WO2016019416A1

WO2016019416A1 - Drilling fluid additive

Info

Publication number: WO2016019416A1
Application number: PCT/AU2015/000464
Authority: WO
Inventors: Ryanto Husodo; Mohammad As'ad
Original assignee: Ryanto Husodo; As Ad Mohammad
Priority date: 2014-08-05
Filing date: 2015-08-05
Publication date: 2016-02-11
Also published as: AU2015299742B2; AU2015299742A8; AU2019204670A1; AU2015299742A1

Abstract

The invention relates to methods for sealing fractures in drilled formations by defining a seal in the formation including a matrix of lost circulation material delivered through a pill of drilling fluid additive. The invention also relates to a drilling fluid additive comprising the lost circulation material that comprises fibres and chips. In a particular arrangement, the fibres may comprise wood fibres such teakwood fiber and the chips may comprise woodchips. In another arrangement, the lost circulation material is an acid soluble material that may comprise calcium carbonate flakes. The lost circulation material may be biodegrdable and may also comprise nut shells. Moreover, the invention also provides for a lost circulation pill and the methods for sealing fractures that extend into a formation from a borehole.

Description

DRILLING FLUID ADDITIVE

FIELD OF THE INVENTION

[0001] The present invention relates generally to drilling fluid additives and, in particular, to drilling fluid additives that include lost circulation material and that are for use in sealing pores and/or fractures in drilled formations to prevent or at least limit lost circulation of drilling fluid. In particular, the present invention relates to such drilling fluid additives that are for use in sealing highly porous/fractured drilled formations.

[0002] Although the present invention will be described with particular reference to sealing pores and fractures inside boreholes for oil, gas, and geothermal wells, it will be appreciated that it may be used to seal pores and fractures in other types of wells.

BACKGROUND ART

[0003] Wells such as oil, gas, and geothermal wells are typically drilled to depths of thousands of metres below the Earth's surface. Such wells are drilled using a drill bit that is positioned at the bottom of the well borehole, and that is connected to a drill string that extends up to the surface. A drilling fluid such as drilling mud is pumped down the drill string, which comprises a plurality of pipes that are secured end to end, to the drill bit, and is then circulated back up an annulus between the borehole and the drill string so that drill cuttings produced by the cutting action of the drill bit on the drilled formation are carried up to the surface by the drilling fluid. At the surface, the drill cuttings are usually removed from the drilling fluid so that the fluid can be recirculated through the borehole.

[0004] The problem of "lost circulation" can occur while drilling such wells. The problem occurs when drilling fluid escapes from the borehole through pores and fractures in the drilled formation instead of returning up the annulus between the borehole and the drill string.

[0005] Lost circulation is a common occurrence when drilling oil, gas, and geothermal wells. The occurrence of lost circulation can add significantly to the cost of drilling a well. Furthermore, if there is a well blowout as a result of lost circulation occurring, the blowout can be fatal to people who are located near the site of the blowout at the time that it occurs.

[0006] In the past, a common practice to combat lost circulation was to add ground coconut shell, ground-up Formica or other inorganic fibres to the drilling fluid to create a "pill" which is then pumped downhole. However, the use of these additives/supplements has been found to be unable to inherently solve the problem of lost circulation, and they can actually cause further damage if they are used in the producing zone of the drilled formation and if they plug the pores of the producing zone irreversibly. In particular, the addition to the drilling fluid of non-biodegradable particles/solids that can penetrate the formation may permanently obstruct the producing zone thus making the well non-productive.

[0007] Organic drilling fluid additive products that are biodegradable have been developed from rice husks, peanut shells, and softwood in an attempt to overcome the problem of permanent producing zone obstruction which is associated with non-biodegradable additives. However, these organic products have been found to have some disadvantages in their application as drilling fluid additives due to their natural properties.

[0008] A particular organic drilling fluid additive that has been found to be effective at sealing pores and fractures in formations and that is therefore able to prevent or limit lost circulation when drilling such formations is disclosed in published Australian Patent Application No. 200172159, the contents of which are incorporated herein by reference. The additive includes a blend of long teakwood fibres and short cotton fibres. It has been found that this drilling fluid additive is only effective in sealing pores and fractures in formations that have permeability in the range of 2000 - 5000 mD (milli Darcy). This is unfortunate, as it is known that severe lost circulation that occurs when drilling highly porous/fractured formations having permeability that is greater than 5000 mD costs oil companies US$2-3 billion per annum.

[0009] International patent application PCT/AU201 1/001209, which was published on 29 March 2012 under International Publication Number WO/2012/037600, discloses a drilling fluid additive that combines the advantages of the drilling fluid additive disclosed in Australian Patent Application No. 200172159, and that is able to seal formations having a permeability that is greater than 5000 mD. The drilling fluid additive disclosed in WO/2012/037600 includes fibres and flakes. The fibres can include long fibres and/or short fibres. Also, the drilling fluid additive may include acid-soluble granules. Although the drilling fluid additive disclosed in Australian Patent Application No. 200172159 is effective at reducing lost circulation in formations that have a permeability in the range of 2000 - 5000 mD, and the drilling fluid additive disclosed in WO/2012/037600 is effective at reducing lost circulation in formations that have a permeability that is greater than 5000 mD, it would nevertheless be desirable to provide an improved drilling fluid additive that is effective at sealing large fractures/vugular in drilled formations.

[0010] It is against this background that the present invention has been developed.

SUMMARY OF THE INVENTION

[001 1] It is an object of the present invention to overcome, or at least ameliorate, one or more of the deficiencies of the prior art mentioned above, or to provide the consumer with a useful or commercial choice.

[0012] Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, a preferred embodiment of the present invention is disclosed.

[0013] According to a first broad aspect of the present invention, there is provided a drilling fluid additive comprising lost circulation material that comprises fibres and chips.

[0014] Preferably, the fibres comprise wood fibres. It is particularly preferred that the wood fibres comprise teakwood fibres. It is also preferred that the wood fibres comprise ground and sized fibres.

[0015] Preferably, the wood fibres comprise wood fibres that range in size from 1 micron to 20 microns, and that comprise 10% to 30% by weight of all of the fibres and chips of the lost circulation material. [0016] Preferably, the wood fibres comprise wood fibres that range in size from 15 microns to 200 microns, and that comprise 15% to 30% by weight of all of the fibres and chips of the lost circulation material.

[0017] Preferably, the wood fibres comprise wood fibres that range in size from 150 microns to 500 microns, and that comprise 10% to 30% by weight of all of the fibres and chips of the lost circulation material.

[0018] Preferably, the wood fibres comprise wood fibres that range in size from 300 microns to 1000 microns, and that comprise 5% to 20% by weight of all of the fibres and chips of the lost circulation material.

[0019] Preferably, the wood fibres comprise wood fibres that range in size from 700 microns to 1500 microns, and that comprise 5% to 20% by weight of all of the fibres and chips of the lost circulation material.

[0020] Preferably, the wood fibres comprise wood fibres that range in size from 1300 microns to 2000 microns, and that comprise 1 % to 20% by weight of all of the fibres and chips of the lost circulation material.

[0021] Preferably, the wood fibres comprise wood fibres that range in size from 1800 microns to 3000 microns, and that comprise 1 % to 15% by weight of all of the fibres and chips of the lost circulation material.

[0022] Preferably, the wood fibres comprise wood fibres that range in size from 2500 microns to 5000 microns, and that comprise 5% to 15% by weight of all of the fibres and chips of the lost circulation material.

[0023] Preferably, the fibres comprise ground and sized cotton fibres.

[0024] Preferably, the cotton fibres comprise cotton fibres that range in size from 1 micron to 150 microns, and that comprise 1 % to 20% by weight of all of the fibres and chips of the lost circulation material.

[0025] Preferably, the cotton fibres comprise cotton fibres that range in size from 100 microns to 250 microns, and that comprise 1 % to 15% by weight of all of the fibres and chips of the lost circulation material. [0026] Preferably, the cotton fibres comprise cotton fibres that range in size from 150 microns to 500 microns, and that comprise 1 % to 15% by weight of all of the fibres and chips of the lost circulation material.

[0027] Preferably, the chips comprise woodchips.

[0028] Preferably, the chips comprise chips that range in size from 25 microns to 20000 microns, and that comprise 1 % to 25% by weight of all of the fibres and chips of the lost circulation material.

[0029] In one preferred form, the lost circulation material is acid-soluble lost circulation material that also comprises calcium carbonate flakes. It is preferred that the acid-soluble lost circulation material is at least 70% acid-soluble. It is also preferred that the fibres and chips comprise up to 30% by weight of all of the lost circulation material, and that the calcium carbonate flakes comprise at least 70% by weight of the lost circulation material. Preferably, the calcium carbonate flakes are derived from mini sea or river shells. It is preferred that the calcium carbonate flakes comprise ground and sized calcium carbonate flakes. It is also preferred that the ground and sized calcium carbonate flakes comprise calcium carbonate flakes that range in size from 50 microns to 5000 microns, and that comprise 1 % to 80% by weight of all of the calcium carbonate flakes.

[0030] In another preferred form, the lost circulation material is biodegradable lost circulation material that also comprises nutshell. It is preferred that the biodegradable lost circulation material is 100% biodegradable. The nutshell is preferably selected from the group of nutshells comprising coconut shell and peanut shell. It is preferred that the nutshell comprises ground and sized nutshell. It is also preferred that the ground and sized nutshell comprises nutshell that ranges in size from 50 microns to 5000 microns, and that comprises 1 % to 40% by weight of the lost circulation material.

[0031] Preferably, the lost circulation material is a single sack lost circulation material, which means that no other lost circulation materials need to be added to it. [0032] According to a second broad aspect of the present invention, there is provided a lost circulation pill comprising a drilling fluid, and a drilling fluid additive according to the first broad aspect of the present invention.

[0033] 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.

[0034] According to a third broad 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 second broad aspect of the present invention; and

circulating the pill through the borehole so that a substantially impermeable seal that includes a matrix of the lost circulation material from the pill's drilling fluid additive forms in the fractures.

[0035] According to a fourth broad aspect of the present invention, there is provided a formation that includes a borehole through which a lost circulation pill according to the second broad 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 pill's drilling fluid additive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings, in which:

Figure 1 depicts a first drilling fluid additive that includes acid-soluble lost circulation material;

Figure 2 depicts a second drilling fluid additive that includes biodegradable lost circulation material;

Figure 3 depicts a barrel containing a lost circulation pill that includes either the first drilling fluid additive or the second drilling fluid additive; Figure 4 is a cross-section of a borehole in a formation when a drill string that has a drill bit secured to a lower end thereof is inserted into the borehole, and when a lost circulation pill containing the first drilling fluid additive or the second drilling fluid additive is circulating through the drill string and the borehole;

Figure 5 is a cross-section of a borehole in a limestone formation that includes fractures that are each sealed by a matrix of the lost circulation material from the first drilling fluid additive after it has been pumped into the fractures; and

Figure 6 is a flow chart of a method of sealing fractures in a formation that extend from a borehole in the formation.

DESCRIPTION OF EMBODIMENTS

[0037] In the drawings, like features have been referenced with like reference numbers.

[0038] Referring to Figure 1 , a first drilling fluid additive 10 comprises acid-soluble lost circulation material 1 1 that includes organic teakwood fibres 12 and cotton fibres 13 blended with calcium carbonate flakes 14 and woodchips 15. The teakwood fibres 12, cotton fibres 13, and woodchips 15 together comprise up to 30% by weight of the lost circulation material 1 1 , and the calcium carbonate flakes 14 comprise at least 70% by weight of the lost circulation material 1 1 .

[0039] Owing to the calcium carbonate flakes 14 comprising at least 70% by weight of the lost circulation material 1 1 , and that calcium carbonate is acid-soluble, the lost circulation material 1 1 is at least 70% acid-soluble.

[0040] The acid-soluble lost circulation material 1 1 actually comprises a blend of ground and sized teakwood fibres 12, ground and sized cotton fibres 13, ground and sized calcium carbonate flakes 14, and sized woodchips 15 that have a range of sizes. The teakwood fibres 12 comprise: teakwood fibres 12 that range in size from 1 micron to 20 microns, and that comprise 10% to 30% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; teakwood fibres 12 that range in size from 15 microns to 200 microns, and that comprise 15% to 30% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; teakwood fibres 12 that range in size from 150 microns to 500 microns, and that comprise 10% to 30% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; teakwood fibres 12 that range in size from 300 microns to 1000 microns, and that comprise 5% to 20% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 11 ; teakwood fibres 12 that range in size from 700 microns to 1500 microns, and that comprise 5% to 20% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; teakwood fibres 12 that range in size from 1300 microns to 2000 microns, and that comprise 1 % to 20% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; teakwood fibres 12 that range in size from 1800 microns to 3000 microns, and that comprise 1 % to 15% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; and teakwood fibres 12 that range in size from 2500 microns to 5000 microns, and that comprise 5% to 15% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 .

[0041] The cotton fibres 13 comprise: cotton fibres 13 that range in size from 1 micron to 150 microns, and that comprise 1 % to 20% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; cotton fibres 13 that range in size from 100 microns to 250 microns, and that comprise 1 % to 15% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 ; and cotton fibres 13 that range in size from 150 microns to 500 microns, and that comprise 1 % to 15% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 .

[0042] The calcium carbonate flakes 14 comprise calcium carbonate flakes 14 that range in size from 50 micron to 5000 micron, and that comprise 1 % to 80% by weight of all of the calcium carbonate flakes 14. The calcium carbonate flakes 14 may be derived from mini sea or river shells.

[0043] The woodchips 15 comprise woodchips 15 that range in size from 25 micron to 20000 micron, and that comprise 1 % to 25% by weight of all of the fibres 12, 13 and woodchips 15 of the lost circulation material 1 1 . [0044] Referring to Figure 2, a second drilling fluid additive 20 comprises biodegradable lost circulation material 21 that includes organic teakwood fibres 12 and cotton fibres 13 blended with nutshell 22 and woodchips 15.

[0045] All of the components of the lost circulation material 21 are 100% biodegradable. Consequently, the lost circulation material 21 is 100% biodegradable.

[0046] The biodegradable lost circulation material 21 actually comprises a blend of ground and sized teakwood fibres 12, ground and sized cotton fibres 13, ground and sized nutshell 22, and sized woodchips 15 that have a range of sizes. The teakwood fibres 12 comprise: teakwood fibres 12 that range in size from 1 micron to 20 microns, and that comprise 10% to 30% by weight of the lost circulation material 21 ; teakwood fibres 12 that range in size from 15 microns to 200 microns, and that comprise 15% to 30% by weight of the lost circulation material 21 ; teakwood fibres 12 that range in size from 150 microns to 500 microns, and that comprise 10% to 30% by weight of the lost circulation material 21 ; teakwood fibres 12 that range in size from 300 microns to 1000 microns, and that comprise 5% to 20% by weight of the lost circulation material 21 ; teakwood fibres 12 that range in size from 700 microns to 1500 microns, and that comprise 5% to 20% by weight of the lost circulation material 21 ; teakwood fibres 12 that range in size from 1300 microns to 2000 microns, and that comprise 1 % to 20% by weight of the lost circulation material 21 ; teakwood fibres 12 that range in size from 1800 microns to 3000 microns, and that comprise 1 % to 15% by weight of the lost circulation material 21 ; and teakwood fibres 12 that range in size from 2500 microns to 5000 microns, and that comprise 5% to 15% by weight of the lost circulation material 21 .

[0047] The cotton fibres 13 comprise: cotton fibres 13 that range in size from 1 micron to 150 microns, and that comprise 1 % to 20% by weight of the lost circulation material 21 ; cotton fibres 13 that range in size from 100 microns to 250 microns, and that comprise 1 % to 15% by weight of the lost circulation material 21 ; and cotton fibres 13 that range in size from 150 micron to 500 micron, and that comprise 1 % to 15% by weight of the lost circulation material 21 . [0048] The nutshell 22 comprises coconut shell or peanut shell that ranges in size from 50 microns to 5000 microns, and that comprises 1 % to 40% by weight of the lost circulation material 21 .

[0049] The woodchips 15 comprise woodchips 15 that range in size from 25 microns to 20000 microns, and that comprise 1 % to 25% by weight of the lost circulation material 21 .

[0050] The fibres 12, 13 may all be produced using the same grinding and/or sizing equipment. For example, they may all be produced using the equipment and/or methods disclosed in published Australian Patent Application No. 200172159 whose contents have, as mentioned above, been incorporated herein by reference.

[0051] Referring to Figure 3, the drilling fluid additive 10 or 20 is added to and mixed with a drilling fluid 30 such as, for example, a water or oil-based drilling mud to form a lost circulation pill 31 .

[0052] With reference to Figure 4, in use, the drilling fluid 30 to which the drilling fluid additive 10 or 20 has been added, and which is called a lost circulation pill 31 , is pumped down a well borehole 40 in 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 .

[0053] Before the pill 31 is pumped down the borehole 40, all drilling as well as the circulation of drilling fluid through the drill string 41 and the borehole 40 is stopped. After drilling and circulation of drilling fluid is stopped, the drill string 41 is opened by releasing all of a bottom hole assembly that includes the drill bit 42 from the drill string. The bottom hole assembly is released by withdrawing the drilling string 41 from the borehole 40 and then removing the bottom hole assembly from the drill string 41 . The open-ended drill string 41 is then inserted back into the borehole 40. Once the drill string 41 has been reinserted into the borehole 40, the lost circulation pill 31 is pumped down the borehole 40. As the pill 31 flows up the annulus 43 between the wall 44 of the borehole 40 and the drill string 41 , the lost circulation material in the pill 31 is able to enter fractures in the formation and plug/seal them up so as to prevent or at least inhibit drilling fluid from being lost from the borehole 40 through the fractures. The pill 31 is only pumped down through the drill string 41 once in an operation that is referred to as 'spotting'. When the pill 31 is pumped down the borehole 40, a 'hesitation squeeze' is applied to the pill 31. The 'hesitation squeeze' involves increasing the pressure on the pill 31 by increasing the rate at which it is pumped through the drill string 41 so that a matrix of lost circulation material that is deposited into the fractures is squeezed/compacted/compressed into the fractures and thereby seals them.

[0054] Figure 5 depicts the borehole/wellbore 40 passing through a fractured limestone formation 50. As can be seen, the formation 50 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 the lost circulation pill 31 includes the first drilling fluid additive 10 that includes the acid-soluble lost circulation material 1 1 , and the lost circulation pill 31 is pumped through the borehole 40 in the manner described above, the lost circulation material 1 1 will be pumped into the fractures 51 where it forms a seal 52 that includes an impermeable matrix 53 of teakwood fibres 12, cotton fibres 13, calcium carbonate flakes 14, and woodchips 15. 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.

[0055] With reference to Figure 6, the above-described steps comprise a method 60 of sealing fractures that extend into the formation 50 from the borehole 40 that extends through the formation 50.

[0056] The inclusion of the ground and sized calcium carbonate flakes 14 in the lost circulation material 1 1 makes the drilling fluid additive 10 particularly suitable for use in sealing a producing formation such that severe lost circulation in the formation is eliminated or at least reduced.

[0057] The seal/plug formed by the matrix 52 of lost circulation material 1 1 can be removed/released without damaging the sealed formation zone by acidizing/adding acid to the material 1 1 so that the acid dissolves the calcium carbonate flakes 14. 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. The inclusion of calcium carbonate flakes 14 in the lost circulation material 1 1 results in an acid-soluble lost circulation material 1 1 that can be used to eliminate or at least reduce lost circulation in a producing formation. Importantly, modifying the lost circulation material 1 1 in this manner enables the material 1 1 to eliminate or at least reduce lost circulation without damaging the formation. This is in contrast to other acid-soluble lost circulation materials that are available and that are prone to damaging producing formations so that the amount of oil or gas that the formations are able to produce is consequently reduced.

[0058] A similar result to that shown in Figure 5 is achieved if the lost circulation pill 31 includes the second drilling fluid additive 20 that includes the biodegradable lost circulation material 21 , and the lost circulation pill 31 is pumped through the borehole 40. The lost circulation material 21 will be pumped into the fractures 51 where it forms an impermeable matrix (not shown) of teakwood fibres 12, cotton fibres 13, coconut shell 22, and woodchips 15. The impermeable matrix of teakwood fibres 12, cotton fibres 13, coconut shell 22, and woodchips 15 is able to substantially plug or seal the large fractures 51 , and is thereby able to substantially stop drilling fluid 30 in the borehole 40 from entering the formation 50 through the fractures 51 and being lost.

[0059] The inclusion of the 100% biodegradable lost circulation material 21 makes the drilling fluid additive 20 particularly suitable for use in sealing a producing formation such that severe lost circulation in the formation is eliminated or at least reduced.

[0060] Due to the fact that the lost circulation material 21 is 100% biodegradable, the seal/plug formed by the matrix of lost circulation material 21 will decompose naturally so that the seal/plug is thereby removed/released without damaging the sealed formation zone. 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. [0061] Whereas most other lost circulation materials are only able to effectively seal highly fractured formations with a maximum success rate in the low 30% range, it is anticipated that the lost circulation materials 1 1 , 21 according to the present invention will be able to effectively seal such formations with a success rate of 90% to 100%.

[0062] 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.

[0063] 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.

[0064] Throughout the specification and claims, unless the context requires otherwise, the term "substantially" or "about" will be understood to not be limited to the value for the range qualified by the terms.

[0065] 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.

[0066] It will be clearly understood that, if a prior art publication is referred to herein, that reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

Claims

CLAIMS:

1 . A drilling fluid additive comprising lost circulation material that comprises fibres and chips.

2. The drilling fluid additive defined by Claim 1 , wherein the fibres comprise wood fibres.

3. The drilling fluid additive defined by Claim 2, wherein the wood fibres comprise teakwood fibres.

4. The drilling fluid additive defined by any one of Claims 2 and 3, wherein the wood fibres comprise ground and sized fibres.

5. The drilling fluid additive defined by any one of Claims 2 to 4, wherein the wood fibres comprise wood fibres that range in size from 1 micron to 20 microns, and that comprise 10% to 30% by weight of all of the fibres and chips of the lost circulation material.

6. The drilling fluid additive defined by any one of Claims 2 to 5, wherein the wood fibres comprise wood fibres that range in size from 15 microns to 200 microns, and that comprise 15% to 30% by weight of all of the fibres and chips of the lost circulation material.

7. The drilling fluid additive defined by any one of Claims 2 to 6, wherein the wood fibres comprise wood fibres that range in size from 150 microns to 500 microns, and that comprise 10% to 30% by weight of all of the fibres and chips of the lost circulation material.

8. The drilling fluid additive defined by any one of Claims 2 to 7, wherein the wood fibres comprise wood fibres that range in size from 300 microns to 1000 microns, and that comprise 5% to 20% by weight of all of the fibres and chips of the lost circulation material.

9. The drilling fluid additive defined by any one of Claims 2 to 8, wherein the wood fibres comprise wood fibres that range in size from 700 microns to 1500 microns, and that comprise 5% to 20% by weight of all of the fibres and chips of the lost circulation material.

10. The drilling fluid additive defined by any one of Claims 2 to 9, wherein the wood fibres comprise wood fibres that range in size from 1300 microns to 2000 microns, and that comprise 1 % to 20% by weight of all of the fibres and chips of the lost circulation material.

1 1 . The drilling fluid additive defined by any one of Claims 2 to 10, wherein the wood fibres comprise wood fibres that range in size from 1800 microns to 3000 microns, and that comprise 1 % to 15% by weight of all of the fibres and chips of the lost circulation material.

12. The drilling fluid additive defined by any one of Claims 2 to 1 1 , wherein the wood fibres comprise wood fibres that range in size from 2500 microns to 5000 microns, and that comprise 5% to 15% by weight of all of the fibres and chips of the lost circulation material.

13. The drilling fluid additive defined by any one of Claims 1 to 12, wherein the fibres comprise ground and sized cotton fibres.

14. The drilling fluid additive defined by Claim 13, wherein the cotton fibres comprise cotton fibres that range in size from 1 micron to 150 microns, and that comprise 1 % to 20% by weight of all of the fibres and chips of the lost circulation material.

15. The drilling fluid additive defined by any one of Claims 13 to 14, wherein the cotton fibres comprise cotton fibres that range in size from 100 microns to 250 microns, and that comprise 1 % to 15% by weight of all of the fibres and chips of the lost circulation material.

16. The drilling fluid additive defined by any one of Claims 13 to 15, wherein the cotton fibres comprise cotton fibres that range in size from 150 microns to 500 microns, and that comprise 1 % to 15% by weight of all of the fibres and chips of the lost circulation material.

17. The drilling fluid additive defined by any one of Claims 1 to 16, wherein the chips comprise woodchips.

18. The drilling fluid additive defined by Claim 17, wherein the chips comprise chips that range in size from 25 microns to 20000 microns, and that comprise 1 % to 25% by weight of all of the fibres and chips of the lost circulation material.

19. The drilling fluid additive defined by any one of Claims 1 to 18, wherein the lost circulation material is acid-soluble lost circulation material that also comprises calcium carbonate flakes.

20. The drilling fluid additive defined by Claim 19, wherein the acid-soluble lost circulation material is at least 70% acid-soluble.

21 . The drilling fluid additive defined by any one of Claims 19 to 20, wherein the fibres and chips comprise up to 30% by weight of all of the lost circulation material, and that the calcium carbonate flakes comprise at least 70% by weight of the lost circulation material.

22. The drilling fluid additive defined by any one of Claims 19 to 21 , wherein the calcium carbonate flakes are derived from mini sea or river shells.

23. The drilling fluid additive defined by any one of Claims 19 to 22, wherein the calcium carbonate flakes comprise ground and sized calcium carbonate flakes.

24. The drilling fluid additive defined by Claim 23, wherein the ground and sized calcium carbonate flakes comprise calcium carbonate flakes that range in size from 50 microns to 5000 microns, and that comprise 1 % to 80% by weight of all of the calcium carbonate flakes.

25. The drilling fluid additive defined by any one of Claims 1 to 18, wherein the lost circulation material is biodegradable lost circulation material that also comprises nut shell.

26. The drilling fluid additive defined by Claim 25, wherein the biodegradable lost circulation material is 100% biodegradable.

27. The drilling fluid additive defined by any one of Claims 25 to 26, wherein the nutshell is selected from the group of nutshells comprising coconut shell and peanut shell.

28. The drilling fluid additive defined by any one of Claims 25 to 27, wherein the nut shell comprises ground and sized nut shell.

29. The drilling fluid additive defined by Claim 28, wherein the ground and sized nutshell comprises nutshell that ranges in size from 50 microns to 5000 microns, and that comprises 1 % to 40% by weight of the lost circulation material.

30. The drilling fluid additive defined by any one of Claims 1 to 29, wherein the lost circulation material is a single sack lost circulation material.

31 . A lost circulation pill comprising a drilling fluid, and a drilling fluid additive according to any one of Claims 1 to 30.

32. The lost circulation pill defined by Claim 31 , wherein the drilling fluid is a liquid drilling fluid.

33. The lost circulation pill defined by Claim 32, wherein the drilling fluid is selected from the group of drilling fluid comprising: a water-based drilling fluid; an oil-based drilling fluid; and a synthetic-based drilling fluid.

34. 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 any one of Claims 28 to

30; and

35. A formation that includes a borehole through which a lost circulation pill according to any one of Claims 31 to 33 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 pill's drilling fluid additive.

36. A drilling fluid additive substantially as hereinbefore described with reference to the drawings.

37. A lost circulation pill substantially as hereinbefore described with reference to the drawings.

38. A method of sealing fractures that extend into a formation from a borehole that extends through the formation, the method being substantially as hereinbefore described with reference to the drawings.

39. A formation substantially as hereinbefore described with reference to the drawings.