SILDRIL System Engineering Guidelines

Transcription

1 SILDRIL System Engineering Guidelines SILDRIL is an inhibitive system formulated with soluble silicate to achieve maximum shale inhibition. The primary shale inhibitor in the system is the Silicate, which precipitates or gels on contact with divalent cations and lower ph in the formation, providing an effective water barrier, which prevents hydration and dispersion. The secondary Shale inhibition is achieved by cation exchange of the potassium with formation clays, reducing the swelling tendency of the clays by direct interference with the swelling mechanism. The shale inhibition mechanism of the SILDRIL system is basically a sealing of the shale surface with a silicate film. This silicate film prevents any further liquid (water) wetting of the Shale. The low ph and divalent cations like Ca 2+ and Mg 2+ are present between the clay layers and when the soluble silicate comes in contact with the low ph shale surface the divalent cations will precipitate the Silicate to form Silicate barrier. The SILDRIL system requires the potassium ion for optimum inhibition performance. Potassium chloride is the inorganic salt normally used in the SILDRIL system with the concentration normally varying from 3-15 w/w%, depending on individual fluid requirements. The system is designed for use with all Xanthan Gum polymers to provide optimum -Shear-Rate Viscosity (LSRV) for superior hole cleaning, Wellbore Stability, and optimum hydraulics. The inhibitive nature of the fluid results in large cuttings consequently the Yield Point and -Shear-Rate values should be higher than conventional polymer fluids to ensure proper hole cleaning. High, flat, fragile gel strengths are maintained to ensure solids suspension. It is important to note that, due to the highly inhibitive nature of the SILDRIL system, it is likely that incorporated drill solids will have little effect on rheology. Assuming MBT is within specification fluid viscosity is obtained almost entirely from polymer addition. Filtration control is readily maintained with all conventional filtration control polymers. Notes: 1. The freshly prepared solids-free fluid will exhibit relatively high fluid loss values, until drill solids have been incorporated. 2. The ph of the system is higher than conventional non-dispersed polymer fluids, ranging from 11.0 to It is important to understand that the high ph and alkalinity values are attributed to the alkaline silicate anion and not a hydroxyl anion. 3. A decrease in the ph or alkalinity is normally the result of the depletion of silicate (via polymerization, precipitation, and adsorption). The ph, alkalinities, and silicate analysis are used to monitor treatment requirements during the drilling. The use of caustic is not recommended to control the ph of the system. Caustic and soda ash, or potassium hydroxide and potassium carbonate, are used to treat pre-mix water prior to the addition of additives to remove hardness. Soda ash or potassium carbonate is also used to treat cement contamination. Silicate mud must be based around drill water or, if seawater must be used, calcium and magnesium ions must be treated out. 4. Due to the nature of the Silicate and its depletion potential when it comes in contact with metals (while shipment), it is recommended to check the Silicate volume % of the spike fluid prior to diluting it with fresh water then adjusting the Silicate to the planned concentration.

2 5. Silicate depletes in the presence of calcium ions. ph from cement is not detrimental to the system and it is recommended not to use sodium bicarbonate or citric acid. The carbonate ion will compete with silicate, removing the calcium ion, and reducing the SILDRIL concentration. Prior to drilling the next interval it will be necessary to restore the system to the required SILDRIL concentration correcting for silicate lost during cementing. 6. Cuttings integrity/quality over shakers should be closely watched for system performance. If tight spots are experienced while tripping or on connections, raising mud weight should be considered immediately. 7. KCl curve should be plotted based on the KCl field sample. The in and out silicate concentrations should be monitored on a regular basis while drilling to determine the rate of depletion and to regulate treatment requirements during drilling. The silicate and KCl concentrations should be monitored, as well as the ph and alkalinity, to ensure optimum product concentration and inhibition. 8. Use all available Solids Control Equipment (SCE) as efficiently as possible to minimize undesirable mud weight increases from drilled solids accumulation and to reduce dilution requirements. As a general guideline the MBT value should be maintained less than 17.5 ppb Bentonite equivalent. Dilution with premix and dumping the sand traps are recommended to reduce the MBT value. 9. Fresh/Drill water will be used for cleaning the shakers screens during connections or before tripping out. The Silicate will be precipitated by the divalent ions in seawater, reducing the concentration of Silicate in the system and blind off the screens surface (sea water must not be used for cleaning purposes). 10. The SILDRIL System will provide a gauge hole and, therefore, it is recommended to utilize wiper trips. The wiper trip is important to ensure the caliper of the wellbore. It is recommended to perform wiper trips every 300 meters drilled and circulate bottoms up before running back to bottom and commence drilling. Once the hole has been wiped it will remain in gauge assuming the correct mud weight is being used. Hazards Identification The ph of the SILDRIL system is buffered in the range (refer to the MSDS) and care in handling is necessary to prevent contact with skin and eyes. Safety goggles must be worn to prevent the occurrence of eye injuries, which can be serious unless treated without delay. Rubber gloves and suitable protective clothing must be worn to avoid contact with the skin. It is important to wear suitable chemical safety goggles when handling the pure Silicate to avoid eye contact with the material. Contact with the eyes can result in serious eye injury unless treated immediately by flushing the eye with fresh water for a period of 20 minutes. If contact with the product does occur, then the contact area should be flushed with large quantities of fresh water. Rubber Compatibility The SILDRIL system is an alkaline fluid and selection criteria for elastomers should be considered. The most commonly used elastomers for alkaline environment is highly saturated hydrogenated nitriles (Hydrogenated Nitrile Butadiene Rubbers- HNBR). HNBR elastomers have a temperature limit of 130 C, but are the most resistant and are preferred for silicate systems.

3 Silicate rig preparedness only Contamination Divalent Brine influx or calcium rich formation Logistics Handling large number of silicate drums Elastomers Incompatibility of surface and down hole elastomers Cementing Chemical incompatibility HSE High ph silicate Brine flow from formation or sweater accidentally added to mud system Silicate is has been shipped in 200lt drums Silicate mud has high ph which is not compatible with all elastomers Sodium Silicate can cause cement to flash set Silicate mud is unusual in having a ph in excess of 11.5 Depletion of silicate NPT Cost HSE issues NPT BOP sealing issues Tool failures - Loss of log data - trips NPT NPT due to premature thickening or setting Flash setting of cement Personnel injury while mixing Maintain overbalance Good drilling practices to avoid swabbing Ensure adequate silicate stocks are available Monitor Silicate % closely Ship silicate in tote tanks or in bulk Consultation with drilling and third party contractors. Regular inspection of elastomers Replace with suitable material (Nitrile) Wash all the equipment s with fresh water when it s not in use Conduct spacer / cement / mud compatibility tests. Run computer simulations to identify effective spacer regime (large volume to ensure adequate separation) Ensure crews are aware of HSE risks Correct PPE MSDS sheets at the AQ base, on work boats and at rig site Med Displacement to KCl/SILDRIL Use sea water or Pad mud to shallow test the LWD. Once firm cement is tagged, the hole will be displaced with xxx ppg KCl/SILDRIL. The displacement will be carried out by pumping xxx bbls fresh water followed by xxx bbls of high viscosity weighted SILDRIL, the mud weight of the spacer will be 1ppg higher than the planned mud weight. The returns will be recovered into separate tanks, then will be dumped overboard after displacement. Once clean mud is observed at shaker house, divert the flow to fill in the sand traps.

4 Continue to circulate and condition the mud to the planned specs. Note: Drilling Fluid Team should submit a displacement plan with full details (Steps, Roles and Responsibilities, Pit Management, Lessons Learned & Safety Requirements), then the displacement plan will be reviewed with Drilling Supv and Senior Tool Pusher before displacement time. Spacers Pump xxx bbls of drill water followed by +/- xxxx bbls (10% of displaced hole volume) high viscosity weighted KCl/SILDRIL ahead of the KCl/SILDRIL. The returns will be recovered into the surface tanks, then will be dumped after displacement. Containment and Surface Flow Paths All involved in the job should have a pre-job toolbox talk and positions and duties assigned. It is the mud engineer s responsibility that any redirection of flow, including switching pumps between pits, be performed as per plan. A pump schedule should be produced and used at the tool box meeting. Contingency shaker screens should be prepared and ready at the shakers in case down screening is required until the mud is properly sheared. Have at least one shaker dressed with coarse screens. Pump Schedule All volumes should be in barrels and strokes. Assumed stroke capacity must be noted. Choke and kill lines should be over displaced by one barrel at start of displacement. Identification A sharp contrast should be experienced between the drill water and the WBM pill (use ph indicator which gives deep red colour with SILDRIL). Engineering Guidelines and Logistics An initial xxxx bbls of double concentrated KCl/SILDRIL spike fluid will be prepared at base and shipped to the rig site. The spike fluid is formulated as follows: o xx % by weight KCl. o xx % by volume Silicate. o xx ppb M-I PAC UL. A 1,000 bbl volume of neat Silicate (80% by volume) will be prepared and shipped as a liquid bulk (Will mix more volume if required). The spike fluid will be diluted with fresh water and the necessary chemicals will be added to achieve the final planned parameters. (Check fresh water for any hardness and treat with Soda Ash if necessary to remove) The StressCage materials will be added at the final stage. Dilution with fresh water is recommended, however if not available, pre-treat any sea water to remove hardness with soda ash. A test of the silicate percentage should be performed when receiving the base fluid at the rig site, at completion of the mixing of the fluid in the pits, and immediately after displacing the well.

5 Start the displacement with 12 % Silicate, to ensure that there will be at least 10 % silicate after the displacement and keep it in the range % while drilling (minimum 10%). Run sieve analysis tests on a regular basis. To maintain the programmed PSD in the active mud, add graphite materials and Safecarb as required per each stand drilled LGS and MBT should be controlled at programmed levels by utilizing all available solids control equipment, and diluting the active system with fresh mud. Use the finest shaker screen size that is compatible with the StressCage application. Rheology will be measured and reported at 120 deg F. It is essential to maintain Yz >10 lb/100 ft 2 to ensure that the large cuttings obtained with the KCL/SILDRIL mud, are removed from the wellbore. The YP and LSYP will be the key factors for the hole cleaning specially with this big hole and large cuttings size. The rheology might increase sharply while drilling due to the increase of the MBT content, dumping and diluting the system with fresh mud is required to maintain the MBT within planned range. If a PWD measurement indicates high ECD due to poor hole cleaning, first response is to circulate bottoms up prior to pumping Super Sweep pills ( ppb). This will minimize the hole packing off risks due to cuttings load at the top of the Super Sweep pill. If losses occur, follow the WBM LCM decision tree (LCM Appendix). If the losses cannot be cured to sustainable levels, consider converting the system to KCl polymer mud which is less logistically demanding than SILDRIL. It is very important to monitor the depletion of the silicate while drilling and to maintain the required level in the active system. Any sudden changes in silicate depletion should immediately be reported to the company man. Maintain good communication with the geologist and mud loggers, to establish whether formation drilled is calcareous. These will cause a higher depletion rate than non-calcareous formations. While drilling, monitor the Silicate depletion rate at least every 6 hours, or every 200m. Recommended system maintenance every drilling hour (15-20mph) based on offset wells depletion rate: o Four drums of neat Sodium Silicate. o One big bag of KCl. Care should be taken when adding silicate directly to the active system to avoid excessive viscosity humps. Rig crew must be made aware of the high ph of the mud system and instructed in proper PPE. Drilling contractor must ensure that for pump-pistons and liners, compatible rubber fittings are used for the duration of this KCL/SILDRIL section. All downhole tools used for drilling this section should immediately be cleaned with drill water when back on surface. Rig floor tools such as slips must be flushed with drill water, NOT seawater, after coming in contact with silicate mud. All spills on deck etc. must be cleaned with drill water (fresh water) NOT seawater. During trips, it is important to keep shaker screens clean with drill water using high pressure gun. Do not let the screens at any time get covered with silicate, this will permanently blind off the screens. Sea water must not be used for cleaning purposes.

6 Ensure all solids control equipment is lined up for handling the overboard dumping of WBM cuttings. M-I SWACO Virtual Hydraulics to be daily run to assess mud properties and drilling parameters effects on hole cleaning. Also it will be used to calculate the maximum running speed to ensure minimal surge pressures prior to running liner. At section TD the hole will be swept with 100 bbls of Super Sweep pill (If required) and circulated clean prior pulling out of hole for running casing. Reserve mud should be checked every day to ensure KCl and silicate levels remaining within specifications. Ensure sufficient shale shaker screens are available for shakers. Ensure enough chemicals, Barite, Drill Water available prior to drilling this section. Make sure to flush head suction lines each interval and between trips as per instructions. Mud engineer will plot the most important properties on a daily basis, and submit the chart together with the mud report. Caution: Silicate fluids tend to precipitate (due to water evaporation and reaction with divalent ions) and become attached to elastomers and metal parts. The crystals that form are very hard and cement-like and can cause abrasive wear of rubber parts when in contact with each other. Pre/post flushing with drill water (not seawater) is recommended for any moving parts in contact with silicate & elastomers and should be included in the preventative maintenance program. Lesson Learned from Offset Wells Lesson Learned Was drilled out at xx ppg as per pore pressure prediction and took losses below xx Shoe. The mud weight was then reduced to xx and drilled ahead with slight dynamic losses until casing 1point where the mud weight ended the hole section xx ppg. Key learning is that Silicate Mud seals and inhibits the wellbore allowing a highly reactive shale to be drilled and cased off in 12 days. Flush all the rig pumps with fresh water when not in use (e.g. on trips) to avoid the formation of a glassy silicate film around the piston. It has been note that if the silicate level drops below 6% the mud rapidly deteriorates and becomes unmanageable in terms of MBT and rheology control. This was particularly evident on Well ZZ when bad weather prevented silicate getting to the rig and drilling was continued with ever decreasing silicate content. Additional lessons: 1. Rig did not have sufficient mud pump spares for the silicate section - had to mobilize more from other Drilling Co. 2. Check the rubbers in mixing system (Pumps, Agitators, etc...) and change to the NHBR seal on hopper, butter fly rubbers, seals of mixing pumps to ensure compatibility with silicate. 3. When using silicate mud and low volume is in the TT, it starts to foam showing an increasing slow gain when it s actually foaming, it s recommended to use one TT on one pump at a time.

7 4. Be careful while pulling big OD items through casing shoe. In more than one occasion, Under-reamer was reported not to be fully collapsed and could not be pulled inside previous shoe. Had to work it up to 2 hours until successfully getting it through. 5. Silicate Mud back flow over the top of casing potentially had caused RMS getting stuck on the casing. 1.5 hrs. were spent with the casing stationary inside the RMS while switching from casing running to HDLS. 6. Lost ~9 hrs. while trying to free casing and had to cut the RMS with gas torch at the end. 7. Try to minimize the time with the RMS stationary around casing. 8. Hang the casing on the elevators rather than slips if the operations had to stop for whatever reason 9. Options include installing manuals whilst picking up hanger or alternatively keep the auto slips in and prepare the equipment prior to picking up to ensure the time in slips is minimized