GB2518441A - Solids in borehole fluids - Google Patents

Abstract

A drilling fluid for use when drilling a borehole includes solid particles of organic polymer as a lost circulation additive. The particles have a size extending at least 0.5mm in each of three orthogonal dimensions and the shape is at least partially bounded by surfaces which intersect at an angle. The particles may be chopped from larger pieces of polymer by granulating machinery, leading to particle shapes with edges and corners assisting the particles in lodging within and blocking a fracture encountered or formed while drilling. The particles preferably have a specific gravity no greater than 1.2.

Description

Solids in borehole fluids

Background

A considerable range of fluids are used in the creation and operation of subterranean boreholes. These fluids may contain suspended solids for a number of purposes. Included within this broad category are drillifig fluids which may contain suspended solids to block fractures in formation rock and mitigate so-called lost circulation.

Lost circulation, which is the loss of drilling fluid into downhole earth formations, can occur naturally in formations that are fractured, porous, or highly permeable. Lost circulation may also result from induced pressure during drilling. Lost circulation may also be the result of drilling-induced fractures. For example, when the pore pressure (the pressure in the formation pore space provided by the formation fluids) exceeds the pressure fri the open borehole, the formation fluids tend to flow from the formation into the open borehole. Therefore, the pressure in the open borehole is typically maintained at a higher pressure than the pore pressure. However, if the hydrostatic pressure exerted by the fluid in the borehole exceeds the fracture resistance of the formation, the formation is likely to fracture and thus drilling fluid losses may occur. Moreover, the loss of borehole fluid may cause the hydrostatic pressure in the borehole to decrease, which may in turn also allow formation fluids to enter the borehole. The formation fracture pressure typically defines an upper limit for allowable borehole pressure in an open borehole while the pore pressure defines a lower limit. Therefore, a major constraint on well design and selection of drilling fluids is the balance between varying pore pressures and formation fracture pressures or fracture gradients though the depth of the well.

Several remedies aiming to mitigate lost circulation are available. These include the addition of particulate solids to drilling fluids, so that the particles can enter the opening into a fracture and plug the fracture or bridge the opening to seal the fracture. Documents which discuss such "lost circulation materials" include US patent 8401795 and Society of Petroleum Engineers papers SPE 58793, SPE 153154 and SPE 164748.

One proposal to use particles of organic polymer as lost circulation material is US 7284611 which mentions ground thermoset polymer laminate. Particle shape is not mentioned. One supplier of such material refers to it as flakes.

This document also mentions an elastomer: again shape is not mentioned.

U57799743 mentions granules of polypropylene, which is a thermoplastic polymer and requires particles to have an average resiliency of at least 10% rebound after compression of a quantity of articles by a pressure of 0.4MPa.

The shape of the particles is not mentioned.

Summary

This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to be used as an aid in limiting the scope of the subject matter claimed.

As now disclosed herein, a borehole fluid comprises suspended solid particles which are particles of organic polymer which meet requirements as to size and shape. The fluid may be a drilling fluid and the particles in the fluid may counteract or mitigate loss of fluid into fractures in the formation being drilled.

If a fracture is created in a formation during drilling or if a natural fracture is encountered, the fluid entering the fracture can carry some of the objects into the fracture, for them to form a bridge or plug which closes the pathway for fluid loss. The particles may themselves block the fracture or they may act jointly with other sofids in the fluid to form a plug which doses the fracture.

The particles have a shape and size such that a particle extends for at least 0.5mm and possibly at least 1mm in each of three orthogonal dimensions and has a shape which is at least partially bounded by surfaces which intersect at an edge. Angles between at least some edges may be not more than 150° and may be less such as not more than 120° or not more than 100°. There may also be distinct corners where three surfaces and three edges meet. A corner maybe such that the included angle in each of two planes intersecting at right angles is not more than 120° and possibly not more than 100°. An aliernative parameter is solid angle: a corner may be such that the included solid angle is not more than 1.7 steradians, which is slightly more than the solid angle (0.5n steradians) subtended by the corner of a cube. Shapes with edges or corners are able to lodge in a fracture by engaging with each other or by engaging with the formation rock.

It is envisaged that the particles will be rigid under surface conditions to allow mechanical handling of them. Rigidity of the particles may be defined as ability of the particles to maintain their own shape under atmospheric pressure at temperahres up to at least 40°C and possibly up to higher temperatures such as up to 60°C. However, the particles may have the property of resiliency which may be such that there is an average of at least 10% rebound after compression of a sample quantity of partides with a pressure of 0.4 MPa as specified in US 7799743.

When carried downhole in a borehole fluid the particles will be subjected to hydrostatic pressure above atmospheric, but this may not distort their shape whilst they are suspended in the fluid. If there is any distortion of their shape by pressure on them after they lodge in a fracture, this may assist in plugging the fracture opening.

The organic polymer may be a material commonly referred to as a plastic which may be a thermopbstic to provide resiliency. Examples of thermoplastic polymers include polystyrene, polyethylene and polypropylene homopolymers and acrylonitrile-butadiene-styrene copolymer. Such polymers may have a specific gravity in a range from 0.7 to 1.3 and possibly in a narrower range from 0.8±0 1.0. Such a specific gravity may be similar to the specific gravity of a borehole fluid. This is useful for particles suspended in a borehole fluid because they will have less tendency to scale out than particles of higher specific gravity and similar size. Settling out of particles can be problematic especially if the circulation of fluid is interrupted. In consequence, the particles may be larger than would be acceptable for particles of higher specific gravity and by reason of larger size they may be suitable for blocking larger fractures.

Solid particles with intersecting surfaces, edges and/or corners may be made using machinery in which larger pieces of the polymer are sheared by cutting parts moving one past another with very small gap between them, so that the pieces of the polymer are cut through. Such machines are often referred to as shredders or granulators and are available from a number of manufacturers.

They have conventionally been used for comminuting scrap plastic. The solid polymer particles for this invention may be made by comminuting rigid plastic objects which have been thrown away as recyclable rubbish.

Particle size can be governed by dimensions of the cutting discs. The partides may have dimensions such that they could fit inside a sphere of 10mm diameter arid possibly inside a sphere of 8, 8 or even 5mm diameter. The particles may be sufficiently large that they could not fit within a sphere of 1mm diameter and possibly not within a sphere of 1.5 or 2mm diameter.

A borehole fluid, which may be a drilling fluid intended to be pumped down a drill string and back to the surface, may contain particles of organic polymer together with another lost circulation material of known type and higher specific gravity, such as graphite particles. Such another lost circulation material may have a mean particle size of at least 10 microns and possibly at least 100 microns. The organic polymer particles may be used in an amount which is tess, by weight and or by volume, than the amount of other lost circdation material(s). For instance the solids incorporated in a drilling fluid to mitigate lost circulation may comprise (i) organic polymer particles having dimensions too large to fit within a 1mm diameter sphere and (ii) other solid particles having a mean particle size of at least 10 microns but less than 1mm, possibly less than 0.5mm with the volume of particles (ii) being at least 5, possibly at least 10 times the volume of particles (i).

Another possibility is to use organic polymer particles which are a mixture of sizes. For instance organic polymer particles incorporated in a drilling fluid to mitigate lost circulation may comprise (i) organic polymer particles having dimensions too large to fit within a 1mm diameter sphere arid (ii) other organic polymer particles small enough to fit within a 1mm diameter sphere, with the volume of smaller particles (ii) being at least 5, possibly at least 10 times the volume of larger particles (i).

Brief description of the drawings

Fig 1 is a cross section through part of a granulator, on line B-B of Fig 2; Fig 2 is a view onto the cutter assembly of the granulator looking in the direction of arrow A of Fig 1; Fig 3 schematically illustrates a particle of organic polymer; Fig 3a shows that angle between surfaces at one edge; Fig4 schematically illustrates another particle of orgEmic polymer Fig 5 diagrammatically illustrates a drill string in a wefibore; and Fig 6 shows an end view of one example of a drill bit.

Detailed description

As stated above, the present invention provides borehole fluid in which suspended particles are particles of an organic (ie carbon-based) polymer. This polymer may be a homopolymer or copolymer. It will have a backbone chain containing carbon atoms and in some polymers, such as polyethylene, the polymer has a continuous chain of carbon atoms. In some other forms of this invention, the polymer backbone may contain oxygen or nitrogen atoms. The organic polymer may overafi contain carbon, hydrogen and possibly also oxygen and/or nitrogen atoms and in some forms of this invention the organic polymer a'so contains a minority proportion (such as less than 10% by number) of other atoms such as sulphur or silicon.

The polymer may be newly mariufachired by polymerisation or it may be recycled material. In some forms of invention the particles are a mixture of polymers provided as recycled rigid plastics.

The polymer particles may be made by arty process leading to particles of required size and shape. However, in some forms of the invention the polymer particles are made by a machine which cuts arger pieces of polymer into particles. The resulting particles may have a plurality of separate surfaces intersecting at edges and corners. Machinery to comminute polymer in this way has been available for many years and can have various designs. Such machinery may have cutters which move past fixed structure with small clearance or cutters which move past other moving cutters with small clearance so that pieces of polymer are sheared through, creating surfaces which are approximately planar. Some forms of machine have a plurality of rotary shafts which each carry a number of spaced cutting wheels which are toothed discs or other shape with parallel faces, with the cutting wheels on one shaft fitting closely within the gaps between cutting wheels on a neighbouring shaft. Such machinery may produce particles with two parallel faces formed by the shearing action of the cutting wheels.

Figs 1 and 2 illustrate a granulating machine of this known type which is suitable for comminuting plastic objects. The machine has a granulating assembly comprising two parallel shafts 42, 44 journalled in plates 46 which are connected by fixed rods 47. This assembly is located within a chute 48. The shafts 42, 44 each carry a series of cutting wheels which are toothed discs 62, 64 spaced axially along the shaft. All the cutting wheels 62 and 64 are of equal diameter and thickness and the dimensions are such that (as shown by Fig 2) wheels 62 on shaft 42 project into the gaps between the wheels 64 on the other shaft 44 and vice versa. Other parts of the gaps between cutting wheels 62 64 discs are partially filled by shaped blocks 50 mounted on the rods 47. The shafts 42, 44 are driven so as to rotate in opposite directions as indicated by arrows in Fig 1. Where the wheels on one shaft project into the gaps between wheels on the other shaft, the clearances are very small, so that the wheels 62, 64 cut with a shearing action.

Pieces of polymer fall down the chute 48 onto the cutting wheels 62, 64 where they are caught by the teeth 66 on the wheels. Pieces of the polymer are then cut off and are carried through the gaps between adjacent wheels, to be discharged into the portion of the chute 48 below this granulating assembly.

Figure 3 schematically illustrates a particle made by cutting with the machine shown in Figs 1 and 2. The particle is approximately cuboidal with two opposite planar faces 30 parallel to each other (only one is visible in Fig 3) formed by the shearing action of the machine. As shosm, the particle has dimensions x, y and z along three orthogonal axes. Each of x, y and z is over 1mm but none exceeds 5mm. The distance x between the parallel planar faces is the distance between adjacent cutting wheels 62 and between adjacent wheels 64. The remaining faces 32 can have other shapes and need not be planar. They may for instance be convex as shown. The surfaces 30 meet surfaces 32 at edges 34. For each edge 34, the angle between the two surfaces at the edge (more precisely the angle subtended in a plane perpendicular to both surfaces) is less than 1000 and may be approximately 90°. The surfaces 32 intersect each other at edges 36. As shown by Fig 3a, the angle 37 included at an edge 36 can be taken as the angle between tangents to the surfaces 32 at the edge 36. In this example, these angles are not more than 120°. Where three edges meet at a corner all the angles between edges are less than 1200 and two are approximately 900.

The surfaces of the particle may have some surface roughness, not shown in the drawing, which may mean that the edges are not sharp, but when viewed as a whole, the particle has visible edges.

Fig 4 schematically illustrates another particle made by cutting with the machine shown in Figs 1 and 2. Reference numerals used for Fig 3 have the same meaning here. The surfaces 32 may be parts of the surface of the larger piece of polymer which is cut by the machine. The slightly concave surface 38 was formed as the tip of a tooth 66 gouged through a piece of polymer and the angles between surfaces 32 and 38 at edges 39 are, in this example, less than 90°.

Fig. 5 shows the drilling of a borehole through rock formations 8. The drill bit is coupled to the lower end of a drill string 4, which typically includes segments of drill pipe (not shown separately) coupled together by means of screw threads at their ends. The drill bit lOis coupled to the drill string 4 through a bottom hole assembly 6 arid 7. The drill string 4 may be rotated by a rotary table (not shown in FIG. 1) or a top drive system 2 which is itself hoisted and lowered by a drilling rig 1. As shown by Fig 2 the drill bit has a body 10 supporting cutters 18. Drilling fluid ("drilling mud") is circulated through the drill string 4 by mud pumps 3. The drilling mud is pumped down the interior of the drill string 4 and through the bottom hole assembly to passages through the drill bit 10. These passages through the body of the drill bit terminate at jets shown by Fig 2. After being discharged through the jets 20, the drilling mud returns to the earth's surface through an annular space 5 around the exterior of the drill string 4 in the borehole.

The circulating drilling fluid provides hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, cools and lubricate the drill string and bit and removes drill cuttings from the bottom of the hole to the surface.

Drilling fluid compositions may be water-or oil-based arid may include weighting agents, surfactants, polymeric thickeners and other materials. In accordance with this invention the drilling fluid includes organic polymer particles as discussed above.

An example drilling fluid contains approximately 100 gram per litre of inorganic solids having a mean particle size above 100 microns and below 500 micons. The fluid also contains (a) 10 gram per litre of organic polymer particles made as described above from recyded plastic objects and having size which would fit in a 2 mm diameter sphere but too large to fit within a 1mm, diameter sphere, together with (b) 10 gram per litre of organic polymer particles made as described above from recycled plastic objects and having size which would fit fri a 5 mm diameter sphere but too large to fit within a 3mm, diameter sphere.

U

The drilling fluid is used in drilling, as illustrated by Fig 5. hi the event that a fracture with a width of 1 to 4mm is encountered, or formed as a result of pressure in a borehole, the organic particles (a) would be carried into the fracture but would form a plug at the fracture mouth. Particle shapes with corners will snag on the rough surface of the rock and assist each other to form a plug to block entry into the fracture. Initially this plug would be porous but inorganic particles hi the fluid would then lodge in the interstices between the organic polymer particles, sealing the plug and blocking further leakage into the formation.

If a larger fracture with a width of 4 to 8 mm is encountered or formed, the organic particles (b) would be carried into the fracture but would form a plug at the fracture mouth. The smaller organic polymer particles(b) would lodge in gaps between the larger particles creating a porous bridge which would then retain the smaller inorganic particles and so form a seal blocking further leakage into the fracture.

Claims (10)

1. Claims 1. A borehole fluid containing suspended solid particles which are rigid particles of organic polymer, wherefri the particles have a size extending at kast 0.5mm in each of three orthogonal dimensions and the shape is at least partially bounded by surfaces which intersect at an edge.
2. 2. A borehole fluid according to daim 1 wherein the partides of organic polymer have a specific gravity which is not more than 1.2.
3. 3. A borehole fluid according to claim 1 wherein the partides of organic polymer have a specific gravity which is in a range from 0.8 to 1.0
4. 4. A borehole fluid according to claim 1, claim 2 or claim3 wherein the particles of organic polymer have shapes where the angle included between surfaces intersectifig at least one edge is not more than 1500.
5. 5. A borehole fluid according to any one of the preceding claims wherein particles of organic polymer have shapes which include three surfaces intersecting at edges which meet at a corner.
6. 6. A borehole fluid according to any one of the preceding claims wherein at least some of the organic polymer partides are too large to fit within a sphere of 1mm diameter, but are able to fit within a sphere of 8mm diameter.
7. 7. A borehole fluid according to daim 5 wherein at least some of the organic polymer particles are too large to fit within a sphere of 1.5mm diameter, but are able to fit within a sphere of 6mm diameter.
8. 8. A borehole fluid according to claim 6 or claim 7 wherein at least some of the organic polymer particles are small enough to fit within a sphere of 1mm diameter.
9. 9. A borehole fluid according to any one of the preceding claims wherein more than 50% by weight of the organic particles consist of thermoplastic polymer.
10. 10. A method of mitigating loss of drilling fluid while drilling a borehole and circulating drilling fluid down and back up the borehole, comprisinig incorporating rigid particles of organic polymer in the drilling fluid.