NO150920B - WATER-BASED DRILLING FLUID CONTAINING A GRINDING, GROWING AGENT - Google Patents

Description

Present opof submission regarding? r water-based drilling fluids r that contain abrasive weighting agents.

Drilling fluids used when drilling oil wells pass bro ri-

ner and similar boreholes in the earth are usually water-based mixtures containing clays or other colloidal materials as well as special additives depending on the borehole conditions. The drilling fluid or mud acts primarily as a medium to bring cuttings formed by the drill bit to the surface. In addition, the drilling fluid serves as a lubricant for the drill bit and the drill string and prevents the ingress of fluids from the formation, such as e.g. oil, gas and salt water into the drilling

the hole while drilling is in progress. The drilling fluid also serves other purposes

measure and have other properties which need not be discussed in detail here because they are well known and described in several <p>publications such as: Walter F. Rogers' "Corrposition and Properties of Oil Well Drilling Fluids, 3rd ed., Houston, Texas, 1963 and Kirk-Dthmer, Encyclopedia of Chemical Technology,

2nd ed., New York, 1965, vol. 7, pp. 287 - 307.

As mentioned, one of the functions of the drilling fluid is to maintain a sufficient hydrostatic pressure in the fluid column in the borehole to prevent the penetration of fluids from the formation into the borehole, and, in extreme circumstances, to prevent blowout in the well. Depending on the drilling conditions encountered, sufficient hydrostatic pressure can be achieved with clean water. However, under normal conditions, and in a Iminde 1 ight as

as a safety measure, it is necessary, in order to achieve the desired hydrostatic pressure, to add to the sludge a suitable weight-increasing agent1 to increase its density. These weight gainers are generally available in the form of finely divided solids of a material with a high specific gravity. The most common weight gain in gsmi dde 1 is finely ground baryte, although iron oxides, celestite, lead luster and other metphiri als have been used in their times. Barytt has been the preferred weight gainer due to its high specific gravity, insolubility, chemical inactivity and

because until now it has been easily accessible. Due to the decreasing availability of baryte, other weight-increasing agents have now been sought.

Apart from the fact that barite supplies are dwindling and in some areas of the world, where drilling activity is high, practically non-existent, barite is relatively soft, with 2.5 to 3.5 on the Mons scale of hardness. This relatively soft nature of barite affects the theological properties of the drilling mud after the barite has circulated in the borehole during the drilling operation. During the drilling operation, the entire circulating system acts as something

in the direction of a liquid mill so that the softer mineral particles are subjected to grinding and are reduced in particle size.

This reduction in particle size causes a direct increase

in the viscosity of the drilling mud, which is due to a much smaller particle size and an increase in the number of particles.

It has been proposed to use bulking materials other than baryte to overcome the difficulties described above in connection with relatively soft bulking materials such as baryte, which are subjected to constant fine crushing.

British Patent No. 1,495,874 covers the use of ilmenite and/or hematite as bulking material for drilling fluids. Also, Colombian patent no. 9396 covers the use of micaceous specular hematite as a weight increasing agent in drilling fluids. The ilmenite used according to British patent no. 1,495,874 is ground to a fineness where at least 85% passes a sieve with a 0.044 mm sieve opening (325 mesh screen), and in general somewhat finer, so that 90 - 92% passes 0.044 mm. Ilmenite also has a sp. v. of at least 4.3, and according to the examples above 4.4. It is also known that ilmenite has a hardness of 5 - 6 according to Mons' scale for hardness.

Specular hematite is a naturally occurring mineral spread all over the world in different forms and different purities. For example the mica-like iron sheen according to the above-mentioned Colombian patent is a relatively soft, mica-like mineral with a clear laminar structure. When this mica in the kn en de iron luster is painted, it tends to form very thin, flat flakes, which when used in drilling fluids are easily ground into very small particles that have the effect of increasing the viscosity of the drilling fluid. The mica-like iron sheen is practically non-soaking under the conditions encountered in boreholes. The iron luster which, however, relates to the present invention is a non-hydrated mineral which, although of general laminar structure, is hard and, when ground, forms particles with irregular, abrasive edges. Such iron luster has a relatively high specific gravity, i.e. higher than 4.3, and a hardness according to the Mohs' scale of 5 - 6. When used as a weight-increasing agent in drilling fluids, the iron luster according to the present invention has no tendency to break into smaller particles which to a particular extent will change the rheological properties of the drilling fluid. However, due to its inherent hardness, this iron luster is quite abrasive and can result in unreasonable wear on the drilling equipment, particularly on the drill string and drill bit. As the search for oil and gas results in ever deeper wells being drilled, wear or erosion of the drill string and drill bit is becoming an ever increasing and serious problem. It is therefore an aim of the present invention to provide a new and improved drilling fluid, a drilling fluid which can use relatively hard, abrasive weight increasing agents.

The weight gain agents which are suitable according to the present invention

in drilling fluids are hard, non-aqueous minerals with a specific weight of approx. 4.3 or greater, e.g. iron luster (specular hematite). Iron luster is a commonly occurring mineral which chemically is ferric oxide iFe2®^' It has been described e.g. in Ivan Kostow: "Mineralogy", (Oliver S Boyd, Edinburg and London) 1.

meadow. ed. (1968). Typical iron luster contains 69.94% Fe and 30.06% 0. Some Ti may be present. Iron luster is often found mixed with hydrous iron oxides, iron carbonates and magnetite as well as j e ms i 1 i cater. However, deposits of i kke-van nho Idi g iron luster are available, so removal of water of hydrate is not necessary.

The weight gain agent according to the present invention will have a specific gravity of at least 4.3, preferably at least 4.5. If necessary, the weight gain medium can be enriched according to well-known metallurgical processes, e.g. flotation, to achieve the desired minimum of sp.v. The weight gainer must have a hardness on the Mohs' scale of at least 4.3. However, many iron lusters have a hardness of 5 - 6. For use according to the present invention, the weight increasing agent must have a particle size such that at least 85% by weight passes a 0.044 mm sieve and at least 98% by weight passes a 0.074 mm sieve. More advantageously, the particle size should be such that 90 - 92% passed a 0.044 mm sieve. The desired particle size of the weight gain medium can be achieved by known grinding procedures.

According to the present invention, an improved water-based drilling fluid is provided containing an abrasive, weight-increasing agent consisting of non-hydrated minerals with a Mohs' hardness of at least 4.5, a specific gravity of at least 4.4, in particular specular hematite and /or ilmenite, which has a particle size where at least 85% by weight passes a sieve with a mesh opening of 0.044 mm and 98% by weight a sieve with a mesh opening of 0.074 mm, and where said weight-increasing agent is present in an amount sufficient to significantly increase the specific of the drilling fluid weight. The drilling fluid according to the invention is characterized in that it contains from 0.1 to 5% by weight of a water-dispersible anti-wear agent1 consisting of one or more fatty acids and/or triglycerides of fatty acids, preferably oleic acid, stearic acid, linoleic acid and/or palmitic acid or their corresponding vegetable oils mixed with long-chain alcohols, sulphonated vegetable oil derivatives and/or alkanolamines.

The amount of weight increasing agent present in the drilling fluid can vary greatly, depending on the desired density of the drilling mud.

This, of course, again depends on the drilling conditions one encounters or expects to encounter. In any case, the amount of baryte that is necessary to achieve a desired mud density is well known and tables showing this are available. From a comparison of the specific gravity of the bulking agent, as used herein, and that of the barite, such tables can be used to determine the amount of bulking agent necessary to provide drilling muds or drilling fluids of any density.

The drawing shows:

Fig. 1 a diagram showing weight loss during grinding caused by different drilling fluid compositions. On the ordinate axis is the weight loss due to wear in % and on the abscissa the stirring time in hours. Fig. 2 is a diagram showing the abrasive wear characteristics for different drilling fluid compositions. On the ordinate axis is the degree of wear in grams/hour and on the abscissa the torsional load in kg.cm.

These diagrams are discussed in more detail after the examples.

As stated earlier, the weight-increasing agents according to the present invention are materials which are relatively hard and therefore less prone to constant grinding down. At the same time, the weight-increasing agents exhibit relatively high abrasive properties which can result in unusually large erosion of the drilling equipment such as pumps, lines and drill bits during the drilling operation. It has now been found that if suitable water-dispersible anti-wear agents are added to drilling fluids that have been prepared with relatively hard weight-increasing agents, according to the present invention, the abrasive effect of the drilling fluid can be reduced to a degree where in reality no greater wear occurs than that which occurs with drilling fluids with the same density and which use the much softer barite as a weight-increasing agent. The anti-wear agent used in the drilling fluid according to the present invention must be dispersible in water to the extent that it forms a colloid, emulsion or other such heterogeneous system in which the anti-wear agent is mostly evenly distributed or dispersed in the water phase . The anti-wear medium must be of a type that allows the desired rheological properties to be achieved in the drilling fluid. Preferably, the anti-wear medium should be a material which, used in sufficient quantities, will provide a usable drilling fluid when using hard weight-increasing agents according to the present invention and which will exhibit wear and/or grinding properties approximately equivalent or practically equal to those obtained with a drilling fluid of the same density or sp.v. when using baryte as a weight gainer. A group of anti-wear agents which have been found to be particularly effective in fresh water-based drilling fluids according to the present invention are those which contain fatty acids such as oleic acid, stearic acid, linoleic acid, palmitic acid etc. Such fatty acids are usually found in vegetable oils such as cottonseed oil, soya oil, corn oil, safflower oil, linseed oil, etc. The fatty acids, either in their pure state or in the form of the vegetable oils mentioned above, are mixed with small amounts of surfactants or other agents so that they become water-dispersible. Such surfactants may include e.g. long-chain alcohols, alkanolamines, sulphonated vegetable oil derivatives such as sulphonated castor oil etc. A particularly effective anti-abrasion

part is a mixture of soybean oil, long-chain alcohols and sulphonated castor oil. This composition and similar materials make ideal anti-wear agents because they only need to be added in relatively small amounts to the drilling fluid to reduce the abrasive action to an acceptable level. Moreover, because only relatively small amounts are needed, there is only a minimal deterioration of the Theological properties.

The amount of anti-wear mite1 in the drilling fluid can be varied within a large range, mEn will generally lie in the range from 0.1 to 5% by weight. However, the anti-wear agent generally need only be present in an amount which will effect a. significant reduction of the abrasive effect of the drilling fluid.

Preferably, the drilling fluid comprises thickeners. Such thickeners can consist of clay-containing formations such as clay-shale or clay-containing sand which are encountered during drilling operations and which are taken up by the drilling fluid or mud and become part of this. Alternatively or in addition, clays can be added to the sludge. Typical non-limiting examples of highly colloidal clays include smectites, particularly bentonite, attapulgite and sepiolite, with bentonite being preferred. A number of other water-dispersible materials, in particular organic colloids can serve as thickeners, e.g. starch derivatives, cellulose derivatives, synthetic polymers such as sodium polyacrylate, natural gums such as guar gum and karaya gum, bacterial gums such as e.g. made up of special Xanthomona species, etc. Mixtures of such suitable thickeners may be used if desired. The technology for using such thickeners in drilling fluids is well developed and well known to professionals. The different thickening media are essentially different when it comes to weight-Efficiency expressed in grams (or kilograms) that must be added per liter of drilling fluid or mud of a certain type to achieve the desired consistency.

In addition to thickening agents, the drilling mud may optionally contain liquid loss-regulating agents, viscosity-regulating agents, anti-foaming additives or freezing-point lowering agents.

□it is understood that by using the drilling fluids herein, also while the borehole is being drilled during drilling operations,! on one, further amounts of the weight increasing agent can be added to maintain or change the specific gravity and/or abrasiveness of the drilling mud, if desired.

To further illustrate the present invention, reference is made to the following non-limiting examples.

The iron glazes used were from the U. S. Steel Corporation and were non-aqueous and had a sp.v. in the range 4.9 to 5.1 and with a particle size where 92% passed a sieve with a mesh size of 0.044 mm. The Norwegian ilmenite concentrates used in the following examples were from Titania A/S, Hauge i Dalane and were of flotation quality with a moisture content of approx. 4 to 5% and a Ti02_content of 44-45%. The American Tahawu's ilmenite was of flotation quality with a TiO2~ content in the range from 45% to 48%.

Example 1

A base drilling fluid or mud was prepared by mixing 25 g of bentonite distillers ("AQUAGEL" from NL Baroid, Houston, Texas) and 50 g of Glen Rose shale into 350 ml of water for 20 min. using

a fast-running mixing device (Dispersator). To the sludge was then added 5 g of lignosulfonate ("Q-BR0XIN" from NL Baroid), 5 g of an organic humic acid material ("CARBDN0X" from NL Baroid),

1.5 g of caustic soda and 4 g of a surfactant for drilling fluids ("AKTAFL0 S" from NL Baroid) with stirring for 10 min. in the above mixer. The prepared sludge was aged overnight at room temperature.

Example 2

Using the base mud prepared in Example 1, various weight increased drilling muds were prepared using a baryte ("BAROID" from NL Baroid), Norwegian ilmenite, Tahawu's ilmenite and iron luster. Assessment of the weight-increased drilling mud produced in this way was carried out for drilling mud with 1.56 kg/l mud (table I),

1.92 kg/l (Table II) and 2.16 kg/l (Table III).

As can be seen by comparing the results from Tables I, II and III, drilling fluids made with iron luster and ilmenite prove to be advantageous over conventional drilling muds made with barite as weight increasing agent.

Example 3

In order to test the proportional reduction rates at high shear forces of different weight-increasing agents in drilling mud, the mud was mixed with 1.92 kg/l. produced according to examples 1 and 2 used. As can be seen from the data in Table IV, the attrition rates for iron luster and ilmenite are significantly lower than that for barite in comparable drilling muds. The resistance to wear is advantageous in order to guarantee that the rheological properties of the drilling fluid are not prone to change during the drilling operation where the mud is constantly being circulated in the borehole.

Example 4

A water-based drilling mud was prepared using 15 g of bentonite clay ("AQUAGEL") and 60 g of Glen Rose shale slurried in 350 ml of water and subsequent addition of 300 g of the weight increasing agent to be evaluated. Sludge made with iron luster was diluted with 12 g/l of a polyphosphate compound ("BARAFOS" from NL Baroid). In order to determine the wearing or grinding properties of the produced drilling mud and the effect of adding anti-wear agents, two methods were used. One method, method A, took into account the phenomenon of wear by the movement of particles in a liquid against a metal surface. With this method, the total weight loss from the entire surface was measured.

Method A

A "Hamilton Beach mixer", model 936 with a rotational speed of 15,000 revolutions per minute (rpm) was used to create a laboratory simulated grinding effect and particle breakdown. For each drilling mud sample, a new, weighed, mixer blade was used as a reference basis for grinding action. Periodically, the leaf was examined for weight loss after different stirring times. The sludge was introduced into a cooling bath during stirring to prevent overheating and evaporation. Thickening of the weight-gaining sludge during agitation was minimized by the addition of a polyphos phate compound ("BARAFOS"). The results of the wear rate tests are shown in Table V. Another wear or abrasive test method, Method B, which was used took into account the movement of metal surfaces against each other while exposed to flowing fluids. In method B, the purpose was to measure the weight loss from a surface that was exposed to stress.

Method B

The test block that was used to measure extreme pressure properties (EP) was modified by turning so that a constant wear surface would be exposed to the test piece during the test. A rotation speed of 432 rpm was chosen because this roughly corresponds to a drill pipe rotation of 130 rpm and is a fairly typical speed for well drilling methods. Varying torsional loads were applied to the ring and block at this speed in 15 min intervals. The sample block's initial weight and final weight were measured and the block's weight loss determined for each sludge sample. The wear rate was determined and calculated for one hour. The results are shown in the tables

VI X.

As can be seen from the data given in tables V - X, ilmenite and iron luster show a much higher abrasive effect than baryte.

From table V it can be seen that the addition of anti-wear mite1 significantly reduces the abrasive effect of drilling mud to which iron luster has been added as a weight-increasing agent. The results are clearly shown in the diagram in fig. 1 where drilling mud increased in weight with iron luster, without the addition of anti-wear mite1, (curve A) is compared to a similar weight-increased mud containing approx. 11.3 g/l of an anti-wear agent, (curve B), and a typical drilling mud that has been increased in weight with barite (curve C).

With reference to tables VI - X and fig. 2 also shows that the degree of wear of mud with iron luster is much greater than conventional mud that has been increased in weight with barite. However, as the data show, which is plotted in fig. 2, the addition of relatively small amounts of anti-wear agent significantly reduces the degree of wear. It should be noted, for example, that the degree of wear for iron luster-added mud (curve''A), which is highly abrasive without the addition of an anti-wear agent, shows an abrasive effect when it is added approx. 22.7 g/ anti-wear agent (curve D) which is equal, or practically equal, to that of conventional mud with added barite (curve C). Even with a relatively small amount of anti-wear agent, i.e. 11.3 g/l, (curve B), the grinding effect has been greatly reduced.

It can thus be seen that greatly improved drilling mud, which shows excellent rheological properties, can be produced with hard or relatively hard weight-increasing agents without excessive wear or grinding occurring when the drilling mud, in addition, contains an anti-wear agent as described above.

Claims (3)

1. water-based drilling fluid containing an abrasive, weight-increasing agent consisting of non-hydrated minerals with a riohs' hardness of at least 4.5, a specific gravity of at least 4.4, especially specular hematite and/or ilmenite, which has a particle size where at least 85% by weight passes a sieve with a 0.044 mm mesh opening and 98% by weight a sieve with a 0.074 mm mesh opening, and where said weight-increasing agent is present in an amount sufficient to significantly increase the specific weight of the drilling fluid, characterized in that the drilling fluid contains from 0.1 to 5% by weight of a water-dispersible anti-wear agent consisting of one or more fatty acids and/or triglycerides of fatty acids, preferably oleic acid, stearic acid, linoleic acid and/or palmitic acid or their corresponding vegetable oils, mixed with long-chain alcohols, sulfonated vegetable oil derivatives and/or alkanolamines. 2. Drilling fluid according to claim 1, characterized in that the wear medium is a mixture of soybean oil, long-chain alcohols and sulphonated castor oil. 3. Drilling fluid according to claims 1-2, characterized in that it contains specular hematite and/or ilmenite as an abrasive weight-increasing agent, and as further weight-increasing agents it may contain barite, celestite, lead sheen or mixtures of these.