Offshore Technology International 2010 volume 2
Managing Mud While Mitigating Impacts to Real Estate, the Environment and the Budget
Michael Rai Anderson, PE - KEM-TRON Technologies, Inc. President & David Reardon, KEM-TRON Technologies, Inc. Performance Chemicals and Applications Director
The same principles that apply when operating liquid-solids separation equipment on an offshore drilling rig with unweighted water-based drilling fluids during surface drilling, should be considered as important as the intervals that require a weighted drilling fluid containing barite and expensive base fluids. The principals are not new, but the specification and use of proper solids control equipment can be easily overlooked, especially on jack-up drilling rigs that have not been built or refurbished during the last ten years.
Recent developments in linear motion shakers and screens have improved separation of drilled solids greater than 74 microns at the flowline. Consequently, these technologies continue to lose operating efficiency when they encounter high solids loading experienced during surface drilling. Even with the addition of hydrocyclone technology, a large percentage of drill solids remain in the drilling fluid during surface drilling. These drill cuttings are ground into colloidal, or clay size solids, unless removed by a decanter centrifuge capable of removing ultra-fine solids greater than two microns. Drilled solids less than two microns will remain in the drilling fluid reclaimed by a “barite recovery centrifuge” and accumulate unless performance chemicals are used. Polymers are required to destabilize their suspension characteristics and create a “floc” that can be removed by means of a “high G-force” decanter centrifuge. The rig’s mud report may not indicate a change in the solids content when particles 2 microns accumulate. Consequently, their influence in mud density, viscosity, bit penetration rates, drill pipe torque-drag, and disposal of whole mud is definitely apparent.
Currently, a popular alternative to the use of decanter centrifuges to manage unweighted drilling fluids is the dumping of whole mud. When this is done, base fluid and chemicals must be added to make up new volume and maintain viscosity and rheology. This method is not only wasteful, but causes well monitoring problems when pit volume totalisers are not recalibrated due to rapid mud loss, or gains.
The goals of solids control encompass much more than simply improving bit penetration rates and down-hole equipment life. It is imperative that the goals include drilling cost and waste minimisation. All of these goals can efficiently and cost-affordably be achieved from a properly specified solids control system incorporating centrifuge technology.
In a recent case study, KEM-TRON Technologies, Inc. installed a barite recovery decanter centrifuge with a variable frequency drive (“VFD”) and a high G-force decanter centrifuge on a 28 year old BMC-200IC jack-up drilling rig operating in the Arabian Gulf. The decanter centrifuge skids were mounted next to mud cleaner/flowline shaker, degasser, and two primary flowline shakers; all of which were installed on top of a partitioned mud processing tank measuring 1,289 cm (504”) x 315 cm (124”) x 264 cm (104”). A multidirectional screw conveyor was located beneath the barite recovery centrifuge. This conveyor allowed the barite to drop into an agitated tank, or allowed the drill solids to be conveyed overboard, when the barite recovery centrifuge was operated as a secondary high g-force centrifuge. The VFD was used to precisely adjust the bowl speed of the decanter centrifuge for barite recovery at 2,000 rpm (795g) or a secondary high g-force centrifuge setting of 2,900 rpm (1,672g). This was done without changing motor sheaves or incurring the high maintenance typically involved with hydraulic drives. The high g-force decanter centrifuge was set up with a mechanical variable drive using a 27.1 cm (10.6”) motor sheave that generated 2,113g at 3,260 rpm.
Prior to commissioning both centrifuges, it was reported by the drilling fluids specialist that dumping and preparation of new weighted synthetic drilling fluid was routine while drilling a 40.9 cm (16”) surface hole with a seawater/XC polymer system. The net result of this method is the limited removal of drill solids at a relatively high cost. This is especially the case when considering the current cost per barrel of a seawater/XC polymer system being approximately $20 USD. Moreover, this cost is independent of the cost incurred for waste disposal and the environmental impact.
The centrifuges were then commissioned to maintain a seawater/ XC polymer mud density between 70 to 72 pounds/cubic foot (9.4 to 9.6 pounds/ gallon), while drilling at rate of 15.2 metres/hour (50 feet/hour).
The periodic operation of the high g-force centrifuge, processing 1.6 litres/ second (25 gallons/minute), was more than sufficient to stop dumping of whole mud into the Arabian Gulf and preparation of new mud. Additionally, this configuration allowed operators to maintain the required mud weight while drilling the remaining portion of the surface hole using the seawater/XC polymer mud system. The high g-force centrifuge was able to efficiently make a 5.5 pound/cubic feet (0.7 pound/ gallon) cut in mud weight. The use of seawater as a base fluid with XC polymer is particularly beneficial in removing drill solids from mud with a high g-force centrifuge due to cation exchange occurring within the clays/silt particles. This effect causes the suspension to become destabilised as a result of the XC polymer’s anionic characteristics.
Due to the sensitivity of performance chemicals, it is highly recommended that a polymer conditioning (i.e. calibrated hydration system) be utilised to inject and monitor dewatering chemicals into drilling fluid prior to entering a high g-force centrifuge. This is particularly the case if the operator’s goal is to maintain a lower mud weight while drilling with an unweighted drilling fluid, or reclamation of seawater, fresh water, or potassium chloride (KCl) brine without any colloidal and ultra-fine solids.
The barite recovery and high g-force decanter centrifuges were operated in tandem after displacement of the seawater/XC polymer drilling fluid with a 79 pound/cubic foot (10.5 pound/ gallon) synthetic base drilling fluid. Both centrifuges were used periodically, during each day, while drilling a 311 mm (12.5”) hole with minimum pump rates. The decanter centrifuges were used whenever the mud weight needed to be cut back from 83 pounds/cubic foot (11.1 pounds/gallon) to 79 pounds/cubic foot (10.5 pounds/gallon). It would take no more than three hours to achieve this cut.
The barite recovery centrifuge was supplied 83 pounds/cubic foot (11.1 pounds/gallon) synthetic base fluid at a pump feed rate of 1.8 litres/second (28 gallons/minute). Recovered Barite was discharged directly into a stirred process tank below. The liquid phase, weighing 67 pounds/cubic foot (9.0 pounds/ gallon), was discharged to a catch tank for supplying the high G-force centrifuge at feed rate of 1.7 litres/second (27 gallons/minute) using a progressive cavity pump. The liquid phase the high g-force decanter centrifuge had a mud weight of 65 pounds/cubic foot (8.7 pounds/gallon) and diverted toward the barite discharged from the barite recovery centrifuge. The ultra-fine solids cut from the high g-force decanter centrifuge slid down a trough and conveyed overboard.
Upon final commissioning, the barite recovery centrifuge and high g-force centrifuge safely maintained the mud density and viscosity values within operating requirements. Of key importance is the fact that their use eliminated the need of dumping of whole mud into the ocean and wasting of chemicals used to prepare new mud when using both unweighted and weighted drilling fluids.
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