Instructor Guide Trinidad Drilling Drillers Program Section 12 1 Objectives Objectives for Mud Systems Upon completion of this section of the course the student will have instruction in: 1. History of Mud 2. Why do we use it 3. Basic types of drilling mud a. Fresh water b. Salt water c. Oil Base d. Air/air mist / stable foam 4. Mud check sheet a. Mud weight b. Viscosity c. Plastic Viscosity d. Yield Point e. Gel Strength f. Filtrate g. Filer Cake h. Retort solids i. Sand Content j. Methylene Blue k. PH l. Mud Alkalinity m. Chloride Measurement n. Total Hardness o. Lost Circulation 1. Preventive Measures p. Shale Problems
Transcript
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
1
Objectives
Objectives for Mud Systems
Upon completion of this section of the course the student will have instruction in:
1. History of Mud 2. Why do we use it 3. Basic types of drilling mud
a. Fresh water b. Salt water c. Oil Base d. Air/air mist / stable foam
4. Mud check sheet a. Mud weight b. Viscosity c. Plastic Viscosity d. Yield Point e. Gel Strength f. Filtrate g. Filer Cake h. Retort solids i. Sand Content j. Methylene Blue k. PH l. Mud Alkalinity m. Chloride Measurement n. Total Hardness o. Lost Circulation
1. Preventive Measures p. Shale Problems
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
2
Introduction
Mud systems The birth of modern rotary drilling was born in the Spindle top area in 1901. Before that time the industry standard was to pump fresh water down hole to bring the cuttings to the surface. In the Spindle top area they encountered down-hole problems. The quick sand would immediately fall around the pipe when they would make a connection. They solved the problem by driving a herd of cattle through the slush pit, thereby creating a thick soup of their drilling water. With that successful action drilling mud was born.
hole and carry them to the surface Wall the hole with an impermeable mud cake Support the wellbore Control subsurface pressures Hold the cuttings in suspension when
circulation is stopped Cool and lubricate the bit and drill string Transmit hydraulic horsepower to the bit using
jet nozzles Transmit hydraulic horsepower to the mud
motor Support part of the weight of the drill pipe and
BASIC TYPES OF DRILLING FLUIDS Fresh water systems Salt water systems Oil based and invert emulsion systems Air/ Air mist/ stable foam circulating systems
Each system has its disadvantages and advantages depending on which area you are drilling. Although oil base and invert is widely used in some areas. By far the most widely use is fresh water and salt water. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
5
Mud properties
Mud check sheet The daily mud engineer’s report is a vital part of the morning report. The information recorded by the mud engineer and the information that the derrick hand inputs on the tour sheet supplies the operating company and the drilling contractor with vital information. But what do the numbers mean? Included in this section is a daily mud engineer’s report. The first four lines indicate where and when the sample was taken, flow-line temperature and depth. All the other items on the sheet are measurements or calculations from the sample. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Mud weight Mud weight is measured with a mud balance scale. The mud balance scale should be calibrated using distilled water and should balance at 8.33 ppg (pounds per gallon). The rig’s mud balance scale should be checked weekly against the mud engineer’s to ensure both are reading the same. Mud weight is the most important property, and you are constantly doing a balancing act between to high of mud weight and to low of mud weight. If it is to low it can cause oil and gas, or water intrusion and possible a well control issue. It also can cause hole instability, tight connections, bridges or hole enlargements. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
8
Mud Weight cont.
The ideal mud weight is as close to formation pressure as possible. Clear fresh water allows the fastest drilling possible of any fluids today. It’s the fastest because it is the lightest. Clear water weighs 8.33 ppg. Anything that weighs more will slow the penetration rate down Mud weight is increased by adding solids to the circulating system. Solids are from two sources. One is addition at the surface the other comes from rock formations that you drill. These solids can be classified as reactive and nonreactive solids. Examples of reactive solids added at the surface to purposely improve mud properties are: Bentonite Gel Fluid loss agents Dispersants Polymers
Reactive solids that can enter the mud by drilling are: Anhydrite Hydratable shales Salt
Building mud weight by drilling solids is bad. Build mud weight out of added barite. Inert solids are those that are chemically inactive. These solids are generally classified as low gravity solids (specific gravity less than 3) and high gravity solids ( specific gravity over 4). The main solid added at the surface is barite, and has little effect on mud properties. It is a high gravity solid and has a specific gravity of 4.2. Drilled inert solids consist of sandstone, limestone, chert and other nonreactive rock chips. Drilled solids are almost always low gravity solids. Unlike barite drilled solids have a adverse effect on mud properties, down hole conditions and surface equipment. Because of this controlling or eliminating drilling solids has a top priority in any mud system. Removing solids is done mechanically in a properly designed mud system, with sand traps, shale shakers, desilters, desanders, centrifuges and by regular cleaning of pits or tanks. Solids can also be controlled by adding flocculants, dispersants, and thinners. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
10
Viscosity Funnel viscosity is measured with a marsh funnel. The calibration of a marsh funnel is done with fresh water at 70 degrees. It should take 26 seconds for 1 quart of water to flow out. You should check the funnel at least every month and should be check against the mud engineer’s funnel to ensure both have the same rate of flow. Viscosity and gel strength increase during drilling operations. Penetration of the bit into the formation adds inert solids, active solids and contaminates to the mud system. This can cause increases in viscosity and gel strength to unacceptable levels. In general water and or chemicals are added to control them. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
11
Viscosity
If viscosity increases are not controlled, many drilling problems can occur: Use of excess pressure to break circulation Higher pump pressure to circulate the well Lost circulation from pressure surges and high
circulation pressure Entrapment to gas in the mud then recirculated
down the hole Contributes to swabbing of the well during trips
and connections Sand and cuttings being carried in the mud
causing abrasive wear on equipment Decrease in penetration rate
Plastic viscosity Plastic viscosity is a more effect way to check the flow properties in drilling mud. However its main disability is that it has to be measured with a Fann-vg meter carried by the mud engineer. However the reading of a V G meter can determine the cause of variations of viscosity and a corrective treatment can be applied. Plastic viscosity is that part of flow resistance caused by mechanical friction. The friction occurs between the solids and the liquid that surrounds them in the mud. For field purposes plastic viscosity depends on the concentration, size and shape of the solids. This rise in plastic viscosity is cause by solids being ground up into finer and finer pieces as the mud is re-circulated. This dehydrates the mud. An increasing plastic viscosity, with little or no increase in yield point can be controlled by dumping tanks and dilution with water or running of solids control equipment. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
13
Yield Point
Yield Point Yield Point is also obtained with a V-G meter. It is an indicator of electrochemical attraction of particles to each other. These forces are the results of negative and positive charges located on mud particles surfaces. High Yield points always results in high viscosity. Yield point is also an indicator of the ability of the mud to clean the hole of cuttings while circulating. In a low solids mud system a relatively high yield point if desirable. This is accomplished by adding polymer to the mud system to achieve higher yield point using lower concentrations. A general rule is for yield point to be equal to the mud weight pounds per gallons. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
14
Yield Point
An increasing yield point, with very little increase of Plastic viscosity may be reduce or controlled by chemical treatment only. This can be done with additives like thinners and dispersants. It is not economical to reduce yield point by adding water. For instance, it’s not practical to reduce contaminated cement. But you can control it with sodium bicarbonate. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
15
Gel strength
Gel Strength Gel strength is contained with a V-G meter. Gel strength differs from yield point. Yield point is an indicator of the ability of the mud to clean the hole with the pump on; gel strength is an indicator of the ability of the mud to suspend solids with the pump off. As solids and yield point measurements increase or decrease gel strength usually increases or decreases in a similar manner. There are two types of gel strengths, fragile and progressive. The most desirable one is fragile. A progressive gel maybe described as low but increases in time. This type of gel strength is strong and firm and hard to break. The progressive gel often occurs because of high concentration of mud solids and can result in a great number of drilling problems. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
16
Gel Strength
At the surface high gels will allow sand and cuttings to remain in the mud, and your solids control equipment will not take it out. These particles are then recirculated, damaging pumps, bits, and mud motors by abrasions. Down hole excessive pump pressures are required to break circulation, possibly breaking down formation and causing lost circulation. Hole swabbing excessive surge pressure can also result from progressive gels. If it is not controlled progressive gels can become so bad that the entire mud system can lock up enough to stick the drill string. Build mud weight and other necessary properties with additives, not containments. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
17
Gel Strength
Fragile gels occur when the initial strength increases slightly with time. This type of gel strength is easily broken resulting in fewer drilling problems. These gels also provide adequate solids suspension cuttings in the bore-hole then allowing proper particle settlings in the mud pits. Gel strength in mud is very important and should be controlled at the lowest practical value. The only rule-of-thumb for gel strengths is to keep them from becoming excessively high or progressive. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
18
Filtrate
Filtrate Filtrate is another very important measurement of chemical and physical properties of drilling mud. A good water loss value and deposition of thin filter cake are often the determining factors of successful performance of drilling mud. Filtrate is measured by an API approved filter press carried by the mud engineer. During drilling operations there is two types of filtrations, dynamic and static. Dynamic filtration occurs while drilling fluid is being circulated. There is no adequate means of measuring dynamic filtration in the field. However increasing fluid loss increases the penetration rate. This may help with increase drilling speed but high filtration rates can spell disaster for other wellbore properties such as hole stability and pay zone penetration. Always follow the mud engineer’s treatment instructions closely each shift. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
19
Filtration
The mud engineer can only measure static fluid loss at the well site. However, the information gained from this test is very important. In static filtration the filter cake becomes the thickest and this is when problems can occur. Excess filtration into a pay zone can cause formation damage by adverse reaction of the formation clays, imbibition of the filtrate water causing water blockage, or by invasion and clogging of the pore throats with solids from the mud. Filtrate invasion of a shale can lubricate the fracture planes and can cause sloughing or facilitate hydraulic erosion of the well bore. From a drilling standpoint, excessive filtration rate can cause differential sticking across any lower pressured porous zone. There are no rules-of-thumb for the fluid loss test run in the field. Filtrate can be controlled by the addition of fresh bentonite, starch, colloids, polymers, oil, and dispersants to the mud systems. Add to system what is needed- if need to control water loss, and then add products to keep water loss down. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
20
Filtration
High Temperature-High Pressure Filtrate (HTHP Filtrate)
Is a much more accurate measurement of the true fluid loss of a drilling mud because it performs the test at temperature and pressures much closer to actual bottom hole conditions. However, it is not used much on moderate depth wells because the equipment is complicated, expensive, and somewhat dangerous for a tired mud engineer to run. However, on some wells where HTHP information is critical, it is run on a daily basis. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
21
Filter Cake
Filter Cake Runs hand-in-hand with filtrate. It is obtained with the same instrument as filtration. A filter cake develops when filtrate is lost to a porous formation. It consists of the solids presents in the mud, both drilled solids and mud additives from the surface mixing operation. In general, poor filtration control results in a thick filter cake. From a drilling standpoint, filter cake accumulation on sand faces causes more problems than the filtration itself. A thick filter cake can cause many problems downhole, such as tight hole, swabbing while tripping, and differential sticking. In some extreme cases, the filter cake can become so thick that is actually reduces the hole diameter to less than the size of the drill bit. In general, filter cake composed mainly of drilling solids is thick and fluffy, and exhibits high fluids loss. It is much more desirable to build a filter cake out of mud products added to the system. Adding chemicals to control fluids loss (gel, drispac, starch, etc.) will serve to toughen the wall cake, and make it thin and tight. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
22
Retort Solids and Retort Liquid
Retort Solids and Retort Liquid Retort Solids and Retort Liquid are derived by calculations performed by the mud engineer. Solids content can only be reduced by either by water or by use of solids control equipment. In a low-solids drilling fluid, the solids content cannot be reduced without lowing the mud weight. If the mud weight needs to be maintained and the solids lowered, the only practical way to do this in a freshwater system is with the additional of barite. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
23
Sand Content
Sand Content Sand content is a measure of sand in the drilling mud. It is measured by the mud engineer during his daily mud check. Sand it extremely abrasive and causes erosion of centrifugal pumps, mud pumps, high-pressure lines and hoses, swivel, drill pipe, drill collars, mud motors, and drill bits. Because of this, it is important to keep the sand content as low as possible at all times. Frequent dumping of the sand trap and mud tanks and constant use of the de-sander should be performed whenever mud is in the tanks. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
24
Methylene Blue Capacity
Methylene Blue Capacity (MBT) Methylene Blue Capacity is a measurement of bentonite in the mud system in lb/bbl. With the results of this test, the mud engineer can estimate how many drilled solids are in the mud and devise a treatment based on this data. The range for bentonite content in a low-weight system would be from 12-18 lb. /bbl., while in a weighed system the range would be 10-14 lb./bbl. Testing MBT is not usually performed on the average well with acceptable mud properties and not every mud engineer carries the kit to test for MBT. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
25
PH
PH Balance The PH of a mud system is determined by testing a sample of mud with litmus paper. Almost all mud systems have a PH of above 7 in order to minimize drill pipe corrosion and maintain mud stability. Extremely high PH is good for corrosion and good for drilling in H2S environments, but bad for mud proprieties, formation clays, and down hole stability. A PH above 11.5 will burn away the polymer in a modern mud system, reducing it to a fresh water gel system. This will drastically lessen the inhibitive effects of the mud and can de-stabilize the well bore. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
26
Mud alkalinity
Mud alkalinity and filtrate alkalinity Mud alkalinity (PM) and filtrate alkalinity (MF/PF) are more detailed and specialized tests of PH in a mud system. In general the Mf should be 3 times the PF. If the ratio gets higher it can be an indication of an alkalinity problem in the mud system. The PF/Mf spread should be low -around 3. Another indication of contamination is skyrocketing PM. Generally salt and anhydrate contamination will cause PM to decrease. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
27
Chloride
Measurement
Chloride Measurements Chloride measurement is a test for salt. Salt content can affect the performance of water-based mud systems. A low solids non-dispersed mud system is an inhibited mud system with very desirable flow properties. However, it is not very tolerant of salt contamination, and the total chlorides approach 10,00 ppm, it will start to clabber and become very thick. At that point, the mud system is much more tolerant of salt than a non-dispersed mud. It does not protect the producing formations or inhibit the hydration of shales in the well bore. When the chloride level reaches about 20,000 ppm, it is time to change over to a true salt water based mud system. All solids in any mud system are considered contaminate a low solid non-dispersed mud system can only be weighted up with barite to about 12.0 ppg. If a higher mud weight is needed, the system must be a dispersed. In a dispersed system can be weighted to about 18.0ppg. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
28
Total Hardness as calcium
Total Hardness as Calcium Total Hardness as Calcium is a measure of the hardness of the water in the mud system. Water containing a large amount of dissolved calcium or magnesium salts is referred to as “hard water”. The most common indicator of hard water is the difficulty of soap to lather up. Hardness in the mud system can come either from the water that you start with or from the formations penetrated while drilling. Some sources for calcium are anhydrite, gypsum, some shales, and cement plugs. Bentonite gel and other drilling clays have low yields when mixed with hard water. Also extensive calcium contamination results in high water loss and fast gel rate. Hardness is usually treated with caustic soda or soda ash. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
29
Lost Circulation
Lost Circulation Lost circulation can be a very troublesome and costly problem in the drilling industry. Yearly the cost of materials, man hours, and lost holes amount to millions of dollars. Lost circulation, or lost returns, is the loss to subsurface formations of drilling fluid used dring drilling operations. The loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returns. Loss of drilling fluids can occur in four types of formations: Unconsolidated or highly permeable formations Natural fractures Induced fractures Cavernous formations
Mud losses to induced fractures are the most troublesome because they can occur in any formation. It is difficult to prevent mud losses to induced formation because further parting of the formation will destroy the seal. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
30
Lost circulation
Preventive Measures When in area of known lost circulation, the driller can increase his chances of success by taking the following preventive measures: Use a light mud weight as possible Lower viscosity, gel strengths, and yield point
within safe limits Slow pumps to minimum rate necessary to
clean the hole Pull and run pipe at slow rates Drill don’t spud bridge Break circulation cautiously after engaging the
rotary or while pulling pipe Don’t speed up the pump until circulation has
been established If lost- circulation zones are expected, pre-treat the mud with lost circulation materials before drilling into the zone. It may be necessary to change your nozzle size to allow the lost circulation material to be pumped. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
31
Lost circulation
Loss circulation zones can be classified as: Seeping losses Partial losses Complete loss with hole full or mud within sight
down hole Complete loss with mud level over 500 ft. down
the hole Match the lost circulation material to the type occurring. For example most seepage losses occur because the lost circulation agents in the mud are not fine enough to complete the seal. The size of material should match the severity of loss. No advantage is gain if the concentration of lost circulation material exceeds 25 lb. per bbl. Mechanical failure of the pump, and plugging of the drill string is likely when you exceed this limit. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
32
Lost circulation
As the severity of the loss zone increases, only the size of the lost circulation increases, not the concentration. If a reasonable amount of lost circulation material is in the mud system and has not helped, it is useless to increase the concentration of the same material. The next step would be to increase the size of the lost circulation material. If necessary you must adjust your jets to pump the material, in extreme cases it might be necessary to run the pipe in open-ended. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
33
Lost circulation
When partial or complete loss occurs, it is usually best to fight the loss zone from above rather than with the drill string on bottom. If the mud column is at surface but the well will not circulate, an accepted procedure would be to trip half way out of the hole then try to break circulation. Continue to tripping half way out until circulation is regained. Once circulation has been established begin staging in the hole and establish circulation at each stopping point until the zone has been sealed. If possible don’t keep lost circulation material in the mud while drilling ahead. Over time the materials grind down to the point where they are useless as lost circulation material. As mud properties deteriorate circulation pressure increases, and a risk of lost circulation returns. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
34
Shale problems
Shale problems Maintaining well- bore stability while drilling shale sections can be tricky. Shales causing trouble have been describe as: Sloughing shale Heaving shale Running shale Bentonitic shale Mud Making shale Plastic flow shale Gas bearing shale Pressured shale
The factors which influence how shale reacts to the drill bit are: Degree of hydration Cementing material binding to the shale Inclination of bedding planes in the shale Pressure acting on or within the shale
Drilling problems associated with shales are: Hole enlargement Hole cleaning Stuck pipe Bridges and unwanted fill Increased mud cost Poor cement jobs Difficulty with logging
Shale problems can’t be solved with drilling mud alone. The mechanical factors which can contribute to shale problems are: Erosion due to high annular velocities Pipe whipping of drill string by rotating to fast While tripping, pressure surges ,swabbing, or
wall contact Internal forces within the formation(tectonics) Gas pressure within the shale Mud filtrate or whole mud invasion of shale
bedding planes or fractures __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
36
Shale problems
Solving a shale problem caused by pressure is simple. Increase the mud weight. There are other drilling practices that will help in this situation: Keep the hole full Low viscosity, low gel strengths, and thin wall
cake will aid in preventing swabbing Pull slowly through the problem interval
Another problem encountered is the effect of mud chemistry on the shale. Mud making and bentonitic shales are examples although there are many other types of clays present in shales. These shale can either hydrate or dehydrate, depending on whether water from the mud moves into the shale, or water from the shale moves into the mud. In either case the shale can become unstable. Field experience has shown that fewer problems occur when fluid loss is kept to a minimum. It should be pointed out that once a problem is created by high water loss it is more difficult to stop. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Instructor Guide
Trinidad Drilling Drillers Program
Section 12
37
Shale problems
Another factor influencing shale stabilization is PH. A PH of 8.5 to 9.5 gives better well-bore stability than a high PH such as found in lime muds. High Ph seems to promote rather than alleviate shale problems of this type. Many problems shales are time sensitive especially the ones affected by mud chemistry. Many shatle problems are not as severe when the hole is drilled in as a short time as possible. To summarize, when drilling shale problem shale: Drill with sufficient mud weight Control the water loss at a low value Keep the mud in the correct PH range Keep the shale exposure as short as possible
