GB2075089A - Drilling method - Google Patents

Abstract

A method of drilling a borehole (1), particularly for oil or natural gas, in which a flushing medium is fed into the borehole, brought into heat exchange relationship with the rock being drilled and/or the drill (2) used, and withdrawn "from the borehole, in which the flushing medium before being fed into the borehole is at a temperature below that of an uncooled circulating flushing medium would assume after its emergence from the borehole in heat exchange with the environment. The flushing medium is preferably a liquefied gas such as liquid nitrogen or CO2 which is at least partially vaporized in heat exchange with the rock and/or drill. The cold flushing medium serves to stabilise unlined sections of the borehole by freezing water contained therein. <IMAGE>

Description

SPECIFICATION Drilling method The present invention relates to a method of drilling in which a flushing medium is fed through the borehole, brought into heat exchange relationship with the rock being drilled and/or the drill used and withdrawn from the borehole.

In the discovery and exploitation of deposits of raw materials, such as petroleum or natural gas, in addition to the selection of a suitable drilling method, it is essential to render the drilling secure during the sinking of the shaft.

Among other reasons, this serves to prevent the borehole from being destroyed by mechanical forces exerted by the rock surrounding it, i.e. to protect the borehole against caving-in and collapsing. In the conventional methods, therefore, a borehole is drilled in stages and each stage is individually made secure. Once a stage has been bored out, it is lined, i.e. steel tubes are lowered into the borehole and the space between the steel tubes and the wall of the borehole is filled with cement. Thus, the set of pipes required for the next stage is passed through the preceding stage. If there is not sufficient solid rock to drill through to complete a stage, the wall of the borehole must be supported in some way until lining is carried out.In conventional methods, this support is achieved by the use of flushing liquids (drilling mud), a liquid being introduced into the borehole, the density of which can be increased by the addition of suitable filling material. Drilling with a flushing liquid, which, if necessary is provided with filling material, means that the hydraulic column, which should actually be pressing only against the unlined walls of the borehole, is also resting its entire weight on the floor of the borehole and in that position is energetically working against the mechanical loosening process involved in drilling. The performance of the drill therefore declines in proportion to increase in the density of the flushing medium. Consequently, the usual dense flushing agents work against the loosening process at the floor of the borehole.

Together with the increasing flushing pressure produced by the advance of the drilling, the resistance to drilling increases and drilling progress is correspondingly slowed down.

Low drilling progress is, however, synonymous with higher drilling costs. At great depths, the hydraulic imbalances that make their appearance in long, unlined stretches of rock must be regarded as another factor leading to increased cost. Allowing for these imbalances demands more intermediate lining and consequently a larger initial diameter for the first-stage drilling tool. The consequences of this are expensive lining and high flushing costs, so that this, too, leads to a considerable increase in the borehole costs.

It is an object of the present invention to provide a method by means of which a borehole can be drilled quickly and inexpensively.

In accordance with this invention, there is provided a method of drilling a borehole in which a flushing medium is fed into the borehole, brought into heat exchange relationship with the rock being drilled and/or with the drill used and withdrawn from the borehole, wherein prior to being fed into the borehole the flushing medium is cooled to a temperature below the temperature which would be assumed by a circulating flushing medium after its emergence from the borehole in heat exchange with the environment.

In contrast to conventional processes using circulating flushing agents which cool down above ground only by some 10 to 20"C in a natural heat exchange with air, for example, in the method of the invention, the temperature of the flushing medium is brought below this temperature (cold flushing). This is advantageous, since the behaviour of all silt and small pieces of the overlying rock is such that they retain their shape more and thus become statically more stable with decreasing temperature. Consequently, if enough heat is removed from the rock by cold flushing by the method of the invention, the result will be a more rigid borehole. Since open, unlined, drilled stretches become more or less stabilized in this way, the hydraulic pressure which must be exerted by the flushing medium on the wall of the borehole can be reduced.This means that conventional dense flushing media can be replaced by flushing agents of lower specific gravity. However, the use of a flushing medium with a low specific gravity results in an easing of the load on the floor of the borehole and thus immediately improves the drilling performance.

A further hindrance to working at greater depths using conventional methods is the increasing temperatures encountered. At temperature gradients of between 25 and 50K/m, temperatures of approximately 520 to 770 K obtain at a depth of 1OKm. The use of a cold flushing medium results in the attaining of a temperature level which is more favourable as regards the drill and the drilling equipment. In particular, the temperature can be reduced to a value at which the drilling components will not suffer any damage beyond normal wear and tear under continuous operation. It is thus unnecessary to search for and try out materials that will remain durable even at the temperatures prevailing at great depths.

In an advantageous development of the method of the invention, the flushing medium is cooled to below 0" C. In this way an open stretch of rock can be cooled to the point at which the water in moist strata of the rock surrounding the borehole freezes. There is thus formed a stable body of frozen material that is impermeable to water and which gives the rock adequate rigidity. The area of the borehole wall that has not yet been secured by lining can thus be temporarily braced and unstable rock can be converted into stable rock by the creation and maintenance of such a body of frozen material.

In comparison with cooling without producing a body of frozen material, freezing the rock moisture considerably increases the stability of the rock. The more stable the rock surrounding the borehole, the lower the specific gravity of the selected flushing medium need be and, owing to the increasing reduction of the pressure on the floor of the borehole, the greater will be the advance of the drilling.

It has been found to be of particular advantage if the flushing medium used is in the gaseous state. An essential advantage of the use of such a medium is the elimination of the retarding effect produced by a column of liquid above the floor of the borehole, as already described. In this embodiment, the medium fed into the borehole is a gas which exerts practically no pressure on the floor of the borehole. Consequently, in comparison with liquid flushing media of relatively high density, a significant increase in the rate of progress of drilling and a reduction in drilling costs can be achieved. The absence, or at least reduction of the supporting pressure on the wall of the borehole means the simultaneous elimination or reduction of all the problems that arise from hydraulic imbalances in long, open, unlined drilling stretces when using high-density flushing media.

Hitherto, when employing such high-density flushing media, it could happen that at one depth a certain flushing pressure had to be maintained which at another depth would have been too high, as it would have led to breaking up of the rock. In such a situation an additional length of pipe was required. Since when a gas is used in the method of invention, no pressure or a much reduced pressure is exerted on the wall of the borehole, it is no longer necessary to have such extra lengths of pipe, thus reducing the costs of lining and flushing. A further advantage of the method of the invention arises from the intensive cooling of the drill and from drilling in a cold environment.As a result of the smaller degree of wear and tear on the materials used for the drilling tool, their service life is longer than in an environment in which the temperature increases with increase in drilling depth. This means that a drilling tool remains longer in technically efficient and economic use. The consequence is a better. utilization of materials and fewer "round trips", i.e. assembling and disassembling the drilling equipment and changing the drill.

When using gaseous flushing media, it is essential that the flushing medium should be cooled to a sufficiently low temperature in order to ensure that a supporting mantle is rapidly formed at an open, unlined stretch of rock. The speed of formation and the loadbearing capacity of the rock mantle formed depend inter alia on the thermal properties ofthe rock, such as its thermal capacity, its thermal conductivity, or the thermal gradients involved. On its part, the speed of formation of the mantle detemines the drilling speed.

Consequently it is an advantage if the highest possible speed of formation of the mantle can be attained.

To achieve this, in an advantage form of the invention, a liquefied gas is used as the flushing agent, which gas is vaporized, at least in part, by heat exchange with the rock surrounding the borehole and/or with the drill.

This embodiment is particularly useful at great depths or in hot rock formations. Additionally, an amount of heat corresponding to the heat of vapourization of the liquefied gas is removed from the rock, so that the rock and the moisture in the strata through which the shaft has been sunk can be cooled or frozen particularly rapidly. A stable body of frozen material is formed with equal rapidity around the borehole. If a liquefied gas is introduced into the unlined area of the borehole, considerably more heat can be carried away and the drill, for example, better cooled.

It is not necessary, nor can it always be achieved, that the liquefied gas should vaporize completely. The only thing that is essential is that the flushing agent should carry the necessary amount of cold to the drilling face, should remove sufficient heat from the rock and should then still be capable both of bringing the drilling debris above ground and maintaining the requisite state of coldness in the stretches already drilled.

It is advantageous if the cooled-down flushing medium is led into the vicinity of the floor of the borehole. The medium that cools the rock reaches the floor of the borehole still at a very low temperature, which encourages the thorough cooling of the drill and of the rock ip the vicinity of the borehole floor. Particularly where a liquefied gas is led to the vicinity of the borehole floor, the fresh borehole floor becomes intensively chilled and can be eroded without difficulty by a mechanically-attacking drill. The vapour formed by thermal contact with the rock splits the debris away from the floor and flows up above ground through the circular space between the drill and the wall of the borehole. As a result of the increase in volume associated with the phase conversion there is a high rate of flow. On the journey between the floor of the shaft and the surface of the earth, the vapour absorbs further heat, thus maintaining the portions of the frozen mantle already formed until the introduction of the casing.

In a variation of the method of the invention, the flushing medium is led into the area of an open, unlined stretch of rock in the borehole through a separate pipe, which may be thermally insulated. It is advantageous if the end of the pipe can be maintained at any desired height, the height being adjustable from above ground, so that the flushing agent can be led to any desired part of the borehole.

It is particularly useful if, in the case of a rotary drilling process, for example, the flushing medium is led through a flow passage within the drill piping by means of which the drill is driven by a drive located above ground, and thence through the drili itself to the floor of the borehole, the medium being returned to the surface outside the drill piping. This does away with the necessity for providing a separate pipe for the feeding-in of the cooling flushing agent. It is of advantage to insulate at least to some extent, the drill against the ingress of heat, particularly when using liquefied, low boiling-point gases as flushing agent.

Particularly effective cooling of the drill and uniform distribution of the cooling flushing medium in the borehole are achieved if the gas is fed through a plurality of openings in the drill in the region of the floor of the borehole.

Because of their physical properties and relatively low cost, nitrogen and carbon dioxide are particularly suitable for use in the method of the invention. It should be noted in this connection that carbon dioxide must be kept under pressure and consequently a drill pipe or feed line is required which can hold the pressure required.

To summarize, it can be said that the method of the invention makes possible an increase in the overall rate of drilling progress.

In order to establish as high a rate of drilling advance as is possible, the values for the specific gravity and the temperature of the cold flushing medium should be optimized.

The adjustment of both of these parameters is dictated by the rock (principally by virtue of its own stability and the speed at which the load-bearing mantle is formed) and by the behaviour of the drilling system, (mainly the drilling equipment and the flushing agent) at various temperatures. The range of flushing agents which can be used in the method of the invention, therefore, extends from flushing agents whose density is considerably lower than that of conventional flushing agents and the temperature of which is brought to a very low value, to flushing agents having moderately low temperatures and correspondingly greater densities, but nevertheless a density that is still less than that of conventional flushing agents.

If a flushing agent at a very low temperature is used, this may, as previously described, be less dense than flJshing agents hitherto used. Such flushing agents contribute only a part of the requisite supporting force.

For the rest, the rock supports itself with the assistance of the borehole mantle produced by cooling. In this, two things are self-evident: a. while every cold flushing agent should display the best possible flushing values at the working temperature (i.e. the circulating temperature),it should retain certain minimum properties at its rest temperature. Cold liquid flushing agents, for example, warm up if they stand for any length of time; in borderline cases they may warm up to the undisturbed temperature of the rock.In that case, they must, above all, not "break", that is to say, they must not lose their intrinsic gel strength, since otherwise the drilling debris suspended in the flushing agent will settle out and block the drilling equipment; aqueous cold flushing agents must be so conditioned that they will remain unaffected by changes of temperature of at least up to 200"C for relatively limited period (for example 8 days); b. the combined stability must be maintained or be capable of being maintained at least for the duration of routine stops (changes of drill, taking of measurements, insertion of pipes and cementing).

The advantages of the method of the invention allow for a significant reduction in the cost of deep and ultra-deep borings. The use of the proposed method is likely to be of particular interest in the search for oil or natural gas. Since in many countries (e.g. in the region of West Germany) the overlying rock mantle has now been so exhaustively prospected down to a depth of some 3,000 m that significant finds are hardly to be expected any longer, drilling must be carried out to increasingly greater depths and super-depths if the supply is to be maintained at present levels or even increased. Another field of use will probably lie in the extraction of geothermal heat.If this alternative source of energy is to make any noticeable contribution to our energy stocks, borings will have to be undertaken into the very deeply situated hot rock, since the few shallow deposits or thermal springs make no contribution worth speaking of. However, the cost of drilling by conventional methods would make such projects unprofitable from the outset.

The invention will now be further described with reference to the drawings, in which: Figure 1 is a schematic side view, partly in section, of a rotary drilling means suitable for use in the method of the invention; and Figure 2 is a graph illustrating some temperature gradients obtained using flushing media under various conditions.

Referring to Fig. 1, a shaft 1 is being sunk using a drill that is driven through a drill pipe 3 and a drill stem 4 by a surface drive means (not shown) through a rotary table F The major part of a derrick 6 and devices for extending the drill piping are also not shown.

At the outlet of the shaft a preventer 8 is fitted as soon as the space between a first section of casing pipe 7, the anchor section, and the wall of the shaft, has been filled with cement 9. In Fig. 1, a further section of casing pipe 10 is shown, the space between which and the wall of the shaft is also filled with cement and through which the casing for the third stage is lowered. In the form of construction illustrated, the wall of the borehole is to be cooled by means of liquid nitrogen, and in the rock surrounding the borehole i.e. in the area 17, which extends from the bottom end 20 of the cementing of the second pipe section 10 to the floor 21 of the shaft 1, a cylindrical body of frozen material is to be formed (indicated by the hatched area).For this purpose, liquid nitrogen from a storage tank 11, located above ground and jacketed with heat-insulating material 12, is withdrawn through a valve 1 3 and fed through an extraction line 14 to a transfer head 15, whose function it is to feed nitrogen in liquid form into the double-walled drill pipe 3, which is designed as a vacuum-insulated liquid nitrogen down pipe. When the heat content of the feed system for the liquid nitrogen has been carried away, liquid nitrogen reaches the borehole floor 21 and the borehole wall adjacent to the borehole floor through a plurality of openings 1 6 in the drill 2. The liquid nitrogen vapourizes in direct thermal contact with the surrounding rock and causes the moisture contained in the silt to freeze, even at high rock temperatures.The vaporized nitrogen flows at high speed into the space between the drill stem 4 and the wall of the borehole 1, drilling smalls being drawn away from the floor of the borehole and carried upwards by the vaporised nitrogen. In its subsequent course the nitrogen, which despite its gaseous state is very cold, withdraws heat from the rock above the borehole floor 21, in which a body of frozen material has already formed, thus maintaining the existing body of frozen material. Thereafter, the nitrogen flows through the region of the two sections of pipe and emerges into the atmosphere by way of a discharge line 18. Drilling smalls which it has carried with it are collected in a bin 19. As soon as the desired third-stage depth has been reached, the cooling can be stopped and the third section of pipe introduced and cemented into place.Using the method of the invention, the individual stages can be made considerably longer than when using conventional methods, which means that with the proposed method, fewer sections of pipe are required for a shaft of a given depth.

When using cold flushing agents it is of advantage to use local drives. This is why in the form of construction shown in Fig. 1 an indication is given by dash-dotted lines of the installation of a local drive 22. Such drives are positioned above the actual drilling equipment (i.e. the drill stem and bit) and only this portion is made to rotate. The drill pipes above it remain stationary and in the main only have to take up the restoring torque. The rotary table is not dispensed with, because experience has shown that it is occasionally useful if there is a slow relative movement in the drill pipes.

Fig. 2 is a graph showing some temperature gradients of circulating flushing agents under various conditions at a depth of up to 10 km and at a temperature gradient of 37"/cm (this temperature gradient and those for 25"/cm and 50"/cm are indicated in the Figure).

The graph illustrates four different cases: Curve 1 shows the use of a low flushing.

velocity and natural re-cooling above ground; Curve 2 shows the use of a high flushing velocity and natural cooling above ground; Curve 3 shows the use of a high flushing velocity and artificial re-cooling above ground to 1 0 C inflow temperature; and Curve 4 shows the use of a high flushing velocity and artificial re-cooling above ground to - 1 00'C inflow temperature.

Claims (7)

1. A method of drilling a borehole in which a flushing medium is fed into the borehole, brought into heat exchange relationship with the rock being drilled and/or with the drill used and withdrawn from the borehole, wherein prior to being fed into the borehole the flushing medium is cooled to a temperature below the temperature which would be assumed by a circulating flushing medium after its emergence from the borehole in heat exchange with the environment.

2. A method as claimed in Claim 1, wherein said flushing medium is cooled to a temperature below 0 C prior to being fed into said borehole.

3. A method as claimed in Claim 1 or Claim 2, wherein said flushing medium is in - gaseous form for at least a part of its passage through said borehole.

4. A method as claimed in any one of Claims 1 to 3, wherein said flushing medium is a liquefied low boiling-point gas which at least partially vaporizes in heat exchange with the rock enclosing said borehole and/or the drill used to drill said borehole.

5. A method as claimed in Claim 4, wherein said liquefied nitrogen or carbon dioxide.

6. A method as claimed in any one of the preceding Claims, wherein said flushing medium is fed through a separate line into unlined regions of said borehole.

7. A method of drilling a borehole substantially as before described with reference to Fig. 1 of the drawings.