Drilling fluid for casing drilling

Abstract

The present invention provides a method of drilling a hydrocarbon well using a drillstring adapted to provide a continuous or quasi-continuous contact with the wall of the well such that at least of section the well a compacting force is exerted on filter cake deposited thereon; circulating a drilling mud through the borehole during the drilling operation such that a filter cake is deposited on the borehole wall by the drilling mud, the filter cake being compacted by contact with the drillstring, including, before the drilling mud enters the borehole a conditioning additive in the drilling mud, the additive conditioning the filter cake upon the contact with the drillstring to increase the yield strength of the filter cake.

Description

The present invention relates to methods to enhance the performance of the filter or mud cake layer on the wall of the wellbore as protective and isolating layer.

BACKGROUND OF THE INVENTION

To obtain fluids, such as oil and gas, from a subterranean reservoir boreholes or wells are drilled from the surface into the reservoir. The most commonly applied method to drill a well uses a derrick or mast structure, in which a drill string is assembled and continuously extended into the borehole as the drilling progresses. Drilling is performed by rotating a drill bit attached to the end of the drill string. During the drilling process pressurized drilling fluid (commonly known as “mud” or “drilling mud”) is pumped from the surface into the hollow drill string to provide lubrication to various members of the drill string including the drill bit. On its way back to the surface through the annulus between drill string and the wall of the borehole, the drilling fluid removes the cuttings produced by the drill bit.

In most cases the pressure exerted by the drilling fluid is above the formation or pore pressure to prevent the entry of formation fluids into the wellbore during the drilling process. As a beneficial side effect, a small amount of pressurized mud enters into porous sections of the formation as it flow across those, thus leaving behind a layer of larger particles on the borehole wall. This layer is referred to as filter or mud cake. The mud cake layer prevents further fluid loss, which can be harmful, damaging formation permeability and lubricating fractures.

The barrier provided by the mud cake can potentially increase the so-called “mud window”. The mud window is a pressure range in which the driller maintains the mud pressure. The mud pressure should be sufficiently high to prevent influx from the formation whilst being low enough to prevent a fracturing of the formation and lost circulation. A wider mud window has the advantage of effectively increasing the distance that can be drilled before the open borehole requires a casing. With an increased distance between subsequent casing shoes or points, the drilling operation can be completed in a shorter time period and at reduced costs.

Considerable efforts have therefore been made to optimize the filter cake as a protective layer—mostly by adding suitable chemical compositions to the base drilling fluid in order to increase the stability of the mud cake and the adjacent formation or to increase its capability of the mud cake layer to isolate the borehole from the surrounding formation.

In a specific branch of drilling techniques normal oil field casing is used as the drill string so that the well is simultaneously drilled and cased. This method is commonly referred to as “casing drilling”.

Under certain circumstances, casing drilling has been shown to reduce the in-hole trouble time significantly below that obtained by conventional drilling, hence reducing overall drilling costs (Fontenot et al. (2003)).

Casing drilling has been identified as a technology which is capable of reducing or minimizing the problems associated with conventional drilling such as stuck pipe, lost circulation, well control, and failure to run casing. Shepard et al. (2002) showed that the incidence of wellbore instability, lost circulation, influx and drag while tripping out were significantly reduced when using casing drilling compared to conventional drilling methods. It has been suggested that the success of casing drilling is at least partly attributable to wellbore plastering. Shepard et al. (2002) suggested that the process of casing drilling mechanically strengthens the wellbore by building and maintaining an impermeable layer on the wellbore. Likewise, Fontenot et al. (2002) suggested that casing drilling provides a wellbore that is more stable and less permeable than when drilling with a conventional drill pipe and collars, and further hypothesised that the casing rotation mechanically conditions the wellbore wall to create a strong impermeable surface finish, possibly accomplished by mechanically plastering the wellbore wall with solids from the drilled cuttings and the mud

In the light of the above, it is an object of the present invention to advantageously condition the interface layer between an open uncased wellbore and the surrounding formation during drilling operations.

SUMMARY OF THE INVENTION

The present invention is at least partly based on a realization that in casing drilling, the interaction between the drill string (i.e. the casing) and the filter cake differs from that which occurs during drilling with conventional drillpipe. For example, the drill string is in much closer proximity to the filter cake than a conventional drill pipe would be. This significantly increases the number and extent of drill string-filter cake contacts and provides a quasi-continuous or continuous interaction, which could be described as compacting or, in analogy, as plastering interaction.

During each contact, the drill string is also less likely to deeply penetrate the filter cake than a conventional drill pipe, because the larger radius of the casing compared to conventional drill pipe reduces the pressure exerted on the filter cake.

It is an aspect of the present invention to make use of these conditions for transferring selected additional material from the drilling mud into the filter cake to condition the filter cake, and hence provide a more stable well bore. Thus, in general terms, the present invention relates to drilling muds containing additives for conditioning the filter cake during casing drilling.

Casing drilling is understood to include casing drilling as such. Also included are methods that rely on the introduction of one or more large diameter tools or sub into a drilling of normal diameter. In these methods the compacting sub has a diameter that ensures a quasi-continuous or continuous contact with the wall of the borehole and the filter cake.

The present invention provides a method of drilling a hydrocarbon well using a drill string adapted to provide a continuous or quasi-continuous contact with the wall of said well such that at least on a section of the well a compacting force is exerted on filter cake deposited thereon

• • circulating a drilling mud through the borehole during the drilling operation such that a filter cake is deposited on the borehole wall by the drilling mud, the filter cake being compacted by contact with the drill string,
  • the method further comprising:
  • including, before the drilling mud enters the borehole a conditioning additive in the drilling mud, the additive conditioning the filter cake upon said contact with the drill string to increase the yield strength of said filter cake.

By introducing such a conditioning additive, it is possible to influence the properties of the filter cake in a selective and optimal manner. For example, preferably, the additive is selected to increase the strength of the filter cake. This is expected to improve the quality of the following cement job.

A typical drilling fluid includes further additives which vary widely depending on the wellbore and drilling conditions. Such known additives include any combination of weight materials such as barite, hematite or calcium, viscosifiers, such as bentonite, xanthan, guar, hydroxyethyl cellulose or mixed-metal hydroxide, dispersants (lignite), shale stabilizers, such as polyacrylamide, glycols, potassium acetate, quaternary ammonium compound, various other salts and lost circulation material as known in the art.

Typically, the additive is particulate. Preferably, the particles are non-spherical, preferably with a ratio of 1:5 or less between thickness and diameter or width. For example, the particles may be lamellar. The highly anisometric particles can have a substantial filter cake strengthening effect.

In some embodiments, the additive is mica flakes. These are suitable when the drilling mud is water-based or oil-based. In other embodiments, the additive is rubber particulates. These are particularly suitable when the drilling mud is oil-based. Both additives have been used as additives for different purposes in known drilling fluids. However, in the present invention, these additives are used to increase the (yield) strength of a filter cake in a casing drilling type operation.

The drilling mud may contain at least 5 volume % of the additive, and more preferably at least 10 volume %. Experience suggests that the proportion by volume of additive in the filter cake will be approximately double that of additive in the mud. If less than 5 volume % additive is included, the additive can be too diluted in the filter cake to have a significant conditioning effect. The drilling mud may contain up to 20 volume % of the additive, and more preferably up to 15 volume %. If more than 20 volume % additive is included, the mud will tend to become too viscous.

A further aspect of the invention provides a drilling mud suitable for performing the method of the previous aspect. One example of such a drilling mud may contain at least 5 volume % of mica flakes, and preferably up to 20 volume % of mica flakes.

Another example of such a drilling mud may contain at least 5 volume % of rubber particles, and preferably up to 20 volume % of rubber particles.

Several subparts in accordance with the above embodiment are advantageously distributed along the length of the bottom section of the drill string, which section is to enter the newly drilled open (uncased) borehole. Thus the action of the first subpart is reinforced by other subparts passing through the same section of the well at a later time. One or more subparts may therefore be located in the drill string above the BHA and/or the drill collar section.

These and other aspects of the invention will be apparent from the following detailed description of non-limitative examples and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a known drilling system;

FIG. 1B shows a detail of the well of FIG. 1A;

FIG. 2A illustrates a casing drilling operations;

FIG. 2B shows a cross-section of a casing drilling arrangement;

FIG. 2C shows a cross-section of a casing drilling arrangement in which the casing is off-centre;

FIG. 3 shows the effect of various particle types on the yield stress of simulated water-based drilling fluid filter cake;

FIG. 4 shows the effect of various particle types on the yield stress of simulated oil-based drilling fluid filter cake; and

FIG. 5 illustrates a subpart of a drill string adapted to extend the casing drilling method to conventional drilling operations.

EXAMPLES

In FIG. 1, there is shown a known well drilling system for rotary drilling operations. A drill string 111 is shown within a borehole 102. The borehole 102 is located in the earth 101. The borehole 102 is being cut by the action of the drill bit 110. The drill bit 110 is disposed at the far end of a bottom hole assembly (BHA) 113 that is attached to and forms the lower portion of the drill string 111. The bottom hole assembly 113 contains a number of devices including several drill collars 113-1 to increase the weight on the bit 110.

The drilling surface system includes a derrick 121 and a hoisting system, a rotating system, and a mud circulation system 130. The hoisting system which suspends the drill string 111, includes the draw works 122, a hook 123 and a swivel 124. The rotating system includes a kelly 125, a rotary table 126, and engines (not shown). The rotating system imparts a rotational force on the drill string 111 during a rotational drilling operation in a manner well known in the art.

A mud circulation system 130 pumps drilling fluid down the central opening in the drill string 111. The drilling fluid is often called mud, and it is typically a mixture of water or diesel fuel, special clays, and other chemicals. The drilling mud is stored in a mud pit 131. The drilling mud is drawn into mud pumps 132 which pump the mud though the surface pipe system 133, the stand pipe 134, the kelly hose 135, and the swivel 124, which contains a rotating seal, into the kelly 125 and finally through the drill string 111 and the drill bit 110.

As the teeth of the drill bit grind and gouges the earth formation into cuttings the mud is ejected out of openings or nozzles in the bit 110 with great speed and pressure. These jets of mud lift the cuttings off the bottom of the hole and away from the bit, and up towards the surface in the annular space between drill string 111 and the wall of borehole 102. At the surface the mud and cuttings leave the well through a side outlet in a blowout preventer 114 and through the mud return line 115. The blowout preventer 114 comprises a pressure control device and a rotary seal. From a cuttings separator (not shown) the mud is returned to mud pit 131 for storage and re-use.

Although a system with jointed drill string 111, a kelly 125 and rotary table 126 is shown in FIG. 1, the invention is applicable to other drilling systems such as in top drive drilling derricks or coiled tubing. Although the drilling system is shown as being on land, it is applicable to marine and transitions zone environments.

In FIG. 1B there is shown a part of an open hole section of the borehole 102. The section shown in FIG. 1B includes a section of the drill string 111 with a tool joint 112 in the center of the open, i.e. uncased, borehole 102. The borehole traverses a porous formation layer 103 embedded within layers of impermeable rock 104. The drilling fluid is circulated through the drill pipe 111 and returns loaded with cuttings through the annulus between the wall of the borehole 102 and the pipe 111 as indicated by arrows.

During the drilling operations, a small amount of the liquid components of the drilling fluid are absorbed by the formation leaving behind a layer of solid particles 105. As indicated in FIG. 1B, the mud cake layer 105 is thicker across the porous formation layers 103 than across impermeable layers 104. The mud cake layer 105 is believed to enhance the stability of the well.

In order to preserve and possibly enhance the stability of the mud cake layer 105, the invention proposes the use of casing drilling or drilling subs that exert force or pressure in a continuous or quasi-continuous manner on the wall of the borehole as the drilling operation progresses. Rather than cutting through the mud cake, casing drilling or subs are designed to slide on the filter cake gently compressing or compacting it, thus forcing more fluid or particles into the surrounding formation and/or solidifying the mud cake layer 105 not unlike wall plastering. The compacting force is exerted in a radial direction, perpendicular to the wall of the borehole.

In FIG. 2A, the drill string 111 of FIG. 1 is replaced by a casing string 211. The larger diameter of the casing narrows the gap between the casing 211 and the wall of the well 102.

As a result the casing 211 exerts at many contact areas a force on the filter cake 105. The following schematic FIGS. 2B and 2C illustrate the effect of this interaction maintaining the numerals as above.

FIG. 2B shows the casing 211 of outer radius OA, coaxially located inside a wellbore 102 of radius OC (=R), the inside of which is coated with a filter cake 105 of radius OB. Drilling mud flows down the well inside the casing 211, and returns in the annulus 102 of width g between the outer radius of the casing 211 and the inside of the filter cake 105.

FIG. 2C illustrates the effect of a lateral force F (per unit length of the bore) causing the casing 211 to penetrate a distance e into the filter cake 105. This force may arise due to gravity in a deviated section, or in a curved section from the axial tension or compression in the casing 211. The force will compress the filter cake 105 between the casing 211 and the inside face of the wellbore 102 away from the region of closest approach of the casing 211 to the face of the wellbore 102. For cake penetrations which are small, we can ignore the viscous force and consider the penetration to be determined mainly by the cake's yield stress τ₀. If the penetration depth e is small compared with the thickness H of the cake, and compared to the annular gap g, cylindrical squeeze flow theory gives: $\begin{array}{cc}e=\frac{\mathrm{FgH}}{4R^2{\tau }_0},& \left[1\right]\end{array}$
hence showing that for a given force F, the penetration e is least for:

• • 1. a small annular gap g;
  • 2. a small filter cake thickness H; and
  • 3. a large yield stress τ₀, i.e. a strong filter cake.

The above model assumes that the filter cake has a uniform yield stress throughout its thickness. However, this is an oversimplification as real filter cakes on a wellbore wall are strongest where the matrix stress is greatest, i.e. closest to the wall However, Equation [1] above is believed to be qualitatively correct.

In casing drilling however, the rotating casing and the filter cake are in closer proximity than in conventional drilling using drillpipe, and according to well known principles of dynamic filter cake formation, the azimuthal shear stress of the mud arising from this proximity will inhibit the deposition of the less compacted part of the filter cake (i.e. that closest to the casing).

Thus, compared to conventional drilling, casing drilling is likely to result in the following differences in filter cake properties:

• • a) a more uniform yield stress over its thickness;
  • b) a smaller thickness; and
  • c) a greater mean yield stress τ₀.

Property a) above means that the approximations in Equation [1] will be less significant. Filter cake penetration is believed to be associated with high torque, drag and the risk of sticking, whereby some advantages of casing drilling may be attributed to the small annular gap g in casing drilling and properties b) and c) above.

Thus, in embodiments of the present invention, additives to the drilling mud augment the mean yield stress τ₀ of the filter cake. Such augmentation is shown by Equation [1] above to decrease the lateral casing penetration, and therefore to decrease torque and drag whilst drilling. A strong filter cake which lines the wellbore is also expected to increase the stability of the wellbore.

In the examples below, we show that granular materials have a moderate strengthening effect when incorporated into filter cake, but that other particulate materials, e.g. those which are highly anisometric in shape or which are deformable, have a much greater strengthening effect. Such materials may be added as additives to standard drilling fluid, or may be deliberately incorporated in the drilling fluid from the start.

Methods which result in increasing the strength and decreasing the thickness of the filter cake are also expected to improve the quality of the cement job.

In the experiments below, the strengthening effect of incorporating particulate materials into simulated filter cake was investigated.

One set of particulate materials used were of granular shape in order to simulate ground cuttings which might result from drilling operations.

Further sets of particulate materials had lamellar and spherical shapes. Deformable particulate materials were also used. These materials would not arise naturally during drilling, but they can be added to the drilling fluid as additives in order to advantageously change the condition of the filter cake.

A filter cake of a water-based drilling fluid was simulated by adding tap water to cuttings of Oxford shale and moulding the stiff mixture by hand until it resembled the consistency of a typical water-based drilling fluid filter cake, similar to that of potter's clay. The shale contained swelling clays such as montmorillonite as well as non-swelling clays and sands, and hence the simulated filter cake approximately resembled the mineral composition of water-based drilling fluid filter cake. Comparative measurements of the weight of the simulated filter cake before and after drying overnight at 105° C. showed the water content to be 25% by weight.

Various particulate materials were moulded into this filter cake simulant, and the resulting strength, expressed as shear yield stress, was measured by an extrusion method, as described in Benbow et al. 1991. All the added particles had a size of less than about 0.3 mm.

FIG. 3 shows the yield stress plotted against the volume fraction of added particles. All particle types are shown to strengthen the filter cake, but mica flake had a much greater effect for a given volume fraction than the rigid granular sand or the granular rubber.

A filter cake of an oil-based drilling fluid was simulated by the modelling clay Plasticine™ (manufactured by Humbrol), which is understood to be a concentrated paste of organophilic clay dispersed in an oil, and therefore similar to oil-based drilling fluid filter cake.

Various particulate materials were moulded into this filter cake simulant and the resulting strength, expressed as the shear yield stress, was measured as described above in relation to the water-based drilling fluid examples. All the added particles had a size of less than about 0.3 mm.

FIG. 4 shows the yield stress plotted against the volume fraction of added particles.

All particles are shown to strengthen the filter cake, but mica flake and granular tyre-rubber had a much greater effect at a given volume fraction than the rigid particles of granular sand or those of near spherical polystyrene.

FIGS. 3 and 4 both show that the yield stress of the filter cake was increased by all the particulate additives used. For a given volume fraction, mica flake had a disproportionately large effect on both types of filter cake simulant, compared to sand and polystyrene. This is believed to be due to the efficient space-filling properties of highly non-spherical particles.

For the oil-based filter cake simulant, tyre-rubber particles also provided a significant yield stress increase. This is believed to result from absorption by the rubber particles of some of the oil from the simulant in proximity to the surface of the particles.

For both water-based and oil-based filter cake simulants, the yield stress data τ_Y(P) can be expressed by the relationship:
τ_Y(φ)=τ_Y(0)+b(exp(cφ)−1)   [2]

in which τ_Y(0) is the yield stress of the filter cake without added particulates and the parameters b and c are independent of φ. Table 1 gives data from the fits of Equation (2) to the experimental results shown in FIGS. 3 and 4. It shows that the magnitude of b and c depend strongly on the filter cake type and on the added particle type.

	TABLE 1
	Added
	particles	τY(0)/MPa	b/MPa	c
	Water-based	Sand	0.122	0.046	5.93
	filter-cake	Tyre-rubber	0.127	0.144	2.30
		Mica flake	0.117	0.084	25.1
	Oil-based	Sand	0.104	0.006	6.96
	filter-cake	Polystyrene	0.100	0.012	6.89
		Tyre-rubber	0.110	0.118	4.97
		Mica flake	0.112	0.027	21.1

A suitable subpart to extend the advantages of known casing drilling to conventional drillstring drilling is shown in FIG. 5. The subpart 530 includes a bottom and upper section 531, 532, respectively, providing box and pin connection to the remainder of the drill string (not shown). A main body 533 of the subpart comprises two frustro-conical sections with a cylindrical middle section similar to a bobbin. The conical sections include the bearings for four hinges 534. Mounted onto each of the hinges is a steel vane or pad element 535 having a flat arcuate shape with rounded edges to reduce forces against any lateral movement of the subpart.

The hinges 534 are spring-loaded to force the four pads to fold tightly around the main section in the absence of hydraulic pressure. The drilling fluid provides the hydraulic pressure as it is pumped from a surface location through the drill string. The pressurized drilling fluid activates internal cylinders (not shown) that rotate the vanes 535 around the hinges thus bringing their distal ends closer to the wall of the borehole. While the drill string remains in a centered position within the borehole, the rollers are designed to provide the first area of contact between the subpart 530 and the formation wall. The hinge-mounted vanes or pads 535 are configured to bend or flex as the radial distance between the drill string and the wall varies during the drilling operations, so as to remain in permanent contact with the wall.

During the drilling process, the drill string including the subpart 530 are rotated from the surface, and the subpart continuously exerts pressure on the formation wall and any mud cake layer on its surface. When the drilling terminates and the pressure inside the drill string drops, the vanes 535 fold back around the main body 533 to facilitate a subsequent tripping operation.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

REFERENCES

The references mentioned herein are all incorporated by reference.

J J Benbow, S H Jazayeri, J Bridgwater (1991) The flow of pastes through dies of complicated geometry. Powder Technology 65:393-401.

K Fontenot, T M Warren, B Houtchens (2002) Casing drilling proves successful in South Texas. IADC World Drilling 2002, Madrid, Spain, June 5-6.

K Fontenot, J Highnote, T Warren (2003) Casing drilling activity expands in South Texas. SPE/IADC 79862. SPE/IADC Drilling Conference, Amsterdam, The Netherlands, 19-21 February, 2003.

S F Shepard, R H Reiley, T M Warren (2002) Casing drilling: and emerging technology. SPE Drilling & Completion March 2002, 4-14.

Claims

1. A method of drilling a hydrocarbon well using a drill string adapted to provide a continuous or quasi-continuous contact with the wall of the well such that at least along a section of the well a compacting force is exerted on filter cake deposited thereon

circulating a drilling mud through the borehole during the drilling operation such that a filter cake is deposited on the borehole wall by the drilling mud, the filter cake being compacted by contact with the drill string,

the method further comprising:

including, before the drilling mud enters the borehole, a conditioning additive in the drilling mud, the additive conditioning the filter cake upon said contact with the drill string to increase the yield strength of the filter cake.

2. A method according to claim 1 wherein the additive is particulate.

3. A method according to claim 2 wherein the particles are non-spherical.

4. A method according to claim 3 wherein the particles are lamellar.

5. A method according to claim 1, wherein the additive is mica flakes.

6. A method according to claim 1, wherein the additive is rubber particulates.

7. A method according to claim 1, wherein the drilling mud contains at least 5 volume % of the additive.

8. A method according to claim 1, wherein the drilling mud contains up to 20 volume % of the additive.

9. A method according to claim 1, wherein the drill string is assembled from casing tubes.

10. A method according to claim 1, wherein the drill string includes one or more subparts with enlarged diameter to emulate casing drilling.

11. A method according to claim 1, wherein the drill string is adapted to engage a wall of an open uncased borehole with a low angle of attack.

12. A drilling mud containing at least 5 volume % of mica flakes.

13. A drilling mud containing at least 5 volume % of rubber particulates.