FIG 2.4 TYPICAL MUD LOGGING SYSTEM
CONTINUOUS CIRCULATING TRIP TANK
The factors affecting swabbing and surging are:
• Pulling speed of pipe.
• Mud properties.
• Viscosity.
• Hole geometry.
Surging
Surging is when the bottom hole pressure is increased due to the effects of running the drill string too fast in the hole. Down hole mud losses may occur if care is not taken and fracture pressure is exceeded while RIH. Proper monitoring of the displacement volume with the trip tank is required at all times.
Figure 2.6
Swabbing is a recognised hazard whether it is “low" volume swabbing or “high”
volume swabbing. A small influx volume may be swabbed into the open hole section. The net decrease in hydrostatics due to this low density fluid will also be small. If the influx fluid is gas it can of course migrate and expand. The expansion may occur when there is little or no pipe left in the hole. The consequences of running pipe into the hole and into swabbed gas must also be considered.
Pulling Speeds
Tripping speeds must be controlled to reduce the possibility of swabbing. It is normal practice for the Mud Logger to run a swab and surge programme and to make this information available to the Driller. This will provide ample information
PRESSURE SURGES SWABBING ACTION
Mud Properties
Controlling the rheology of mud is important. Controlling water-loss to avoid thick wall cake will also help.
Trip Margin
A safety factor to provide an overbalance to compensate for swab pressure can be:
Trip Margin Factor APL psi ––––––––––––––––– = –––––––––––––––––
(psi/ft) True Vert. Depth. ft APL = Annulus Pressure Loss
If swabbing has been detected and the well is not flowing a non return valve should be installed and the bit returned to bottom. Flow check each stand. Once back on bottom the well should be circulated and the bottoms up sample checked for contamination.
If the well is flowing or the returns from the well are excessive when tripping in then the following should be carried out:
• Install a non return valve. If there is a strong flow then a kelly cock may have to be installed first.
• Shut the well in.
• Prepare for stripping.
• Strip in to bottom.
• Circulate the well, check bottoms up for contamination.
Continuous monitoring of replacement and displacement volumes is essential when performing tripping. A short wiper trip and circulating the well before pulling completely out of the hole will provide useful information about swabbing and pulling speeds.
Useful formulae for calculating the psi reduction per foot of drill pipe pulled are as follows:
(mud grad. (psi/ft) x metal disp. (bbls/ft)) Pulling Dry Pipe: psi/ft or dry pipe pulled = (casing cap. (bbls/ft) - metal disp. (bbls/ft))
(
–––––––--––––––––––––--–––––––––––––––)
(mud grad. (psi/ft) x metal disp. + cap. (bbls/ft) Pulling Wet Pipe: psi/ft or wet pipe pulled = (casing cap. (bbls/ft) - metal disp. + cap. (bbls/ft))
(
–––––––--–––––––-–––––-–––––––––-––––––)
2.3.3 LOSS OF CIRCULATION
Another cause for a kick to occur is the reduction of hydrostatic pressure through loss of drilling fluid to the formation during lost circulation. When this happens, the height of the mud column is shortened, thus decreasing the pressure on the bottom and at all other depths in the hole.
The amount the mud column can be shortened before taking a kick from a permeable zone can be calculated by dividing the mud gradient into the overbalance at the top of the permeable kick zone.
Overbalance (psi) H (ft) = ––––––––––––––––––––––
Mud Gradient (psi/ft)
2.3.4 INSUFFICIENT MUD WEIGHT
A kick can occur if a permeable formation is drilled which has a higher pressure than that exerted by the mud column. If the overpressurised formations have low permeability then traces of the formation fluid should be detected in the returns after circulating bottoms up. If the overpressured formations have a high
permeability then the risk is greater and the well should be shut-in as soon as flow is detected.
2.3.5 ABNORMAL PRESSURED FORMATIONS
A further cause of kicks from drilling accidentally into abnormally pressured permeable zones. This is because we had ignored the warning signals that occur, these help us detect abnormal pressures. Some of these warning signals are: an increased penetration rate, an increase in background gas or gas cutting of the mud, a decrease in shale density, an increase in cutting size, or an increase in flow-line temperature, etc.
In some areas, there were adequate sands that were continuous and open into the sea or to the surface. In these areas the water squeezed from the shale formations, travelled through the permeable sands and was released to the sea or to a surface outcrop. This de-watering allowed the formations to continue to compact and thereby increase their density.
Figure 2.7
In other areas, or at other times, the sands did not develop or were sealed by deposition of salt or other impervious formations, or by faulting such as we have indicated here. Although the shale water was squeezed, it could not escape. Since water is nearly incompressible, the shales could not compress past the point where the water in the shale started to bear the weight of the rock above. This section caused a condition in which the weight of the formation that is, the overburden -was borne not by the shale alone, but assisted by the fluids in the shale. In this situation the shale will have more porosity, and a lower density, than they would have had if the now pressured water had been allowed to escape. These
formations, both sand and shale, are then overpressured. If a hole is drilled into an overpressured formation, weighted mud will be required to hold back the fluids contained in the pore space.
Figure 2.8
Abnormally high formation pressure is defined as any formation pressure that is greater than the hydrostatic pressure of the water occupying the formation pore spaces. Abnormally high formation pressures are also termed surpressures, overpressures and sometimes geopressures. More often, they are simply called abnormal pressures.
PERMEABLE ZONE