little to no deviation from the conical liner axis. For example, it is critical that the liner thickness and density be precise around the cone at any given point away from the apex. Figure 7b is an example of a less desirable jet due to poor quality control.

Related Links

Read More