Project report on a deep well construction

Transcription

1 Project report on a deep well construction The Project The client GCO is in the process of assembling the world s biggest dredge mining operation at Senegal s coast. The Mining Concession comprises of a 106 km long and 4,5 km wide section at the coast. The goal is to extract zircon (ZrSiO 4 ), ilmenite (FeTiO 3 ) and small quantities of rutile (TiO 2 ). BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

2 Arrangement from left to right: collecting module, feed line, dredger and concentration unit. Mining concession and planned infrastructure BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

3 Work Scope BAUER Technologies was contracted to drill, construct and test the first 11 production boreholes with a depth of m. A water-rich aquifer was found at depth and should be exploited up with the deep wells. The production boreholes are located along the dredging path. Wells design All wells were drilled and constructed by using PRAKLA RB40 drilling rigs. Initially on each location a small well ( supply well ) with 8 inch diameter was drilled down to 50 60m depth in order to access a shallow ground source. The water supply is required for compounding the drilling fluid for the actual well. For the drilling mud of the supply well water is also required and is delivered in tanks. Above: Air rods; below: drill rods Excavation of pump sumps Storage of chemicals Drill head, stabilisers and collar; Lifting and breaking device in the back (blue) Assembly of generator BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

4 The first meters through the loose dune sand are drilled with a 26- inch drag-bit where 24-inch permanent casings are installed. The space between the wall and casing (annulus) is filled with concrete. Concrete is pumped into the drill mud via small tremie pipes. Due to the fact that the casings do not seal off in loose sands, concrete enters the inside of the casings from the bottom. After approx. 10h of hardening process the drill head will be changed to 23inch and drilling will continue. The concrete layer inside the casings has to be penetrated. Now drilling will continue to the final depth approx. 60m where a first aquiclude layer of clay is reached. The drill rods are removed from the borehole and casings of 20-inch diameter will be installed. Screen (filter pipe for water inlet from Aquifer) with centraliser and do collar. Visible also the different diameter casings. Dog collar and installation of casings The boreholes will be stopped when 60m depth are reached and the casings will be installed. The casings are penetrated into the clay layer as deep as possible so that a minimum amount of concrete enters the inside of the casings during the concreting process. The concrete is pumped into the annular space by means of a tremie system and 12 hours setting period are observed. The concrete grouting serves for: The stabilization of the borehole and the installed casings. Separating the top water from the water of deeper aquifers Separation of different water qualities Keeping the water pressure inside the aquifer itself Protection of water from contamination from the surface res from inferior water quality of different layers. Avoiding the deepening of the local ground water level BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

5 The drill bit will be exchanged at 60m. From there the drilling will continue with a diameter of 17.5-inch. During the concreting process a layer could possibly be formed inside the casings. When drilling through this concrete layer the drill mud will be contaminated by the chemicals inside the concrete and has to be exchanged. This happens after every casing run. Furthermore, the quantity of circulating drill mud has to be continuously increased with increasing depth as the drill hole volume increases. The drilling continues to -200m. The pump chamber will be constructed from - 60 m to - 200m. From - 200m onwards the entire drill rod will be installed, the drill head exchanged and a casing with inch installed. The annular space will be concreted and after the hardening is completed the drilling will continue with inch. It is not essential to concrete the entire annulus. It is quite often sufficient to concrete the uppermost and lowermost casing section and to fill up the other space with gravel or similar. The diameter of inch will be kept until the final depth is reached. The final depth depends on the fact at which depth the required aquifer is found. When recovering the drill string the drill bit get stuck repeatedly. This can be explained by the existing geology. Several dry clay layers were drilled through which start to swell immediately once they get in touch with the water from the drill mud. The composition of the drill mud should avoid exactly that, but since the quality of the water for mixing changes due to bacterial and mineral contamination, Screens with diameter 0.8mm there are always some small swellings. When the drill bit is now pulled up past the clay tapers, some material gets shaved off from the walls. This material then falls down to the ground of the borehole and changes the final depth. Due to the fact that the final depth is very important for the calculation & well installation, the drill string has to be reinstalled in order to flush out the residual material with the drill mud. After the borehole flushing, the drill bit is brought up for a second time it has enough space and does not spoil the well any further. From this drill section onwards the casings are left in the ground as part of the lining. They have the same diameter as the screens. The lowest 5m are filled with gravel (back filling) in order to have a solid underlayment for the well installation. The gravel is just poured into the borehole from above. The necessary volume can easily be calculated by circular area multiplied by the height with the diameter of the borehole end. The filter screens are put onto the gravel later. These pipes with 0.8mm wide slots allow the water from the aquifer layer to enter the inside of the casing. A total of 62m of these screens is installed in order to give the water enough space to enter. No further casings will be installed in this section. Screens with centraliser and Teflon-sealing tape BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

6 The well installation First the sump pipe is run into the borehole. It serves as spacer which should avoid that rests of clay adhere to the screens. After that the screens are attached to the installation string. The centralizers are installed at the changeovers in order to ensure an even distance to the wall. The screens consist of stainless steel and have a diameter of inch. They are screwed onto each other and are sealed off by a Teflon sealing tape. Due to the friction of the upcoming water at the screen there is a charge that could reach the pump chamber via the pipes and could create complications for the pump operation. That s why a dielectric crossing made of fiber glass is screwed onto the last screen in order to keep the charge away from the pump chamber. The module has one connection for the screens and a bigger thread in order to connect the casings to the drill string. For this small diameter the casings consist of solid plastic (PVC-U) and are connected by screw threads. The water can move upwards inside these casings. The opened-up aquifer is under enormous pressure and produces artesic wells to -80m before pressure equalization is reached. The pump can be installed relatively far up on -160m although the aquifer is only reached at -550m. The pressure of the aquifer is constant and sufficient for longer pump operation. The ground water of the aquifer has a temperature of approximately 55 degrees. The sequence of screens, casings and modules described above is approx. 350m long, from the bottom of the borehole at -550m to the lower end of the pump chamber (-200m). In order to put this installation onto the bottom of the borehole, the drill string is extended with normal drill rods without rotation until it reaches the borehole bottom. Now all pipes and casings are in place and remain there whilst the well installation is carried out. The space with the borehole wall and the screen (annulus) will be filled with filter sand. The filter sand has a grain size of 1 2 mm and is adequately selected for the openings of the screens (0,8 mm). The sand filters impurities from the water and avoids that clay gets adhered to the openings and causes the decrease of the screen permeability The filter gravel volume calculates between 6.5 inch screens and inch borehole over the screen length plus 20% safety factor. BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

7 After the filter sand has had time to set, a clay seal with a layer of compactonit pellets will now be put on top of the filter gravel. This serves for sealing off the annulus to the top and to protect the production zone of the well from any negative influences from above. After sufficient swelling time the annulus will be further filled with coarse gravel until the filling level reaches the under 30m mark below the pump chamber (- 230 m). The relevant filling level is being measured with a dipper. The remaining 30m of the annulus are filled with concrete by means of tremie rods. The back filling with gravel and the concreting provide the necessary stability of the borehole. As soon as the concrete is hard, the installation of the well is completed and the phase of well development can start. Well Development Well citric acid is poured into the pump chamber for disinfection purposes as the water in it is still contaminated from previous works with drill mud, concrete and lubricants as well as surface dirt. The water is then slowly pumped to create a flow through the screens via the drill string to the surface. The filter sand and the geology in the area of the aquifer first have to adjust to the new flow conditions and align accordingly. This procedure takes approximately 2 days whereby the pump capacity / permeability is permanently increased. During the well development the undocking of the lower well installation from the drill string finally takes place. BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

8 Borehole DW#04 during upcoming sandstorm BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.

9 Design of first well BAUER Technologies (PTY) Ltd. BAUER Technologies (PTY) Ltd.