Overpressure VS Underpressure Evidence During Exploration Drilling

One of the risk in  petroleum exploration drilling is a blowout. A blowout can cause a collapse of the rig hence to cause the fatality to people and  million dollar property loss. One cause of blowout is the formation overpressure that is late to anticipated. One way of anticipating this is by the work of wellsite geologist, a geologist who is  assigned in the drilling site to monitor geological data  or phenomena that appears on the borehole that is being drilled. The wellsite will monitor all the drilling parameter from the mudlogging unit and the borehole cutting that come out from the borehole.

Evidence of overpressure.
When the formation fluid pressure is greater than the mud pressure, the cuttings tend to explode into the well bore. Cuttings are liberated promptly into the mud with a minimum of abrasion by the bit. Consequently, the cutting have sharp edges and look fresh; they may even be somewhat larger than normal. This case also applies to shale , so we do not have to wait for mud kick from a permeable zone to anticpate significant overpressure. Cuttings tends to be large and fresh looking when they are from overpressured, low-permeability rocks because the pore pressure cannot readily bleed out of the pore system when permeability is low.

Evidence of underpressure
The formation underpressure  does not cause a well blowout but when it does not manage well the targetted oil reservoir may be damage and futher the oil can not be optimally produced. This is also a loss in the business investment.

When mud pressure exceeds formation fluid pressure, the mud tends to plaster the hole ( one of the things mud is designed to do ). An undesirable byproduct of this attribute is that cuttings are held on place by the pressure differential between the well bore and the formation such cuttings, even if broken away fram the host rock, are likely to be struck several time by the bit teeth before being carried away in the mud. This process is more likely to take place in permeable zones where ther is a finite flow of mud filtrate into the formation.

From the short story above, as a leader of exploration projects, I would appreciate and give credit to my operation and wellsite geologist who take care our  exploration well during the drilling. They are awake almost 24 hours a day when our drill bit approaching the exploration target or the anticipated abnormal pressure zone.  My wellsite geologist has to notice carefully the drilling parameter and well cuttings to see if we encounter the such cutting shape above.

JGA Wibowo


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