Deep drilling into the earth’s crust is carried out both for commercial reasons, such as exploration and production of natural resources like crude oil and natural gas. And also for scientific reasons, such as investigating the composition of the earth’s crust or to answer geothermal and geological questions. Dr Thorsten Arnhold reports.
Development of crude oil or natural gas deposits is always preceded by long drawn-out systematic investigations. After a geophysical investigation of large areas, which frequently extend over several thousand square metres by seismic measurements, the promising structures discovered are further investigated by deep drilling into the geological structures. Frequently, hundreds of speculative deep drilling operations are required in order to discover only a few deposits worth developing.
Since the raw materials sought are flammable liquids and gases, the above-ground installation is considered as a hazardous area and must be equipped with explosion protected equipment in accordance with the zone classification. Owing to the hardness of the rock, the immense pressure and the high temperatures in the lithosphere, extreme requirements are made of the tools for drilling into our planet’s lithosphere. Ultimately however, drilling into several hundred metres of rock functions on the basis of the same principle as drilling into a piece of metal with a pillar drill. The drilling derrick is the drilling machine: it accommodates the drive for the drill bit and supports the drill stem. This stem must be withdrawn fully from the bore hole each drilling in order to change the drill bit.
Two types of drill bits are used: firstly, cone bits on which several steel cones tipped with carbide studs rotate against each other and which are used chiefly to achieve drilling depths. The second type is the diamond drill bit, which looks like a toy castle, the ‘crenellated’ turrets crowned with diamond chips. The carbide metal of the crenellation wears away over time and diamond chips break off, but these are replaced immediately by new sharp chips which emerge. One important component of the drilling system is the well fluid. The task of conditioning the well fluid will be discussed in the context of explosion protection.
The job of the well fluid
When drilling, particular attention is paid to the well fluid, since it is required to perform a number of tasks. The most important are as follows:
* Driving, lubricating and cooling the drill bit;
* Transporting the drilled rock to the surface;
* Maintaining the stability of the bore hole;
* Sealing the bore hole against the penetration of substances such as oil, gas or water;
* Transmitting information on the rock.
The well fluid must have quite specific properties matched precisely to the rock, changing as little as possible during use in order to be able to perform these tasks precisely. The well fluid must thus be conditioned in such a manner that its composition and properties remain stable.
The well fluid is influenced only by the drilled solids which it takes up – apart from thermal or, under certain circumstances, chemical influences, eg when drilling into salt. Well fluid can be constantly replaced, it can be conditioned continuously, ie cleaned to remove the drilled solids, in order to reverse changes to the fluid composition.
In basic terms, the fluids used can be subdivided into oil-based and water-based fluids and these, in turn, can be subdivided into unweighted or weighted fluids. Since the fluid contains a wide variety of additives for achieving specific properties, such as specific gravity, viscosity and water loss, the rock drilled can be removed only by precise classification of the solids contained in the fluid. Continuous conditioning of the well fluids is thus a complex, multi-stage process.
Normally, so-called mud pits are used as a storage area for storage of the well fluid for onshore drilling. However, today’s environmental protection legislation makes it more and more difficult to provide adequately large mud pits for storage of well fluids. In addition, it must be borne in mind that these pits ultimately need to be eliminated fully again. Consequently, storage of well fluid incurs substantial costs. In the case of offshore installations, creating a storage area is, by the very nature of things, far more difficult. Frequently, it is necessary to transport the entire well fluid in tankers over large distances and to finally dispose of it after use. This incurs high costs.
Centrifuge installations which recycle the well fluid and greatly reduce well fluid consumption are therefore employed to reduce the need for storage space. This represents a substantial saving in storage and transport costs.
Solids are monitored and filtered using the centrifuge installation, improving the safety and speed of the drilling process, by reducing wear on the drill bit and avoiding of unnecessary entrainment of drilled rock. The centrifuge also helps provide more precise information on the composition of the drilled rock strata.
With all types of well fluid, the task of the centrifuge installation can be described as separating off a specific solids fraction from the fluid and thus restoring the characteristic parameters of specific gravity and viscosity.
The liquid phase is recovered and solids rejected for low weighted or unweighted fluids. In the case of highly weighted fluids, the ballast material can be recovered by and rejecting remaining solids rejected. Conditioning well fluids is a multi-stage process, which is shown in simplified form in Fig.1.
Some of the drilled solids are separated off via vibratory screens. Currently, vibratory screens are available which can be used industrially with screen plates approximately up to 125µm. The efficacy of these screens for separation of solids lies in the order of about 10–30percent for drilled solids.
For many years Hydrocyclones have proven successful for removal of further solids from the fluid. These installations allow the entire fluid stream to be treated. Centrifugal force in the hydrocyclones separates off solid particles with a size down to 10µm dependent on grain size, specific gravity of the particles, viscosity and specific gravity of the fluid in the underflow. One disadvantage of process stage is that a substantial quantity of fluid is also separated off together with the solids.
The disadvantages of a conventional desilting installation can be eliminated by downstream connection of a solid-bowl scroll-type centrifuge (Fig. 2). The desilting underflow is separated in the centrifuge into the recovered fluid and the relatively highly concentrated solid.
Central to the decanter centrifuge is the rotating part which consists of a cylindrical-conical bowl (2) containing a conveyor scroll (6) which rotates at a differential speed.
The rotating part is belt – driven by electric motors. Feed enters the bowl through a central feed pipe (1). The feed enters into the scroll body through the ports and passes into the bowl where separation by centrifugal force takes place.
Inside the bowl the product is separated into a liquid phase (3) and a solid phase (5). The discharge of the liquid is driven by gravity (4). The separated solids are conveyed by the scroll to the conical end of the bowl (7) where they are spun off into the solids housing (8).
A highly weighted fluid, separated off by the centrifuge contains the highest share of valuable barite which determines the fluid properties. This barite should be fed back to the fluid. For a weakly or unweighted fluid, the solid separated off by the centrifuge contains almost only drilled rock and is thus discarded.
The liquid separated off is fed back to the fluid at this point. Flocculation aids are increasingly used as a separation aid for cleaning well fluids. Using flocculation aids all solids contained in the fluid can be removed. The decanter is able to achieve separation of the particles down to 2µm from the fluid.
Centrifuges are used on the highly weighted fluids to recover the highest share of the valuable barite for recycling into the circulating fluid. This is achieved by using two consecutively connected centrifuges. The heavy spar is separated off as recyclable solid in the first stage of the centrifuge installation.
The liquid phase is then cleaned and fed back to the fluid in the second stage of the centrifuge installation. The fine particles separated off with the smallest grain share of heavy spar are discarded from the solids. These installations can also be used for unweighted fluids.
The entire above-ground section of the production installation, including the installation for conditioning the well fluid, must be classified as a hazardous area. Fig.3 shows a centrifuge (decanter) manufactured by Flottweg GmbH & Co. KGaA. Flottweg is a Bavarian company founded in 1932 which has been developing centrifuges, including decanters for the chemical and petrochemical industries for over 50 years.
The electrical control interface for the installation, including the decanter bowl speed, the differential speed and the pit depth, can be seen in the foreground. The lower section is the flameproof enclosure, accommodating the motor protection switches, control transformers, contactor relays and miniature circuit breakers.
A 100W heating plate prevents formation of condensation inside the flameproof enclosure, which may be caused by major fluctuations in ambient temperature, controlled by a thermostat. The flameproof enclosure also contains a level tester, and associated electrical apparatus, with intrinsically safe inputs and outputs for determining the fluid height inside the barite conditioning installation.
The control box above the flameproof enclosure is increased safety 'e' protected, and contains the flameproof main switch, various control switches and push buttons, in addition to the emergency stop pushbutton. Indicator lamps and ammeters signal the operating states of the installation.
The overall control panel has the explosion protection marked EExde[ia]IIBT6. A motor with the explosion protection marking EExdeIICT4 is used to drive the hydraulics. After production at a location is complete, the entire installation is dismantled and loaded into separable individual modules onto trucks for further transportation to the next drilling location.
In addition to the withstanding the rough operating conditions during drilling, the installations, the installation must be easily transported. In order to reduce assembly and dismantling times, all electrical installations are interconnected using means of explosion protected plugs and sockets.
The individual installation modules must fit within the typical overall dimensions of 20 individual 40ft containers. This saves costs because normal transport can be used. The weight also plays a particularly critical role. It was thus possible to save space and 350kg weight in comparison with the previous solution using the new CubExIIB modular flameproof enclosure system made by R Stahl because of the compact design of the control panel.
At drilling locations operated with unweighted fresh water or salt water well fluids, most of the drilled solids that were not separated by the vibratory screen were successfully removed from the fluid with the desilter – Flottweg centrifuge system.
About 1.5 metric tons of drilled solids were separated off per cubic metre centrifuge discharge, depending on the size of the centrifuge and the throughput. In general, the volume concentration of the centrifuge discharge was around 50–60% by-volume of solids.
The specific gravity of the centrifuge discharge was in the order of about. 2.0kg/l or above. At various drilling locations, it has been possible to date to dispense either partially or entirely with transporting away exchanged well fluid owing to the desilter and downstream fluid centrifuge combination. Only the solids occurring in the centrifuge discharge with a remaining share of adhering well fluid had to be transported away into the mud pits.
Dr Thorsten Arnhold is with R Stahl Schaltgeräte GmbH, Waldenburg, Germany. www.stahl.de