Drilling Fluids Functions

–They  

 

1-Cool the drill bit and lubricate its teeth
2-Lubricate and cool drill string
3-Control formation the pressure
4-Carry cutting to the surface
5-Stabilize the well bore
6-Help in the well logs
7-limitting corrosion

1-Cool bit and lubricate its teeth

  

•Drilling action requires mechanical energy in form of weight on bit, rotation and hydraulic energy

•Large part of energy dissipated as heat

•A Heat must be removed to allow drilling

•Mud helps remove heat

•Mud helps remove cutting between the teeth and prevent bit balling

2-Cool and lubricate drill string

•Rotating drill string generates heat

•Mud help dissipate heat from hole

•Mud absorbs heat by convection and release it by radiation

•Mud lubricate drill string and reduce friction

 

 3-Control formation pressure

•For safe drilling, high formation pressures must be contained within the hole to 

prevent damage and injury

•This achieved by hydrostatic pressure of mud

•Mud pressure should be higher than formation pressure

•An overbalance of 100-200 psi is used

•Pressure overbalance id referred as chip hold down pressure

    (CHDP)

•Penetration rate decreases as CHDP increases

•For abnormal pressure CHDP becomes negative and kick will happen

 

4-Carry cutting to surface

 

•Cutting generated by the bit must be removed

•Drilling mud carries cutting form bottom to surface

•A Carrying capacity depend on annular velocity, plastic viscosity and yield 

point of mud and slip velocity of generated cutting For power law fluid

 

 5- Stabilize the wellbore  

•Good mud cake stabilize the hole

•Differential pressure between mud and formation keep hole stable

•Reduce drilling time also help keep hole stable

 

 6-Help in the evaluation 

 
and interpretation of the well logs

•During logging mud fill the hole

•Logs used to detect hydrocarbon, measure porosity, formation ,pressure

•Mud should posses properties that help evaluate these properties

•Water base -mud  is  better  for  electric  logging  

7-Limiting corrosion

•The drilling  fluid in most cases  will have water which contain dissolved salts

•This serves as a medium for corrosion

•Precautions should be taken  to prevent  damage  of the  equipment

•Mud containing oil as the continuous phase may decrease

   or  prevent corrosion  completely

 

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Perforation (6)(SAFETY PROCEDURES)

2.7. GENERAL SAFETY PROCEDURES

The following comments are applicable to both TCP and wireline conveyed methods.

Additional comments are given in section specific to wireline conveyed perforating.
 

a) All perforating operations, since they involve the handling and use of explosives and possibly radioactive materials, require special safety procedures to be strictly observed at all times.

b) Perforating operations should be carried out strictly according to the safety policies of Eni-Agip and the perforating Contractor. In the event of any inconsistency between policies, the most conservative policy will apply.

a) Operations involving the use of explosives shall only be performed by

Contractor’s specialized personnel responsible for perforation and similar operations. The number of persons involved shall be as low as possible.

b) Only perforating Contractor’s personnel are allowed to remain in the hazardous area (gangway, rig floor etc.) during arming of guns. The number of personnel should be limited when the guns are within 500ft of surface when tripping in and out of the hole.

c) Any operation involving the use of explosives is not allowed in the presence of thunder, lighting and thick fog, as these are sources of electric potential.

d) Explosives shall be kept on site for the shortest possible time, any remaining at the end of the operation shall be removed from the installation.

e) Explosives shall be stored on site in proper containers, within a confined area on the rig. Detonators shall be stored in separate boxes, in the same area as explosives.

f) Warning signs must be placed around the hazardous area where explosives are used.

g) All radio transmitters, radio beacons included, within a radius of 500ft from the well, shall be turned off, (since they may detonate blasting caps), starting from gun arming until perforating guns are 500ft below the sea bottom (similarly, when pulling guns out of hole and guns above 500 ft). All portable transmitters (both Eni-Agip’s and Contractors) shall be placed inside the Eni-Agip office and turned off to avoid accidental transmission. Avoid critical periods of perforating coinciding with arrival and take-off of helicopters.

h) Cranes and welding machines shall be put out of service starting from gun arming till gun pulling out and unloading.

i) District Office shall be advised by the Well Operations Supervisor on the estimated time of radio silence two hours before starting operations. The Radio Operator shall communicate actual timing.

j) Casing perforating can be performed during daylight or at night. However, the first series of shots must be carried out in daylight hours. Before perforating casing, the acceptable cement job quality shall be ascertained by means of CBL/VDL and/or by squeeze jobs.

k) Explosives are to be transported unarmed and clearly labeled to the site in secure and protective containers. Extreme care must be applied during loading and off-loading.

l) At the rig it is the responsibility of the Installation Manager to ensure that these precautions are taken.
 

2.7.1.Firing Systems for TCP Operations

It is normal practice to run the TCP guns with two firing systems, whenever possible, to improve the chance of a successful operation especially when running the guns on the bottom of a completion. A common combination is to use a tubing pressure actuated system as the primary means of detonating the TCP guns with a mechanical system as the back up. There are four main types of firing mechanisms for TCP guns. Only top down firing mechanisms should be used for safety when arming the guns. The operation of each firing mechanism is:

2.7.2. Tubing Pressure Activated

The guns are fired by pressuring up the test string and then bleeding off the pressure immediately. A time delay device is incorporated to allow time to bleed off. This device can be either hydraulic or a slow burning fuse. Some of the firing heads for this system are wireline retrievable which gives another back up option. However, this would preclude the

use of the drop bar system as a back up. Although this technique could be expensive on nitrogen, it is well suited to the use of a nitrogen cushion but the time delay on the system will have to be increased to allow time for the nitrogen cushion to be bled off.

2.7.3. Mechanical Impact

The TCP guns are detonated by the mechanical impact of a firing bar, which for safety must be run on wireline. (This system is colloquially known as the drop bar system.) Since the system can be affected by debris in the tubing, the completion fluid must be kept clean. The system is preferred as a back up instead of the primary firing mechanism because of the need to use wireline.

2.7.4.Electrically Activated

With this system, the guns are fired with an electrically-initiated detonator which must be run on a logging cable. Therefore the pressure control system must be rigged up. Since an inductive or wet electrical connection must be made at the firing head, the system is also susceptible to debris. This system is rarely used on well tests as the only is that the firing heads for this system are wireline retrievable, hence the guns can be run unarmed and, in the case of a misfire, the firing head can be recovered on wireline to determine the cause of the misfire.

2.7.5. Retrievable Slick line Firing Head

This type of head was primarily designed to overcome the concerns over about the possibility of guns being denoted by stray pressure or tools/debris/unnamed articles which could fall down through the tubing string and force the detonating pin into the initiator. With this type of head, this possible problem has been completely eliminated due to the design of the system. The guns are run in the hole without the firing head. Then, when ready to arm the guns, the head is run to depth on slick line and latched on to the firing stem or stinger. This system

provides its own back-up in that if the firing head does not work, it can be retrieved and a replacement run.

Retrievable firing heads are available with mechanical, hydraulic or electric triggering.

Safety

Working with explosives is one of the most dangerous professions. While working with explosives you must concentrate on what you are doing. You must perform each step carefully and correctly. Because when shortcuts are taken, when concentration is broken, when communication fails, when respect for explosives is ignored, when instructions in the book are ignored, accidents can happen and they do happen.

Safe operating practices are critical to the long-term success of perforating.

Any deviation from these procedures can put lives and properties in danger. If precautions are not taken, the danger of premature detonation may occur!

Oil and gas are our main sources of energy and in all probability will be for a long time. The oil and gas industry is involved in finding and exploiting underground deposits of oil and gas in addition maintenance of the equipment used to provide a passage for hydrocarbon to flow from reservoir to the surface is also critical.

Due to the nature of work involved, hazards typical to the oil and gas industry operations exist. Therefore, in the oil and gas industry work and safety must go hand –In -hand.

Safety measurement includes:

Properly designed, constructed and tested equipment

Well-trained, highly qualified personnel

All perforating crew members receive training on the characteristics of the explosives they use and proper techniques for handling and transporting these explosives .perforating engineers and technicians also need to be proficient in the specialized process of gun arming and disarming. They should thoroughly understand procedures and applicable local regulations. In addition, only the engineer or technicians is permitted to arm or disarm the perforating guns on a perforating job.

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perforation (5)(WIRELINE DEPTH CONTROL)

WIRELINE DEPTH CONTROL

Depth control for perforating is almost universally obtained through radioactivity instruments run in the cased hole in conjunction with the Casing Collar Locator (CCL). The Gamma Ray Log is generally used (Figure B28) though, in some cases, the Neutron Log or both Gamma Ray and Neutron are run. Accurate correlation of radioactivity logs with open hole logs establishes the position of casing collars with respect to the formation to be perforated. A short sub in the casing string is highly desirable to eliminate ambiguities with CCL identification, particularly when all joints of casing are about the same length. If the depth control log is made on a separate trip in the well, the proper shooting depth is determined on the perforating run by recording a second collar log with the collar locator attached to the perforator.

2.6.1.1. Gun-Gamma Ray Tool

If the combination Gun-Gamma Ray Tool is used, the entire equipment for depth control and perforating is run on a single trip in the well. The Gun-Gamma Ray Tool includes a rugged, shock proof gamma ray detector. A casing collar locator and a perforating gun can all run together. This offers greater assurance of accuracy and considerable saving of rig time. Depth control should always be used to accurately position TCP guns. A reference radioactive collar is run in the work string and its distance from the top shot is measured. Once on bottom, a through-tubing GR/CCL log is run and compared to open hole logs to establish how guns should be moved for exact positioning opposite the target formation. A variation of this procedure has been used from floating vessels in sand control completions. A sump packer is positioned and set with a wireline and becomes the locating device. The TCP gun string then is run with a locator and collet assembly on bottom. The distance from bottom gun shot to the collet latch is selected to place guns on depth. A radioactive collar should still be run to allow adjustment by logging in case of pipe tally discrepancies or slippage of the sump packer downhole.

2.6.2.2. Precision Identified Perforations

P.I.P. tags are used to provide a record of the position of perforations with respect to casing collars and/or formation boundaries. Special shaped charges fired at top and bottom of the perforated section leave traces of radioactive material within the perforations. The top and bottom perforations are then identified by sharp peaks on a Gamma Ray curve after perforating. Small size, low activity and short half-life of radioactive material used in the special charges prevent significant contamination of produced fluid. When run with Gun-Gamma Ray tool and Hollow Carrier perforators, no additional rig time is required other than that needed to log through the perforated interval.

2.6.2.  TCP DEPTH CONTROL

Four main techniques are used to ensure that the guns are at the correct perforating depth:

- Running a through-tubing gamma ray collar locator to locate a reference point in the string and tie into openhole logs.

- Setting the packer on wireline at a known depth, and stinging through the guns and completion string.

- Setting the packer and guns on wireline at a known depth, and stabbing the completion string in the packer.

- Tagging a fixed and accurate reference point such as a bridge plug. The first method is the most accurate. It relies on a radioactive marker sub in the string, and the distance from the radioactive marker sub to the top shot being precisely measured at surface. The string is run in the hole to approximately the correct depth and a short section of GRCCL (Gamma Ray-Casing Collar Locator) log is run over the zone where the sub is located. The gamma ray log indicates the position of the sub (a short radioactive peak anomaly) relative to the formation gamma ray as shown in Figure B30. As the distance from the sub to the top shot is known, the position of the guns can be calculated, and corrected if necessary by spacing out the string at surface. After the packer is set, the gamma ray may be rerun to ensure that the guns are at the correct depth. Fig. B30: TCP Depth Control Log.

As the cased hole gamma ray log can be considerably attenuated, a low logging speed will achieve better correlation results between the cased hole and the open hole gamma ray logs. If the formation gamma ray curve does not show much activity, a radioactive pip tag may be placed

As the cased hole gamma ray log can be considerably attenuated, a low logging speed will achieve better correlation results between the cased hole and the open hole gamma ray logs. If the formation gamma ray curve does not show much activity, a radioactive pip tag may be placed in or below one casing joint. (Placement of the pip tag must be included in the casing setting program.) Alternatively, a TDT or a neutron log can be run in place of the gamma ray log.

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Perforation (3) (Explosives Classifications)

Explosives Classifications

Explosives were invented first by the Chinese in the 10th century, then independently by the Arabs in the 13th century. The low explosive, or black powder, was characterized by slow reaction rates, 500 to 1500 m/sec, and relatively low combustion pressure. Much later, in 1846, the first high explosive was discovered by an Italian, Ascanio Sobreto, and then made commercially by Alfred Nobel in 1867 with the development of dynamite, a combination of nitroglycerin and clayey earth. High explosives, unlike the earlier low explosives, detonate at very rapid rates of 5000 to 9000 m/sec and generate tremendous combustion pressures. The terms low and high explosive are still used to characterize chemical explosives.

2.4.1.Low explosives (propellants)

 Are used in modern oilfield applications as power charges for pressure setting assemblies, bullet perforators and sample taker guns as well as for stimulation (high-energy gas fracturing, perf wash, etc.). High explosives are found in shaped charges, the detonating cord and detonators, and blasting caps.

2.4.2.High explosives 

 are further classified by their sensitivity or ease of detonation. 

2.4.2.1. Primary high explosives 
 are very sensitive and easily detonated by shock, friction or heat. For safety reasons, primary high explosives, such as lead azide, are used only in electrical or percussion detonators in Schlumberger gun systems.

2.4.2.2. Secondary high explosives  

are less sensitive and require a high-energy shock wave to initiate detonation (usually provided by a primary high explosive). Secondary high explosives are used in all other elements of the ballistic chain (detonating cord, boosters and shaped charges). PETN, RDX, HMX and HNS are secondary high explosives used in oilwell perforating. The rate of reaction, combustion pressure and sensitivity of chemical explosives are affected by temperature. Consequently, maximum safe operating temperatures are defined for all explosives. Exceeding temperature ratings may result in autodetonation or reduced performance. The table below lists the 1- and 100-hr temperature ratings and uses for the various explosives in gun systems.
 

2.4.3. Effect of temperature

Temperature affects the rate of reaction, combustion pressure and sensitivity of chemical explosives. Consequently, maximum safe operating temperatures are defined for all explosives. Exceeding the optimum temperature rating may result in autodetonation or reduced performance.

The table lists the 1-hr and 100-hrtemperature ratings and uses for the various explosives in schlumberger gun systems.

 

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perforation(2) (Detonators)(guns)

Detonators

Perforating guns carry explosive charges to the borehole where they are detonated creating cylindrical holes in the casing and the cement. This allows oil and gas to flow from the formation into the well.

  The critical parts of the perforating guns are:

·        Detonating cord

·        Detonators

Detonators are divided into two main types:

·        Electrical detonators

·        Percussion detonators

Electrical detonators are known as blasting caps and are typically used in wireline operations.

Percussion detonators are generally used with tubing conveyed, coil tubing conveyedd and non-electric wireline systems

2.3.4.3. The S.A.F.E system

There are numerous safeguards implemented in both the electrical and percussion detonation systems. However, these conventional systems may explode accidentally when exposed to electric magnetic fields or other voltages found around the field.

The S.A.F.E. Slapper-Actuated Firing Equipment system was developed to be immune from the potential differences created by the radio-frequency(RF) radiation, impressed current from  corrosion cathodic protection, electric welding, high-tension power lines and induction motors such as top drives on drilling rigs.

S.A.F.E. equipment is available for most types of perforating / explosive assemblies run on wireline .

2.3.4.4. Key components of the safe system

The S.A.F.E. system initiates a gun firing without the use of any primary detonating material.

The key components of the S.A.F.E. System are the EFI or exploding foil initiator and the ESIC or the electrical secondary explosive initiating cartridge. The ESIC generates a unique signature of high voltage and current and rapidly discharges the pulse. The pulse is necessary to fire the EFI.

2.3.4.5.Operation mechanism of the S.A.F.E SYSTEM 

Let's now look at the detailed operation mechanism of the S.A.F.E system. Here we see an internal view of the EFI with the components identified

2.3.4.6.Secure Detonator

The secure detonator is a third-generation on S.A.F.E type device that also uses an EFI. it does not contain primary high explosives or a down-hole electronic cartridge. A microcircuit performs the same function as the electronic cartridge and EFI together in a package. It is similar in size to a conventional electric detonator. The secure system has all the technical advantage of S.A.F.E detonator, but is more reliable and fully expandable. It is also smaller in size and therefore allows the gun strings to be shorter.

Both secure and safe system fire using high voltage and current. Their electronic circuits are protected and they don't fire accidentally in case of malfunction. 

2.3.5. Casing guns

Casing guns are a type of carriers. Traditionally, casing guns were run on wireline to perforate wells before completion is run. 

There are two types of casing guns:

·      Reusable-carrier Port Plug Gun (PPG)

·      Expandable High-Efficiency Gun System (HEGS)

Both types of casing guns are fully retrievable.

Casing guns are designed as systems comprising specific carriers, detonating cords and boosters to provide maximum perforator performance. To ensure the performance meets design specifications, charges are quality control-tested during production in actual gun carriers.

Loaded casing guns contain only secondary high explosives (detonating cords, boosters and charges), allowing safe transport and handling when safety procedures are being observed.

2.3.6. Parameters of gun selection

After deciding to use wireline casing guns in the completion, the selection of the most appropriate gun depends on several parameters. These are:

·        Casing internal diameter

·        Bottomhole temperature and pressure

·        Deep penetrator or big hole application

·        Required shot phasing and density

·        Perforator performance and value

2.3.7. High shot density guns

High Shot Density (HSD) guns are another type of carrier guns. They comprise of specific carriers, charges, detonating cords and boosters to provide maximum perforator performance. HSD guns provide increased shot available for natural, stimulated or sand control completions.

HSD are the most flexible guns in the field. They are expandable, retrievable carriers and can be run on any type of conveyance (tubing, completion, slick line, TCP, wireline, coil tubing, etc.)

HSD gun features:

·        Expandable carriers

·        Shot density

·        Helical shot pattern

·        Automatic ballistic connection

·        Firing modes

·        Mechanical connections

·        Exclusive use of secondary explosives

·        Quality assured

HSD perforating guns incorporate shaped charges, detonating cord and detonators. Standard operating procedures must be followed when loading or running these gun systems. Loaded guns should be enclosed in protective tubes during storage to protect the exposed explosives.

Three critical tests are performed on HSD perforating guns for reliability testing. These are:

·        Mechanical / Pressure / Temperature test

·        Perforating gun Split / Swill test

·        Drop Test

2.3.8. Through-tubing Guns

Through-tubing guns are used primarily for underbalanced initial or subsequent completions that have the tubing and bottomhole assembly already in place. Optimal underbalance can be applied to achieve clean, productive perforations while maintaining absolute well control.

The through-tubing guns are designed as systems comprising of specific carriers, charges, detonating cords and boosters to provide maximum perforating performance.

Loaded through-tubing guns contain only secondary high explosives (detonating cords, boosters and charges), allowing safe transport and handling when standard safety procedures are being observed.

Through-tubing guns include:

1. The hollow carrier guns

2. The exposed guns

Types of hollow carrier guns

They include Scallop gun systems .these guns are fully retrievable and are the most rigged through-tubing guns, capable of withstanding the highest temperatures and pressures .the hollow carrier guns can be run at very high speeds .enerjet gun are wireline conveyed ,capsule charge -type ,perforating guns in the enerjet gun systems each shaped charge is encapsuled and loaded on a strip carrier rather than being enclosed in a hollow tube carrier.

This permits larger charges for the same overall gun diameter .Also, due to the greater charge size; enerjet guns outperform hollow carrier guns of the same diameter.

enrjet guns are classified into two main types

1.with retrievable carriers

2.fully expendable carriers

The retrievable system is designed for rugged conveyance while running down-hole .it provides shot verification.any charge that does not retrived from the well along with the carrier strip .as the carrier is retrieved there is less debris in the well. Expendable systems are useful for applications where well components or conditions make the retrievable of the carrier strip difficult after the gun is shot.

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