Oil-based drilling fluids have long been recognized as a sound technical answer to problems encountered in deep-hot holes and, more recently, to stabilize boreholes with thick intervals of reactive clay formations. However, oil-based mud product development was slowing down until the introduction of low toxicity base oils in the late seventies created a need for new investigations. This innovation sparked a resurgence of oil mud research and development which has resulted in oil-based mud (OBM) systems being used routinely in many active drilling areas worldwide.
Concerns for conservation, both economic and environmental, persist even after low toxicity oils have been accepted, based on field performance, as a suitable diesel replacement. These concerns are important to both the operator and contractor alike. Control of intangible costs related to mud expenditures is important to the operator, but rig modification and installation of auxiliary equipment to process the oil-based drilling fluid are equally important to both companies.
This paper discusses a variety of equipment that is currently available for both onshore and offshore locations to improve oil-based drilling fluid conditioning, to aid oil recovery, to reduce pollution, and to maintain a safe working pollution, and to maintain a safe working environment. Minor modification of most of these techniques will allow their utilization on land, inland barge, platform, jack-up, semi or floating drilling units.
Introduction
Field development of drilling fluid research efforts has always been oriented toward improving the financial and, more recently, environmental objectives of drilling mad production operations. New product and total system development combined with improved application of mud engineering technology will continue to have a positive impact on this cost containment effort well into the late eighties.
Future drilling activity in harsh environments will require improved drilling and logistics support equipment, better performance from consumables, and rigorous application of engineering principles to reduce the trouble time cost of each project. Selective application of oil-based muds has already contributed to this cost improvement on several recent projects in both the North Sea and the Gulf of projects in both the North Sea and the Gulf of Mexico.
Replacement of diesel with low toxicity oils as a babe in these mud systems has provided an unexpected benefit of increased rates of penetration with no appreciable increase in total mud cost. This benefit would be lost very quickly if efforts to reduce surface losses, to eliminate contamination to improve treating methods and to ensure a safe work site are not successful.
The following discussion will itemize various pi of equipment and techniques that can be utilized to handle these oil-based drilling fluids. Item selection for a specific application should be based upon the size and scope of that project.
SURFACE EQUIPMENT
Most drilling rigs require only slight modification of their rig floor equipment to be able to operate with oil-based muds WBML Mud buckets and pipe rack drains that can be directed back to the active system and textured (non-skid) metal surfaces around the rotary table are generally standard equipment on most land and offshore drilling units. [source]
Potable water wash-down systems have replaced high Potable water wash-down systems have replaced high pressure washers and the steam hose system on some pressure washers and the steam hose system on some rigs operating with low toxicity oil mud (LTOM), even in cold weather environments. This is due to the improved surface tension characteristics of the base oil. Excess mud can be removed easily leaving a thin film that dries much faster than the residue left by conventional diesel based systems. The result is a safer work area with less risk of minor injuries from slips and falls.
An alternative to handling a large volume of oil contaminated wash water on an offshore rig might be to utilize coarse ground oil absorbent material for mud accumulations not accessible by present drain systems. Removal is more labor intensive but the residue can be placed in containers designed to transport cuttings back to the shore base. Since this method offers no fluid recovery it may become cost prohibitive.
Drip pans and, in certain cases, installation of a pneumatic operated two piece U-slotted rubber wiper can be cost effective because the fluid saved can be returned to the active system without any chemical treatment necessary. If it is impractical to plumb the drain system back to the mud pits: then a buoyed internal drill string wiper will reduce the amount of OBM draining out of the drill pipe onto the pipe rack during a trip.
Active system processing equipment technology improvements have also contributed to cost effective drilling optimization. A standard set up should include high efficiency primary shakers, hydrocyolones, a low pressure mixing system, and properly sized agitators. Supplemental equipment may also include mud cleaners, centrifuges, and cuttings processing machinery.
Recent experience has shown that proper screen selection with a cascading arrangement of two shale shakers in series with two additional shakers has reduced the need for downstream solids separation equipment during normal drilling rate intervals. Fluid conductance and separation efficiency are greater with the low viscosity LTOM systems compared to mud prepared with the higher viscosity paraffin or diesel base oil.
This improvement combined with a 105 layered mesh screen in hard rock areas has eliminated the use of the sand trap. The mud flows directly into the degasser compartment. This procedure reduces the surface volume and, therefore, mud treating cost while maintaining new hole and trip volume in the active system. When the cascade arrangement is impossible, for high performance shaker can be used.
Mud tank arrangement and design are important. Although conversion of present rectangular tank systems on land rigs to a more efficient circular mud tank scheme might be cost prohibitive on a one well basis, long term usage will benefit the operator by reducing treatment costs for both water-based and oil-based drilling fluid systems. Furthermore, the contractor will also benefit from this design with potential reduction in down time due to maintenance problems during drilling operations, as well as with increased efficiency during rig moves. Trip tanks should be utilized and installed as suggested by Young.15 Floating units will obviously require covered tanks.
Operation of conventional vacuum degassing equipment has not been affected by the use of OBM systems. Recent experience with a unique design capable of removing gas from the mud in the flow line land rigs prior to the shakers has been successful. The only problem that can occur is the loss of water phase from the OBM whenever the flow-line temperature is above 150 degrees F (65.6 degrees C) and humidity is very low. Efficient operation of this equipment upstream of the shaker will prevent conventional measurements of mud gas in the possum belly. A successful alternative is to quantitatively measure the amount of gas with a differential pressure detector as the gas expands in the annulus at the surface.
Daily mud treating costs depend upon several Independent var1ables. These variables Include mud design and property specification, rock type and contaminants present, well depth and geothermal gradient, penetration rate and primary solids separation, pore pressure profile and mud density, surface volume and mixing equipment, and, finally, personnel treating philosophy and product quality. Controlling a majority of these items without maximizing the use of the mixing equipment may not reduce the daily cost to an acceptable level.
Agitator size and placement are ‘important. Variable position subsurface guns are very effective on OBM systems because they contribute to the emulsion stability by shearing the mud without aeration as with the chemical hopper. The addition of a high shear, low pressure pin mixer downstream of the conventional chemical hopper will increase dry and liquid chemical mixing efficiency with only minor rig modification.
All surface pits should be covered to prevent contamination from heavy rains or wash down runoff. Each inch (25.4 mm) of water in an 8 X 40 foot (2.4 X 12.2 m) tank represents 4.8 barrels (0.76 m3) of fluid that mud is a 16.0ppg (1917.2 kg/m3), 85/15 0/W ratio LTOM, Then it will require 27.2 barrels (4.3 m3) of oil, 23,000 pounds (10.432.7 kg) of barite, and chemicals at a cost of $4,665.00 to mix the water into system.
Another source of contamination is the water-lubricated packing on centrifugal pumps. Packing conversion may not be practical for system where LTOM use is intermittent; therefore, a short term solution might be to loosen the seal just enough to allow a very small amount of mud to weep past the packing around the shaft. Care must be exercised to contain this fluid (1.59m3) of mud in a 24 hour period is not as costly as treating the addition of 10barrels (1.59m3) of water.
A similar concern on land location is the type of storage tank required when the program calls for short distance rig moves and repeated use of an OBM after intermediate casing is run and cemented. The use of circular skid-mounted lay down tanks fitted with a gun line have proven to be cost effective in south Texas.
These gun lines can facilitate the conditioning of the mud with a cement pump truck circulating the fluid at least one hour for each mud tank prior to transfer to the rig pits. In certain cases the pump truck should also be used to transfer the mud rather than involve the time consuming operation of switching lines on vacuum trucks five different times per tank. The high shear also reduces the need for chemical treatment of the mud to restore the properties to pre-storage values prior to the displacement.
Selection of the proper elastomers is extremely important to both the operator and the contractor. The OBM should be specified with an aniline point above 150 degrees F (65.6 degrees C)and should be tested by the mud engineer before it is sent to the rig. This test procedure is described in API RP 13B. most OBM systems are above this value and present no problem to oil resistant elastomers but repeated use through formations containing light hydrocarbons may alter this value on the mud due to contamination. There is no practical way to raise the aniline point except through dilution with fresh base oil or removal of these components by evaporation.
Oil resistant elastomers should always be used when drilling with any LTOM system. The operating environment can also have a large influence on elastomer service life. High failure rates reported by rigs drilling in remote hot end arid climates could be the result of long term storage and exposure to Ultraviolet rays from the sun and not the type of base oil being used. Elastomer inventory shelf life can be improved by storing it in a paraffin content oil bath away from direct sun light. For convenience, use the same oil as the one utilized to formulate the LTOM.
Additional savings have been realized from the use of an industrial size vacuum cleaner designed to remove the volume of liquid mud from the mud tanks at the completion of the oil mud interval. The unit will efficiently pick up even high density or viscous fluids and collect them in a 10 barrel (1.59 m3) tank that can discharge the fluid into a permanent storage container.
CUTTINGS CLEANING SYSTEMS DESCRIPTION
The following is a brief description of various system designs that have been considered or actually used by the industry to process the drilled cuttings generated with oil-based drilling fluids. Whole mud disposal will not be considered here because Nesbitt and sanders have already presented a detailed discussion on the various methods of disposal for actual drilling fluid systems.
Spray Washing
This system begins with the cuttings being gravity fed onto a vibrating screen (shale shaker) where they are sprayed with a solvent or water-surfactant wash fluid. The cuttings are then dumped either overboard or through a flume pipe positioned to discharge just above the sea floor.
Immersion Washing
Cuttings processed by this method are dumped into a tank containing the wash fluid and agitated before being pumped over a screen separator. The wash fluid is usually treated mechanically with hydrocyclones or a centrifuge to remove the fines before it it returned to the main reservoir tank.
Vacuum distillation
This system removes the oil by batching the particles into a chamber where the temperature is increased to 700 degrees F and the internal pressure is lowered by a vacuum pump causing the oil to vaporize as the particle is ground down below a 100 micron size. The separator oil is condensed and reclaimed.
Briquetting
A prototype of this system has been tested but the results have not been published. This operation is based upon mechanically compressing the cuttings with an absorbent, Draining the excess raw mud off, and containing the remaining oil in a briquette sized particle for appropriate disposal.
Solvent Extraction
This method is still experimental. The theory involves reduction of the cutting particles size by grinding to expose a high percentage of the oil. The ground material is then conveyed to a vessel where it is treated with a liquid solvent under pressure. Next, the solvent and oil mixture is processed in a separate vessel where the solvent is evaporated off leaving the oil behind to be returned the active mud system. The solvent is then condensed for reuse.
Bacterial Degradation
The bacterial degradation process naturally occurs with organic material deposited on the ocean floor. Induced biochemical degradation of the oil associated with the drill cuttings allows the local discharge of the “oil frees” particles. The limiting factors of this disposal system are surface storage space end time.
Centrifugal separation
Formation particles are separated from the mud by the primary shakers, re-fluidized with the base oil and conveyed to a decanting centrifuge where the particles are separated and shipped to shore.
The immersion washing system is currently being used on LTOM in the Gulf Coast area with a water solution containing a high electrolyte content. It requires the least amount of rig modification to install and operate. Cuttings are put into 20 barrel containers that can be sealed and returned to a shore base for processing at an approved disposal site.
EGINEERING TECHNIQUES
The OBM displacement operation is critical to the successful use of these systems in any well. Improper handling that result in sever contamination from either the water-based mud (WBM) or formation fluids will be costly.
Only two scenarios are considered in this discussion. In each case the drilling operation will be resumed after the change out is complete. One is an open hole displacement. Both methods utilize a few common techniques and assume the pipe is not stationary and circulation is unrestricted.
Inside Casing (Continuous Displacement)
- After a successful leak of test, position the drill pipe on bottom and condition the WBM to reduce the viscosity.
- Stop circulation, clean the pits and fill them with OBM.
- Fill the slugging tank with WBM make-up water (volume required to cover 500 to 1000 feet of the annulus).
- Pump this water down the drill string, followed by OBM, at a pump rate designed for the drilling operation. Discard the returning WBM.
- When the OBM makes the turn at the bit, slow the pump rate down to 70 percent of the drilling pump rate.
- Reciprocate the string one joint every 15 minutes. Rotate the string at the string at 10 to 20 RPM.
Open Hole (running displacement)
- With the pipe on bottom, condition the WBM to reduce the viscosity.
- Clean one pit and fill it with OBM.
- Continue to circulate with the WBM.
- Fill the slugging tank with OBM and add 1 pound per barrel of oil wetting agent. Increase the yield point on this spacer to 35-4 lbs/100ft2 with a mixture of gallant and lime material
- Pump the spacer down the drill string at the normal drilling pump rate. Store or discard the returning WBM.
- When the spacer leaves the bit, slow the pump rate down to 70 percent of the drilling pump rate.
- Reciprocate the string one joint every 5 to 10 minutes while the spacer is in the open hold annulus. Do not rotate the pipe.
- When the spacer enters the casing, slow the reciprocating pipe movement down to every 25 to 30 minutes.
- When the spacer appear at the flow-line, shut the pumps down and short trip to the casing shoe. (A slug may be necessary if the spacer contains a large amount of WBM wall cake from the open hole.)
- Clean the pits and fill the active system with OBM.
In either case it is more efficient if the oil base mud is 1 or 2 pounds per gallon heavier than the mud being displaced.
Another useful technique that help analyze oil-based mud performance is to prepare a daily material balance worksheet on the system volumes.
General categories for the entry of data include interval footage and estimated volume of cuttings generated, system volume at the time of a trip (pits, hole, etc.), losses observed since the last reference point (trip), and gains to the system.
Gains can be from whole mud additions or liquid/chemical mixing for new volume. Liquids can be measured or metered and volume increases from dry materials can be calculated. It takes 1,470 pounds of barite or 900 pounds of dry chemical to make one barrel of volume.
The volume reduction category should be subdivided into several items. These include solids control equipment operation, down-hole seepage or filtration volume transferred to the reserve pits, or, the largest factor, that lost with the cuttings.
The following relationships are useful for planning purposes. With every barrel of 12.25inch rock bit cuttings you lose approximately 0.75 barrel of mud. The ratio increases to 1.25 for an 8.5 inch rock bit. PDC bit cutting are on the low side of these figures. Actual values for your situation can be verified in the field by retorting a standard weight sample and calculating the amount of mud on the cuttings from these results.
Our North Sea records indicate that it requires 1,000 barrel of LTOM to coat 9,000 feet of cuttings from a 13 inch hole. If diesel is used as the base oil, the coating volume discarded with the same amount of outings can increase to 2,000 barrels of OBM. Keeping accurate records on the mud system volume has contributed to a better understanding of the overall cost of using an oil-based drilling fluid.
SUMMARY
Preparation and modification of a drilling rig to handle oil-based drilling fluids does not require a large investment without any potential return. The type of expenditure, purchase or rental, should depend upon the length of time the additional equipment is needed. The proper selection of equipment and engineering techniques tailored for the scope of the project can optimize intangible cost, materials and labor, whether the rig is operating on a footage or day rate basis. Equipment installation during the rig fabrication phase can save even more money because it can also be utilized on water-based drilling fluids to reduce the total well cost.
