GB2185507A - Shear thinning fluids - Google Patents

Abstract

A shear thinning fluid suitable for use as a drilling mud comprising a suspension of ferromagnetic particles having at least 50% by weight in the size range 0.5 to 10 micrometres.

Description

SPECIFICATION Shear thinning fluids The present invention relates to novel non-Newtonian fluids.

In fluids exhibiting pseudoplasticity the apparent viscosity decreases with an increase in the rate of shear.

Fluids which have a high viscosity when in an undisturbed state but whose viscosity can be decreased by the application of shear are useful forvarious purposes. Thus in the fluids (known as "muds") used in drilling gas and oil wells it is desirable for the fluid to have a relatively low viscosity when subject to shear so that it can be pumped readily butfortheviscosityto be high in the undisturbed state, in orderto transport rock cuttings to the surface and to preventthem from sinking through the fluid if pumping is stopped. Circulation may be stoppedforseveral days.It is therefore important for any fluid intended for use as a drilling mudto resist any tendency for solids present in the fluid to sedimenttoform a hard packed solid around the drill string.The preparation of fluids have shear-thinning properties often involvesthe use of organic additives e.g. surfactants.Thus drilling muds are normally prepared from clays often stabilised by organic chemical additives. However in deep, high temperature, wells these additives can decompose leading to a deterioration in drilling mud quality. In addition to heat, the stability ofa chemical additive may be affected bychemicals present in the environment in which it is used.

It is possible to prepare drilling muds containing clays without additives but even these have problems resulting from clay gelation at temperatures above 1 500C.

In the course of drilling a well it may enter rockformations containing gases or liquids under pressure.

Drilling fluids may contain weighting fluids to increase their density to assist in maintaining a sufficient hydrostatic pressure in the well to control it. The weighting material commonly used is barytes.

South African patentspecification 847953 discloses the use of magnetite as a weighting fluid. The magnetite has substantially all the particles with a size less than 75 micrometres, and approximately 86% with a particle size less than 45 micrometres. The magnetite is heat-treated to destroy its magnetic properties. There is no suggestion that the magnetite has any function otherthan as a weighting fluid.

US 4008775 discusses the undesirable breakdown of conventional drilling mud additives of temperatures commonly encountered at the bottom of wells. It discloses a drilling mud and packerfluid which contains high porosity iron oxide having the composition Fe304.The oxide is added to scavenge H2S and also to improve the rheological properties and thermal stabilityofthe drilling mud and to serve as aweighting agent. The iron oxide is prepared by a special process from iron borings. The particle size is measured by a Coulter counter.This is an indication that the oxide particles were not magnetically attracting one another, as the Coulter counter is not suitable for determining the sizes of particles which are mutually attracted into agglomerates. The particles were in the range 1.5 to 60 micrometres.

US 2276075 discloses a drilling fluid containing ground magnetite. The major proportion of the drilling fluid is stated to consist of particles in the size range usually employed for barytes, namely less than 325 mesh (44 micrometres). The specification also refers tothe use of materials passing a coarser sieve (200 mesh) (74 micrometres) and thus having a larger particle size. The specification refers to the possibility of including in the drilling fluid a minor proportion of relatively fine material preferably a chemicallyprecipita- ted ferro-ferric oxide with a particle size of about 0.5 micrometres. US 2276075 states that the resulting drilling fluid has quasi-thixotropic properties and that such properties cannot be obtained by ground magneticalone.We have made drilling fluids (drilling muds) in accordance with US 2276075. We have found that such drilling fluids are not satisfactory by modern standards, particularly in respect of their responseto water-loss control additives. Such additives are commonly added to drilling muds to control loss of water from the mud into porous rock formations.

Itwould be desirable to find a shearthinning fluid whose properties were not dependent on chemical additives.

According to the present invention there is provided a non-agitated fluid having shearthinning properties comprising a suspension offerromagnetic particles having at least 50% by weight oftheferromagnetic particles in the size range 0.5 to 10 micrometres in a liquid medium.

According to a further aspect of the present invention a process for making a bore in a rock formation by drilling while passing a fluid through the boreto remove rockfragments is characterised in thatthefluid is a shearthinning fluid comprising ferromagnetic particles having at least 50% by weight of the size range 0.5to 10 micrometres in a liquid medium.

Preferably at least 99% by weight of the ferromagnetic particles have a size less than 20 micrometres.

In the specification the term "ferromagnetic" is used to include materials which some writers describe as "ferrimagnetic".

The quantity offerromagnetic particles in the size range 0.5 to 10 micrometres is preferably at least 60% by weight.

The quantity of ferromagnetic particles having particle sizes above 10 microns is preferably not morethan 40% by weight, more preferably not more than 30% by weight, most preferably not more than 20% byweight.

An excessive amountoflarge particles will tend to reduce the shearthinning effect and will tend to cause wear on pumpsetcusedtomovethefluid.

The presence offerromagnetic particles having particle sizes below 0.5 micrometres is less objectionable than the presence offerromagnetic particles above 10 micrometres. Thus the presence of an increased numberoffiner particles will allow a more viscous fluid to be produced art a given volumefraction of particles (and thus density). The presence of surfactant (as is used in the production offerrofluids) will not significantly affect the ability offerromagnetic particles in the size range 0.5 to 10 micrometres to form a gel structure in the absence of shear. The presence of surfactantwill however have an adverse affect on the gelling ability of finer ferromagnetic particles.Where substantial amounts offine ferromagnetic particles are present sur- factants and stabilisers (used in the production of ferrofluids) are preferably substantially absent.

As the quantity offerromagnetic particles in the size range 0.5 to 10 micrometres is increased the viscosity ofthe fluid under shear is increased which will make it more difficultto pump or otherwise transportthefluid.

The volume offerromagnetic particles in the fluid is therefore preferably not greaterthan 0.6. More prefer- ably the volume fraction offerromagnetic particles in the fluid is in the range 0.3 to 0.1.

The density and rheology of the fluid may be modified by incorporating finely divided non-magnetic solid particles. These particles preferably have a size range substantially the same as that oftheferromagnetic particles.

The ferromagnetic particles are preferably particles of magnetite, which is cheap and readily available. The magnetite must not of course have been treated so asto destroy its ferromagnetic properties, eg by heat treatment. The fluids ofthe present invention may be prepared by grinding larger particles offerromagnetic materials until the required size range is obtained. The ferromagnetic particles used as starting material may for example have particles in the range 10 to 90 micrometres. An example of a readily available material having these characteristics is commercially available magnetite powder for example as used in sink-float beneficiation of coal.The grinding is conveniently carried out in the fluid in which the ferromagnetic particles areto be suspended intheshearthinning fluid. An example of a suitable liquid medium iswater.

Various grinding techniques give different inherent minimum sizes and the grinding technique used must be one which is capable of producing particles in the size range 0.5 to 5 micrometres. Examples of suitable grinding apparatus are tumbling ball mills and stirred ball mills. The duration ofthe grinding step will depend upon the desired viscosity under shear. The longer the grinding time the higher will be the viscosity. In general the grinding time will be in the range 1 to 100 hours, preferably 5to 24 hours. No additives are required in the mixture being ground to give stability.

The invention will now be described by reference to the following Examples.

In these Examples all viscosity measurements, exceptwhere indicated otherwise, were made using a constant stress rheometer manufactured by Deer Rheometers Limited fitted with a 5cm diameter cone. A lubricating oil moat was used to prevent drying out ofthe specimen being tested. The torque was increased linearly to 1 m N.m. The viscosity was measured at a shea r rate of 1 000 sec-'. Measurements were made at room temperature (ca 209C).

Example 1 Magnetite powder and water were introduced into a grinding mill in quantities such that the magnetitewas 0.3 ofthetotal volume of material introduced. No other materials were added. The magnetite was atypical commercially available powder having particles in the range lOto 90 micrometres.

The grinderwas a5 litre tumbling ball mill charged with about2 1 of 6 mm steel balls.

Grinding was carried outfor24 hours.

The slurry formed by grinding was washed from the mill and allowed to settle over several days. The excess water was decanted and the sediment was evaporated to dryness. The dried product was then redispersed in water using a high speed vortex mixer made by the firm of Silverson with a shear head attached.

Thevolumefraction %,the relative densityandtheviscositywere determined fora numberofsamples diluted to differing extents with water. The resu Its are shown in Table 1.

Example 2 An experiment was carried out as in Example 1 exceptthatthe magnetite was ground for about100 hours.

Experiments were carried out as in Example 1 with the product diluted with varying degrees of water. The results are shown in Table 2.

Example 3 Asample ofthe aqueous slurry prepared in Example 2was allowed to standfor3 days. Asedimentformed.

The supernatantwater was discarded. The sediment was found to have a volume fraction of 19.6% and a relative density of 1.80 This sediment was a shear-thinning fluid but its low shear viscosity was outsidethe range of the measuring instrument used.

Comparative Example A This is a comparative Example not according to the invention. An aqueous suspension was produced as in Example 1 exceptthat coal was used in place of magnetite. The coal was pulverised fuel from the Herrington Collieryofthe National Coal Board UK.

The coal/water slurry produced formed only a very weak gel.

Samples ofthe slurry in various dilutions were tested as in Example 1 are given in Table3.

Example 4 to 7 Experiments were carried out as in Example 2 i.e. using a grinding time of 100 hours butwith various amounts of magnetite and coal in the initial feed to the ball mill. The products obtained all showed a gel-like characterwhen at rest.

The relative density and the viscosity at 1000 sec1 were tested (without dilution with water). The results are given inTable4.

Table 1 Volume fraction Relative Density Viscosity (%) (mPa.s) 18.7 1.76 35.1 15.8 1.65 24.1 15.6 1.64 27.5 15.5 1.63 21.7 14.2 1.58 18.3 12.3 1.50 14.4 Table2 Volume fraction Relative Density Viscosity (mPa.s) 17.3 1.71 47.3 15.4 1.63 29.6 13.8 1.56 23.2 11.3 1.46 15.3 Table3 Volume fraction Relative Density Viscosity (mPa.s) 15.8 1.050 14.5 14.4 1.046 12.4 13.1 1.042 10.7 10.7 1.034 9.8 Table4 Example No. 3 4 5 6 Mag.wt.fraction% 47.6 43.1 31.8 17.2 Coalwtfraction% 3.4 7.1 12.8 19.2 Mag.Volfraction% 15.4 13.3 8.8 4.2 Coal Vol fraction% 4.2 8.4 13.6 17.8 Solidwt.fraction% 51.0 50.2 54.6 36.4 Solid Vol fraction% 19.6 21.7 22.4 22.0 Relative density 1.64 1.57 1.40 1.23 Viscosity(mPa.s) 55.0 44.9 33.9 19.5 Example 8 An experiment was carried out as in Example 1 exceptthatthe magnetite was milled for 7 hours.

The magnetite particle size was determined by converting it to non-magnetite haemetite and then using a commercial particle size counter as described below.

The dried powder was redispersed in water as in Example 1, but in addition 6.7% by weight of kaolin, based on weight of magnetite, and about2%wtof a commercially available water loss control starch were also added to the magnetite. The kaolin was added to simulate the presence of the solid particles which would be present in a drilling mud as a result of drilling. The purpose of the starch is to reduce water loss into the rock formation being drilled.

The particles size distribution, the fluid loss characteristics, filter cake thickness, and the viscosity characteristics are given in Table 5.

The fluid loss was determined by Standard American Petroleum Institute Test Procedure RP13B Section 3 with a pressure of 6.8 atmospheres (0.68 MPa) for 30 minutes.

The filter cake thickness was determined by direct measurement and is inversely related to fluid retention.

It gives an indication of mud performance (thickfiltercakes reduce borehole gauge and increase the likelihood of differential sticking).

The suspension had shear-thinning properties and had a gel-like characterwhen at rest.

Comparative Tests, CandD Experiments were carried out as in Example 8 but using magnetite ground for 1,2 and 4 hours respectively.

The drilling fluids obtained had a water loss equivalent to that for Example 8. However they did not have satisfactory viscosity or stability characteristics. Thus these materials showed poorsuspension stability (be- tween 40% and 60% in 2 hours). They were apparently weakly shearthinning but contained too many large particles to allow realistic viscosity measurements to be carried out. Particle size analysis by sieving magnetite ground for 1,2 and 4 hours indicated that the proportion of magnetite having particle sizes less than 10 micrometres was less than 50% by weight, and the proportion of magnetite with particle size greaterthan 20 micrometres was greaterthan 1 % weight.

The variation of particle size with grinding or milling timefora sample of magnetite is given in Table 5.

Table 5 Sieve Size Analysis of Magnetite- Effectof Milling Size Range HRS Milling -wt% 0 1 2 4 7 12 24 116 +3011m 48.1 30.5 11.4 1.3 Trace 0.3 0.3 0.1 20-30 Am 12.2 16.8 17.3 4.6 1 0.1 0.3 0.1 10-20 > m 24.2 33.7 46.6 54.1 39.2 19.1 2.2 0.3 -lOl-Lm 15.4 19.0 24.8 39.9 59.8 80.5 97.2 99.5 Example 9 An experiment was carried out as in Example 8 but using a 24 hour grinding time.

The results obtained are given in Table 6. The suspension showed shear-thinning properties and had a gel-like character at rest.

Table6 Example 8 9 milling time 7 24 d50 micrometres 8.2 4.6 d80micrnmetres 13.0 7.0 less than 10 micrometres 70% 98% less than 30 micrometres 100% 100% 20-30 micrometres 1% 0% 10-20 micrometres 28% 1% lessthan6micrometres 38% 30% fluid loss 6ml 25ml filtercake 2mm 5mm viscosity characteristics (MPa.s) 20.3 28.3 Respectively 50% and 80% by weight of the particles have sizes below the values given ford50 and d80.

Comparative TestE Adrilling mud was prepared in accordance with the disclosure of US 2276075.

Commercial magnetite was wet sieved to provide magnetite with a particle size less than 44 micrometres.

The precipitated magnetite was laboratory reagent grade supplied by BDH Chemicals Ltd, and mixed with the dried sieve magnetite and redispersed in water as in Example 1. The weight ratio of sieved magnetiteto precipitated magnetite was 7:1.

The volume fraction of magnetite in water was 0.3.

The viscosity was determined using a Brookfield viscometer, and the sedimentation stability and fluid loss were determined as before. The results are given in Table 7.

The material was shearthinning. However as can be seen from Table 7 its other characteristics make it unsuitable for useas a drilling mud.

Comparative Tests Fand G Test Ewas repeated but with 0.2%wt and 1.2%wt respectively of a commercially available starch sold as a water loss reduction additive under the trade name "ldflo".

The results are given in Table 7.

Comparative Tests Handy Tests were carried out as in Comparative Test E and G but with a magnetitevolumefraction of 5.

The results are given in Table 7.

Example 10, 11 and 12 Experiments was carried out as in Example 1, with magnetite being milled for 24 hours and with varying amounts of water-loss control additive (0,0.4 and 2% by weight respectively).

The resultsaregiven inTable7.

These suspensions all showed shearthinning behaviour and had a gel-like character at rest.

Table 7 Experiment E F G H 1 10 11 12 V magnetite volumefraction 0.3 0.3 0.3 0.15 0.15 0.15 0.15 0.15 Viscosity (mPa.s) Brookfield6rpm 5328 - - - - 2331 - Deer rheometer at 1000 s-l (mPa.s) - - - - - 16 13.4 26 Relative density 2.25 2.25 2.25 1.6 1.6 1.6 1.6 1.6 Fluid loss (ml) Total Total Total 94 Total Total 9 Sedimentation set stability (%) 79 - solid 38 47 85 79 96 Sedimentation stability is the sedimentation height of the undisturbed suspension after 2 hours as a percentage of the initial height. The material prepared according to US 2276095 contained a substantial propor tion of material with a particle size greaterthan 10 micrometres.It was therefore impossible to obtain realistic viscosity measurements using the cone and plate arrangement on the Deerviscometer because the clear- ance between the parts is too small. A Brookfield viscometer was therefore used to provide a comparison between Comparative Test E and Example 10.

Acomparison ofthe results shown in Table 7 showsthata drilling mud made according to US 2276075 has a very high viscosity and shows a total water loss in the absence of a water-loss control additive. If a water loss additive is added howeverthe mud sets solid and cannot be used.

If a diluted version of the mud of US 2276075 is madethen it is possible to add water loss control additive without causing the mud to set solid. However the water loss is still high, and the stability of the mud is relatively low.

The drilling mud according to the invention has a very much better response to water-loss control addit ives. The sedimentation stability of the mud according to the present invention is much betterthan thatof materials made in accordance with US 2276075.

All the shear-thinning fluids of the above examples ofthe invention contained a substantial proportion of ferromagnetic particles in the size range 0.5 to 10 micrometres as is shown by the particle size data below obtained from two samples of magnetite suspension made as described above.

All measurements were made using a Malvern 3600E particle sizerfitted with a 63 mm lens. The Malvern is a light scattering deviceforwhich the particle size distribution quoted is that which has a calculatedtheoreti- cal diffraction pattern that fists closes to the actual measured pattern. Forthese experiments a model independent program was used (ie a model distribution such as log-normal or Rosin-Ramlerwas notassumed).

The technique used in orderto measure the particle size distribution was as follows:- The suspension was dried and heated overnight at815 C.This converted the magnetite to haematite,thereby elliminating particle aggregation by magneticforces.The resultant cake was broken up using a mortarand pestle and the powder dispersed in waterfor3 hours in an ultrasonic bath.

The Malvern was set up using the stirred flow cell with the ultrasonics turned on. A dispersant (Tamol NH) was added, although this appeared to have little effect on the results, which are given overleaf.

Although in all cases the fit between the theoretical data and the measured data was poor, the results quoted are consistent with electron microscope observations.

Table8 Size ( > m) 7hourgrind 24 hour grind 100 hour grind Haematite Haematite Haematite 13.6 82.3 100 100 10.5 72.0 100 100 8.2 49.8 100 99.8 6.4 30.3 98.7 98.3 5.0 23.6 90.5 97.9 3.9 13.6 69.8 94.7 3.0 3.8 40.6 87.2 2.4 0.8 16.4 72.5 1.9 0.3 46.1 1.5 0.0 6.4 1.0 0.0 0.6

Claims (10)

1. A non-agitated liquid having shearthinning properties which comprises a suspension of fer- romagnetic particles having at least 50% by weight of the ferromagnetic of particles in the size range 0.5to 10 micrometres.

2. A liquid according to claim 1 wherein the suspension offerromagnetic particles has atleast60% ofthe ferromagnetic particles in the size range 0.5 to 10 micrometres.

3. A liquid according to claims 1 or 2 wherein not more than 40% by weight ofthe ferromagnetic particles have particle sizes greaterthan 10 micrometres.

4. A liquid according to anyone ofthe preceding claims wherein not more than 30% by weight ofthe ferromagnetic particles have particle sizes greaterthan 10 micrometres.

5. A liquid according to any one ofthe preceding claims wherein at least 99% by weight ofthefer- romagnetic particles have a size less than 20 micrometres.

6. A liquid according to any one of the preceding claims wherein the volume fraction offerromagnetic particles in the liquid is not greaterthan 0.3.

7. Aliquid according to claim 6whereinthevolumefraction is in the range 0.1 to 0.3.

8. A liquid according to anyone ofthe preceding claims wherein the ferromagnetic particles are particles of magnetite.

9. A liquid according to any one of the preceding claims wherein the liquid contains a water-loss prevention material to prevent loss offluid from the liquid into porous rockformations.

10. A method of making a borehole in a rockformation by drilling while passing a liquid through the bore to remove rockfragments characterised in thatthe liquid is a liquid according to any one of claims 1 to 8.