E-Line Perforating: Cable Upward Force vs Tool Weight
You are preparing to perforate a 12-ft interval at 3,100 m with a 22-ft e-line gun string on 0.32-in cable. The well has 3,500 psi shut-in tubing pressure and is filled with 9.0 ppg brine. The gun string weighs 220 lb in air. The lubricator is 25 ft long and rated 5,000 psi. The cable has a breaking strength of 1,800 lbf, and the wireline company requires a minimum 100 lb net downward force on the tool string before the cable is disconnected from the gun. The tool string includes a 3-ft cable head and a 2-ft firing head. The wellhead is a 7-in, 5,000 psi system. The pressure will be 3,500 psi at the wellhead when the gun is at the perforating depth.
1. Calculate the upward force on the cable at 3,500 psi and the net downward force on the tool string. Is the 100 lb minimum met?
Read the worked answer
Cable cross-sectional area = π/4 × 0.32² = 0.0804 in².
Upward force from wellhead pressure = 3,500 psi × 0.0804 in² = 281 lb.
Buoyancy factor for 9.0 ppg = 1 − 9.0/65.5 = 0.8626.
Buoyed tool weight = 220 lb × 0.8626 = 190 lb.
Net force = 190 lb down − 281 lb up = −91 lb, i.e. 91 lb upward.
The 100 lb minimum net downward force is not met. The tool string would tend to be pushed up the lubricator by wellhead pressure, which is a serious control problem when the cable is disconnected or the tool is near the wellhead.
2. How much additional weight must be added to achieve at least 100 lb net downward, and what other precautions are needed before running?
Read the worked answer
Required buoyed weight = upward force + 100 lb = 281 + 100 = 381 lb.
Required air weight = 381 / 0.8626 = 442 lb.
Additional weight = 442 − 220 = 222 lb. In practice, add at least 250 lb of weight bars so the minimum is met with margin.
Precautions:
- Check lubricator length. The tool string is 22 + 3 + 2 = 27 ft, which exceeds the 25-ft lubricator. A longer lubricator or a staged make-up is required so the whole string can be contained.
- Confirm the lubricator, BOP, and wellhead are rated above 3,500 psi and tested.
- Check cable strength and weak-point rating; 1,800 lbf breaking strength is adequate for the expected loads but the weak point must be set correctly.
- Use grease injection and a proper stuffing box for 3,500 psi.
3. If the wellhead pressure drops to 1,500 psi during the run, how does the net force change?
Read the worked answer
At 1,500 psi, upward force = 1,500 × 0.0804 = 121 lb.
With the original 220 lb air-weight string, buoyed weight is 190 lb, so net force = 190 − 121 = 69 lb downward. This is positive, but still below the 100 lb Minimum.
With 250 lb of added weight, air weight becomes 470 lb, buoyed weight = 470 × 0.8626 = 405 lb, and net downward force = 405 − 121 = 284 lb.
The lesson is that the net force changes with wellhead pressure. The string must be designed for the highest pressure expected during the job, not the pressure at the start of the run. A pressure drop makes the string heavier; a pressure increase makes it lighter and can push it upward.