Borehole assembly and method

Abstract: A method of accessing water from an aquifer which underlies an impervious stratum without contamination of the aquifer by polluted water from above the stratum by drilling a bore hole to the aquifer; comprises the steps of (a) drilling a first bore hole into, but not through the impervious stratum and a second bore hole of smaller diameter to the aquifer; (b) inserting therein a protective tube of diameter intermediate between those of the bore holes and which extends to the ground surface; and (c) adding a protective mortar to the annular space between the protective tube and the larger diameter bore hole; there being circumferentially wound around the tube at the level of the protective mortar, transfer tubing which comprises radial perforations which permit passage of fluids, e.g. sealing composition from the interior of the transfer tubing to the mortar. The method works best with certain reinforced mortar formulations.

Description: Field of the Invention This invention relates to a process of extracting water from an aquifer which is overlaid by an impervious stratum, and to a bore structure for use therein. Background of the Invention It is often found that it is necessary to bore through an impervious stratum to reach an aquifer lying below it ("the lower aquifer"). One reason for doing this is to instrument the aquifer. This can be done when there is also an aquifer above the stratum ("the upper aquifer"), and a technique for instrumenting two aquifers in such a relationship by means of a single boring operation is described in United States Patent 4,548,266. It may also be desired to extract water from the lower aquifer. This may occur because the upper aquifer has become polluted or depleted or both. The drilling operations carried out to access the lower aquifer are often in themselves a source of pollution because of oil, grease, drilling mud and bacteriological contamination. In addition, polluted water may seep into the lower aquifer via the annulus created between the sides of a bore hole and a bore tube inserted therein. It is normal practice to fill this annulus with grouting based on quick-setting cement, but it has been frequently observed that, very soon after drilling, the lower aquifer becomes polluted because the grouting currently used is often not completely water-tight. The perforation of an overlying impervious stratum thus risks polluting an aquifer lying beneath it. This risk is increased when the bore hole is decommissioned, in which, conventionally, the top part of the bore tubing is pulled out and the bore hole is filled with non-watertight material, such as light concrete, gravel, and earth. At the present time, this practice is not strictly regulated. Summary of the Invention It has now been found that it is possible to create a bore for the extraction of water from an aquifer underlying an impervious stratum, which not only prevents pollution from seeping into said aquifer from above the impervious stratum, but which also permits the bore to be decommissioned without adverse effect on the underlying aquifer. There is therefore provided, according to the present invention, a method of accessing water from an aquifer which underlies an impervious stratum by drilling a bore hole to the aquifer, comprising the steps of (a) drilling a first bore hole into, but not through, the impervious stratum and a second bore hole of smaller diameter to the aquifer; (b) inserting therein a protective tube of diameter intermediate between those of the bore holes and which extends to the ground surface; and (c) adding a protective mortar to the annular space between the protective tube and the larger diameter bore hole; characterised in that the watertightness of the mortar is monitored and if necessary enhanced by the injection of appropriate fluids through transfer tubing which is circumferentially wound around the protective tube at the level of the protective mortar and which tubing comprises radial perforations which permit passage of fluids from the interior of the transfer tubing to the mortar. The invention additionally provides a means for the extraction of water from an aquifer which underlies an impervious stratum, comprising a bore hole extending to the aquifer, a bore hole of larger diameter extending into, but not through, said impervious stratum, a protective tube which is inserted therein, which protective tube is of diameter intermediate between the diameters of the bore holes and which extends from the stratum to the ground surface, a protective mortar placed in the annular space between larger bore hole and tube and radially-perforated transfer tubing wound circumferentially around the tube at the level of the protective mortar, the tubing allowing the passage of fluids from the transfer tubing to the mortar, for testing and enhancing the watertightness of the mortar. The invention further provides a method of re-establishing the imperviousness of an impervious stratum through which a bore hole has been drilled in order to reach an aquifer beneath said stratum, wherein the protective tube, protective mortar and transfer tubing are left in place when the borehole is decommissioned, the watertightness of the mortar being tested and maintained by means of fluids injected through the transfer tubing. The invention further provides a bore hole assembly adopted to draw water from an aquifer lying beneath an impervious stratum, comprising (a) a water withdrawal tube extending from the surface to the aquifer; and (b) a protective tube concentrically located around the water withdrawal tube and extending from the surface into, but not through, the impervious stratum and held in place at its lower end by protective mortar inserted into the annular space defined by the protective tube and a bore hole into which it fits; further characterised in that there is wound around the protective tube, at the level of the protective mortar, transfer tubing comprising radial perforations which permit the passage of fluid from the transfer tubing to the protective mortar, for testing and enhancing the watertightness of the mortar. Detailed Description of the Invention The bore hole of this invention thus comprises (a) a relatively narrow bore hole extending to an aquifer which lies beneath an impervious stratum, (b) a broader bore hole which extends into, but not through, the impervious stratum, (c) a protective tube which has a diameter intermediate between those of the two bore holes and which extends from the surface to the stratum, (d) a protective mortar which is added to the annular space defined by the sides of the larger bore hole and the tube, and (e) perforated transfer tubing wound around the tube at the level of the protective mortar and adapted to permit fluids to be injected from the surface into the mortar. It is important that the protective tube be seated at its lower end on the impervious stratum, hence the requirement that the appropriate bore hole extend into, but not through, the stratum. It must naturally extend a sufficient distance into the stratum to permit the establishment of an annulus of protective mortar of an appropriate size. This will depend on factors such as size of bore hole and depth of stratum, but the skilled person can readily ascertain this. Preferably the bore hole for the protective tube extends nearly to the bottom of the stratum. Thus, the protective tube with the protective mortar will prevent water from above the stratum from entering the aquifer. When such a bore hole is decommissioned, the protective tube is capped and left in place, and the imperviousness of the stratum is therefore assured. The other bore hole equipment used in conjunction with this invention is conventional. It is typical, for example, to insert into the bore hole extending to the aquifer a perforated tube which is then surrounded by gravel. Through this tube water from the aquifer is withdrawn in the normal manner. The transfer tubing with the radial perforations is available, for example, under the trade mark FUKO (Master Builders Technologies). This particular transfer tubing is of neoprene and comprises multiple radial perforations over its entire periphery. It is wound around the protective tube in a tight helix. Naturally the perforations should all lie within the mortar, such that any fluid passed therethrough will pass only into the mortar. The separation of the perforations from the top and bottom of an annulus of mortar established between the sides of a bore and those of a protective tube depends very much on the relative dimensions of bore and tube, the depth of the mortar annulus and the composition of the mortar, but it has been found that the perforations should be no closer than 0.5 metre, preferably one metre, from either the top or the bottom of the mortar annulus. The perforated transfer tubing solves two problems, namely, how to ascertain whether a protective mortar annulus is waterproof, and if not, how to make it so. Detection of water impermeability is a simple matter of injecting water under pressure through the transfer tubing. An excessively rapid drop in pressure will reveal an unacceptable degree of permeability on the part of the mortar. This situation can then be rectified by the injection under pressure of a fluid water-reactive sealing material which will seal any interstices and cracks and render the mortar water-impermeable. This then makes impossible the percolation of polluted water into the aquifer from above the impervious stratum. A typical example of a suitable water-reactive sealing material is "Master Flex" (trade mark) 601 ex Master Builders Technologies, a solvent-free injectable resin. One of the benefits of this invention is that both the monitoring and repair of the mortar can be carried out continually, even when the bore hole has been decommissioned, thus ensuing the long-term imperviousness of the stratum. The mortar for use in the process of this invention may be any suitable mortar known to the art. However, a mortar composition which gives particularly good results is described in United States Patent 5,370,188. Description of the Drawings The invention will now be further described with reference to the drawing which is a schematic representation of a preferred embodiment of a bore hole assembly in place, shown in vertical cross-section. Description of Preferred Embodiments In particular geological strata, two beds of sand, 2 and 3, which are aquifers, are separated by an impervious stratum of marl 1. The lower sand bed 3 rests on a bed of limestone 5. The upper bed 2 is located just below a bed of natural soil 4 which also forms the surface. A bore for the purposes of extracting water from the lower bed 3 without admitting polluted water from the upper bed 2 to leach thereinto basically comprises two bore holes, a larger diameter bore hole 6 which extends from the surface nearly to the base of the marl layer 1, and a smaller diameter bore hole 7, concentric with bore hole 6 and extending from the surface to the limestone bed 5. Into bore hole 6 is fitted a protective tube 10, this resting at its bottom end on the marl and protruding above the surface at its top end. Into bore hole 7 is inserted a water collector tube 8, this extending from above the surface at its top end to a concrete base 11 established for it in the limestone at its lower end. This tube 8 is perforated at that part thereof which extends into the sand bed 3, from which it is intended to draw water. The annular space formed between tubes 10 and 7 is filled with grit 9 which acts as a filter. At the base of tube 10, in the annular space defined by the outer surface of tube 10 and borehole 6, is placed a layer of an impervious material 12 (a quick-setting mortar or "Sobranite" (trade mark), a commercial material which creates a plug of clay). Above this is placed a protective mortar of the type hereinabove described (the mortars actually used will be further described hereinunder). This is capped with a rapid-setting mortar 12a and the remainder of the annular space is filled with a normal mortar 15. Helically wound around the protection tube 10 at a level corresponding to the protection mortar 14 is a radially-perforated transfer tubing 13. This is connected to the surface by means of tube 16. The transfer tubing is arranged such that the perforations lie completely within the mortar 14, being no closer than one metre to the top or base of the protective mortar annulus. This tube allows the water impermeability of the protective mortar to be tested, and if found deficient, to be rectified easily. An example of a suitable tube is FUKO neoprene tube, previously described. A useful water-reactive material is "Master Flex" (trade mark) 601 (MBT), which is a solvent-free, injectable resin of low viscosity, with two components whose principal constituents are polymers which swell in water. This resin hardens whilst remaining flexible, even at low temperature. It is hydrophilic and impregnates the moist surfaces thoroughly. It prevents oxidation of the steel, is unaffected by grease, metal or concrete. It is resistant to weak acids, saline solutions, oils, fats and hydrocarbons. Two preferred compositions of mortar to be used in the process of this invention and described in the abovementioned US 5,370,188 are as follows (given here as dry compositions prior to addition of cast-iron fibres). First composition : sand 2.0-3.15 mm quantity to make up 100 % sand 0.7-1.4 mm 14.6 % ± 5 % sand 0.0-0.4 mm 18.95 % ± 5 % sand 0.5-1.0 mm 15.32 % ± 5% gravel 6-8 mm 0.0 % calcium beta-naphthalene sulphonate in powder form 0.8 % ± 0.3 % cetyl alcohol (foam inhibitor) 0.07 % ± 0.03 % silica fume 2.0 % ± 1 % calcium oxide (crystalline expansion agent) 2.1 % ± 0.5 % aluminium powder (gaseous expansion agent) 0.0035 % ± 0.001 % Portland cement CPA 55 Type I 35.0 % ± 10 % Second composition: sand 2.0-3.15 mm quantity to make up 100 % sand 0.7-1.4 mm 13.35 % ± 5 % sand 0.0-0.4 mm 8.18 % ± 5 % sand 0.5-1.0 mm 11.46 % ± 5 % gravel 6-8 mm 20.0 % ± 5 % calcium beta-naphthalene sulphonate in powder form 0.55 % ± 0.3 % cetyl alcohol (antifoam) 0.07 % ± 0.03 % silica fume 1.2 % ± 1 % calcium oxide (crystalline expansion agent) 1.8 % ± 0.5 % aluminium powder (gaseous expansion agent) 0.0025 % ± 0.001 % Portland cement CPA 55 type I 30.0 % ± 10 % The fibres of the preferred type hereinabove described are added at a rate of about 42Kg/M 3 . The mortar has sufficient resilience to allow the driller to work in the drilling area soon after the injection (after half a day or one day at most). For this, the mortar 14 is rapid-hardening, but non-brittle. The addition of fibres, preferably cast-iron fibres, fundamentally modifies the mechanical behaviour of the mortar in that they permit expansion to be controlled and contained. Whereas a mortar or grout without fibres breaks in a brittle manner (as soon as a crack appears, the rupture is complete), the expanded mortar which is reinforced with fibres has ductile behaviour: when the mortar receives shocks from a tool communicated through the protective tube 10, the stresses are taken up by the fibres and cracking is stopped. It would be necessary for the stress to increase or for the shocks always to be always reproduced at the same spot for cracking to continue and to lead to rupture of the mortar. This ductile behaviour is translated into very good resistance to shocks and excellent tenacity of concretes which are reinforced with cast-iron fibres. A minimum of 3,000,000 and preferably 3,500,000 fibres per M 3 of cast mortar are necessary in order to obtain good resilience after 24 hours with 20 mPa of compression. The characteristics of fresh mortar are as follows: particle size range: 0/4 or 0/8 mm depending on the nature of the annular space, plasticity: 1' to 3' with fibres in a Marsh cone of 12.5mm diameter, cohesive, non-dilutable and thixotropic to be resistant to water circulation, pumpable at low pressure and incapable of giving up water when subjected to normal water reduction treatments, such as centrifugation, exudation at 3 hours: nil, no sedimentation, density of fresh mortar: 1.950 ± 0.15, initial set ≥ 6 hours depending on the size of the borehole, volumetric expansion in the plastic phase ≥ 2%, but capable of modification in accordance with the surrounding stratum, yield by volume: 25 kg of special mortar + 3 litres ± 0.5 l of water = 15 litres of grout ± 1 litre of grout. The characteristics of hardened mortar are as follows: resistance to compression at 24 hours with fibres > 25 mPa, no shrinkage primary thrust of crystalline expansion from initial set to 2 days, secondary from days 1 to 28 and longer, ≤ 5 bars if the container is not ductile very good resilience at 24 hours (Rc at 24 hours ≥ 25 mPa), with more than 3,000,000, in particular more than 3,500,000 cast-iron fibres (ex Pont à Mousson) per M 3 of cast mortar pH ≥ 12 very good resistance to corrosive water (choice of cement according to the nature of the corrosive ions) intrinsic permeability to water with constant flow below 1 bar lower than 10 -10 m/s, preferably lower than 10 -11 m/s. The mortar is put into place by the methods normally employed in the art. The mortar is typically pumped, and a suitable pump is, for example, an endless-screw pump of the type commercialized by the company Master Builders Technologies under the name MEYCO (Registered Trademark) DEGUNA 20. Preparation of the mortar is effected by mixing the cement conveyed by an endless screw and air with aggregates (sand and gravel) on the one hand and silica fume and powdered admixtures on the other hand. The water employed for mixing with the mortar is preferably clean and free from organic matter. Mixing is carried out in a temperature range of between 5 and 35°C. After placing two-thirds of the quantity of mixing water in a mixer, the ready-to-use mortar is added, and is mixed at low speed for two minutes, the remaining one-third of water then being mixed in, mixing being carried out at high speed for three minutes. This is then added to the annular space between the sides of the borehole 6 and the protection tube 10, after the bottom of this space has been filled with a plug of "Sobranite" or rapid-setting mortar 12, so that the protection mortar 14 does not go back into the protection tube 10 or migrate into the underlying bed 3. There is approximately constant feed from of the pump, care being taken to ensure that air entrainment is excluded. Cementation of the annular space is carried out from the bottom to the top by means of a plunger pipe. In order to obtain a uniform loading, the metallic tip is continuously plunged from a height of 1 or 2 m into the mortar, preferably with continuous addition. Cementation of the top plug 12a, and of the part 15 which does not have to be rendered water-tight, is carried out with the same material and with the same technique, but using a traditional mortar 0/5 mm, measured at a minimum to 650 kg of CLC or CLK, optionally containing admixtures. The traditional cementation is carried out immediately, so as to counteract the expansion of the underlying mortar during the plastic phase and to effect horizontal expansion efforts towards the stratum 1, in order to fill in all the crevices, especially on difficult ground. The upper plug 12a of rapid-setting mortar plays the role of a wedge, and prevents the expanding mortar from lifting the column of normal mortar 15, while this latter is in the plastic phase. There are many other possible embodiments which fall within the scope of this invention, as defined by the appended claims, which are not mentioned here, but which are well within the skill of the art and will be readily be appreciated as such by persons skilled in the art. The skilled person will also appreciate that any dimensions of apparatus given herein are purely exemplary, and that these may be readily varied, should the need arise.

Claims: A method of accessing water from an aquifer (3) which underlies an impervious stratum (1) by drilling a bore hole to the aquifer, comprising the steps of (a) drilling a first bore hole (6) into, but not through, the impervious stratum (1) and a second bore hole (7) of smaller diameter to the aquifer; (b) inserting therein a protective tube (10) of a diameter intermediate between those of the bore holes and which extends to the ground surface; and (c) adding a protective mortar (14) to the annular space between the protective tube and the larger diameter bore hole; characterised in that the watertightness of the mortar is monitored and if necessary enhanced by the injection of appropriate fluids through transfer tubing (13) which is circumferentially wound around the protective tube at the level of the protective mortar and which tubing comprises radial perforations which permit passage of fluids from the interior of the transfer tubing to the mortar. A method of re-establishing the imperviousness of an impervious stratum through which a bore hole has been drilled by a method according to claim 1 in order to reach an aquifer beneath said stratum, wherein the protective tube, protective mortar and transfer tubing are left in place when the borehole is decommissioned, the watertightness of the mortar being tested and maintained by means of fluids injected through the transfer tubing. A means for the extraction of water from an aquifer (3) which underlies an impervious stratum (1), comprising a bore hole (7) extending to the aquifer, a bore hole (6) of larger diameter extending into, but not through, said impervious stratum, a protective tube (10) which is inserted therein, which protective tube is of a diameter intermediate between the diameters of the bore holes and which extends from the stratum to the ground surface, a protective mortar (14) placed in the annular space between larger bore hole and tube and radially-perforated transfer tubing (13) wound circumferentially around the tube at the level of the protective mortar, the tubing allowing the passage of fluids from the transfer tubing to the mortar, for testing and enhancing the watertightness of the mortar. A bore hole assembly adopted to draw water from an aquifer lying beneath an impervious stratum, comprising (a) a water withdrawal tube (8) extending from the surface to the aquifer; and (b) a protective tube(10) concentrically located around the water withdrawal tube and extending from the surface into, but not through, the impervious stratum and held in place at its lower end by protective mortar (10) inserted into the annular space defined by the protective tube and a bore hole into which it fits; further characterised in that there is wound around the protective tube, at the level of the protective mortar, transfer tubing (13) comprising radial perforations which permit the passage of fluid from the transfer tubing to the protective mortar, for testing and enhancing the watertightness of the mortar.

Inventors: COTTRELL GABRIEL MARIES-JOSEPH, COTTRELL, GABRIEL MARIES-JOSEPH JEAN
Applicants: MBT HOLDING AG
Application year: 1993
Main classification: C04B28
IPC codes: E21B43, E21B47, C09K8, C04B28, E21B33, C04B24