Before discussing drilling parameters practices, and guidelines for drilling performance optimization, a review of the basic concepts of Weight Indicator, Torque, and Limits is beneficial.
Basic Concepts Of Drilling Parameters Optimization For Oil & Gas Wells Operations
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The weight indicator, often referred to as the `Martin Decker‘, is the most important instrument for the driller in terms of drilling optimization, and avoiding Pipe Sticking. It measures the weight hanging from the crown block, provides clues to what is happening downhole, and allows him to operate within the limits of the equipment.
The following definitions are very important:
The value shown on the drillers weight indicator is defined as Measured Weight and includes the weight of the Traveling Equipment.
The weights observed when moving the String down and up are shown in the diagram below:
The Down Weight is the measured weight under Normal Hole Conditions when moving the string in a downwards direction, without rotation, and with the mud pumps shut off. The converse is Up Weight, i.e. when moving the drill string in an upwards direction.
Every driller is aware of the fact that the up weight reduces and that the down weight increases when he starts rotating the string. Torque and Drag In Drilling Optimization). The measured weight while rotating off the bottom without reciprocating is called Rotating Weight.
Down Drag is the difference between the down weight and rotating weight, whilst the difference between the up weight and rotating weight is called Up Drag.
Particular care is required when the measured weight is lower than the down weight or higher than the up weight because some form of restriction is then being encountered. In the former case, the difference between the measured weight and the down weight is called Setdown, while in the latter case the difference between the up weight and measured weight is referred to as Overpull.
The value shown on the driller’s torque indicator is defined as Measured Torque. Off the Bottom, Torque is measured when rotating with the string off the bottom.
Drilling Torque is the measured torque while drilling under normal hole conditions. Increases over the drilling torque are referred to as Incremental Torque.
Maximum Pull Limits
The upper limit of the measured weight is called Maximum Allowable Measured Weight. It is determined by the minimum of:
- The strength of the string (85% of pipe body yield strength corrected for torque).
- The pulling limit of the hoisting equipment.
Additional forces when jarring must be taken into account.
Minimum Pull Limits
The lower limit of measured weight is called Minimum Allowable Measured Weight, which is achieved:
- At the onset of plastic buckling deformation of the string.
- By slacking off the entire weight of the string if the plastic buckling limit is not reached.
Note that the limit for buckling in a stuck pipe condition, i.e. without rotation, is much lower than with rotation. In the latter case, a large allowance is made for fatigue.
The upper limit of right-hand measured torque is determined by the lowest of:
- Maximum allowable torque on drill pipe corrected for pull.
- Minimum make-up torque of any connection in the string.
- The maximum torque that can be generated by the rotary equipment.
Monitoring of Drilling Parameters & Performance
- Monitor closely penetration rate, rotary speed, torque, standpipe pressure, and pump stroke rate. Do this while reaming as well as when drilling a fresh hole. Ensure that the mud logging unit (or rig data system if appropriate) records all relevant parameters when drilling and when reaming.
- For better optimization, continuously compare the observed drilling performance (penetration rate, torque, standpipe pressure) and drilling cuttings interpretation with the prognosis for the well. Any discrepancies between the observed and anticipated performance should be evaluated and explained.
- Use the characteristics for different drillability problems to diagnose the probable cause of any lower-than-anticipated penetration rate. Changes in torque can aid the diagnosis of many drillability problems.
- Irregular torque can indicate interbedded formations, drilling stabilizers hanging up, keyseats or doglegs, excessive weight on drilling bit, bit going under gauge, and junk in the hole.
- Increased torque may be due to higher weight on bit, formation change (softer, higher porosity), increased inclination, thicker filter cake (potential for differential sticking), roller cone bit bearing failure, bit going under gauge or kick.
- Reduced torque may be due to lower weight on the bit, formation change (harder, lower porosity), running on a dislodged drill pipe rubber, cutting structure wear/damage, bit balling, or bottom balling.
- Standpipe pressure should normally be steady and should increase slowly with increasing hole depth. Increases in standpipe pressure may be due to annulus packing-off / stabilizers balling up, global balling of the bit body, ring-out of a PDC bit, inadequate hole cleaning, plugged nozzle or fluid passageway in a bit, PDM running at higher torque, mud viscosity increasing.
- Decreases in standpipe pressure may indicate a pipe washout in the drill string, a lost nozzle, lost circulation, aeration of the mud, lower torque at PDM, and wear/damage to the downhole motor/turbine.
- However, increases in pump rate can have the same causes as falling standpipe pressure, decreases in pump rate can have the same causes as increasing standpipe pressure.
General Drilling Parameters Optimization In Oil & Gas
- Operate at the weight on bit and rotary speed that give the highest penetration rate in the drill-off tests, unless significant drill string vibrations are seen.
- Avoid operating drilling parameters that lead to drill string vibrations. Use a rotary speed that is high enough to give smooth rotation of the drilling string, but not so high that axial and/or lateral drill string vibrations or bit whirl occur for better performance optimization.
- Do not apply excessive weight on bit if bit or bottom balling have been identified as potential problems, or if a shale is to be drilled with a water-based mud.
- Keep the drilling parameter “applied weight on drilling bit” smooth once it has been optimized. Feed weight continuously to the bit using the Driller’s electric brake (Elmago, Baylor etc) and avoid “slack-off / drill-off” – this can contribute to damaging torsional vibrations.
- The maximum allowable penetration rate, beyond which the ECD increases due to cuttings loading in the annulus exceeds the leak-off pressure, should be calculated if the penetration rate is high. Reduce the applied weight on bit and/or rotary speed as necessary to ensure that the penetration rate does not exceed the maximum allowable.
Monitor in Mud Gas
Be aware and fully appreciate that it is imperative that the gas levels in a drilling mud are correctly interpreted and the definitions are strictly adhered to. This is particularly critical during pipe tripping operations.
Pore pressure, or certain potential kick causes, can only be assessed accurately based on observations of trip gas, connection gas, swab gas, and pump-off gas. If any of these are observed then pore pressure levels are close to mud hydrostatic.
Increasing background gas levels can indicate increasing pore pressure if correctly determined and analyzed – it is important that the drilled gas level content of background gas is understood.
For definition purposes, the level of gas in the mud is due to one or a combination of the following:-
- Background Gas: The general level of gas carried by the mud purely as a function of circulating in an open hole.
- Drilled Gas: Gas which has entered the mud due to the actual drilling of the formation, i.e. the gas contained in the matrix of the rocks which have been drilled.
- Connection Gas: The gas which enters the mud when a connection is made due to reduction in hydrostatic due to loss of ECD and due to swabbing while pulling back.
- Swabbed Gas: The gas which enters the well due to swabbing . This may be caused by tripping or by simulating tripping.
- Trip Gas: The gas which enters the mud during a trip which is measured after a trip has taken place.
- Pump Off Gas: The gas which enters the mud due to turning off the mud pumps and removing ECD from the hydrostatic pressure on the bottom of the well.
Monitoring of Drilling Cuttings For Better Performance Optimization
- Cuttings samples should be collected regularly at the shakers.
- The collected cuttings should be examined and interpreted promptly after collection.
- They will confirm an expected formation change or indicate an unexpected formation change, can aid the interpretation of penetration rate and torque changes, and can help discriminate between different causes of reduced penetration rate, e.g. bit wear versus harder rock.
- The presence of large cavings can indicate mechanically induced wellbore instability problems and may indicate an increase in pore pressure gradient. The shape of cavings can help identify the mechanism that caused them to enter the well. The appropriate corrective action is influenced by the caving mechanism.
- The presence of rubber in the cuttings often indicates problems with the downhole motor. The presence of metal in the cuttings can indicate problems with the bit or downhole motor, or damage to the wellhead, casing or downhole equipment.
- The lag time is the time for cuttings to be transported from the bit to the surface. The lag depth is the depth from which the cuttings originate, and not the bit depth when they reach the shale shakers. Both the lag time and depth should be calculated and updated as often as feasible, to avoid misinterpretation of the depth of the formation from which the cutting samples originated.
- The effects of high penetration rates and borehole instability on the depth resolution of cuttings should be considered when concluding cuttings.
- The impact of over-gauge hole on lag time and depth should be considered if there is any potential for hole enlargement.
- Closely monitor shakers and drilling cuttings volume trend, in order to assess hole cleaning for better performance optimization.
Reference : ABC of Stuck Pipe, Shell U.K. Exploration and Production.