SDMM - Operations I

SDMM manufacturers provide a certain guidelines which is to followed while its operation.

The operational limit of parameters such as

1. Flow Range
2. Bend Set
3. Critical RPM
4. Max. Differential Pressure & Torque
5. Temperature Ranges
6. Suitable bit sizes to be run along-with

must be strictly adhered. Servicing and parts replacement must also be done with strict adherence to the laid down procedures by manufacturer; these steps if followed can give maximum performance and enhance SDMM life-cycle.

Following are the set of Operating Procedures that's common in the operation of most SDMM:

Run Preparation

• Perform a visual check prior to running in the SDMM.
• Check connection threads and face at both ends for any damage.
• Take Fishing Diagram (It's a detailed physical description of the tubular in terms of OD, ID & length of the tubular).
• Use proper protectors and make sure they are properly/ completely in place while pick-up or lay-down. Sometimes half open or half closed metal protectors can damage threads while picking-up or laying-down SDMM.
• Use proper lifting subs and apply sufficient dope while torquing threads.
• Arrange for proper size of safety clamps and slips to be used for the SDMM.

Rig Site Surface Testing

1. Make up SDMM with suitable bit size by applying recommended torque. Set the motor in the slips and install a safety clamp. Remove the lift sub and make up the Kelly/top drive (use x-overs if required). Remove the safety clamp and slips.
2. Check for the SDMM bearing gap. It's the gap difference between off-load (SDMM hanging) and on-load (SDMM fully rested on rig floor). 
3. Record the readings. Measure and compare the same after the SDMM is pulled out of hole. This will give you an idea of the bearings and will help in deciding if SDMM can be re-run or not.
4. Lower SDMM just below rotary table and just above riser such that bit and bit-box is visible. 

(a). For SDMM with no Dump-Sub

• Switch on mud pumps slowly and as required so as to avoid excessive vibrations. 
• Check if bit is rotating.
• Check if mud is flowing from nozzles or from both, nozzles and bearing gap. (For SDMM with oil sealed bearings it must flow from only nozzles and for Mud-lubricated bearings some amount of mud must come from the bearing gap section above the bit box).
• Note circulation rate and pressure.

(b). For SDMM with Dump-Sub

      In addition to above perform steps given below.

• Start the rig pumps slowly; fluid should flow out of the dump sub ports.
• Increase the pump rate slowly until the dump sub closes.
• Leave the pumps running and make note of the circulation rate and stand pipe pressure when the dump sub closes. 
• With the pump running and the dump sub closed, check to ensure that there is no drill fluid leakage through the ports. It is advisable to increase the pump speed in two or three steps, to the maximum circulation rate expected down hole, and note the circulation rate and standpipe pressure in each case.

Running In

• If the Surface testing is OK. Resume running in.
• When using a bent sub angle in the adjustable housing, be careful in passing the motor through the blowout preventer, casing shoes, liner hangers, ledges, or key seats to ensure that the motor or drill bit does not hang up. Do not run into bottom, or bottom fill, as it could plug the bit or damage the motor.
• If there's a long section of open hole then, it's advisable to make up Kelly/Top drive near casing shoe (preferably bit to be out side the shoe) and establish circulation. Note the stand-pipe pressure at different circulation rates. 

Note:

1. In doing so string get filled completely. Compare the stand-pipe pressure at a circulation rate with those calculated from hydraulics. If found OK this makes sure that none of the nozzles were choked while running-in through casing and there's no restriction in the drill string.
2. When ever there's an abrupt pressure drop or increase, bit can be pulled out up to casing shoe and then SDMM can be tested at the same circulation rates and stand-pipe pressure can be compared with the previous test that was done near the casing shoe to make sure that there's no problem with SDMM. This avoids any open-hole complications. The problem can be analyzed and further course of action can be decided. 

 Always remember PRECAUTION IS BETTER THAN CURE!!!

Running SDMM in a High Bottom-Hole Temperature

When running into a hot hole, it is important to gradually warm the motor during its descent.

Manufacturers provide with the max temperature rating within which SDMM can be operated without damaging the elastomers.

1. Run in hole and stop at the depth where the expected down hole temperatures are in the range of 115°C to 125°C.
2. Stop and pump drilling fluid to cool the motor.
3. Pump for about three minutes every 400-500ft until you reach bottom.
4. At bottom start at around half the de-rated pressure and work up to the max de-rated pressure over thirty minutes. 

Note: Avoid long periods without circulation if possible.

Starting SDMM

• Make-up Kelly/ Top Drive at least 70ft - 100ft above bottom.
• Start at the slowest circulation rate (similar to that at surface testing) to establish circulation with string in reciprocation (Up-Down movement). The pressure will be higher due to the restrictions of the drill string components added. The off bottom pressures noted may be higher than calculated. This is caused by bit drag on the side of the hole due to the bent sub, adjustable housing angle, and stabilization.
• Avoid rotation of drill string unless circulation is established.
• After establishing circulation, increase it further slowly. Circulate 15-20 mins.
• Now, increase the circulation rate along-with reciprocation and rotation as required.

Drilling

After a short hole-cleaning circulation period, slowly lower the bit to bottom. When bottom is tagged, the standpipe pressure gauge will show an immediate increase. Increase the bit weight slowly to achieve the desired build up rate and/or rate of penetration. Do not exceed the recommended maximum differential pressure across the motor.

Differential Pressure = (Off-bottom pressure) - (On-bottom pressure)

The off-bottom pressure is the total system pressure (read on the stand pipe gauge), from the standpipe, through the drill string, the annulus, and back to the drilling nipple, while circulating with the bit off-bottom (i.e. zero weight on bit).

The on-bottom pressure is the total pressure system read when some weight on the bit is applied. As weight on the bit increases, pressure also increases. The pressure increase is due to the restriction in front of the bit nozzles.

The torque applied to the bit while on-bottom is directly proportional to the differential pressure (difference between the on-bottom and off-bottom pressures). An increase in the weight on bit produces an increase in torque. As the bit drills off, the weight on bit decreases and correspondingly the pressure and torque decrease. The standpipe pressure gauge can therefore be used as a torque indicator.

There are two modes of drilling with SDMM

1. Rotary Drilling
2. Sliding Drilling

In Sliding mode, drill string remains stationary and only bit box along with bit revolves. Sliding is done to Build or Drop or for Course-correction.

Since, flow rate causes bit rotation; bit rpm can be controlled by the flow rate. Refer to the manufacturer's specification for the operating range of flow rate.

Higher Flow Rate means Higher RPM and better ROP.

Bit RPM (sliding) = {Flow rate (in GPM) X RPG (Revolutions Per Gallon)} 

Sliding requires better attention and precaution on the part of directional driller. 

Since, the drill string is stationary you have a great chances of getting stuck!!! So, it's a good practice to reciprocate the string once or twice if there's a reducing trend in the differential pressure and ROP.

All parameters such as Flow rate, WOB, Drill-off rate, Differential Pressure, amount of sliding, Standpipe pressure, Drag and many more must be monitored and recorded closely.

In Rotary mode, drill string along with bit box and bit rotates.

Bit RPM (rotary) = {Flow rate (in GPM) X RPG (Revolutions Per Gallon)}  + Rotary RPM

Note:

Practically combination of both, rotary and sliding is used for drilling directional wells. Be it a building, holding or a dropping section.

Rotary RPM

• While rotating the drill string can provide benefits such as improved hole cleaning and reduced drag but, rotating a bent motor creates high bending stresses in the motor housings, and can eventually lead to fatigue fracture. This risk grows as the hole curvature increases and higher bend settings are used.
• Note that the practice of alternating between rotating and sliding through a curve will create micro-doglegs that are higher than what may be shown on a survey report. Rotating the drill string while subjected to bending loads produces fatigue loading on the motor. These bending loads can be produced even when using adjustable bend settings that are within the recommended values.
• The maximum recommended drill string rotational speed for most of the motors is 50-60 RPM. This 50-60 RPM limit helps extend the life of the tool by limiting fatigue cycles and helping to avoid excessive vibration and whirl. Rotary speed should be decreased as necessary based on local conditions. It may be feasible to exceed the 50 RPM limit if using an MWD system to monitor vibrations and collar RPM in real-time at the rig, so down hole vibrations can be mitigated.

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