For the stability of mud fluids of prime importance is the magnitude of the potential at the AB boundary of slippage during movement of the liquid phase respective the solid one, which is commonly known as the zeta-potential.
The surficial dissociation results in the formation of an ionic cloud around each clay scale. Since the water molecules appear as dipoles they become oriented in the electric field of the scale and are attracted to it by their positive charges, while to the negative dipole charges are attracted new water molecules which become oriented in a manner similar to the former. At the same time, subject to hydration are cations broken, away from the surface of the scale, with Avater dipoles getting oriented around them as well.
Thus, a specific cloud of oriented water, molecules, inclusived cations, forms around the clay scale. Such a cloud of
a tented water molecules is called hydration envelope, while the aggregate consisting of the clay particle, double* electric layer and the hydration envelope represents the micelle, which is electrically neutral.
The water in hydration envelopes is physically combined. In the inner layers of the envelope this water has a structure and properties of ice, viz. it is resilient, possesses an elevated viscosity and mechanical strength. With increasing distance from the scale surface the bonding forces get weaker, the orientation of water molecules becomes less marked, the strength of the hydration envelope decreases and its properties approach those of ordinary water. The thickness of the hydration envelope depends in a great measure on the magnitude of the clay particle charge and the valency of counterions. The greater the particle's charge and lower the valency of counterions, the more intensively develops the ionic cloud and the stronger becomes hydration of the clay scale.
Between adjacent hydrated clay scales there exist forces of molecular attraction, that diminish parallel with increasing distance between the scales by following the power lav/ and forces of electric repulsion between the double electric layers which also diminish with increasing distance, but by following the exponential law. The geometrical addition of attraction and repulsion energies demonstrates that at great distances between the scales the energy of molecular attraction surpasses somewhat that of electric repulsion. With medium distances of approximately 1()'5 cm the odds are in favour of the electric repulsion forces, while at short distances of about 10"7 the forces of attraction have the upper hand again. Consequently, to have two scales stick together some outside energy must be imparted to them, the one that woul d be in excess of the repulsion energy at medium distances, e.g. capable of overcoming the energy barrier represented by the positive peak. Should the outside energy imparted to the scales be insufficient to overcome this energy barrier, the approximation of the scales so as to achieve their complete fusion is impossible. The higher the zeta-potential and the energy barrier, the more stable is the clay suspension. This appears to be the key factor in the stability of clay suspensions.
The thickness of hydration envelopes on the surface of clay scales is dissimilar, being maximal on plane faces and minimal on edges and at the scale apices. With irregular thermal motion, water molecules impinging on the scales impart some energy to them, make them move. When moving, the hydrated scales collide with one another. It is obvious that the rupture of hydration envelopes on colliding is more probable in places where they are the thinnest and, consequently, where less energy is required for their rupture. Hence, the hydration envelopes are the second factor contributing to the stability of clay suspensions.
During mechanical grinding of the clay, the crystal lattice can be destroyed in such a manner as to have concentrated at the apices and on the edges of the scales not only fairly high-power negative but also positive charges. Upon collision of the scales on their apices and edges, the rupture of hydration envelopes and partial adhesion occur, above all, under the effect of attraction forces between opposite charges. At rest, the number of clay scales sticking together with their apices or edges gradually increases. With the passage of time they form a specific honeycomb structure that spreads throughout the whole of clay suspension volume. The structure gradually gains in strength owing to adhesion of ever new clay scales and approaches asymptotically a certain limit. Thanks to such a structure the mud is capable of keeping suspended fairly large particles of the solid phase, including fragments of drilled-out rocks.
On stirring the mud solution, its structure disaggregates and it becomes fluid again. The ability of fluids to thicken at rest as a result of the structure formation and become mobile afresh on its agitation or shaking is known by the term of thixotropy.
The particles can stick together in two ways. Firstly, the particles can agglutinate on colliding with their apices and edges and thus form a honeycomb structure whose cells contain free water and inert particles of the solid phase. Then the hydration envelopes remain intact on the most of the particle surface (and above all on plane faces). This phenomenon is known by the name of hydrophilic coagulation.
Secondly, under the effect of certain factors the particles can lose their charge and become devoid of hydration envelopes. In this case, on colliding even with their plane faces they will agglutinate and form bigger aggregates which under the effect of gravity settle out from the mud which then splits up into two phases. Such a coagulation is known as hydrophobic, or as flocculation.
As the degree of the drilling fluid mineralization increases, the coagulation phenomena get more intensive and the stability of the fluid deteriorates. To maintain the preassigned properties of the mud it is subjected to treatment with chemical reagents. The higher the degree of mineralization, the more difficult it is to keep stable the properties of the mud, the more complicated its treatment, the greater consumption of reagents and, consequently, the more costly is the drilling fluid itself.
Hence, when running the risk of a heavy mineralization of the drilling fluid it is advisable in preparing the latter to use salt-tolerant palygorskite clay as a source of the colloidal fraction. Solutions with palygorskite are prepared with fresh water, for then the clay better breaks up into unit scales, salt being added thereafter.
T ext 2
Lignosulphonates Mud Fluids and Their Derivatives
The incrusting material of wood and vegetable masses is lignine. When boiling wood with aqueous solutions of sulphurous acid and its salts for the purpose of obtaining wood pulp (cellulose) there appear by-products containing lignosulphonic acids and their salts, along with sugar, tannides, proteins, resins and other components. Following neutralization of these products, fermentation of the bulk of sugars and their separation for production of alcohol and yeasts there remains so-called sulphite cellulose liquor. It contains a large amount of calcic and other salts of lignosulphonic acids and may conventionally be designated as calcium lignosulphonate. The industry delivers this product to drilling enterprises as a concentrated liquid or paste of dark-brown colour, or as a light-brown powder with pH ranging between 4 and 7.
Lignosulphonate is introduced into fresh water-base muds together with alkali and often also with lime, and into saline solutions, without alkali. Lignosulphonate effectively forces down the funnel viscosity and gel strength of muds, as well as water loss of heavily saline fluids (brine muds). On addition of this agent to fresh water-base muds their filtration loss goes up. The agent is less effective with declining salinity, while in poorly mineralized and fresh water-base solutions its effect diminishes also with decreasing pH. Its optimal addition in primary treatment of salinized drilling fluids comprises from 1 to 5 per cent (calculated in terms of dry substance) and during secondary treatment, less than 1 per cent.
Modified lignosulphonates. The effectiveness of the protective action produced by lignosulphonates and, consequently, their ability to keep down filtration losses can be augmented if the agent's molecules are made bigger. The enlargement of the molecules or their condensation is realized as a result of the lignosulphonates interaction with formalin in the presence of sulphuric acid and at an elevated temperature. To avert corrosion and raise the salt sistance, phenol (phenylic acid) is added to the reaction mixture, along with chromat.es, to increase its thermal stability. Such modifications of lignosulphonates are known by the name of condensed lignosulphonates available in three brands both as liquids and powde rs. The liquid product has a density of roughly 1100 kg/m3 and pH - 8-9.
The condensed lignosulphonate of brand 1 contains small amounts of phenols and serves the purpose of reducing water loss and viscosity of fresh-water base, poorly-mineralized and lime-base muds at temperatures not exceeding 100°C. An optimal amount of the additive during primary treatment should be from I to 3 per cent, cal culated to the value of dry substance.
The condensed lignosulphonate of brand 2 carries a much greater proportion of phenols and is distinguished by a higher salt- resistance or salinity tolerance. It serves to reduce filtration losses and viscosity of fresh-water, poorly and medium-mineralized muds and also of calcium-chloride drilling fluids at temperatures of up to 130°C.
The condensated lignosulphonate of brand 3 contains, apart from phenols, also chromâtes. It is used to keep down the water loss and viscosity of fresh-water and poorly-mineralized muds at temperatures of up to 200°C. At temperatures of up to 150°C its optimal addition is from 1 to 2 per cent and at higher temperatures up to 3-3.5 per cent.
At higher temperatures the protective action of lignosulphonates decreases quite significantly with the rising degree of mineralization.
The condensation of lignosulphonates molecules can also be achieved through their oxidation with chlorine, nitric acid and chromates. Wide application have found such products of condensation as chromolignosulphonates and ferrochromo- lignosulphonates.
Chromolignosulphonate is put out in the USA under the trade name of Oksil. It is intended for reducing the funnel viscosity and rheological properties of fresh-water, mineralized, lime- and gypsum-base drilling fluids and also for keeping down the water loss of nonmineralized mud fluids and magnesium hydrogels. In treating mud fluids Its optimal addition is up to 1 per cent (calculated to the value of dry substance). The agent is introduced into the drilling fluid treated with alkali to raise pH. The treatment is most effective with pH r: 9-10.
The heat resistance of nonmineralized drilling fluids after their treatment with Oksil reaches 200°C and that of brine- and gypsum-base ones up to 160°C.
Ferrochromolignosulphonates differ from
chromolignosulphonates in that some of the chromium atoms therein are substituted by iron. They bring down effectively the funnel viscosity of fresh-water, little- and medium-mineralized muds, and also gypsum and calcium chloride drilling fluids. The thermal stability of mud fluids treated with ferrochromolignosulphonates is as high as 180-200°C.
Lignosulphonates are compatible with nearly all chemical agents. Their common shortcoming is foaming of drilling fluids, especially of nonmineralized ones. Therefore, they should be employed together with froth breakers.
Lignine derivatives. These are obtained by treating lignine hydrolysate, a bulk waste product of hydrolytic industry, with oxidants. They effectively depress the funnel viscosity and gel strength and, quite often, also reduce the filtration loss of mud fluids
with pH = 8-10, but, unlike lignosulphonates, produce no foaming thereof.
Nitrolignine is a light-brown coloured powder, insoluble in water. For treating the drilling fluid it is used in the form of an aqueous-alkaline solution of a 5-10% concentration. Depending upon the composition and pH of the fluid the nitrolignine-to-alkali ratio varies from 1: 0.1 to 1: 0.5 (calculated to the value of dry substance). It is designed for the treatment of nonmineralized and lime-base mud fluids, the optimal amount of the additive comprising from 0.2 to 0.5 per cent. Nitrolignine can be also used in treating brine mud fluids, stabilized with water-loss reducing agents. It is recommended that pH of the drilling fluid be kept at about 10.
Sunil - a product of nitrolignine reduction with sulphites, is a darkish-brown liquid with pH of around 8, water-soluble. It is used for the treatment of both fresh-water and mineralized drilling fluids, its optimal addition comprising from 0.2 to 0.5 per cent (calculated to the value of dry substance). With additions of 1.5-2 per cent, sunil lowers water loss of nonmineralized mud fluids.
Starchy agents. These tend to effectively lower filtration losses and viscosity of medium- and heavily-mineralized drilling fluids, including these containing polyvalent cation salts, the ones that act as strongest coagulants. They can also be employed for keeping down water losses of little mineralized mud fluids, but then their viscosity increases.
Natural starches are poorly soluble in cold water. Therefore, in bating mud fluids use is made of aqueous alkaline solutions of a 5 to 10 per cent concentration with a NaOH-to-starch ratio of 1:10 to 1: 2.5 (with respect to the dry starch mass). The starchy is prepared directly at the drill rig before proceeding with the treatment of the mud fluids, for lengthy storage adversely affects its quality. Treatment is most effective when pH of the drilling fluid filtrate is higher than 10. In primary treatment of a highly mineralized drilling fluid optimal addition of the agent amounts to 1.5-3 per cent (calculated to the value of dry starch). In cases of repeated treatment the amount of the agent spent varies from 0.5 to 1.5 per cent.
The thermal stability of the stairchy agent does not exceed 120°C. An important shortcoming of the starch is its tendency to rot under the effect of bacteria and enzymes. The decay of starch is attended by liberation of a great quantity of gaseous substances having an offensive odour and by foaming of the drilling-fluid. The fluid in which the process of decay has begun is to be fiilly replaced, for to improve its properties is practically impossible. To avert fermentative destruction it is recommended that the starch pH be brought up to 11.5-12 and salinity maintained at not less than 20 per cent. It is also advisable to introduce antiseptics (chlorinated lime, formaldehyde, formalin, paraformaldehyde, phenylic acid, catapine, etc.), which suppress the vital activity of bacteria. The use of antiseptics yields the greatest effect.
Modified starches. These are prepared by treating natural starches with antiseptics and sodium carbonate, desiccation at 150- 160°C and subsequent grinding, in production of modified starch potassium chrome alums are used as an antiseotic. The ready-made product is a cream-coloured powder, readily soluble in water. To reduce water loss the modified starch may be introduced into the drilling fluid in its powder form without preliminary dissolution. The modified starch is distinguished by its higher fermentation resistance within a broad range of the mud fluid pH. In primary treatment its optimal addition does not exceed 3 per cent and in the repeated treatment, varies from 0.3 to 0.7 per cent. It is fully compatible with other agents.
The thermal stability of starchy agents can be raised somewhat through addition of Oksil.
The effect of starchy agents is greater when they are used in combination with other reagents (with carboxymethyl cellulose or hypan, for example).
Humates. The base of these agents form sodium salts of huffiic acids contained in lignites (brown coal) and peat. These salts come in consequence of interaction of an aqueous solutionol caustic soda with finally ground lignite (or peat). For obtaining humate agents, brown coals containing no less than 35 per cent of humic substances are utilized. The quality of the prepared agent depends upon the proportion of humic substances in M initial raw material, on the alkali-coal ratio, temperature, time and on other factors. The alkali-coal ratio varies in the range of from 10:1 to 4:1. The optimal compounding formulation of the agent for treating mud fluids is found by experimentation.
At the manufacturing plants the powdered humate agent is prepared through irrigation of dried brown coal with concentrated (40 ± 45 per cent). In the USA wide use finds also pasty hamate agent which is obtained through mixing of humidified brown coal with concentrated alkali, subsequent briquetting and air drying. The moisture content of briquettes is about 50 per cent.
The humate agent is used most widely and is one of the cheapest and most accessible compound. It is fully compatible with the majority of reagents employed for the treatment of drilling fluids. Its purpose is to reduce water loss and, often, viscosity of fresh-water and low-salinity mud fluids at temperatures of approximately up to 140°C. At higher temperatures 0.01-0.25 per cent of sodium or potassium chromates or bichromates are added to mud fluids together with the humate agent. Sodium humates become adsorbed on the surface of clayey particles in the mud fluid and prevent mutual cohesion of these particles.
Should a drilling fluid be repeatedly treated with a humate agent it can become structureless, its gel strength then falls down to zero and filter cakes formed by such fluids display a high degree of clamminess. In order to avoid this the treatment with a humate agent should be alternated with that involving some other reagents. Especially effective is combined use of a humate agent with chromolignosulphonates.
With a higher salinity the effect of humate agents falls drastically even at not too high temperatures, this being its major disadvantage.
The nitrohumate agent is obtained through treatment of brown coal with nitric acid and alkali. The agent is supplied in powdery form. It is used for reducing viscosity and water loss of fresh water-base, little- and medium-salified mud fluids.
Polyphenol agents. This group embraces substances of vegetable (quebracho, fir, pine, oak extracts) and synthetic origin, sulphurization products of natural substances and also condensed phenol.
Quebracho is a finely ground brown-coloured powder obtained from the bark of the South-American tree of the same name. It contains 60 to 70 per cent of tannins, largely pyrocatechol ones. The agent is employed together with alkali in a proportion of 1: 1 to 1: 5. It is apt to effectively depress viscosity and gel strength of fresh water-base and little-mineralized and also lime-base muds at temperatures ranging from 120 to 140°C. At higher temperatures the efficiency of the agent drastically decreases, even in cases of low salinity. It is well fit for use with all other agents, being particularly often employed together with humates.
Sulcor is a sulphurization product of alkaline fir-tree bark extracts. The agent has an appearance of a dark mass readily soluble in water. It effectively brings down fuimel viscosity, gel strength and cuts down filtration loss of fresh water-base muds and is well suitable for use with all other agents. Its optimal addition amounts to 0.1-0.5 per cent with pH = 8-11.
Lignitic polyphenol is obtained through condensation of phenol contained in by-products of wood processing with formaldehyde in an alkaline medium and subsequent sulphurization. It effectively reduces funnel viscosity, gel strength and freshwater muds filtration loss at pH = 8-11. Its optimal addition is 0.2- 0.5 per cent. Its effect quickly falls off in muds even of little salinity, especially at elevated temperatures.
The polyphenol agents are capable of foaming the drilling fluid and therefore they are introduced together with froth breakers.
Synthetic agents with acrylic polymers as their base. These are distinguished by their high heat resistance (up to 200-250°C) and complete fermentative stability. In the USA most popular are hypan, metas and polyacrylamide.
Hypan or hydrolyzed polyacrylonitrile is a viscid liquid of a yellow to darkish-brown colour, smelling of ammonium. It is used to cut down water loss of fresh water, salified (with sodium sulphate or chloride) and lime-base mud fluids, funnel viscosity of fresh water muds then going up. The manufacturers supply hypan ol two brands: of high and medium viscidity. The former lowers the water loss much stronger, but produces greater thickening of the drilling fluid treated with it, especially when there is an elevated concentration of the solid phase. With growing mineralization the gel strength of the mud fluid, following its treatment with hypan, can be brought down to zero
Optimal addit ion of hy pari at 120°C varies from 0.5 to 0.75 per cent, and at highe:* temperatures may be 2-2.5 times higher.
The major shortcoming of hypan is its high sensitivity to salts of calcium and other polyvalent metals and metalloids.
Metas is a powdered compound of white or yellowish-gray colour, difficultly soluble in fresh or brine water, but readily soluble in a slightly alkalized water. As concerns its capacity to keep down filtration loss of fresh-water mud fluids or of the ones carrying sodium chlorides or sulphates at high temperatures, it is equivalent and, possibly, even superior to hypan. Like all agents with acrylic polymers as their base, metas is highly responsive to salts of polyvalent cations.
Polyacrylamide is a highly viscid liquid and a strong coagulant. It is used for the treatment of mud fluids with a low solid- phaseconcentration. The agent effectively reduces water loss and raises the viscidity of such muds.
Inorganic electrolytes. In treating mud fluids wide use have found electrolytes, such as caustic and calcined soda, water-soluble glass phosphates, chlorides, calcium sulphates and lime.
Text 3 Mud System
The mud system is used on a vast majority of wells. This is a cycling process which starts at the slush pump. The pump picks up the fluid from the suction tank and delivers it through the stand-pipe and mud hose to the swivel. Then it paisses through the kelly, drill pipe, drill collars and bit. The fluid passes upward around the drill string, over a shale shaker where the cuttings are removed. It drops into a settling tank and then goes to the suction tank to start a new cycle.
The term «fluid» means anything which flows. In the drilling fluids, this could mean either liquid., or air, or gas. The drilling fluid is now recognized as one of the major factors of the drilling operation. There are the following drilling fluid types:
1 Clay water-base muds and hydrocarbon-base muds.
2 Aerated muds.
3 Oil-base muds.
4 Emulsion muds.
5 Natural muds (carbonate, sulphate).
6 Water - fresh or salt.
7 Air and gas.
The specific gravity of the fluid may range from 0.8 to 2.3 gr per cm3. To maintain the desirable quality chemical agents are added to the circulating fluid. The following parameters determine the quality of the fluid: the specific gravity, the viscosity, the water loss, the gel strength, the sand content, the alkali ne percentage.
Mechanical and hydraulic mud mixers as well as chemical agents are used for the preparation of the circulating fluid at the surface. Cuttings, different salts, formation water, gas and oil enter the drilling fluid in the process of drilling and as a result the parameters of the fluid may alter.
Special devices and machinery - grooves, sediment boxes, shakers, hydrocyclones and etc. are used for cleaning the fluid at the surface.
The hydrocone provides a simple and efficient method of removing sand or solid from a light density drilling fluid. The fluid is pressured by a pump and introduced into the cone at a tangent to the cone wall. This tangential introduction causes the fluid to follow a swirling or spiraling path down the cone wall. The change in direction caused by the conical shape of the cone creates a centrifugal force. This force settles the sand,or light solid toward the wall where they collect and spiral down to-a bottom apex of disc harge valve.
Meanwhile, the spiral effect has created in the; cone a voitex or vacuum analogous to the eye of a cyclone. The lighter fluid moves toward the center and is picked up in the updraft, carried out and discharged free from sand solids. These cones operate with any feed pressure from 1 to 40 PS I and are easily adjusted to handle any amount of sand.
The composition of drilling mud will depend upon the requirements of the particular drilling operation. For some drilling operations dirty water may be the ideal drilling fluid. Holes must be drilled through a large number of different types of formations so it is natural to expect that various improvements in the drilling fluid will be necessary to meet the various conditions encountered as the hole is drilled into the earth. In some areas drilling can be started with water and, as drilling progresses, the clays and shales picked up from the formations will be dispersed in the water, resulting in a good drilling mud. In other areas formations such as limestones, sand, or gravel may be encountered which do not form mud. Under such conditions it will be necessary to add clays.
The drilling fluid must perform six primary functions:
1 Bring the cuttings to the surface.
2 Cool and lubricate the bit.
3 Wall up the hole.
4 Keep the wall under control, that is create a counter pressure on the formation.
5 Reduce friction on the drill string.
6 Drive the turbodrill hydraulically.
The choice of mud type for a specific instance is governed by those functions which are the most critical to the well in question.
Ability to carry cuttings up the hole and into a settling pit depends on the velocity of the fluid stream in the annulus between the drill pipe and the wall of the hole.
Text 4
Cooling and Lubrication of Bit
The heat generated by friction at the bit and other contact points between the drill string and the hole wall is absorbed by the drilling fluid. Improper lubrication of the bit and drill string will cause excessive torque and reduce bit bearing life.
Lubricants coat metal surfaces such as bit bearings with a high strength lubricating fjlm, thereby reducing metal-to-metal friction and reducing wjear..
The addition of oil such as in oil emulsion muds, also improves down hole lubrication and bit wear.
Text 5
U se of Compressed Air or Gas as the Circulating Medium in
Rotary Drilling
It is possible to use compressed air or gas as the circulating medium for removing drill cuttings instead of the usual mud fluid. With sufficient gas pressure and volume, the drill cuttings are readily swept out of the drilling zone and lifted to the surface through the annulus between the drill pipe and the wall of the well while suspended in the stream of flowing gas. Definite advantages are possible by this method, including greater drilling speed and longer bit life. However, there are certain disadvantages in the use of compressed air or gas: higher abrasive wear of pipes, the necessity of additional surface equipment for well head hennetization, higher capacity compressors as well as the increase of the rig load capacity.
No provision can be made for consolidating and sealing the walls of the well in soft formations, and it is difficult to control high formational fluid pressures when they are encountered.
If the strata penetrated yield water to the well, the drill cuttings become wet and sticky and are likely to cling to the walls and drill pipe.
If natural gas is used as the circulating medium, a considerable fire hazard is presented about the well head and in surface facilities.
[1] Read Text 2:
Text 2 D ensity
By the density of rock is meant the mass contained in the unit volume of its solid phase. It depends upon the density of mineral grains entering into composition of the rock, upon the density of cementing substances and the proportionate content of minerals and cement. The density of rock-forming minerals varies largely within a range of 1900-3500 kg/mJ, but rocks may also contain even heavier minerals. The density of rocks is habitually som ewhat inferior to that of the rock-forming minerals. The density of magmatic rocks is, as a rule, greater than that of sedimentary rocks, this being due to the presence of heavy ferromagnesian silicates. It declines with increasing proportion of quartz in the rock. The density of sedimentary rocks
[2] Learn the meaning of the following words, word-combinations and word groups:
triaxial, uniform, compressibility, volumetric, boundary, dwindle, permeability, submontane, crumble, dissimilar, shifting, gliding, displacement, enhancement, lattice, schistosity, segregate, ambient, massif, strata, acceleration, gravity, fluid.
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