METHOD OF AND SYSTEM FOR FLUSHING BOREHOLES

Abstract: A system (15) for flushing boreholes (11) during drilling, includes a flushing circuit comprising a reservoir (18) containing a flushing liquid (17) having sufficient density to oppose the formation pressure of fluids in the geological strata through which the borehole is drilled and sufficient viscosity to entrain detritus particles from the drilling operation, pumps (16) and appropriate pipelines for circulating the flushing liquid from the reservoir to the borehole, for receiving the return flow from the borehole and for directing a major proportion of the return flow from the borehole to centrifuges (22) of the system capable of separating detritus contained in the flushing liquid from the return flow, the cleansed flow from the centrifuge and any return flow bypassing the centrifuge, being returned to the reservoir (18) for reconditioning and recycling to the borehole. A method of flushing a borehole during drilling using the above system is also described.

Description: METHOD OF AND SYSTEM FOR FLUSHING BOREHOLES The present invention relates to the flushing of bore holes particularly but not exclusively for drilling operations for hydrocarbons such as oil and gas. During the drilling of a bore hole detritus from the drilling operation accumulates in the bottom of the bore hole and must be pumped to the surface in a flushing liquid. In practice, this liquid is pumped down the bore hole through the drill string, which is hollow, exits into the bore hole around the drill head which is of larger diameter than the drill string and is pumped up through the annular channel between the wall of the bore hole itself and the exterior of the drill string. The flushing liquid must be carefully formulated to fulfil various tasks effectively. It should have sufficient density to balance the formation pressure exerted on it in the bore by the surrounding geological strata, particularly so as to prevent bore-hole collapse and the influx of fluids from the strata into the bore hole during the drilling operation. The additional density required is achieved by the addition of weighting solids to the flushing liquid as well as by the accumulation of detritus particles in the liquid as drilling progresses. A second task of the flushing liquid is to entrain detritus from the drilling operation to flush it from the bore hole. For this purpose the liquid must be relatively viscous, the viscosity usually being achieved by the addition of further particles,* clay and clay derivatives have been used for many years for this purpose but polymeric materials are now sometimes used. Although it is necessary for the flushing liquid to contain suspended particles, it is desirable for its solids content to be minimised for various reasons. Firstly a large solids content makes the liquid more difficult to pump and therefore increases the energy used by the pumps. Secondly the solids interfere with the drilling operation itself in various ways so as to reduce the drilling efficiency and also erode the drilling equipment. Furthermore, once the flushing liquid enters the bore itself, it can be forced into porous rocks or fractures in the bore hole wall where the solids are, effectively, filtered out and left blocking the cracks and also forming a lining or filter cake on the wall itself. A thin lining has advantages, as will be explained below, but thick cakes cause problems which will also be discussed below. A particularly burdensome task in current drilling operations is that of removing the drilling detritus from the flushing liquid once this has been brought to the surface so that the flushing liquid, which contains a significant quantity of expensive materials, can be reutilised. The cleansing and reconditioning of the flushing liquid involves several stages. At present, the entirety of the liquid flow is first passed through sieves, or so-called shale shakers, which constitute the primary separating equipment and, depending on flow-rates, can remove particles down to about 75 microns. It is then usually passed through hydrocyclones, of desilter, desander and/or mud cleaner type, which remove particles down to about 45 microns. Part of the flow, generally about 10%, may then be passed to decanting centrifuges which remove particles down to about 5 microns while the remaining flow is passed directly to a treatment pit or tank where it is subsequently joined by the centrifuged flow. In the treatment pit, liquid and solid additives are added to restore the flushing liquid to the condition required in the bore hole to which it is to be recycled. In view of the considerable quantity of detritus remaining in the liquid returned to the treatment tank, considerable quantities of clean liquid in fact need to be added to dilute the flushing liquid to the desired density. The overall quantity of reconditioned liquid in the treatment tank thus often exceeds that required in the flushing circuit, even though the requirement increases as boring progresses and the bore hole deepens. This frequently means that some of the reconditioned liquid must be dumped, which is extremely wasteful of materials and burdensome with regard to costs. A major problem with this current system is that the separating sieves are extremely expensive to operate and, if it is wished to separate greater proportions of the detritus particles from the return flow in order to improve the drilling efficiency and reduce dumping, then more sieves and/or hydrocyclones must be added to the appropriate stages. This is burdensome in terms of equipment, operating costs and space since each sieve and hydrocyclone outputs the flushing liquid into a separate collecting pit or tank from which the liquid is fed to the next stage. Moreover, the separation achieved is, in any case, limited by the cut-points of the equipment used. An additional problem, which is aggravated the greater the proportion of detritus removed from the return flow, is that some flushing liquid, including its weight- contributing solid additive content, is removed in the separating stages together with the detritus and must be replaced in the treatment pit, thus adding to the operating costs. In view of the fineness of particles removed in the centrifuging stage, the loss of weighting additive particles is particularly serious here since the centrifuges cannot distinguish between detritus and additive particles and, it is for this reason in particular, that only a small proportion of the flow is passed through the centrifuges. A further problem, which is again aggravated the greater the proportion of solids removed from the return flow, is that the drilling detritus, which is a waste material, is contaminated with the flushing material and its discharge into the environment, particularly into the sea in underwater drilling operations, is now seen as unacceptable. The object of the present invention is to provide a flushing system and method for flushing bore holes during drilling operations which does not require such a burdensome cleansing process as the prior art method described above while enabling waste detritus to be produced with an acceptably low pollutant content for discharge into the environment. Accordingly, the present invention provides, in one aspect, a system for flushing bore holes during drilling, comprising a reservoir containing a flushing liquid having sufficient density to oppose the formation pressure of fluids in the geological strata through which the bore hole is drilled and having sufficient viscosity to entrain detritus particles from the drilling operation, means for circulating the flushing liquid from the reservoir to the bore hole, means for directing at least a major proportion of the return flow from the bore hole to centrifuge means of the system capable of separating detritus contained in the flushing liquid, from the return flow, and means for directing the cleansed flow from the centrifuge, and any return flow by-passing the centrifuge, to the reservoir. The flushing liquid must achieve the required density without the use of particulate matter except in exceptional circumstances indicated below and preferably comprises an aqueous solution including one or more soluble salts, preferably selected from potassium, sodium and caesium formates, sodium and potassium silicates, potassium carbonate, calcium, sodium and zinc bromides and ammonium and chloride salts, particularly potassium, sodium and calcium chlorides and calcium-magnesium chloride complexes. The flushing liquid may alternatively be an oil, an oil in water (direct) emulsion or a water in oil (indirect) emulsion, with or without dissolved salts and appropriate emulsifiers and stabilisers. The density of such mixed liquids may be varied by varying the relative proportions of dissolved salts and other constituents. Oils suitable for the purpose would preferably be chosen from those used in the art and generally having a viscosity at 40°C of up to about 6 centistokes. The viscosity of the flushing liquid is preferably controlled by the addition of solid particles, which may be selected from the polymers currently used in flushing solutions, such as hydroxyethyl cellulose (HEC) , XC polymer (a xanthomonas gum product made by Kelco) , hydroxypropyl guar (HPG) , a partially hydrolysed polyacrylamide (PHPA) and guar, in quantities such as to give a yield point of up to about 50lb/100ft 2 . If an emulsion is used, this may itself contribute to the viscosity. In another aspect, the invention provides a method of flushing a bore hole during drilling including circulating to the bore hole a flushing liquid having sufficient density to oppose the formation pressures of fluids in the geological strata through which the bore hole is drilled and having sufficient viscosity to entrain detritus particles from the drilling operation and centrifuging a major proportion of the return flow of flushing liquid from the bore hole to separate detritus particles therefrom. The method preferably further includes the steps of reconditioning the flushing liquid and recycling it to the bore hole, the reconditioning including readjusting its dissolved salt concentration or readjusting oil/water proportions to give the required density and adjusting the content of viscosifying additives. The invention lies particularly in the realisation that, by avoiding the use of particulate material to achieve the required density for the flushing liquid, it is possible to simplify the detritus-separating stages enormously and thereby achieve savings both in equipment and in operating costs. At the same time this simplification leads to the possibility of achieving much greater control over the flushing process as a whole, giving further advantages which will be discussed below. In particular, the system and method of the invention eliminate the need for the hydrocyclones and their auxiliary equipment usually used in the prior art and in many circumstances may also eliminate the need for sieves, especially fine-screen sizes, the separation being achieved entirely by the centrifuge stage. The entire return flow may be passed through the centrifuges but, if a certain proportion of detritus is acceptable in the recycled flushing flow, part of the return flow may by-pass the centrifuge stage. Preferably at least 75% by volume of the return flow is centrifuged and it is envisaged that, in most circumstances, all of the flow will be centrifuged. The return flow may be passed through sieves, as in the prior art, or through a settling tank or tanks to remove coarser particles before the centrifuging stage if the drilling conditions are likely to produce large particles or large volumes of particles. Sieving may, however, be much less rigorous than in prior-art arrangements, requiring fewer stages with less equipment and less loss of flushing liquid. In particular, it is necessary to separate particles down to a size of about 200 microns only whereas, in the prior art, it is usual to try to separate particles down to about 75 microns. In cases where the flushing liquid contains gas entrained from the bore hole, the fluid may be passed through a degasser before entering the particle-separating stages but, even in such cases, the complex of separating equipment required is less than in the prior art . The reduction in equipment and operating costs achieved by the invention is offset to some extent by the cost of the fluids themselves, particularly dissolved salts, which are more expensive than the particulate matter used in the prior art, but there need be very little loss of materials in the present method so that their purchase is effectively a one-off cost. In particular less flushing liquid, is lost in the centrifuging stage than in the combination of shakers, hydrocyclones and centrifuges of the prior-art since the detritus separated by the centrifuges alone is dryer. This has the dual advantage of reducing expense due to losses and the need to replace materials continuously and of facilitating the reconditioning of the flushing liquid in the treatment pit to restore its viscosity and density to the values required for return to the bore hole. There is yet a further advantage of reduced contamination of the detritus, enabling its return to the environment with little if any further anti-pollution treatment. A particularly important advantage of the method and flushing system of the invention is that a far greater proportion of the detritus is removed from the return flow than in the prior art because of the ability to centrifuge substantially the entire flow. Indeed, it is estimated that the overall solids content of the flushing liquid pumped into the bore hole may be reduced to approximately 1% by volume whereas, in the prior art, the recycled liquid contains from 5 to 50% solids by volume, depending on the density required. The reduced solids content of the flushing liquid reduces pumping costs, reduces wear on the drilling equipment and speeds up the drilling operation. It is also relevant to note that oil companies may specify that the flushing liquid should contain no more than a given maximum percentage of solids, and hence of drilling cuttings, in the reservoir or treatment pit, of the flushing system. In the prior art system, if the cuttings proportion rises, it is necessary to dilute the flushing liquid by the addition of fresh clean liquid, with the possible need for dumping and concomitant costs as indicated above. The system of the present invention, however, is effectively a fixed volume circulating system substantially without a need for dilutions (apart from the necessary additions required as drilling proceeds) since a very high proportion of drill cuttings can be removed and little flushing liquid is lost. The use of the method of the invention has yet further advantages. Sometimes, during drilling, sudden increases in formation pressure are experienced, for example when a more porous, permeable, overpressured stratum is broken into. This can result in a net influx of fluids into the bore hole with an associated increase in the volume of the circulating flushing liquid. Flow meters are generally attached to the pipes to indicate any changes in the flow rates, whether in magnitude or direction, but these are unreliable, especially in environments where large quantities of entrained material of varying constitution are present. In the prior art, the continual liquid losses and the number of different processes to which the flushing liquid is subjected, with the associated complexity of pipework and storage pits, as well as the continual additions of liquid to the treatment tank due to the need for dilution, make it extremely difficult to monitor flow and volume changes due to formation pressure changes and these are usually picked up only at a relatively late stage as a result of an unacceptably large gain in volume in the treatment pit. The remedial action is to increase the density of the flushing liquid until the positive pressure differential in the bore hole is reinstated. In the system of the present invention, which has less liquid loss, no hydrocyclones and few, if any, sieves and subsidiary pits and pipework, and much smaller additions required to dilute the flushing liquid because less detritus is retained, the volume of liquid in the "treatment pit" or "reservoir", between the output from the centrifuge stage and the pump which returns the liquid to the bore hole, will change much more quickly in response to changes in formation pressures and such changes may be detected much more readily and identified with changes in drilling conditions so that these can be remedied as appropriate. In general it can be said that process control with the prior art system is difficult and comprises visual inspection and intervention by skilled staff at regular intervals, up to five times per day. Fully automatic process control ha ' s not been achieved as tests on the properties of the flushing liquid cannot be made readily since the substantial solids content of the flushing liquid interferes with the readings produced by the monitoring equipment. Moreover the process of adjusting the density of the flushing liquid is itself complicated since the liquid is weighted both by the weight-adjusting additives and by the retained detritus. In general these have very different specific gravities, the additives chosen usually having specific gravities of about 4.2 and the detritus about 2.6 Bearing in mind that the quantity of retained detritus increases as boring progresses, the difficulty of determining the relative proportions of detritus and weighting additives at any time, and then calculating the amount of liquid (or additives) required to adjust the density to the desired value will be appreciated. The flushing liquid of the present invention, however, has a very much lower solids content and therefore allows the use of automatic monitoring equipment, at least in the line between the output of the centrifuge stage and the pump input, including the treatment pit. In addition, since nearly all the detritus is removed from the liquid returned to the treatment pit, weighting of the flushing liquid is achieved substantially wholly by the dissolved salts. The measurements and calculations required are thus far simpler than in the prior art and again enable the introduction of automation to effect the necessary adjustments. This enables the high manning levels previously required to be reduced and, bearing in mind that drilling takes place 24 hours a day, any automation which reduces manning levels can constitute a very high cost saving, especially when the cost of maintaining staff on offshore oil rigs is taken into account . Another factor involved in bore-hole drilling and which concerns the flushing liquid, is that of "caking" on the wall of the bore-hole itself. As mentioned above, a thin wall lining with low permeability is advantageous as it reduces the escape of liquid into the porous rock strata whereas the build-up of a thick lining cake causes various problems. One problem is that it reduces the cross-section of the bore hole, thus increasing the pumping pressure needed to pump the liquid. In some places the cake can even grow to such a thickness that it contacts the drill string itself and increases its frictional resistance to movement, thereby increasing the energy used in drilling. In the worst cases the differential pressure between the flushing fluid and the surrounding geological formation forces the drill string against the bore hole wall and, if a thick filter cake has built up, the drill string will contact the cake over a large area and cause what is known as "differential sticking", in which the drill string is jammed completely against movement. After a bore hole has been completed and before it is commissioned for production, various tests on the geological formations are conducted by means of instruments lowered down the bore hole, in a procedure known as wireline logging. The presence of a thin filter cake rather than a thick filter cake makes such tests easier to perform and more reliable since there is less insulation between the interior of the bore hole and the surrounding geological formations. Also, differential sticking can occur in this phase and, whereas considerable force can be exerted on a drill string and can sometimes release it, mush less force can be exerted on the supporting wire for instruments without danger of this snapping. Once these tests have been completed, a casing is introduced into the bore hole and a cementitious infill is pumped into the annular space between the casing and the bore hole wall. This cement makes a better bond with a thinner filter cake than with a thick filter cake and prevents seepage of fluids such as hydrocarbons between the casing and the wall from one geological stratum to another. It will be appreciated that the flushing liquid of the present invention will have a lesser tendency to produce a thick cake than prior-art, aqueous flushing liquids since it has a much smaller particle content, while similar cake permeabilities can be achieved. It is also advantageous to make use of the pumping of the cementitious infill into the bore hole to force out the flushing fluid, this being returned up the casing itself. To achieve this, much better results are obtained if the viscosity of the flushing liquid can be reduced to a very low level. This cannot be achieved with the prior art system because the density of the liquid must be maintained and the presence of the viscosity-adjusting particles is to some extent responsible for keeping any density-contributing particles in suspension. The removal of the former therefore allows the latter to fall out of suspension, thereby placing a lower limit on the viscosity which can be achieved. In the flushing liquid, of the present invention, however, viscosity-adjusting particles may be removed without such problems. To this end, a polymer breaker may be added to the flushing liquid to break down the polymers to reduce the viscosity. Once a bore hole has been drilled and lined, it must then be commissioned. This may involve the carrying out of a controlled explosion at the bottom of the bore hole to create cracks and fissures in the surrounding strata through which oil or other hydrocarbons can seep into the bore itself. If, in the prior art, any flushing liquid were allowed to remain in the bottom of the bore hole at this stage, its solid content could sometimes be forced into the cracks and fissures, thus blocking or partially blocking them as they are formed. The prior-art flushing liquid is therefore preferably replaced by an alternative liquid which does not have such a solid content. It is known to use a salt solution for this purpose but such solutions have been used largely for this very limited purpose to date, that is, for balancing the formation pressure on the bore hole, because of their expense. When the flushing liquid of the present invention is used, there is the added advantageous possibility of using the same liquid during commissioning of the bore hole as is used for the flushing, possibly after cleansing by passage through cartridge, diatomaceous earth or other filters possibly after the addition of flocculants to make the particles coalesce. The present invention further provides a method of drilling and commissioning a bore hole, in which the bore hole is flushed during drilling with a flushing liquid as described above and, once drilling is complete, a quantity of this same salt liquid, possibly the flushing liquid itself cleansed of particles is used to oppose the formation pressure from geological strata surrounding the bore hole during lining and/or commissioning of the bore hole. Although the method and flushing system of the invention have many advantages, it must be admitted that they also have some disadvantages. The use of soluble salts can create scale which must be dealt with: this may be reduced or removed chemically or the salts used in a flushing solution may be selected for their inertness to dissolved ions in formation water surrounding the bore hole under consideration. Corrosion rates may also be increased by the presence of salts but this may again be reduced by the particular choice of salts or by the introduction of corrosion inhibitors. In addition, the effectiveness of separation in the centrifuge stage reduces as the density of the flushing liquid increases and there may be a density ceiling above which the method of the invention ceases to be cost-effective: it is envisaged that the centrifuges will be effective with liquids having a specific gravity of less than about 2.4, although the limit may be as low as about 2. It should be appreciated that specific gravities as high as 4 are sometimes required, and could not readily be provided in the system of the invention. Storage and transport of the circulating liquid presents logistical problems because the expense of the liquid, particularly of dissolved salts implies that it is essential to try and preserve them rather than simply disposing of the flushing liquid at the end of the drilling operation. Moreover, should the drilling operation encounter a natural or induced fracture or fissure leading to loss of the pumped flushing liquid into the surrounding geological formations, this could constitute an expensive loss although the reduction in volume in the treatment pit would be detected fairly promptly allowing steps to be taken to minimise the loss. Also the likelihood of a fracture being induced is lower in use of the present invention than in the prior art since the pumping pressure losses are lower and the flushing liquid does not gel as readily as prior art fluids when left static. One embodiment of the invention will now be more particularly described with reference to the accompanying drawing which is a schematic representation of a flushing system according to the invention for flushing the bore hole created in a drilling operation. With reference to the drawing, a rig 10 is arranged for drilling a bore hole 11 into potentially hydrocarbon- bearing strata 12. The drilling is effected by means of a drilling head 13 supported for rotation at the bottom end of a hollow drill string 14. The detritus which collects at the bottom of the bore hole 11 as a result of the drilling operation is flushed out and brought to the surface by means of a flushing system of the invention generally indicated 15. The flushing system 15 includes pumps 16 for pumping a flushing liquid 17 from a collecting reservoir, or treatment pit 18, down the hollow drill string 14 to the drilling head 13 where it exits into the bore hole 11 itself. The flushing liquid 17 is so formulated that it entrains detritus formed at the bottom of the bore hole 11 by the drilling operation and carries it up to the surface through the annular channel 19 surrounding the drill string 14. Once the flushing liquid 17 has been returned to the surface, it is cleansed of detritus particles by apparatus described below before being returned to the treatment pit 18 for reconditioning and recycling to the bore hole 11. The flushing liquid 17 used in the flushing system 15 comprises an aqueous salt solution containing, for example, one or more of the following salts: potassium, sodium and caesium formates, potassium and sodium silicates, calcium and zinc bromides, ammonium and chloride salts. The solution is formulated to give it sufficient density to counteract the formation pressures from the geogical formations surrounding the particular bore hole 11 being drilled so as substantially to prevent seepage of liquids from these formations into the bore hole 11 during drilling. The choice of salts may be determined by the density requirements and also the sensitivity of the formations to swelling in contact with the liquid, or the risk of insoluble salts forming when the solution comes into contact with formation salts. Alternatively, the flushing liquid could be an oil/water emulsion, with or without dissolved salts. The flushing liquid 17 further contains fine particles of a polymer, such as HEC, XC, PHPA or HPG or guar, in such quantities as to give the solution a yield point of up to about 25lb/100ft 2 or, in some cases, up to about 50lb/l00ft 2 , sufficient for it to entrain the detritus particles from the bore hole 11. The composition of the flushing liquid which is recycled to the treatment pit 18 differs slightly from that required for the flushing processes because of losses in the flushing circuit and the presence of some contaminants. In addition, further water, oil, emulsifiers, other additives salts and polymer may be added continuously to the treatment pit 18 through separate lines or a single line, not shown, to increase the volume of liquid in the flushing system 15 as the bore hole 11 is deepened. Monitoring equipment 20 is preferably associated with the treatment pit for monitoring factors such as the salt composition, solids content, density, conductivity and viscosity of the flushing liquid. The monitoring equipment may be checked at intervals by operators who add the necessary ingredients to the pit to restore the flushing liquid to the desired composition or the monitoring equipment may be arranged to activate automatic equipment (not indicated) for adjusting the composition. The apparatus for cleansing the flushing liquid 17 returned from the bore hole 11 may include a degasser 9 for removing excessive gas should this be entrained in the liquid from the bore hole 11 but the degasser may be by-passed as shown by the line 9a, or omitted should circumstances allow. The apparatus then includes one or more shale shakers, or sieves, 21 which remove particles down to a size of about 200 microns, but again these may be by-passed, as shown by the line 21a, or omitted if not required for a particular drilling operation or replaced by a settling pit. The major part, preferably the entire flow from the bore hole 11 is, however, passed through one or more centrifuges 22 which remove particles down to a size of about 5 microns. Centrifuges manufactured by Broadbent specified by a bowl diameter of 900mm and length of 1800mm are suitable for most applications but for small bore holes 11, Broadbent centrifuges having a 600mm diameter and length of 950mm or 550 mm diameter and length of 1375 mm may be more suitable. As drilling progresses, the content of particles less than 5 microns in the flushing liquid increases and, there may come a point at which even these particles are unacceptable. When this stage is reached, a flocculant, such as a polyacrylamide or acrylamide copolymer, or a polyamine known in the drilling field may be introduced in the return flow through a line 23a before the entry to the shale shaker or through the line 23b between the shaker 21 and the centrifuges 22. The flocculent aggregates the fine particles so that at least some are removed in the centrifuges 22. The output of cleansed liquid from the centrifuges may also be monitored by automatic equipment and the readings used to control the operation of the centrifuges themselves to maximise the removal of detritus particles from the liquid. The flushing liquid leaving the centrifuges 22 will in most cases have upwards of 95% of the freshly drilled detritus removed before its return to the treatment pit 18 for reconditioning. Indeed, with the use of the flocculant, almost 100% of the drilled solids may be removed although there will be a concomitant loss of polymer which must subsequently be replaced if flushing is continued. It may be noted that the flushing liquid returned to the treatment pit in prior art systems generally contains more than 5% by volume of solid material and may contain as much as 50% by volume or more. Moreover the detritus leaving the centrifuges 22 is almost dry and contains little salt or polymer contaminant, whereas the solids discarded in the prior art techniques are usually very wet, with loss of large quantities of valuable and environmentally unfriendly fluid. The flushing system 15 described above may be operated with a flushing liquid having sufficient viscosity to lift all the drilled cuttings to the surface throughout the operation or it may be operated with alternating, intermitent sweeps of liquid with higher and lower viscosities, the majority of the flow being of lower viscosity with the higher viscosity used to entrain remaining larger particles. In use of the flushing system described above, anomalous changes in the volume of liquid in the treatment pit may be noted by the monitoring equipment, requiring various responses. An anomalous increase may indicate the influx of geological fluids into the bore hole 11 which may be stopped by increasing the density of the flushing liquid by adding an appropriate quantity of salts. Alternatively, if the bore is being drilled through a particularly porous stratum, it may be preferred to add particulate material which will be sacrificed partly in forming a bridging filter cake in the pores and fissures and partly by removal in the centrifuges. It is envisaged that 0.1 - 1% by volume quantities of particles less than 75 microns would suffice in most cases, this being considerably less than the volume of such particles employed in prior - art flushing liquids. In cases where a decrease in formation pressure is noted, it may be appropriate to add oil and emulsifiers to a flushing salt solution or to an oil/water emulsion to reduce its density. In addition, polyols, such as glycols and glycerols, may be added to reduce absorption of water by clays present in the geological formations. It is also envisaged that, in bore holes where large losses of flushing liquid into the geological formations are expected or noted, or where high densities are required which cannot be achieved economically with dissolved salts, it may be appropriate to alternate the circulation of a salt-containing flushing liquid with a prior-art flushing liquid in different sections of a bore hole. Once the bore hole 11 has been completed and is ready for commissioning, flocculants may be added to the return flow from the bore hole before it enters the centrifuges 22: these may, for example, be added through a line shown at 23a prior to the shale shocker 21 or through a line 23b after the shale shaker 21. The flocculant causes the aggregation of detritus and polymer particles in the flow so that all of these may be removed in the centrifuges 22 or, if necessary, the flow may be filtered. The substantially particle-f ee flow may be returned to the treatment pit 18 and recycling continued in the usual way until substantially all the solids have been removed from the flushing circuit and the bore hole is filled with clean liquid and can be brought on line. Alternatively polymer breakers may be added to the flow or the flushing liquid may be replaced by a clean flow of the salt solution, or a water/oil emulsion if used, without the polymer particles. It is further envisaged that, when very dense flushing liquids are required, it may be necessary to add solid particles to the salt solution, but particles having a size of less than 5 microns are used such that they are less likely to be removed in the centrifuges 22 than prior art particles which have sizes up to about 75 microns. Alternatively particles having sizes up to 20 microns may be used if the G forces and/or residue times of the liquid in the centrifuge are reduced. It may also be advantageous to use such solid particles when it is found necessary to adjust the density of the flushing liquid very quickly in use of the system described above. Alternatively solid particles may have advantages when extremely dense liquids are required, partly because of the relative cheapness of the insoluble solids usable.

Claims: CLAIMS 1. A method of flushing a bore hole during drilling including circulating to the bore hole a flushing liquid having sufficient density to oppose the formation pressures of fluids in the geological strata through which the bore hole is drilled and having sufficient viscosity to entrain detritus particles from the drilling operation, and centrifuging a major proportion of the return flow of flushing liquid from the bore hole to separate detritus particles therefrom. 2. A method as claimed in Claim 1, in which the flushing liquid is weighted to the required density by the addition of soluble salts. 3. A method as claimed in Claim 2, in which the flushing liquid comprises or includes an aqueous solution including one or more salts selected from potassium, sodium and caesium formates, sodium and potassium silicates, potassium carbonate, calcium, sodium and zinc bromides and ammonium and chloride salts, particularly potassium, sodium and calcium chlorides and calcium-magnesium chloride complexes. 4. A method as claimed in any one of Claims 1 to 3 , in which the flushing fluid is an oil and water emulsion . 5. A method as claimed in any one of Claims 1 to 4, in which the flushing liquid is partly weighted by the addition of particles, the particles being selected or the centrifuging being effected in such a manner that the particles are not removed in the centrifuging stage. 6. A method as claimed in any one of the preceding claims, in which the centrifuging is carried out so as to separate particles down to about 5 microns. 7. A method as claimed in any one of the preceding claims, in which the viscosity of the flushing liquid is controlled by the addition of solid particles to give a yield point of from about 0.25 to about 2.5 kg/m 2 . 8. A method as claimed in any one of Claims 1 to 7, including the further step, after the centrifuging step, of reconditioning the flushing liquid returned from the bore hole to give it the requisite density and viscosity to oppose formation pressures and entrain detritus and recycling the reconditioned liquid to the bore hole to effect further flushing. 9. A method as claimed in Claim 8, including the step of monitoring the viscosity and density of the flushing liquid for return to the bore hole after the centrifuging step on a continuous basis and using the monitored results to determine the materials to be added to the flushing liquid to recondition it. 10. A method as claimed in any one of the preceding claims, in which upwards of substantially 75% by volume of the flushing liquid returned from the bore hole is centrifuged. 11. A method of flushing a bore hole as claimed in any one of Claims 1 to 10, further including the step of commissioning the bore hole, once drilling is complete, in which the liquid used during the flushing is also used to oppose the formation pressure from geological strata surrounding the bore hole during commissioning of the bore hole. 12. A system for flushing bore holes during drilling, including a flushing circuit comprising a reservoir containing a flushing liquid having sufficient density to oppose the formation pressure of fluids in the geological strata through which the bore hole is drilled and having sufficient viscosity to entrain detritus particles from the drilling operation, means for circulating the flushing liquid from the reservoir to the bore hole and for receiving the return flow from the bore hole, means for directing at least a major proportion of the return flow from the bore hole to centrifuge means of the system capable of separating detritus contained in the flushing liquid from the return flow, and means for directing the cleansed flow from the centrifuge, and any return flow bypassing the centrifuge, to the reservoir. 13. A system as claimed in Claim 12, in which the flushing liquid comprises or includes an aqueous solution including one or more salts selected from potassium, sodium and caesium formates, sodium and potassium silicates, potassium carbonate, calcium, sodium and zinc bromides and ammonium and chloride salts, particularly potassium, sodium and calcium chlorides and calcium-magnesium chloride complexes. 14. A system as claimed in Claim 13, in which the flushing liquid is an oil and water emulsion. 15. A system as claimed in any of Claims 12 to 14, in which the flushing circuit further includes apparatus selected from settling tanks and sieves for receiving the flushing liquid from the bore hole and removing entrained coarse particles before its delivery to the centrifuge means. 16. A system as claimed in any one of Claims 12 to 15, further including means for monitoring selected properties of the liquid in the reservoir and for supplying an indication of the values detected so that the liquid can be brought to a desired condition for supply to the bore hole. 17. A system as claimed in Claim 16, in which the monitoring means are arranged to monitor properties selected from the density, viscosity and volume of the liquid in the reservoir. 18. A system as claimed in Claim 16 or 17, further including programmable means connected to receive the indications of properties of the flushing liquid and to control the activation of respective metering means for supplying constituents of the flushing liquid to the reservoir in dependence on the values detected to bring the liquid to the desired condition for flushing. 19. A system as claimed in Claim 18, further including means for supplying the control means with information relating to conditions in the bore hole or in geological formations surrounding the bore hole as drilling progresses, the control means being programmable to control the metering means to vary the supply of constituents to the reservoir so as to vary the condition of the flushing liquid therein in dependence on conditions prevailing in the bore hole.

Inventors: ASTLEFORD JOHN, ASTLEFORD, JOHN
Applicants: ASTLEFORD JOHN, ASTLEFORD, JOHN
Application year: 1995
Main classification: E21B21
IPC codes: E21B21