Mud Related Drilling Problems Differential Sticking

•Differential pressure sticking of the drill pipe can be defined as the force that holds the pipe against the wall of the borehole due to the differential pressure between the hydrostatic pressure of the mud column and the formation pressure. •The pressure differential acts in the direction of the lower pressure in the formation. •This pressure pushes the pipe toward the permeable formation. •As the pressure differential gets larger, the force exerted on the pipe gets larger. •Differential stuck pipe occurs most often at a point next to the drill collars. •This is due to the drill collars being larger; hence more surface area is in contact with the side of the wellbore. •The following are major factors in differential pressure sticking: –The pipe becomes stuck opposite a permeable formation. –The sticking occurs after an interruption of pipe movement. –The pipe comes in contact with a soft, mushy or non-resilient type wall cake. •If the pipe is differentially stuck, as opposed to other types of sticking, the following will occur: –Circulation, if interrupted, will be restored and maintained after sticking is noticed. –The pipe cannot be raised or lowered. –No large amounts of cuttings are circulated out The force required to move differentially stuck pipe could exceed the strength of the drill pipe. •Several preventative steps can be taken to minimize the chances of becoming stuck: –The mud density should be maintained as low as practical, taking into consideration wellbore stability and potential well control problems. –Keep the pipe moving or rotating. •Avoid undue shutdowns and/or slow connections. •Use spiral drill collars to reduce the contact area against the well bore. –Maintain a low fluid loss and pay particular attention to the filter cake; i.e.: it should be thin, tough and resilient. •In areas where differential sticking is prevalent, the high temperature / high pressure fluid loss should be maintained below 20 ml. •Adding 2-8% lubricant to the mud system gives preferential oil wetting to the drill string, thereby allowing better lubricity and minimizing the possibility of stuck pipe. •When the drill string become stuck, it is imperative to act quickly as the sticking coefficient increases with time. •To avoid costly and time consuming wash over operations, a couple of methods are generally used to free the pipe.

Mud Related Drilling Problems
Differential Sticking -Spotting Fluid

•Spotting crude oil or diesel oil with a surfactant around the drill collars has gained wide acceptance. –There are many surfactants available are arecommonly called spotting fluids. –If a surfactant is not available on location, a straight diesel oil pill should be spotted across the collars as quick as possible. –If differential sticking is suspected in an area, always keep a supply of a differential sticking surfactant on location in the event it may be required. •Generally enough pill is mixed up to cover the entire length of the drill collars, plus an excess of 1.5 m3(10 bbls) to be left on top of the collars, and another 3.0 m3(20 bbls) to be left inside the drill collars. •Normally 20-25 litresof surfactant is recommended per cubic metreof diesel oil (1-2 gal/bbl). •The pill should be spotted leaving 3 m3(20 bbls) inside the drill string. –The pipe should then be worked by pulling up to a predetermined over pull weight, applying torque and releasing the weight at regular intervals. –The pill across the collars has a tendency to migrate up the hole; therefore approximately 0.1 m3(1/2 -1 bbl) of excess fluid in the pipe should be pumped every half hour. •An average waiting period is generally 10-12 hours. –If the pipe does not come free in a reasonable period of time (maximum of 2 pills), mechanical methods may be required to free the pipe. –If the spotting pill has to be weighted due to an abnormally pressure zone, or to increase the pill density to that of the mud weight to minimize migration, the spotting procedure would be the same although some of the products may be different.

Mud Related Drilling Problems Stuck Pipe

•The drill string can be stuck for many reasons including poor hole cleaning due to inadequate mud carrying capacity, sloughing shale, key seating and/or differential pressure sticking.
•Bridges can be caused by poor cleaning or by sloughing of the walls into the wellbore.
–The key to a muds lifting capacity is indicated by the appearance of formation solids coming over the shale shaker.
–An unusually large amount of shale indicates that the hole is washing out.
–Rounded edges on large cuttings show that these pieces have been tumbling in the hole for a long time and are not being lifted out effectively.
–Long splinters or fissured shale may indicate that the shale is "popping" into the wellbore, indicative of overpressuredshale.
–At times large amounts of material can remain in the hole without any surface indication that a hole cleaning problem exists.
•Large pieces of rock, which are not removed from the hole often, become lodged between stabilizers or reamers and the hole.
–If this occurs while drilling, the torque required to rotate the drill string will increase rapidly.
–If pieces of rock become lodged while making a connection or during a trip, the additional pull of the hook will appear as a drag.
–A sudden increase in pump pressure can sometimes be observed, as bridges form and restrict mud flow up the annulus
•Prevention of stuck pipe is often the best remedy


•Methods of preventing stuck pipe due to sloughing shale or inadequate hole cleaning may include the following:
–Increase the viscosity and particularly the Yield Point of the mud.
•There is no exact yield value that can be specified, as every situation is unique, but generally an upper Yield Point of ±30 lb/100ft2should clean most cuttings or cavingsfrom the wellbore.
•Again watch the shale shaker closely to determine the characteristics of cuttings.
–If possible annular hydraulics should be improved, to provide faster cuttings transport.
•Pump liners may have to be changed or larger bit nozzles utilized so that more fluid may be circulated without excessive pump pressure buildup.
•Critical velocities should be calculated to avoid turbulent flow that could increase shale problems by tearing up or eroding the hole.
–Use viscous pills to sweep the hole when drilling. This is a common and effective practice when drilling
–Increasing the mud density may be beneficial in some cases to balance the pore pressure of the shale, and to help hold formations in place to stabilize the wellbore.
–Reducing the water loss may help to minimize the hydration of shales and wetting along bedding planes with could disperse and slough into the wellbore.
–The drill string itself should be evaluated to minimize flexure of the string against the sides of the wellbore, which might tend to physically knock shale from the walls of the borehole.
–Keep the hole full at all times.
•Avoid excessive surge or swab pressures by tripping slowly, especially if a float is utilized in the string.
–Use invert mud or inhibitive water base mud.

Oil Muds –Acid Gas

•Hydrogen sulfide (H2S) is a poisonous and dangerous acidic gas encountered in many formations and produced fluids. –It can quickly deaden senses and can be fatal even at low concentrations. –Personal protection and the appropriate safety measures should be taken any time hydrogen sulfide is suspected. •Oil and synthetic muds provide good protection from hydrogen sulfide corrosion and hydrogen embrittlement. –The continuous oil or synthetic phase of the mud is non-conductive and does not provide an electrolyte for the corrosion process. –If the mud has adequate wetting agents, the drill pipe will be preferentially oil-or synthetic-wet. –If the emulsion becomes unstable, and the mud water-wets the drillstring and casing, the corrosion protection provided by oil and synthetic muds will be lost. •Use personal safety protection and the utmost caution if hydrogen sulfide is encountered. –When hydrogen sulfide is expected or encountered, the oil mud alkalinity (POM) should be maintained at >5.0 cm3of 0.1 N H2SO4at the flow line with additions of lime. –Addition of Zinc oxide is also recommended •When hydrogen sulfide is encountered, the mud may require large additions of lime, emulsifier and wetting agents to stabilize its properties. –The mud should be watched for indications of water-wetting. •When Hydrogen Sulfide is encountered –Mud may turn black –The excess lime will drop rapidly –The mud should be tested for H2S with the Garret Gas Train and treated with scavenger and lime as required

Oil Muds –Oil/Water Ratio–Electrical Stability– Gas Solubility

Oil/Water Ratio

•The oil-or synthetic-to-water (O/W or S/W) ratio relates only to the liquid portion of the mud and is not affected by the solids content.
–The oil-or synthetic to-water ratio relates the oil and water fractions to the total liquid fraction.
–Generally, higher mud weights require higher ratios.
–Different conditions favor the use of different ratios, so there is no ratio that must be used for any set of conditions.
•The calculation of the oil-to-water ratio requires retort values as follows:
–Oil ratio (O) = (vol% oil)/(vol% oil + vol% water) x 100
–Water ratio (W) = 100 –oil ratio
•The O/W ratio remains constant when the mud is weighted up or solids are incorporated into the mud, even though the volume percent liquid is decreased significantly.
–A rapid decrease in the O/W or S/W ratio indicates an influx of saltwater from the formation, and a pit volume increase should have been observed.
•When using oil or synthetic muds, all water hoses on the pits should be disconnected or plugged to prevent accidental contamination with water
•The viscosity and HTHP filtrate will change with changes in the oil-or synthetic-to-water ratio.
–Changing the ratio is not used to alter either of these properties.

Electrical Stability

•The electrical stability is an indication of how well (or tightly) the water is emulsified in the oil or synthetic phase.
–Higher values indicate a stronger emulsion and more stable fluid.
–Oil and the synthetic fluids do not conduct electricity. In the electrical stability test, the voltage (electrical potential) is increased across electrodes on a fixed-width probe until the emulsified water droplets connect (i.e., coalesce) to form a continuous bridge or circuit.
•The stronger the emulsion, the higher the voltage required to break down the emulsion completing the electrical circuit to conduct electricity.
•The unit of measure for recording the electrical stability is volts.
•Factors that can influence the electrical stability are:
–Water content
–Water wet solids
–Emulsion strength
–Temperature
–Salt concentration
–Saturation
–Weight material
•Freshly mixed, invert-emulsion muds usually have low electrical stabilities when shipped from the liquid mud plant, even though they are adequately treated with emulsifiers.
–The emulsions of these systems will tighten as they are exposed to downhole temperatures and sheared through the bit.

Gas Solubility

•Oil and synthetic fluids are soluble to methane and other gases encountered while drilling.
•They have high gas solubility to natural gas, carbon dioxide and hydrogen sulfide, see pic.
•This can interfere with kick detection and well-control procedures.
–This soluble gas does not begin to come out of solution until the pressure is reduced as the mud is circulated up the annulus.
–The majority of the gas expansion occurs in the last 1,000-ft interval below the surface.
–For this reason, extra care should be taken to monitor pit levels with these systems and when handling the influx of wellbore fluids.
–It is important to be able to monitor and detect kicks to a level of about 5 bbl

Oil Muds – Osmotic Theory and Borehole Stability-Water Activity– Water Phase Salinity

Osmotic Theory and Borehole Stability

•Osmosis is the net movement of water across a selectively permeable membrane driven by a difference in solute concentrations on the two sides of the membrane •In reference to Invert emulsion oils muds the water phase is saturated with Calcium Chloride –While drilling occurs and new shales are exposed to the fluid, the water captured in the shale will move with osmosis into the fluid effectively drying out the shale –Using this theory, the shale in the borehole cannot hydrate therefore it becomes more stable

Water Activity

•Water activity (AW) is a measure of the chemical potential for water to be transferred between mud and shales. –Activity is measured using the vapor pressure (relative humidity) of shale or mud. –Activity can also be estimated based on the chemical composition of the brine (salinity). –Pure water has an AWof 1.0. –Calcium chloride brines used in most non-aqueous emulsion muds have an AWbetween 0.8 (22% wt) and 0.55 (34% wt). –Lower values for activity are more inhibitive.

Water Phase Salinity

•Calcium chloride is added to increase the emulsified water phase salinity to provide inhibition of shales and reactive solids. •The range for calcium chloride content is usually 25 to 35% by weight. •The CaCl2content should be determined by titration and can be calculated by: •% CaCl2 (wt) =(Ag x 1.565)/((Ag x 1.565) + %H2O) x 100 •Where: –Ag = cm3 0.282 N silver nitrate per cm3of mud –% H2O = Volume % water from retort •The concentration can be adjusted by adding powdered calcium chloride over several circulations. –Powdered CaCl2is preferred over flake CaCl2, because the larger flake particles do not readily dissolve in oil and synthetic muds. –Flaked salts must first be dissolved in water before being added to a non-aqueous system. –The powdered form is generally available as 94 to 97% active material.

Oil Muds

•The origin of non-aqueous drilling fluids can be traced to the 1920s when crude oil was used as a drilling fluid.
•The advantages of oil as a drilling and completion fluid were obvious even then:
–Clays do not hydrate and swell.
–Wellbore stability is improved.
–Production is improved from sandstones containing clays.
–Problems are reduced when drilling evaporites(salts, anhydrite, etc).
–Wellbore enlargement is reduced.
–Mud properties are more stable.
–Contamination resistance is increased.
•Oils also have certain characteristics that are undesirable.
–They are flammable and may contain compounds that cause the failure of rubber goods such as hoses, O-rings, gaskets and Blowout Preventer (BOP) elements.
–Oils lack gel structure and are difficult to viscosifyso they can be weighted.
–Many oils contain toxic or hazardous compounds that cause Health, Safety and Environmental (HSE) concerns.
–They have high gas solubility for many of the gases encountered when drilling wells (natural gas, carbon dioxide and hydrogen sulfide).
–This can interfere with kick detection and well-control procedures.
–Oils may not degrade readily under certain conditions.
–Oils also float on water and can migrate a significant distance from their source
•Today, an invert emulsion mud is a fluid with diesel oil, mineral oil or synthetic fluid as the continuous phase and water or brine as an emulsified phase.
–The emulsified water or brine is dispersed within the oil
–This is the internal phase.
–Calcium chloride salt is used to increase the emulsified water phase salinity to a level where it does not influence (soften or swell) water-sensitive formations and cuttings.
•Invert emulsion muds should be used when conditions justify their application.
•Environmental acceptability, disposal, initial makeup cost, daily maintenance cost, anticipated hole problems, formation evaluation and formation damage issues should all be considered.

Oil Muds -Applications–Emulsion Fundamentals–Additives

Applications

•Troublesome shales. •Salt, anhydrite, carnalliteand potash zones. •Deep, hot wells. •Drilling and coring sensitive productive zones. •Extended-reach drilling projects. •Difficult directional wells. •Slim-hole drilling. •Corrosion control. •Hydrogen sulfide (H2S) and carbon dioxide (CO2) bearing formations. •Perforating and completion fluids. •Casing pack or packer fluids. •Workover fluids. •Spotting fluids to free stuck pipe.

Emulsion Fundamentals

•Invert emulsion drilling fluids are mixtures of two immiscible liquids: oil (or synthetic) and water. –They may contain 50% or more water. –This water is broken up into small droplets and uniformly dispersed in the external nonaqueous phase. –These droplets are kept suspended in the oil (or synthetic) and prevented from coalescing by surfactants that act between the two phases. •To adequately emulsify the water in oil, there must be sufficient chemical emulsifier to form a film around each water droplet. –The emulsion will be unstable if there is not sufficient emulsifier. –As the water content increases, the required concentration of emulsifier increases. •From the standpoint of stability, the smaller the droplet, the more stable the emulsion since large droplets will coalesce more easily than smaller droplets •Uniform droplet size also makes the emulsion more stable –Shear is required to reduce the droplet size, the fluid through the bit, mud guns and shearing devices will aid in reducing droplet size •The importance of sufficient shear and small droplet size and their relationship to mud stability cannot be overemphasized. –Small, uniform water droplets generate viscosity and gel strengths that help support weight material and aid in the reduction of fluid loss by becoming trapped in the filter cake. •Increasing water content (internal phase) of an invert emulsion: –Increases the size of water droplets. –Increases the chances of water droplets coalescing. –Increases the emulsion plastic viscosity. –Increases the amount of emulsifier required to form a stable emulsion. –Decreases the emulsion stability. •The incorporation of solids into a water-in-oil or synthetic emulsion can have either a positive or negative effect on mud properties, depending upon the manner in which they are wetted. –As long as the solids are maintained in an oil-wet condition and do not coalesce or deplete the required surfactant concentration, they will form a stable emulsion. •Non-aqueous drilling fluids are formulated using additives based on a broad group of chemicals called surface-active agents or surfactants. –These chemicals include emulsifiers, soaps and wetting agents. –They act by reducing the interfacial tension between two liquids or between a liquid and a solid. –Surfactants have a hydrophilic (water-loving) polar head and an organophilic (oil-loving or lipophilic) non-polar tail, •Non-aqueous systems contain wetting agents that coat surfaces and solids to alter the contact angle (wettability) of the solid-liquid interfaces, –These materials allow preferentially wetting of solids by the oil or synthetic. –If a fluid is over treated with wetting agents so that solids are totally wetted, the solids may tend to settle or sag. –Solids must be maintained in the preferentially oil-wet condition to maintain a stable fluid. •The preferential oil-wet condition can be disrupted by contamination with water, increased solids loading and insufficient treatments of wetting agents. –When water-wet solids occur: –Solids tend to adhere to shaker screens. –The appearance of the mud becomes “grainy,”losing its glossy sheen. –The Electrical Stability (ES) will decrease. –The rheology will increase. –Barite settling will be observed in mud cup, heat cup and pits. –The High-Temperature, High-Pressure (HTHP) fluid loss will increase and may contain free water.

Additives

•Emulsifiers. –Emulsifiers are surfactants that reduce the surface tension between the water droplets and oil (or synthetic). –They stabilize the mixture by being partially soluble in water and partially soluble in oil –They are usually long-chain alcohols, fatty acids or polymers and can be anionic, cationic or non-ionic. •Soaps. –Some emulsifiers are soaps that are formed by the reaction of a fatty acid ester with an alkali (such as lime) where the hydrogen on the fatty acid is replaced by a metal, such as calcium from lime. –Soaps made with sodium are water-soluble and form oil-in-water emulsions. •Wetting agents. –A wetting agent is a surface-active agent that reduces the interfacial tension and contact angle between a liquid and a solid. –This causes the liquid to spread over the surface of the solid –Wetting agents have one end that is soluble in the continuous-phase liquid and the other that has a strong affinity for solid surfaces, •Viscosifiers. –Although emulsified water increases viscosity, viscosifiers and gelling agents are also required. –Untreated clays cannot be used as viscosifiers because they do not hydrate and yield in oil or synthetic fluid. –If the clays are first coated with an amine, so that they are organophilic, then they will yield and viscosifyin oil and synthetic fluids. •Viscosifiers cont. –Organophilic clay still needs a polar activator (water or alcohol) to produce the maximum yield. –Therefore, their yield decreases as the oil-or synthetic-to-water ratio increases. •Alternative non-clay viscosifiers are available to increase viscosity. –They include asphaltic materials, fatty acid gellants and polymers. •Developing viscosity is a particular problem when mixing new fluids in mud plants where low shear mixing and low temperatures do not allow amine-treated clays to yield. –Freshly prepared muds should not be treated with more organophilic clay than will be required when drilling. •Weight material. –Barite is the most common weight material used in oil and synthetic-base muds. –Calcium carbonate is also used, particularly in lower-density packer fluids, where it is easier to suspend than either barite or hematite. –Hematite may be used in high-density muds where its high specific gravity helps minimize the total solids content of the mud. –Alternative weight materials may require different wetting agents. •Filtration-control additives. –HTHP filtration control of invert emulsion muds is affected by the viscosity of the continuous fluid phase, the oil or synthetic-to-water ratio, the tightness of the emulsion, water-wetting of the solids, the solids content, and the amount of amine-treated clay in the system. –Gilsoniteor asphalt, amine-treated lignite (and polymers are the most common filtration-control additives.

Solids Control –Centrifuges

•As with hydroclones, decanting-type centrifuges increase the forces causing separation of the solids by increasing centrifugal force.
•The decanting centrifuge consists of a conical, horizontal steel bowl rotating at a high speed, with a screw-shaped conveyor inside.
–This conveyor rotates in the same direction as the outer bowl, but at a slightly slower speed.
–The high rotating speed forces the solids to the inside wall of the bowl and the conveyor pushes them to the end for discharge.
•Centrifuges are capable of making a sharp cut point.
–The ideal cut point is the particle size at which all larger particles are separated and all finer particles are retained.
–This is, however, not possible, so the actual stated percent of cutpoint(D number) should be included when comparing centrifuge performance characteristics.

Solids Control –Centrifuges Discharge




Solids Control –Centrifuges -Applications

•Weighted mud
–Dual centrifuges can be rigged up for barite recovery.
–The first centrifuge recovers barite and send it back to the mud system.
–The overflow is sent to the second machine to remove LGS and send clean mud back to the mud system
•Un-Weighted mud
–Centrifuges set up in a parallel configuration, both remove low gravity solids and send cleaned fluid back to the mud system
•De-watering
–Whole mud is treated to form dry solids for disposal and clear water for recycling.
–For this application, the solids content of the mud is brought to a very low level.
–Then, chemicals are added to encourage the particles to coagulate and flocculate.
–Once the fluid is properly treated, it can be processed through a centrifuge, with mostly dry solids and water being recovered
•Reduction of mud costs, without sacrificing control of essential mud properties, is the main purpose of, and justification for, using a decanting centrifuge.
•Although it helps control undesirable fine solids, the centrifuge’s principal function is to minimize dilution and maintain acceptable properties in the mud system

Solids Control –Centrifuges –Internal Parts