Softwares are used to produce Drag charts on which is shown the measured and predicted weights vs. depth. The purpose is to provide the crew with the expected torque and drag during the drilling of each successive section of the well. This chart enables the driller to see at a glance what the torque and drag should be at a given depth. By comparing with the actual, problems can be identified early.
1. Data requirement
Every 90 ft Tripping out, tripping in, rotating off bottom weights and torque values should be collected. For each assy the data must be collected both in open hole and inside the previous casing.
Survey data are obtained daily from the Directional Driller.
Accurate description of the BHA and drill pipes is essential (grade, weight, etc.)
2. Wellplan Torque and Drag programme
For each set of hookload and torque data collected at the wellsite, and sent daily to the office, an average friction factor for hookload and an average friction factor for torque is calculated for this BHA.
3. Drag Charts
Drag charts are produced based on the average friction factors obtained from the previous assy. When a new assy is run in hole, a new chart is prepared based on the new configuration, to predict the expected hookloads and torques for this BHA in the next section of the well.
Drag charts present the predicted and measured weights versus depth for tripping in and out and rotating off bottom, but should also contain the minimum and the maximum allowable weights. The minimum weight corresponds to the onset of plastic deformation and the maximum to the onset of tensile failure.
4. How should Drag charts be used?
As the hole is drilled, the actual hookload and torque should be plotted on the chart by the wellsite engineer. Although it is unlikely that the prediction will match perfectly the actual, the deviations and the trends will provide valuable information on the hole condition.
The graph can be used to remind the driller of where cuttings could build up and provide valuable information for the following shift and can be used to assist the drillers in their handovers.
The drag chart do not explicitly identify a specific hole problem, but they remind the crew that a problem is developing. The data available should be used to identify what the problem is.
5. Interpretation
The hookload should be recorded while drilling ahead, but also while tripping. If the hookload does not decrease as expected, increasing amount of overpulls are being experienced and it is likely that the BHA is being pulled into something that could cause it to become stuck.
57% of stuck pipes occurs while pulling out of hole.
Effect of cuttings bed.
Angle less than 50-60 degree: Avalanching will probably occur. The hook load and torque will probably be higher than for the previous stand. The assy is being run in the hole and the cuttings are acting like roller bearings. It is run with less resistance and more weight is seen at surface than expected.
Angle above 50-60 degree: The cuttings bed is fairly stable, and the assy will have the tendency to push the cuttings in front of it. The weight will be lower than expected, and the friction factor higher.
Stabilisers
Torque is affected in a similar way, if the stabs are gouging into the formation, or are packed into a cutting bed, the torque can become erratic or higher than normal. This is because the stabs are in contact with a softer formation.
6. Friction factor calculation:
Torque and drag are affected by the following mechanisms:
- Wellbore geometry
- Cuttings bed
- Stabiloser gauge
- Key seats
- Mud
Generally, the friction factors for hookload and torque are different. Friction factors corresponding to tripping in and out are also different. To draw the hookload chart the friction factor to be used is the average of the friction factor found for tripping in and out.
By comparing the actual and calculated weights, the Torque and Drag Analysis programme will calculate a friction factor which will account for the difference in the two values. This value should be representative of the downhole conditions at the time the data was recorded.
Well plan Torque & Drag programme will calculate all the force imposed on the drillstring. For each component of the BHA the contact force is calculated and summed from the bit to surface and this is the value which represents the expected hookload at surface.
The drag due to normal downhole contact forces is known and any change from this calculated value is due to specific downhole conditions.
The friction factors used range from 0 to 1 and represent the portion of the drillstring weight that is converted to drag in the hole.
The programme compares the calculated and actual off bottom rotating weights. They should be identical. If they are different, there is either an error in the input of the data or a discrepancy between the actual weights/length of the components and the one used in the model. To solve this problem it is suggested to alter the block weight. This will make the free rotating weights the same without altering the weight downhole which would affect the drag.