GB2577937A

GB2577937A - Drilling method

Info

Publication number: GB2577937A
Application number: GB1816666.0A
Authority: GB
Inventors: Henriksen Sverre; Alexandru Merciu Ioan; Lopez Olivier; Gabriel Hanea Remus; Kittilsen Pål
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2018-10-12
Filing date: 2018-10-12
Publication date: 2020-04-15
Also published as: GB201816666D0

Abstract

The invention relates to a method of drilling a wellbore 8 in a formation 4. The method comprises introducing into said formation a drilling fluid which comprises graphite and activating the drilling fluid whereby to convert at least some of the graphite to graphene within fractures 22 in the formation. The graphene may be generated in-situ in the fractures. The graphite may be contained within a drilling fluid 16 and the drilling fluid exposed to energy 20 to convert at least some of the graphite into graphene.

Description

Drilling method The present invention relates generally to oil and gas well drilling and, more specifically, to oil and gas well drilling in situations where there is a loss of drilling fluid into the geological formation ("lost circulation"). In particular, the invention relates to drilling of depleted oil and gas reservoirs.

In the drilling of onshore and offshore wells to recover oil and/or gas, a wellbore (or borehole) is formed using a drill bit at the lower end of a string of hollow steel drill pipes (a "drill string"). In a rotary drilling operation the drill string is connected to a rotary drill bit which grinds and gouges the rock as it is rotated by the drill string. Rock cuttings are continuously removed from the wellbore through an annulus between the drill string and the borehole, for example using a drilling fluid (typically referred to as a "drilling mud" or simply "mud") which is pumped down through the hollow drill string and which exits holes in the drill bit. The drilling fluid also serves to increase the hydrostatic pressure at the bottom of the wellbore thereby controlling the flow of formation fluids into the wellbore, as well as functioning to cool, clean and lubricate the drill bit.

If the pressure within the drilled rock or formation strength is less than the mud pressure acting on the borehole, a loss event can occur in which the drilling fluid or mud flows into the formation instead of returning up the annulus. Loss events (also known as "lost circulation") can be a serious problem in drilling of oil and gas wells. They can lead to a well collapse or an influx of gas or fluid higher up the wellbore, known as a "kick" which can lead to a blowout. Minor losses of drilling fluid may be tolerated or controlled during drilling by taking certain action, such as increasing the viscosity of the drilling fluid by the addition of so-called "lost circulation materials" (LCMs). If significant losses occur and circulation cannot be regained, it may still be possible to continue drilling while still pumping drilling fluid (or water) into the borehole. This is known as "drilling with losses".

Lost circulation may be encountered in various situations, such as when drilling formations that are inherently fractured or which have high permeability. In some cases, the formation may be unstable (e.g. sandstone formations) or it may be non-consolidated due to the geology of the rock. Loss of drilling fluid is a particular -2 -problem in the drilling of reservoirs in which the oil or gas content has been depleted by previous drilling operations. In such "depleted reservoirs", losses may occur as a result of a mechanical disturbance of the interconnecting forces between the grains of the formation which can arise where the rock has been cooled by production or water injection. Loss of drilling fluid typically occurs along relatively thin zones of weakness in the formation, such as unconformities or zones of heterogeneity.

Many new wells need to pass into or through zones in a formation that have already been produced from, i.e. depleted reservoirs. One reason for this could be that an existing reservoir needs to be drained from a different location in order to enhance oil or gas recovery, or it could be that a new reservoir is located below an older one. New oil and gas wells are therefore becoming more challenging to drill. A lower pressure within the formation gives rise to problems and potential risks when drilling depleted reservoirs. Drilling requires passage through very large pressure differential barriers and it is essential that safety is maintained whilst advancing through depleted zones of the formation. Drilling of depleted reservoirs without encountering excessive mud losses or wellbore collapse is a considerable challenge. During conventional drilling, well stability is ensured by using a complex chemical mud system which interacts with the formation and serves to provide a borehole pressure higher than the formation fluid pressure inside the "drilling window" (between the borehole stability curve and the fracturing pressure). As a well becomes depleted, however, the fracture initiation gradient often lowers to less than the mud weight required to prevent wellbore breakout, i.e. the weight of fluid required to prevent formation fluids (naturally occurring liquids and gases within the formations) from entering the wellbore is higher than the mud weight at which the wellbore fractures. This means that the operating "drilling window" either no longer exists or is diminished to such an extent that it is no longer possible to drill. The area in a formation where this occurs is known as a "depletion barrier".

Several oil and gas fields have a limited, or in some cases non-existent, operating window due to high depletion and/or a combination of high and low pressure within the formation. There are a variety of models which can be used to estimate formation strength and which provide some flexibility for drilling and completion of wells with differential depletion. In some cases, new technologies, muds, drilling -3 -practice and extensive data acquisition from the field have enabled an increase in the drilling window. However, for some fields, this approach is not sufficient and new technology is needed to enable further drilling.

Strengthening of a wellbore can aid in drilling of a depleted formation. Methods conventionally used to strengthen wellbores include installing equipment to support the borehole, such as conventional and expandable casing, and chemically consolidating the geological formation, e.g. using chemicals or combinations of chemicals which physically plug any fractures and instabilities. WO 2017/066757, for example, describes a method for enhancing the stability of a formation in which a nanomaterial and a cross-linkable polymer component are introduced into a formation and subjected to microwave irradiation. The nanomaterial generates heat upon the application of microwave energy which aids in crosslinking and thermosetting of the polymer compositions. The resulting polymer composite becomes embedded within the formation thus enhancing its stability.

Graphene-containing fluids are known for use in oil and gas production. In WO 2012/129302 the addition of graphene nanoparticles to a base fluid is used to improve various properties of the fluid, such as its rheological properties and stability. In US 8,183,180 the addition of derivatised graphenes to drilling fluids reduces penetration of the drilling fluids into rock pores of a formation. The use of graphene in drilling applications can offer several advantages over conventional additives which are generally spherical. Graphene has a layered or sheet-like' structure and can exist in very thin, yet strong and flexible layers, for example ranging from 1 nm to 1 pm in thickness. As such, these are able to able to block pores in rocks more effectively than additives which have a spherical geometry. The flexibility of graphene sheets also permits a slight deformation under pressure which further aids in sealing of the graphene sheets around pore edges.

Due to its interesting properties (e.g. electrical conductivity, high specific surface area and excellent mechanical strength), graphene has the potential for use in a wide range of applications, including the production of electronic components. However, known methods for the synthesis of graphene are time-consuming and the yield of high quality graphene is poor. Graphene is therefore expensive and not -4 -readily available in large quantifies. This has limited its widespread use, particularly in the oil and gas industry where large quantities may be required.

There remains a need for other methods of drilling oil and gas wells with minimum loss of drilling fluids into the formation and, in particular, a need for improved methods for drilling of depleted reservoirs so that existing reservoirs might be accessed from a different location, or oil and gas can be recovered from new reservoirs located below older ones.

In one aspect the invention provides a method of drilling a wellbore in a formation, the method comprising: introducing into said formation a drilling fluid which comprises graphite; and activating the drilling fluid whereby to convert at least some of the graphite to graphene within fractures in the formation.

The methods herein described can be used in the drilling of any formation, but are particularly suited to drilling in situations in which lost circulation would be expected during a conventional drilling process. In particular, the methods may be used in the drilling of depleted reservoirs.

The inventors thus propose a method of drilling into a formation in which graphene is generated in-situ in fractures in the formation. Such fractures may be an inherent part of the natural geology of the rock formation. In other cases, fractures may be generated in the formation during the drilling process herein described (so-called "drill-induced fractures"). In the drilling of depleted reservoirs, the fractures will generally be drill-induced as a result of the pressure conditions in the areas of depletion and the rotational force of the drill bit. In one embodiment, the invention thus relates to a method in which low cost graphite is introduced into the formation and exfoliated to form graphene in fractures induced when drilling through a depleted reservoir, thereby serving to strengthen the formation and positively influence the drill margins. Such a method has a direct impact on the drilling of depleted reservoirs by increasing the drilling window ahead of the drill bit and increasing the chance of achieving the drilling target. It also provides for full control during drilling operations and is simple and cost-effective. -5 -

In the following description, various details are provided such as quantifies and concentrations, etc. in order to aid in understanding of the various embodiments of the invention disclosed herein. However, it will be apparent to those of skill in the art that the present disclosure may be practiced without such specific details and these are not necessary to obtain a complete understanding of the disclosure.

Variations and modifications are within the skill of those of ordinary skill in the art.

Most of the terms used herein will be recognised and understood by those of skill in the art. However, the following definitions are provided to aid in understanding of

the present disclosure:

As used herein, the term "drilling fluid" refers to a fluid used during the operation of drilling of a wellbore.

The term "fluid" refers to gases, liquids and combinations of gases and liquids. It also includes combinations of gases and solids, and liquids and solids (e.g. a dispersion of a solid in a liquid).

The terms "wellbore" and "borehole" are used interchangeably herein and refer to a hole in a formation made by drilling. A wellbore may be on land (i.e. on-shore) or off-shore and may be cased, or un-cased. A wellbore may have a substantially circular cross-section or any other cross-sectional shape.

The term "formation" refers to any geological region below the earth's surface or strafigraphic interval. The formation may contain one or more hydrocarbon-containing layers, e.g. layers containing crude oil and/or natural gas.

The term "depleted reservoir" refers to an area of a formation having a reduced fluid content as a result of production of that fluid. For example, it may have a reduced content of crude oil and/or natural gas.

The term "depletion barrier" refers to a region in a formation in which the fracture pressure intersects the stability curve (e.g. pore pressure) resulting in a reduced or non-existent drilling window. In this region it is impossible to effectively drill through the formation other than with losses. The term "depletion barrier" is used herein to -6 -define the boundary between a depleted reservoir and an area in the formation which is non-depleted (i.e. which has not already been produced from).

The terms "lost circulation" and "drilling with losses" are used interchangeably herein and refer to a drilling operation in which the volume of drilling fluid (or "mud") which is pumped into the wellbore is greater than the volume of fluid returning up to the surface. Loss of drilling fluid into the formation means that the borehole pressure, which is controlled by gravity and the length of the fluid column, may not be able to balance the influx of fluids into the wellbore from the formation. The term "lost circulation zone" refers to an area in a formation which, when drilled, gives rise to a loss of drilling fluid, i.e. drilling with losses occurs.

"Graphene", as defined herein, is an allotrope of carbon composed of sheets of carbon atoms that are only one layer thick. In each layer the carbon atoms are arranged in a 2-dimensional hexagonal lattice. The term "graphene" as used herein includes materials (e.g. particles) that contain more than one atomic plane whilst still retaining a layered morphology. Graphene may, for example, comprise less than about 100 monatomic-thick carbon layers, more typically less than about 50 such layers, e.g. less than about 10 carbon layers.

"Graphite", as defined herein, is the 3-dimensional version of graphene. Graphite has a layered, planar structure in which the individual layers consist of graphene. In each layer, the carbon atoms are arranged in a hexagonal lattice. Bonding between layers is via weak van der Waals bonds which allow layers of graphite to be separated, or to slide past each other. Exfoliation of graphite, as referred to herein, refers to separation of one or more layers of graphite to produce "exfoliated graphite". Exfoliated graphite, as referred to herein, may be considered to contain at least a proportion of graphene, but need not be pure graphene.

In the method of the invention, a graphite-containing drilling fluid is introduced into fractures within a lost circulation zone in the formation. Once inside the fracture space, activation of the graphite within the fluid causes its exfoliation to separate one or more of the graphite layers. Without wishing to be bound by theory, exfoliation of the graphite is believed to disrupt at least some of the van der Waals forces between the individual layers of the graphite to convert at least some of it to -7 -graphene which serves to strengthen the formation. Depending on the extent of exfoliation, the resulting material may be pure or impure graphene. Impure graphene will still contain a proportion of graphite. In some cases, exfoliation may result in the formation of pure graphene in which substantially all graphite is converted to monoatomic-thick carbon layers. It is postulated that the graphene produced by exfoliation of the graphite is able to coat the granular components of the rock matrix to strengthen the formation. It may also serve as a bridging agent to connect the granular components of the rock and, in some cases, may act to effectively seal the pores within the formation. The invention is not intended to be limited by any mode of action herein described.

In one embodiment, the method may comprise the following steps: (a) drilling into a lost circulation zone within the formation using a drill bit to form the wellbore; (b) introducing into the wellbore a drilling fluid which comprises graphite; (c) allowing at least some of the drilling fluid to enter fractures in the lost circulation zone; and (d) exposing the drilling fluid to energy whereby to convert at least some of the graphite to graphene within the fractures.

If required, steps (a) to (d) may be repeated one or more times until the drill bit has passed through the entire lost circulation zone, for example through a depleted reservoir within the formation. At that point, normal drilling operations may be resumed.

The lost circulation zone may comprise fractures due to the inherent characteristics of the rock formation and into which the graphite-containing drilling fluid may pass following its introduction to the wellbore. However, in some embodiments of the invention, drilling into the lost circulation zone of the formation may itself give rise to fractures in the rock, i.e. so-called drill-in or drill-induced fractures are formed during the drilling process. In one embodiment, the lost circulation zone is a depleted reservoir defined by a depletion barrier as herein described. Drilling through the depletion barrier and into the depleted reservoir will generally give rise to a series of fractures in the formation extending from the wellbore. In such cases, graphene -8 -may be generated in-situ in these drill-induced fractures following the introduction of the graphite-containing drilling fluid.

In the method of the invention drilling may be carried out using any conventional drilling equipment. Typically, drilling may be performed using a rotary drill which comprises a drill string provided with a drill bit at its distal end. The drill bit will be capable of cutting and/or crushing the rock as it penetrates the formation. For example, this may induce compressive fractures, or apply a shear force to the rock. It may be made from any suitable material and provided in any conventional shape or configuration.

Generally, the drill bit will be attached to a drill string which can be gradually lengthened as drilling progresses and the wellbore is formed. The drill string will generally consist of a series of lengths of drill pipe coupled together. At its distal end are heavier walled lengths of pipe, known as drill collars, which help to control the weight on the drill bit. The drill bit can be retracted (either partially or fully) and lowered as required using any conventional technique such as adding or removing lengths of drill pipe (i.e. drill string) above the drill bit.

During drilling of the formation, a drilling fluid (also known as a "drilling mud" or simply "mud") may be circulated (e.g. pumped) down the annulus of the drill string and out of orifices in the drill bit. The drilling fluid may contain a mixture of fluids, solids and chemicals which aid in cooling the drill bit, lifting rock cuttings to the surface, and overcoming the pressure of the fluids inside the rock formation to ensure that these fluids do not enter the wellbore during drilling. Any known drilling fluid may be used during normal drilling. The mud used during drilling may be the same or different to the graphite-containing drilling fluid also used in the method of the invention, although generally the fluids will be different.

The method of the invention involves the step of drilling through a lost circulation zone in the formation, e.g. through a depleted reservoir whose boundary is defined by a depletion barrier. In some embodiments, the method may include the step of detecting the lost circulation zone in an area of the formation ahead of the drill bit. For example, it may involve detection of the depletion barrier prior to drilling into the depletion reservoir. Various methods, such as "logging while drilling", are known in -9 -the art for data acquisition from the field during drilling and these may be used to detect the presence of lost circulation zones or depletion barriers.

Drilling into a depleted reservoir may cause fracturing of the formation, i.e. the production of one or more fractures in the rock ("drill-induced fractures") ahead of and/or lateral to the drill bit. Fracturing of the formation may be achieved by drilling at a high circulation rate, for example at a higher circulation rate than that used to drill the non-depleted areas of the formation. Depending on the particular situation (e.g. the nature of the rock formation, etc.), the skilled person would readily be able to select suitable drilling parameters to give rise to fracturing.

The extent (i.e. depth) of drilling into the lost circulation zone or depleted area of the formation prior to introduction of the graphite-containing drilling fluid may vary depending on factors such as the nature of the rock formation, its extent of depletion, etc. but can readily be determined by those skilled in the art. Drilling through the entire lost circulation zone may be carried out in a single (i.e. continuous) drilling step. Generally, however, it is envisaged that drilling will be carried out in several drilling stages in which each stage of the drilling process will involve partially drilling into the wellbore and may, for example, progress the extent of the borehole a relatively short distance, for example from about 1 to about 10 m, into the lost circulation zone or area of depletion, e.g. from about 2 to about 7 m. During each stage of the drilling process, it is envisaged that additional graphite-containing drilling fluid will be introduced downhole and graphene will be generated in-situ in further fractures in the rock in order to strengthen the formation.

When drilling through the lost circulation zone is carried out in a single drilling step, introduction of the graphite-containing drilling fluid into the wellbore may be carried out simultaneously with the drilling procedure, i.e. without terminating the drilling process. More typically, however, drilling may be interrupted and, in some embodiments, the drill bit may be retracted from the bottom of the well prior to and/or during introduction of the drilling fluid into the wellbore. This allows a volume of fluid to be introduced into the bottom of the well and below the drill bit. In this situation, the drilling fluid can readily penetrate fractures within the rock formation which are adjacent to the wellbore. Although the drill bit may be fully retracted prior to the introduction of the drilling fluid into the wellbore, generally it will be partially -10 -retracted. In cases where the drill bit is retracted, this is then lowered into the graphite-containing drilling fluid at the bottom of the well prior to exposing the fluid to energy to exfoliate the graphite. The extent of retraction of the drill bit may vary depending on factors such as the extent of drilling into the lost circulation zone, the volume of graphite-containing drilling fluid to be introduced into the wellbore, etc. but can readily be selected by the skilled person according to the situation. Typically, the drill bit will be retracted just beyond the area of the formation in which fractures are present, or in which fractures have been induced during drilling, such that the graphite-containing drilling fluid which is introduced into the wellbore can readily penetrate the fractures.

Introduction of the graphite-containing fluid into the wellbore may be carried out by conventional means known for introducing drilling fluids. For example, the fluid may be introduced by pumping into the wellbore, e.g. through the drill string. In some embodiments, pumping may be achieved using a pump. Typically, the fluid will be introduced through the drill string and drill bit. In this case, the fluid is ejected out of one or more openings in the drill bit to fill the space between the bottom of the borehole and the drill bit. The volume of drilling fluid will vary according to factors such as the extent of drilling into the lost circulation zone, the nature of the formation and extent of fracturing of the formation, etc. The skilled person would readily be able to select an appropriate volume depending on the particular situation.

Any form of graphite may be used in the drilling fluid for use in the method of the invention. For example, any of the following may be used: crystalline graphite (e.g. microcrystalline graphite, nanocrystalline graphite), amorphous graphite, lump graphite, highly ordered pyrolytic graphite, or graphite particles. The graphite may be chemically or naturally expanded. Typically, the drilling fluid will comprise particles (e.g. nanoparticles) of graphite. Graphite-containing materials are known and supplied for use in the field of oil and gas production, e.g. for use in pore blocking, and any such known materials may be used in the methods herein described. Graphite for use in the invention may be obtained from suppliers such as Halliburton, for example as STEELSEAL8-398 lost circulation material (available as fine, medium or coarse granular, carbon-based material).

In one embodiment the graphite may be used in the form of graphite particles, e.g. graphite nanoparticles. These may be graded as fine, medium or coarse and may, for example, have at least one dimension which has an average D50 in the range of from 30 to 600 nm, e.g. 60 to 450 nm. Such materials are widely commercially available.

For use in the invention, the graphite will generally be dispersed as a solid in a drilling fluid. Fluids used in the drilling of subterranean oil and gas wells are well known. Drilling fluids (also known as drilling muds) are generally either oil-based muds (0BMs) or water-based muds BMs), although emulsions containing both oil and water may also be used. The graphite-containing drilling fluid (or mud) for use in the invention may be based on any conventional drilling fluid. The fluid may thus be a water-based or an oil-based fluid. It is envisaged that water will generally form the base of the fluid, however, it is possible that an oil-based or oil-emulsion based fluid may be employed. In the case of an oil-emulsion both oil and water are present. Water-based fluids are, however, generally preferred since these are most cost-effective and environmentally friendly. The aqueous component of the fluid may be any water-based fluid that is compatible with other components of the fluid. For example, the aqueous component may be fresh water or sea water. Preferably it will be fresh water.

Drilling fluids which may be used as the base to which graphite (e.g. graphite particles) may be added are well known in the art. Non-limiting examples of drilling fluids include HYDRO-GUARD® water-based fluids, BOREMAX® freshwater fluid, baraECDO, BaraPuree, AND PERFORMADRILO. Any of the graphite materials described herein may be added to any of these drilling fluids.

The amount of graphite provided in the drilling fluid may vary according to factors such as the nature of the formation, the extent of fracturing, etc. but may readily be determined by those skilled in the art. In some embodiments, the graphite may be present in a concentration range of about 5 to 350 kg/m3 (based on the total weight of the drilling fluid). In other embodiments, it may be present in a concentration range of about 50 to about 100 kg/m3 (based on the total weight of the drilling fluid).

-12 -The graphite-containing drilling fluid may also comprise other components known for use in oil and gas drilling operations. Additional materials which may be present include those conventionally used in drilling fluids. Other materials include bridging agents, filtration control agents, lubricants, de-foamers, fluid loss control agents, pH adjusting agents, shale inhibitors, and anti-bacterial agents. These may be used in conventional amounts known to those skilled in the art or in an amount which could readily be determined by those skilled in the art.

Generally, the graphite-containing drilling fluid will contain additives to increase the fluid density. The overall density of the fluid can be varied as required. In one embodiment the density of the fluid may be adjusted such that this is greater than about 1.80 g.cm-3.

Certain graphite-containing fluids are known for use in oil and gas drilling and any of these may be used in the invention. However, in some embodiments, a drilling fluid may be customised for use in the methods herein described, for example this may be formulated to have a graphite content higher than that in conventional graphite-containing drilling fluids. Customised drilling fluids containing graphite can be prepared based on any water and/or oil-based liquid. In certain embodiments, the drilling fluid may further contain a dispersant, e.g. a surfactant. Surfactants may include, for example, anionic surfactants, cationic surfactants, amphoteric surfactants, non-ionic surfactants, and any blends thereof.

Methods of producing a drilling fluid comprising graphite for use in the method of the invention may comprise the step of dispersing graphite in a drilling fluid.

Dispersion may be achieved using known methods including, for example, stirring, sonication, or a combination thereof. In some embodiments, the methods may include suspending the graphite in the drilling fluid. For this purpose, a surfactant may be used.

The amount of graphite-containing drilling fluid employed in the method of the invention will vary depending on factors such as the concentration of graphite, but may readily be determined by those skilled in the art.

-13 -Once the drilling fluid containing graphite has been introduced into the wellbore, this is activated to cause exfoliation of the graphite. As described herein, exfoliation involves disruption of the forces which bond the individual graphene layers together to convert at least a portion of the graphite to graphene which is effective in plugging of the fractures in the formation, e.g. the drill-induced fractures in a depleted reservoir. In some embodiments, the formed graphene enhances the stability of the formation in which it is provided.

Activation of the graphite converts it, at least in part, to graphene. In some cases, activation may be effective to convert substantially all of the graphite to graphene.

Although not wishing to be bound by theory, it is considered that the produced graphene is able to stabilise and/or "plug" any fractures induced in the depleted formation. Activation of the graphite may be achieved in various ways including, but not limited to, any of the following: drill movement (i.e. the mechanical force provided by rapid rotation of the drill bit), the difference in pressure at the depletion barrier (i.e. when drilling into a depleted reservoir), exposure to electromagnetic radiation, and any combination thereof. A combination of drill rotation (sufficient to induce an effective level of turbulence and/or shearing forces in the drilling fluid) and exposure of the graphite-containing drilling fluid to electromagnetic radiation is considered to be particularly effective.

In some embodiments, activation of the graphite may be achieved by rotation of the drill string within the graphite-containing drilling fluid. If the drill string has been partially retracted, this is lowered into the fluid and rotated. The speed and duration of rotation should be sufficient to give rise to turbulence and shearing forces within the fluid in the wellbore and/or in the fractures which are effective in separation of the individual layers of the graphite. Suitable rotation speeds can readily be determined by those skilled in the art. In some embodiments, the drill string may be rotated at between about 30 to about 120 rpm to achieve exfoliation of the graphite.

Suitable time periods for rotation can similarly be readily determined.

In some embodiments, the graphite-containing fluid is exfoliated by exposure to electromagnetic radiation. The fluid may be exposed to electromagnetic radiation using various known devices. For example, the fluid may be exposed to electromagnetic radiation using an electromagnetic radiation emitting device (e.g. -14 -an electromagnetic radar tool) which forms an integral part of the drilling assembly. As described herein, the drill string will generally comprise, at its distal end, thicker walled lengths of pipe (i.e. drill collars) which provide rigidity and add weight to aid the drilling process. In one embodiment, one or more electromagnetic radiation emitters may be built-in to the drill collars. For example, these may be mounted in an external face of one of the drill collars.

In one set of embodiments, one or more electromagnetic pulse transmitters and transmitting antennas may be built-in the drill collars. Each transmitter may be capable of generating an electromagnetic pulse wave (e.g. a short electromagnetic pulse wave) and emitting it into the formation through an associated transmitting antenna mounted on one of the drill collars. In one embodiment, an array of transmitters may be provided together with a corresponding array of transmitting antennas.

Suitable operating powers, frequencies and wavelengths of the electromagnetic pulses may be appropriately selected by those skilled in the art according to the situation. In one embodiment, one or more electromagnetic shock waves may be employed. Intensity of the electromagnetic radiation pulses may also be increased by focusing of the radiation, for example by passing it through a converging lens.

Drilling assemblies which contain electromagnetic transmitters are generally known in the oil and gas field. For example, these may be known for use in obtaining data relating to conditions downhole while drilling ("logging while drilling" (LWD)).

Examples of electromagnetic logging apparatus which may be used in the invention are described in US 2011/0251795 and in US 4,704,581, the entire contents of which are incorporated herein by reference.

Depending on the extent of the lost circulation zone (e.g. the depleted reservoir), the various steps of the method herein described may be carried out multiple times in order to traverse (i.e. drill through) the entire zone. Methods of monitoring traversal of lost circulation zone include any of the methods described above in relation to detection of the depletion barrier. Once the wellbore has passed through the lost circulation zone, normal drilling may be re-commenced.

-15 -Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figures 1A-E schematically illustrate a method according to the invention in which a section of a depleted formation is drilled.

For the purposes of explanation and illustration, and not limitation, an illustrative embodiment of a method for drilling a depleted reservoir according to the invention is shown in Figures 1A-E. The wellbore and associated drilling apparatus shown in these figures is a typical arrangement which is briefly described herein in the background to the invention. It comprises a conventional drill string (8) attached to a rotary drill bit (10). An electromagnetic radiation emitter (20), e.g. an electromagnetic radar tool, is built-in to one of the drill collars and forms an integral part of the drilling apparatus. The operation of such an apparatus is well known and will not be described further herein.

In Figure 1A, a wellbore (6) is drilled in a formation (4) using a conventional drill string (8) attached to a rotary drill bit (10). A drilling mud (not shown) may be circulated through the drill string (8) to the drill bit (10) during drilling which then returns up the annular space between the drill string (8) and the side walls of the wellbore (6) being drilled. A "depletion barrier" (2) defines the boundary between a zone within the formation (4) having a lower pore pressure (P2) than that of an adjacent overlying zone having a higher pore pressure, P1 (i.e. P1»P2). During drilling, the depletion barrier (2) may be detected ahead of the drill bit (10), for example in the range of 30 to 100 m ahead of the drill bit (10).

In Figure 1B, drilling at a high circulation rate continues a short distance through the depletion barrier (2) and into the depleted reservoir (having a pore pressure P2), e.g. 1 to 10 m into the depleted reservoir. Drilling-induced fractures (12) form in the depleted reservoir due to the mechanical action of the drill bit (10). Borehole fluids and/or drilling mud (14) used during the drilling process are naturally driven into the fracture space due to the pressure differential within the different zones of the formation (4) (P1 >> P2). The direction of the flow of fluids and/or drilling mud is from the area of high pressure (P1) to that of low pressure (P2).

-16 -In Figure 10, drilling is stopped and the drill bit (10) is partially retracted from the bottom of the well. A graphite-containing drilling fluid (16) is introduced to the wellbore through the drill string (8) and injected ahead of the drill bit in the space drilled through the depleted reservoir (e.g. 1 to 10m). The graphite-containing drilling fluid (16) enters the drilling-induced fractures (12). At this stage, the graphite particles in the drilling fluid remain substantially inert (i.e. inactive).

In Figure 10, the drill bit (10) is lowered into the graphite-containing drilling fluid (16). Drilling is resumed by rotation of the drill string (8) (depicted by arrow "17") whilst simultaneously subjecting the graphite-containing drilling fluid (16) to a series of shock pulses of electromagnetic radiation (depicted by "18") generated by electromagnetic radiation emitter (20). A combination of the rotation of the drill string (8), the pressure differential in the formation (P1 » P2) and the application of electromagnetic impulses (18) exfoliates the graphite around the drill string (8) and in the induced fractures (12) to convert it, at least partially, to graphene. This increases the formation strength deep inside the formation (4). For example, it may strengthen the borehole wall to a distance of 10 m or more into the formation. Further rotation of the drill string (8) gives rise to a further set of drill-induced fractures (22).

The process shown in Figures 1A to 10 is repeated until the drill string has passed through the entire depletion reservoir. This is shown in Figure 1E in which further drilling-induced fractures (22) are strengthened using the same method steps.

The methods herein described will generally be performed in the context of an operation intended to recover oil and/or gas. They may be conducted on-shore or off-shore. Although primarily intended for use in drilling of depleted reservoirs, it will be apparent that the graphite-containing fluids herein described may also be used in other down-hole operations. For example, these may be used in any operation in which it may be desirable to prevent a fluid from entering a rock formation, e.g. in drilling of wellbores in non-depleted formations (e.g. in unstable formations), in hydraulic fracturing of rock formations, etc. Whilst the method of the present disclosure has been shown and described with reference to certain embodiments, those skilled in the art will readily appreciate that -17 -changes and/or modifications may be made thereto without departing from the scope of the present disclosure.

Claims

-18 -Claims: A method of drilling a wellbore in a formation, the method comprising: introducing into said formation a drilling fluid which comprises graphite; and activating the drilling fluid whereby to convert at least some of the graphite to graphene within fractures in the formation.
2. A method as claimed in claim 1 wherein said formation comprises a lost circulation zone and said method comprises the step of drilling the wellbore into said lost circulation zone.
3. A method as claimed in claim 1 or claim 2, which comprises the following steps: (a) drilling into a lost circulation zone within the formation using a drill bit to form the wellbore; (b) introducing into the wellbore a drilling fluid which comprises graphite; (c) allowing at least some of the drilling fluid to enter fractures in the lost circulation zone; and (d) exposing the drilling fluid to energy whereby to convert at least some of the graphite to graphene within the fractures.
4. A method as claimed in claim 3, wherein steps (a) to (d) are repeated until the drill bit has passed through the lost circulation zone.
5. A method as claimed in any one of the preceding claims, wherein the step of drilling the wellbore induces fractures in the formation.
6. A method as claimed in claim 5, wherein graphene is generated in-situ in said fractures. 30
7. A method as claimed in any one of claims 2 to 6, wherein the lost circulation zone is a depleted reservoir.
8. A method as claimed in any one of claims 2 to 7 which further comprises the step of detecting the lost circulation zone prior to drilling into said zone.-19 -
9. A method as claimed in any one of claims 2 to 8, wherein drilling through the entire lost circulation zone is carried out in a plurality of drilling stages.
10. A method as claimed in claim 9, wherein graphene is generated in-situ in fractures in the formation during each of said drilling stages.
11. A method as claimed in any one of claims 3 to 10, wherein drilling is interrupted and the drill bit is partially retracted from the bottom of the well prior to or during the step of introducing the drilling fluid into the wellbore.
12. A method as claimed in any one of the preceding claims, wherein the graphite is provided in the form of graphite particles, e.g. graphite nanoparticles.
13. A method as claimed in claim 12, wherein said nanoparticles have at least one dimension which has an average D50 in the range of from 30 to 600 nm, e.g. 60 to 450 nm.
14. A method as claimed in any one of the preceding claims, wherein the drilling fluid comprises graphite dispersed in a water-based or oil-based fluid, or in fluid which comprises an oil-in-water emulsion.
15. A method as claimed in any one of the preceding claims, wherein the step of activating the drilling fluid is effected by one or more of the following: drill rotation within the drilling fluid, a pressure differential within the formation, and exposure of the drilling fluid to electromagnetic radiation.
16. A method as claimed in claim 15, wherein the step of activating the drilling fluid is effected by a combination of drill rotation and exposure of the drilling fluid to electromagnetic radiation.