WO2021084387A1

WO2021084387A1 - High pressure drilling fluid additive

Info

Publication number: WO2021084387A1
Application number: PCT/IB2020/059914
Authority: WO
Inventors: Mohammad Asad; Ryanto Husodo
Original assignee: Pt. Obm Drilchem
Priority date: 2019-10-31
Filing date: 2020-10-22
Publication date: 2021-05-06
Also published as: EP4051751A1; CN115038772B; EP4051751A4; CN115038772A

Abstract

The present invention relates to a drilling fluid additive, the drilling fluid additive comprising a blend of cellulosic fibres, wherein the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the cellulosic fibres have a particle size of less than 75 microns.

Description

High Pressure Drilling Fluid Additives

TECHNICAL FIELD

[0001] The present invention relates to an additive for drilling fluids. More specifically, the additive of the present invention is intended to seal the walls of drilled boreholes to prevent or at least limit lost circulation of drilling fluid.

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Drilling for oil, gas or geothermal wells usually occurs in a depth of thousands of meters. Such wells are drilled using a drill bit that is connected to a drill string that extends up to the surface. The drill string comprises a plurality of pipes, also known as casings, that are secured end to end. The drilling operation will form a borehole in the formation and more pipes are added to the drill string as the drilling operation continues. A drilling fluid, such as drilling mud, is pumped down the drill string to the drill bit and is then circulated back up the annulus between the borehole and the drill string. The circulating drilling fluid will carry any drill cuttings from the drill bit up to the surface. The drill cuttings are typically removed from the drilling fluid at the surface and the drilling fluid is recirculated through the borehole.

[0004] A further function of the drilling fluid is to provide a hydrostatic head that counters the pressure of the drilling gas or oil in the reservoir. Drilling mud pressure is usually maintained above formation pressure to prevent the reservoir fluid from flowing into borehole, which can cause well blowout conditions.

[0005] As a drill bit penetrates a petroleum bearing formation, the drilling mud invades the formation due to the positive differential pressure between the mud and reservoir fluids. Pores and fractures in the formation provide an outlet through which drilling fluid may flow into. Adequately sized solid particles in the drilling fluid will enter the pores and fractures and a filter cake will eventually form. The formation of a low-permeability mud cake on the entire sand face effectively prevents additional drilling fluid solids from entering the formation, but does not stop the liquids in the drilling fluid. This loss of liquids into the formation is known as “lost circulation” which at the very least will cause the loss of drilling fluid, but can also lead to more serious problems. If the amount of fluid in the wellbore drops due to lost circulation, hydrostatic pressure is reduced, which can allow a gas or fluid which is under a higher pressure than the reduced hydrostatic pressure to flow into the wellbore. This leads to a blowout. Another consequence of lost circulation is dry drilling. Dry drilling occurs when fluid is completely lost from the well bore without actual drilling coming to a stop. This can cause damage to the drill, may cause the drill bit to be lost or could require a new well to be drilled.

[0006] In order to control lost circulation, a range of drill fluid additives have been used to plug up and/or seal the pore and fractures of the borehole. These additives are typically formed into a pill that may be added to the drilling fluid when lost circulation occurs. Whilst a number of cross-linking polymers have been found to be effective in sealing the boreholes, these substances are not typically biologically or chemically degradable. This can lead to the producing zone being permanently obstructed. Biodegradable additives are therefore preferred and a number have been used including fibres and granules from rice husks, peanut shells, and woods. These materials have only been effective in sealing formations that have a permeability in the range of 2000 - 5000 mD. Furthermore, the formed cake still exhibits a degree of permeability and this essentially provides a cap on the pressure at which the drilling fluid may be circulated.

[0007] In AU 72159/01 , which shares a common applicant to the present invention, the inventors attempted to seal the borehole using a drilling additive that contained teakwood fibres. These teakwood fibres were subjected to a grinding process to a target average particle size of mesh 80 to mesh 325. Whilst this additive did plug fractures of standard formations to some extent, the seal was limited.

[0008] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. SUMMARY OF INVENTION

[0009] In accordance with the present invention, there is provided a drilling fluid additive, the drilling fluid additive comprising a blend of cellulosic fibres, wherein the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the cellulosic fibres have a particle size of less than 75 microns.

[0010] Preferably, the blend of cellulosic fibers comprises hardwood fibres. More preferably, the hardwood fibres comprise teak wood fibres. As would be appreciated by a person skilled in the art, teak (Tectona grandis) is a hardwood tree species in the family Lamiaceae.

[0011] Throughout this specification, unless the context requires otherwise, the term "blend of cellulosic fibres" or variations, will be understood to refer to a mixture of cellulosic fibres with different particles sizes or particles size fractions.

[0012] In one embodiment of the present invention, the blend of cellulosic fibres comprises fibres that have a modulus of rupture of at least 80 MPa. Preferably, at least 90% of the cellulosic fibres have a modulus of rupture of at least 80 MPa.

[0013] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the fibres have a particle size of between 75 microns and 250 microns.

[0014] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 15% and 24% of the fibres have a particle size of less than 40 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 17% and 22% of the fibres have a particle size of less than 40 microns.

[0015] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 12% and 20% of the fibres have a particle size of between 40 microns and 50 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 14% and 18% of the fibres have a particle size of between 40 microns and 50 microns.

[0016] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the fibres have a particle size of less than 60 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 45% and 57% of the fibres have a particle size of less than 60 microns

[0017] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 24% and 36% of the fibres have a particle size of between 80 microns and 160 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 26% and 33% of the fibres have a particle size of between 80 microns and 160 microns.

[0018] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 29% and 44% of the fibres have a particle size of between 80 microns and 220 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 33% and 41% of the fibres have a particle size of between 80 microns and 220 microns.

[0019] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 8% and 12% of the fibres have a particle size of between 160 microns and 350 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 9% and 11% of the fibres have a particle size of between 160 microns and 350 microns.

[0020] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 3% and 5% of the fibres have a particle size above 220 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 3.5% and 4.5% of the fibres have a particle size above 220 microns.

[0021] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that less than 1% of the fibres have a particle size above 350 microns.

[0022] In one form of the present invention, at least 80% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 82% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 84% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 86% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 88% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 90% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 92% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 94% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 96% of the drilling fluid additive comprises the blend of cellulosic fibres. In one form of the present invention, at least 98% of the drilling fluid additive comprises the blend of cellulosic fibres.

[0023] In accordance with a second aspect of the present invention, there is provided a drilling fluid that comprises the drilling fluid additive of the present invention.

[0024] In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 1 pounds per barrel of oil (Ibs/bbl). In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 2 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 3 Ibm/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 4 Ibm/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 5 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 6 Ib/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 7 Ibm/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 8 Ibm/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 9 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 10 Ibs/bbl.

[0025] In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is between 1 Ibs/bbl and 50 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is between 5 Ibs/bbl and 50 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is between 10 Ibs/bbl and 50 Ibs/bbl.

[0026] According to a third aspect of the present invention, there is provided a method of treating a borehole that extends into a formation, the method comprising the steps of: providing a drilling fluid according to the second broad aspect of the present invention; and circulating the drilling fluid through the borehole to form a substantially impermeable layer on a wall surface of the borehole.

[0027] Preferably, the substantially impermeable layer comprises a matrix of the cellulosic fibres. In one form of the present invention, the matrix further comprises solid particles from the formation.

[0028] According to a fourth aspect of the present invention, there is provided a lost circulation pill comprising drilling fluid additive according to the first aspect of the present invention.

[0029] According to a fifth aspect of the present invention, there is provided a method of sealing fractures that extend into a formation from a borehole that extends through the formation, the method comprising the steps of: providing a lost circulation pill according to the fourth aspect of the present invention; and circulating the pill through the borehole to form a substantially impermeable layer on a wall surface of the borehole.

[0030] Preferably, the substantially impermeable layer comprises a matrix of the cellulosic fibres. In one form of the present invention, the matrix further comprises solid particles from the formation.

[0031] According to a sixth aspect of the present invention, there is provided a formation that includes a borehole through which a lost circulation pill according to the fourth aspect of the present invention has been circulated, a fracture that extends into the formation from the borehole, and a substantially impermeable seal that is formed in the fracture and that includes a matrix of the lost circulation material from the pills drilling fluid additive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a cross-section of a borehole in a formation where a drilling fluid containing the drilling fluid additive is circulated through the borehole;

Figure 2 is a cross-section of a borehole in a formation where the cellulosic fibres from a drilling fluid additive of the present invention has formed a layer of wall filter cake;

Figure 3 is a cross-section of a borehole in a formation that includes fractures that are each sealed by a matrix of the cellulosic fibres from a drilling fluid additive of the present invention; and

Figure 4 is a flow chart of a method of sealing fractures in a formation that extends from a borehole in the formation;

Figure 5 shows the results of a particle size analysis performed on the drilling fluid additive of the present invention;

Figure 6 shows the results of a particle size analysis performed on a comparable drilling fluid additive

Figure 7 shows the results of a permeability plugging test conducted at 5000 psi using the drilling fluid additive of the present invention

Figure 8 shows the results of a permeability plugging test conducted at 8000 psi using the drilling fluid additive of the present invention

Figure 9 shows the results of a fluid invasion test using the drilling fluid additive of the present invention.

DESCRIPTION OF EMBODIMENTS

[0033] The present invention relates broadly to a drilling fluid additive that is intended to reduce the loss of drilling fluid to the surrounding subterranean structure. The drilling fluid additive comprises a blend of cellulosic fibres with a particular particle size distribution. When incorporated into a drilling fluid, the cellulosic fibres will form a barrier across pores or fractures in the surrounding subterranean structure to form a seal. This barrier has been found to limit the loss of the drilling fluid to the surrounding subterranean structure.

[0034] The blend of cellulosic fibres contains a particular blend of cellulosic fibres of different particle sizes. The inventors have found that the blend of cellulosic fibres needs to contain a specific amount of particles below a particular particle size to form a sufficiently impermeable layer on the walls of the borehole to substantially seal the borehole. In preferred embodiments, the inventors have found that a minimum amount of particles in a number of different particle size ranges will increase the seal. In particular, the inventors believe that both an adequate amount of particles below a certain particle size and above a certain particle size are required to form the seal.

[0035] Without wishing to be bound by theory, it is believed that when the blend of cellulosic fibres encounters void in the surrounding subterranean structure that the larger cellulosic fibres will start to accumulate and build up over the void. As more fibres are deposited across the void, a mat of cellulosic fibres will form. As this mat grows, the space between the fibres will eventually lessen to a point where fibres that are much smaller that the void size will begin to accumulate on the fibre mat. The smaller fibres of the blend will continue to collect on this mat, thereby sealing the mat.

[0036] The cellulosic fibres are preferably derived from hardwood sources. The inventors have found that hardwood fibres are particularly useful for use in the drilling fluid additive of the present invention as they demonstrate particular physical properties. In particular, such fibres are both flexible and strong. This allows the fibres to intertwine and penetrate the pores and/or fractures, whilst being able to support the build-up of a fibrous mat that is capable of withstanding significant hydrostatic pressure from the fluids encountered in the well bore. In a preferred embodiment, the cellulosic fibres are teak wood fibres.

[0037] The inventors believe that an important physical property of cellulosic fibres that are suitable for use in the present invention is the modulus of rupture. This is also known as flexural strength or bend strength. This property is a measurement of the stress in the material at the moment of yield. In one embodiment of the present invention, the blend of cellulosic fibres comprises fibres that have a modulus of rupture of at least 80 MPa. In one embodiment, the blend of cellulosic fibres comprises fibres that have a modulus of rupture of at least 85 MPa. In one embodiment, the blend of cellulosic fibres comprises fibres that have a modulus of rupture of at least 90 MPa. In one embodiment, the blend of cellulosic fibres comprises fibres that have a modulus of rupture of at least 95 MPa.

[0038] An important advantage of the use of cellulosic fibres is that that they are biodegradable. This provides an environmentally friendly solution as the additive does not rely on long chain polymers which do not readily decompose. A further advantage is that the formed fibrous mat does not irreversibly seal the surrounding formation. Whilst some prior art technologies utilise seals that may be reversed, this will typically require the addition of a further chemical agent, for example and acid, to reverse the seal. As the cellulosic fibres will naturally degrade over time, the need to introduce a further chemical agent is avoided.

[0039] As discussed above, the blend of cellulosic fibres comprises two distinct distributions of fibres lengths, a short fibre component and a long fibre component.

[0040] The short fibre component has an average particles size less than 75 microns. The short fibre component is present at an amount of between 40% and 60% of the drilling fluid additive as a whole. In one embodiment at least 90% of the fibres in the short fibre component have a particles size less than 75 microns.

[0041] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 15% and 24% of the fibres have a particle size of less than 40 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 17% and 22% of the fibres have a particle size of less than 40 microns.

[0042] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 0.10% - 0.20% of the fibres have a particle size of between 8 microns and 15 microns. In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 0.10% - 0.15% of the fibres have a particle size of between 8 microns and 15 microns.

[0043] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 15.50% - 23.40% of the fibres have a particle size of between 15 microns and 40 microns. In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 17.50% - 21.50% of the fibres have a particle size of between 15 microns and 40 microns.

[0044] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 12% and 20% of the fibres have a particle size of between 40 microns and 50 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 14% and 18% of the fibres have a particle size of between 40 microns and 50 microns.

[0045] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 12.60% - 19.10%of the fibres have a particle size of between 50 microns and 60 microns. In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 14.20% - 17.50% of the fibres have a particle size of between 50 microns and 60 microns.

[0046] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 40% and 62% of the fibres have a particle size of less than 60 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 45% and 57% of the fibres have a particle size of less than 60 microns.

[0047] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 6.60% - 10.00% of the fibres have a particle size of between 60 microns and 80 microns. In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 7.40% - 9.20% of the fibres have a particle size of between 60 microns and 80 microns.

[0048] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 13.30% - 20.10% of the fibres have a particle size of between 80 microns and 130 microns. In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 15.00% - 18.40% of the fibres have a particle size of between 80 microns and 130 microns.

[0049] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 24% and 36% of the fibres have a particle size of between 80 microns and 160 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 26% and 33% of the fibres have a particle size of between 80 microns and 160 microns.

[0050] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 29% and 44% of the fibres have a particle size of between 80 microns and 220 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 33% and 41% of the fibres have a particle size of between 80 microns and 220 microns.

[0051] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 10.60% - 15.90% of the fibres have a particle size of between 130 microns and 160 microns. In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 11.90% - 14.60% of the fibres have a particle size of between 130 microns and 160 microns.

[0052] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 8% and 12% of the fibres have a particle size of between 160 microns and 350 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 9% and 11% of the fibres have a particle size of between 160 microns and 350 microns.

[0053] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that between 3% and 5% of the fibres have a particle size above 220 microns. Preferably, the particle size distribution of the blend of cellulosic fibres is such that between 3.5 and 4.5% of the fibres have a particle size above 220 microns.

[0054] In one form of the present invention, the particle size distribution of the blend of cellulosic fibres is such that less than 1% of the fibres have a particle size above 350 microns.

[0055] For purposes of the present invention, the term "particle size" is defined as the dimension of a particle which is determined by a sieve size analysis according to the Sieving Test described in greater detail herein. A sample of particles is sieved as described, and the results are recorded. The results of such a sieve size analysis sufficiently define the size of the particles for the purposes of the present invention. The results of the sieve analysis may be expressed by two equivalent conventions in terms of the characteristics of the sieves used. [0056] One way to express the size of the particles is in terms of the size of the openings in the sieves. For instance, in principal, a particle that is retained on a sieve with 75 micron openings is considered to have a particle size greater than or equal to 75 microns for the purposes of the present invention. A particle that passes through a sieve with 90 micron openings and is retained on a sieve with 75 micron openings is considered to have a particle size between 75 and 90 microns. A particle that passes through a sieve with 75 microns is considered to have a particle size less than 75 microns. The other way to express the size of the particles in terms of the results of a sieving analysis, is in terms of the designation used for the sieves. Two main scales are used to designate the mesh sizes, the US Sieve Series and Tyler Mesh Size.

[0057] The results described in either of the foregoing manners can be easily described in the other way by referring to a sieve size chart and locating the corresponding value. Such a sieve size chart is found in Table 21-6 of Perry's Chemical Engineers' Handbook, Sixth Edition, (McGraw-Hill Book Company, 1984) at page 21-15.

[0058] As would be appreciated by a person skilled in the art, for non-spherical particles, the sieving test may determine the size of only certain dimensions of a specific particle. In the case of the fibrous particles of the present invention, the fibres may pass vertically through the apertures of the mesh, resulting in particles with larger cross sections passing through the mesh. Because of this, the test results are generally expressed in terms of the percentage of particles, by weight, which will ordinarily pass through a sieve of one dimension and be retained on a sieve of a second dimension. Whilst the major proportion of the fibres in the cellulosic fibre blend with fall within the ranges disclosed, it should be understood that any the blend of fibres will comprises a broad distribution of fibre sizes. The existence of some proportion of fiber outside the claimed range does not avoid the claims. Preferably, in the present invention, no more than about 20%, more preferably no more than about 10%, and most preferably no more than about 5% by weight of the particles should have any dimension larger than the sieve they have passed through.

[0059] The specific particle size distributions described above can be prepared by any suitable method. The specific particle size distributions can be prepared, at least in relatively small amounts by a sieving operation. [0060] In one embodiment of the present invention, cellulose fibres undergo one or more size reduction steps to produce a blend of cellulosic fibres with an appropriate particle size distribution. Preferable, the one or more size reduction steps include one or more mechanical size reductions step. Preferably, at least one of the size reduction step is a grinding step. In one embodiment of the present invention, a size reductions step is performed using a hammer mill. Additionally or alternatively, a size reduction step is performed using a disc mill.

[0061] In one embodiment, one or more sizing steps are used to separate out fractions of the cellulosic fibres that require additional size reduction. In one embodiment, a cyclonic classification apparatus is used in the one or more sizing steps.

[0062] It is envisaged that the most effective way in which to achieve the desired particle size distribution is to produce fractions of fibres within a particular particle size range. Means for bulk classification of fibres may be used initially to produce rough fractions which may then be further separated into further size fractions downstream. The fraction may then be appropriately blended to produce an additive with the required particle size distribution.

[0063] The present invention further relates to a drilling fluid that comprises a drilling fluid additive as described above.

[0064] In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 1 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 2 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 3 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 4 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 5 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 6 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 7 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 8 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 9 Ibs/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is at least 10 Ibs/bbl.

[0065] In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is between 1 Ibm/bbl and 50 Ibm/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is between 5 Ibm/bbl and 50 Ibm/bbl. In one form of the present invention, the concentration of the drilling fluid additive in the drilling fluid is between 10 Ibm/bbl and 50 Ibm/bbl.

[0066] Preferably, the drilling fluid is a liquid drilling fluid. For example, the drilling fluid may be a water-based drilling fluid, an oil-based drilling fluid, or a synthetic-based drilling fluid. In a particular preferred form the drilling fluid is a water or oil-based drilling mud.

[0067] Preferably, the drilling fluid comprises hydrated clay. It is preferred that the clay comprises bentonite. Alternatively, the drilling fluid comprises a polymer

[0068] The present further relates to a method of drilling a well borehole in a subterranean formation. The method generally comprises the steps of: providing a drilling fluid additive comprising a blend of cellulosic fibres, mixing the sealant with a drilling fluid; and pumping the sealant and drilling fluid mixture down the borehole to seal the borehole pores and fractures.

[0069] With reference to Figure 1 , in use, the drilling fluid 30 to which the drilling fluid additive of the present invention has been added is pumped down a well borehole 40 that extends through a formation. In particular, the drilling fluid 30 is pumped down the borehole 40 through a drill string 41 and a drill bit 42 that is attached to the drill string 41 , and is then circulated back up an annulus 43 between a wall 44 of the borehole 40 and the drill string 41. In addition to the additive, the drilling fluid 30 which is circulated back up the annulus 43 also includes cuttings and other fine solid particles. The overbalance pressure (i.e. the extent to which the hydrostatic pressure of the drilling fluid 30 in the borehole 30 exceeds the pressure of the formation through which the borehole 30 is being drilled) of the drilling fluid 30 results in the fibres of the additive combining with the cuttings and other fine solid particles in the drilling fluid 30 to form an impermeable layer 45 comprising a thin wall/filter cake 46 that lines the wall surface 47 of the borehole 40.

[0070] Figure 2 depicts the borehole/wellbore 40 passing through a sand formation 50. Drilling fluid 30 in the borehole 40 can be lost through the pores in the formation 50. Also, because the formation 50 is a sand formation, it is not as strong or as stable as rock formations. The impermeable layer 45 on the wall surface 47 inhibits the drilling fluid 30 in the borehole 40 from being lost in the porous formation 50. In addition, it strengthens and stabilises the borehole 40 to inhibit the wall 44 of the borehole 40 from collapsing. The fibres form an impermeable matrix in the layer of wall filter cake 46. The large fibres first agglomerate over the pores or fractures to form a loose fiberous mat. The smaller fibres then agglomerate on the fiberous mat to reduce the spaces between the large fibres in the matrix. As the fibres in the mat build up, the wall cake 46 matrix becomes denser and the permeability reduces. The drill cuttings and other fine solid colloidal particles in the drilling fluid 30 further enhance the matrix by filling the spaces between the fibres and assist in reducing the wall cake 46 permeability. The wall cake 46 is then able to substantially plug or seal the porous/fractured formation 50 and is thereby able to substantially prevent drilling fluid 30 in the borehole 40 from entering the formation 50 and being lost.

[0071] Figure 3 depicts the borehole/wellbore 40 passing through a fractured limestone formation 60. As can be seen, the formation 60 includes a plurality of large fractures 51 that extend into the formation 50 from the borehole 40. Drilling fluid in the borehole 40 can be lost through the fractures 51 resulting in lost circulation of the drilling fluid. When a drilling fluid that contains the first drilling fluid additive is pumped through the borehole 40 in the manner described above, the lost circulation material 11 will be pumped into the fractures 51 where it forms a seal 52 that includes an impermeable matrix of cellulosic fibres and cuttings. The seals 52 are able to substantially plug or seal the large fractures 51 and are thereby able to substantially stop drilling fluid in the borehole 40 from entering the formation 50 through the fractures 51 and being lost.

[0072] With reference to Figure 4, the above-described steps comprise a method 70 of treating a borehole that extends into a formation. [0073] The inclusion of both the small and large fibres in the drilling fluid additive makes the drilling fluid additive particularly suitable for use in sealing a producing formation such that severe lost circulation in the formation is eliminated or at least reduced.

[0074] After the seal/plug has been removed/released in this way, the formation zone is able to produce an amount of oil or gas which is the same as or more than that which was originally intended.

Example 1

[0075] A drilling fluid additive in accordance with the present invention was prepared (New Fracseal) and a particle size analysis was performed. The results of the particle size analysis are shown in Figure 5. For comparative purposes, a drilling fluid additive was prepared in accordance with the method taught in AU 72159/01 (Reg. Fracseal). A particles size analysis of this reference drilling fluid additive is shown in Figure 6.

[0076] In the reference sample, the target average particle size was mesh 80 to mesh 325. As can be seen from the particle size distribution curve, this results in a fairly normal distribution of particle sizes with a mean particles size of approximately 80 urn and a fairly large standard deviation. The particles size distribution of the New Fracseal sample exhibits multiple distinct modes that is indicative of a blend of separate particle size factions. It is clears that there is significant portion of the particles in a range of 15-50 urn and another significant portion of the particles in a range of 80-160 urn

Example 2

[0077] A series of tests were undertaken to determine effectiveness of the sealing capability of the drilling fluid of the present invention and to compare it to prior art drilling fluid additives. The testing methodology employed was as follows:

Prepare 350ml of pre-hydrated bentonite.

Mixture of a predetermined amount of a drilling fluid additive with the prepared bentonite.

Fill an API cylinder cell (without any filter paper) with 20/40 gravel pack sand to about 1/3 of the cylinder. Pour the mixed cellulose fiber or bentonite into the cylinder.

After the cylinder is closed then apply pressure to the cylinder.

[0078] After 30 minutes the depth of invasion and fluid loss is measured to determine the effectiveness of the seal of the pores of the sand based formation. The fluid invasion (cm) and fluid loss (ml) was then measured.

[0079] Separates tests were conducted for of the samples from Example 1 at pressures of 5000 psi and 8000 psi. Sample is placed in the stainless steel cylinder and the ceramic disk is used instead of sand, then 5000 psi and 8000 psi pressure are applied. The permeability plugging test (PPT) value (ml) then was calculated. The results of these tests are shown in Figure 7 and Figure 8 respectively. As is evident from the tests, the drilling fluid additive of the present invention provided a greater seal than the drilling additives of the prior art, resulting in a larger PPT value.

Example 3

[0080] Samples of the drilling fluid additives of Example 1 were subjected to a fluid invasion test. As would be appreciated by a person skilled in the art, measurement of the filtrate invasion behavior and wall-cake building characteristics of a mud are fundamental to drilling fluid control and treatment. A thin and proper wall cake will form an impermeable filter cake resulting in a very minimum fluid invasion (loss) into the formation. These characteristics are affected by the types and quantities of solids in the fluid and their physical and chemical interaction which in turn, are affected by temperature and pressure.

[0081] The American Petroleum Institute (API) Recommended Practice 13B-1 (RP 13B- 1) establishes recommended practices for field testing water-based drilling fluids using a standard API filter press. In order to measure the degree of fluid invasion, a standard API filter press was modified to replace the filter paper with a 20/40 gravel sand to emulate a down hole formation. In 20/40 sand, a 2 - 2.5cm invasion across gravel sand is generally accepted as the bench mark for a good wellbore stability. The following test procedure was adopted: i. The cell was filled with 20/40 sand/gravel until approximately half full. ii. The sand bed was levelled by manually shaking the cell. iii. The drilling mud sample was gently poured into the cell up to the upper mark for maximum fluid level. iv. The cell was placed into its support and the top cap was secured with the “T- screw”. A dry graduated cylinder is placed below the cell. v. The relief valve was closed and the regulator was slowly adjusted to a pressure of 100 psi. The test period begins at the time of pressure application. vi. The fluid was allowed to filtrate through the sand for 10 mins. vii. At the end of 10 minutes the volume of filter is measured and the cell is opened to measure fluid invasion into sand in centimeters.

The results of the fluid invasion test are shown in Figure 9. As can be seen in the results, the drilling fluid additive of the present invention demonstrated significantly less fluid invasion over the comparative drilling fluid. Furthermore, the recorded 1 cm was well below acceptable standards.

Example 4

[0082] The drilling fluid additive of the present invention will act to seal the borehole to limit the loss of the drilling fluid. Whilst this is advantageous during the drilling process, the introduction of the additive to the formation may also impact the flow properties of the formation at the intended production zone.

[0083] A series of return tests were undertaken to determine any detrimental impact the drilling fluid of the present invention had on the formation. The testing methodology employed was as follows: i. Prepare 350ml of pre-hydrated bentonite. ii. Mixture of a predetermined amount of a drilling fluid additive with the prepared bentonite. iii. Fill an API cylinder cell (without any filter paper) with 20/40 gravel pack sand to about 1/3 of the cylinder. iv. Pour the mixed cellulose fiber or bentonite into the cylinder. v. After the cylinder is closed then apply pressure of 200 psi to the cylinder. vi. After 30 minutes the cylinder was opened and excess drilling fluid was discarded leaving the formed filter cake vii. A portion of 20/40 gravel sand was inserted into the cylinder and a 40 micron mesh screen was inserted into the cylinder viii. The top of the cylinder was sealed with a lid comprising a single outlet ix. Water was pumped into the bottom of the cylinder at a pressure of 10 psi and allowed to flow through the sand, the filter cake and the outlet port

[0084] The tests showed a return permeability of 100%. The test showed that the under positive pressure, the drilling fluid additive forms a filter cake across the gravel pack sand. Once the positive pressure is reversed, the formed filter cake was removed from the gravel pack sand, allowing the reverse flow of liquid through the gravel pack sand. The results of the return permeability measurements indicated that the filter cake formed did not have any detrimental effects on the flow properties of the formation.

[0085] It will be appreciated by those skilled in the art that variations and modifications to the invention described herein will be apparent without departing from the spirit and scope thereof. The variations and modifications as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth.

[0086] Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0087] Also, future patent applications may be filed in Australia or overseas on the basis of, or claiming priority from, the present application. It is to be understood that the following provisional claims are provided by way of example only and are not intended to limit the scope of what may be claimed in any such future application. Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions. Moreover, additional claims may be added at a later date so as to claim other aspects of the invention.

Claims

1. A drilling fluid additive, the drilling fluid additive comprising a blend of cellulosic fibres, wherein the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the cellulosic fibres have a particle size of less than 75 microns.

2. A drilling fluid additive according to claim 1 , wherein the blend of cellulosic fibers comprises hardwood fibres.

3. A drilling fluid additive according to claim 2, wherein the hardwood fibres comprise teak wood fibres.

4. A drilling fluid additive according to any one of the preceding claims, wherein the blend of cellulosic fibres comprises fibres that have a modulus of rupture of at least 80 MPa

5. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the fibres have a particle size of between 75 microns and 250 microns.

6. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 15% and 24% of the fibres have a particle size of less than 40 microns.

7. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 12% and 20% of the fibres have a particle size of between 40 microns and 50 microns

8. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 40% and 60% of the fibres have a particle size of less than 60 microns

9. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 24% and 36% of the fibres have a particle size of between 80 microns and 160 microns.

10. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 29% and 44% of the fibres have a particle size of between 80 microns and 220 microns.

11. A drilling fluid additive according to any one of the preceding claims, wherein, the particle size distribution of the blend of cellulosic fibres is such that between 8% and 12% of the fibres have a particle size of between 160 microns and 350 microns

12. A drilling fluid additive according to any one of the preceding claims, wherein the particle size distribution of the blend of cellulosic fibres is such that between 3% and 5% of the fibres have a particle size above 220 microns

13. A drilling fluid, the drilling fluid comprising the drilling fluid additive of any one of claims 1 to 12.

14. A drilling fluid according to claim 13, wherein the concentration of the drilling fluid additive in the drilling fluid is between 1 Ibm/bbl and 50 Ibm/bbl

15. A method of treating a borehole that extends into a formation, the method comprising the steps of: providing a drilling fluid according to the second broad aspect of the present invention; and circulating the drilling fluid through the borehole to form a substantially impermeable layer on a wall surface of the borehole.

16. A lost circulation pill comprising a drilling fluid additive according to any one of claims 1 to 13.

17. A method of sealing fractures that extend into a formation from a borehole that extends through the formation, the method comprising the steps of: providing a lost circulation pill according to the fourth aspect of the present invention; and circulating the pill through the borehole to form a substantially impermeable layer on a wall surface of the borehole.