GB2055412A - Heavy minerals for drilling fluids - Google Patents

Abstract

A weighting agent for use in a drilling fluid for oil and gas wells is formed by mixing together fine particles of one or more heavy minerals of specific gravity greater than 4.0 and a flux and sintering the mixture to form a particulate product. Suitable heavy minerals include compounds of barium, iron, titanium, lead, tin, copper, manganese and tungsten and the flux may be, for example, boric acid or a borate, a phosphoric acid or a phosphate or an alkali. The particulate product is preferably formed or comminuted to consist predominantly of granules having sizes in the range from 50 microns to 75 microns, and the fine particles are preferably substantially all smaller than 50 microns. The sintering step is preferably performed at a temperature not greater than 1200 DEG C and for the minimum time required to cause the fine particles to cohere.

Description

SPECIFICATION Heavy minerals for drilling fluids This invention relates to drilling fluids for oil and gas wells and is particularly concerned with drilling fluids which have a high initial specific gravity and retain this high specific gravity in use. In the rotary method of drilling wells for oil or gas, a drill bit is rotated at the bottom of the hole by a drill pipe which extends to the surface and which has an outer diameter which is substantially smaller than the diameter of the hole being drilled. A drilling fluid, commonly called a drilling "mud" is pumped down through the hollow bore of the drill pipe, whence it emerges through bores in the drill bit and rises through the annular space between the drill pipe and the wall of the hole.The primary function of the drilling fluid is to raise to the surface the cuttings which are dislodged from the rock formation as the drill bit cuts through it, but the drilling fluid also serves to cool the bit, to lubricate the cutting edges and to act as a seal to prevent fluids such as oil, gas or water in the rock formation from passing into the hole.

It is the last named function with which this invention is principally concerned. The rock formation fluids are often at a high pressure and if the column of drilling fluid rising up the annular space between the drill pipe and the wall of the hole is to form an effective seal its hydrostatic pressure at the level of the formation fluid should be greater than the pressure of the formation fluid itself. The effectiveness of the seal is improved by using a drilling fluid of high specific gravity which is achieved by incorporating in the drilling fluid one or more ingredients which themselves have high specific gravities. These high specific gravity components of drilling fluids are commonly known as "weighting agents" and are generally naturally occuring mineral materials.

In order to be effective as a weighting agent a material should have a specific gravity of a least 4.0 and should be capable of being incorporated in a drilling fluid in an amount not less than 40 to 50% of the total weight of the drilling fluid. A drilling fluid containing this amount of weighting agent should still be sufficiently fluid to permit circulation of the drilling fluid through the bores of the drill bit, bearing in mind that colloidal additives are generally incorporated in the drilling fluid to impart plastic and thixotropic properties so that if drilling fluid circulation is interrupted the cuttings held in suspension in the column of drilling fluid in the hole do not quickly settle to the bottom. The particle size distribution of the weighting agent is also important.Firstly the particles of the weighting agent should not be so large that they settle through the drilling fluid at an undesirable rate. This requirement imposes an upper size limit on the particles of the weighting agent of about 75 microns. Secondly the particles should not be so fine that they are similar in size to the particles of clay or silt which accumulate in the drilling fluid. If a drilling fluid emerging from the top of a bore hole is to be recovered and recycled it is necessary first to remove from it the waste material dislodged from the rock formation as the drill bit cuts through it.

This waste material generally comprises relatively large chips of rock, particles of sand which are generally larger than 75 microns in size and particles of clay or silt which are generally smaller than 50 microns and often of the order of a few microns or smaller. The larger chips and the sand can be separated by centrifugal sedimentation and/or sieving and the silt can also be separated by means of sieves.

However, if the weighting agent contained a substantial proportion of particles smaller than 50 microns, these would be removed with the silt and lost from the drilling fluid. The ideal weighting agent therefore consists of particles almost all of which are between 50 microns and 75 microns in size. It is also important that the particles of the weighting agent should be sufficiently resistant to crushing, abrasion and impact to avoid being broken down to an undesirable extent by the pumps and by the drill bit.

On the other hand the particles of the weighting agent should not be so hard they they abrade the components of the pumps, the drill pipe and the bit.

Hitherto the most commonly used weighting agent has been barytes or naturally occurring barium sulphate. This mineral has a specific gravity of about 4.2 to 4.5 but is relatively soft, having a Moh hardness of 2.5 to 3.5, and its particles are therefore relatively easily broken in the drilling operation.

Other weighting agents which have been used include haemetite and other iron oxides and hydroxides, galena or lead sulphide and ilmenite or naturally occurring ferrous titanate.

Drilling fluids may have as the suspending liquid either oil or water or either liquid may contain the other as an emulsified phase. The object of this invention is to provide a weighting agent for a drilling fluid which satisfies more completely than the weighting agents currently in use the conditions set out hereinabove.

Accordingly, a first aspect of the present invention provides a weighting agent for a drilling fluid for an oil or gas well said weighting agent comprising granules formed by sintering a mixture comprising fine particles of one or more heavy minerals of specific gravity greater than 4.0 and a flux.

The fine particles advantageously comprise an oxide or hydroxide of iron, a barium mineral or a compound comprising iron ortitanium, but other fine particles such as those comprising minerals of lead, tin, copper, manganese and tungsten may also be used.

Suitable fluxes for use with oxides and hydroxides of iron and with barium minerals include boric acid, phosphoric acids and salts thereof, and, for compounds comprising iron and titanium, such as ilmenite, alkalies are suitable as the flux.

Preferably the weighting agent consists predominantly of granules having sizes in the range from 50 microns to 75 microns.

The fine particles are preferably substantially all smaller than 50 microns.

According to a second aspect of the invention there is provided a process for manufacturing a weighting agent for a drilling fluid wherein fine particles of one or more heavy minerals of specific gravity greater than 4.0 are mixed with a small quantity of a flux, the mixture is sintered and the sintered material discharged in particulate form. The mixture of fine particles and the flux is preferably sintered at a temperature not exceeding 12000C and for the minimum time required to cause the fine particles to cohere in order to conserve energy.

The flux may be mixed with the fine particles in substantially dry form or in an aqueous medium, for example in the form of an aqueous suspension or solution.

The mixture may be formed by grinding relatively coarse pieces of the heavy mineral(s) together with the flux in substantially dry form or in an aqueous medium in order to prepare an initmate mixture of fine particles of the heavy mineral(s) with the flux.

For example barytes in substantially dry, lump form may be ground with borax to form an intimate mixture in which chemical reaction takes place between the borax and the surface of the barytes freshly exposed by grinding to form a thin film of barium borate on the surface of the barytes. By this method only a very small amount of flux is required to provide the desired effect in the sintering step.

The procedure described in the preceding paragraph make it possible to make granules for use as weighting agents for drilling fluids from raw materials which would normally be considered too hard and thus too abrasive to be used with conventional drilling equipment. Thus dense but hard materials such as ilmenite and haematite may be ground in the presence of the flux until the particle size distribution of the material is such that the abrasiveness is acceptable, which particle size distribution would generally be finer than the optimum specified for a weighting agent, and the mixture of finely ground hard material and flux sintered to form granules of the required particle size and specific gravity.

When the flux is mixed with the fine particles in -an aqueous medium the mixture may be in the form of a paste or slurry. If the mixture is in slurry form the slurry may be introduced into a spray drier to form a substantially dry product comprising minute spheres of the mixture. These spheres may then be sintered by exposure to a temperature not exceeding 1 200 C for a very short time, as, for example, in a flash calcining operation.

Alternatively the flux may be added to the furnace in which the mixture is sintered so that the flux is vaporised and reacts chemically with the surface of the fine particles.

As a further alternative the mixture of fine particles and flux may be sintered to form relatively large solid lumps which are then ground to the desired particle size.

According to a third aspect of the invention there is provided a drilling fluid comprising a weighting agent in accordance with the invention.

Claims (19)

1. A weighting agent for a drilling fluid for an oil or gas well, said weighting agent comprising granules formed by sintering a mixture comprising fine particles of one or more heavy minerals of specific gravity greater than 4.0 and a flux.

2. Aweighting agent according to claim 1,wher- in the fine particles comprise an oxide or hydroxide of iron or a barium mineral.

3. Aweighting agent according to claim 1, wherein the fine particles are of a compound comprising iron and titanium.

4. Aweighting agent according to claim 1, wherein the fine particles comprise one or more minerals chosen from the group consisting of lead, tin, copper, manganese and tungsten minerals.

5. Aweighting agent according to claim 2, wherein the flux is boric acid, a phosphoric acid, a borate or a phosphate.

6. Aweighting agent according to claim 3, wherein the flux is an alkali.

7. A weighting agent according to any one of claims 1 to 6, wherein the weighting agent consists predominantly of granules having sizes in the range from 50 microns to 75 microns.

8. A weighting agent according to any one of claims 1 to 7, wherein substantially all of the fine particles are smaller than 50 microns.

9. A process for manufacturing a weighting agent for a drilling fluid wherein fine particles of one or more heavy minerals of specific gravity greater than 4.0 are mixed with a smaller quantity of a flux, the mixture is sintered and the sintered material is discharged in particulate form.

10. A process according to claim 9, wherein the mixture is sintered at a temperature not exceeding 12005C and for the minimum time required to cause the fine particles to cohere.

11. A process according to claim 9 or 10, wherein the flux is mixed with the fine particles in a substantially dry form.

12. A process according to claim 9 or 10, wherein the flux is mixed with the fine particles in the form of an aqueous suspension or solution.

13. A process according to any one of claims 9 to 12, wherein relatively coarse pieces of the heavy mineral(s) are ground together with the flux in order to prepare an intimate mixture of fine particles of the heavy mineral(s) with the flux.

14. A process according to any one of claims 9 to 12, wherein the flux is added to the furnace in which the mixture is sintered so that the flux is vaporised and reacts chemically with the surface of the fine particles.

15. A process according to claim 12, wherein the mixture of fine particles and flux is introduced into a spray drier to form a dry product comprising minute spheres of the mixture and the spheres are sintered by exposure to a temperature not exceeding 1 2000C for a very short time.

16. A process according to any one of claims 9 to 14, wherein the mixture of fine particles and flux is sintered to form solid lumps which are then ground to the desired particle size.

17. A drilling fluid comprising a weighting agent in accordance with the invention.

18. A weighting agent for a drilling fluid for an oil or gas well substantially as herein described.

19. A process for manufacturing a weighting agent for a drilling fluid substantially as herein described.