What is a drilling break?
Another name for a drilling break is a drilling jump. When drilling any well, the rate of penetration tells the driller just how fast or how slow the drilling process is going. For example, if we were drilling at 50ft per hour and suddenly we start drilling a new section at 200ft per hour, this sudden jump or increase in drilling rate is called the drilling break. It is important to note, all things being equal, that the deeper we drill vertically into the earth, the harder the formations are supposed to be since the overburden pressure from overlying formations increases with depth. Harder formations are more difficult to drill as compared to less compact formations. This is why it is important to take note when we suddenly begin drilling faster as we go deeper. Notice, that a drilling break is not a gradual process, it happens suddenly. This sudden jump is trying to tell us something about the section we are drilling through, if we pay close attention.
Why do we have drilling breaks?
A drilling break could occur just at the intersection between a hard rock and a soft rock. This is called a lithology change, meaning that there is a change in the type of rock we were drilling a while ago, and now we are drilling into a different rock type. The new rock being drilled may even be naturally fractured which is why we are drilling through it this fast. When a rock is naturally fractured or has cracks, it is like half the work has already been done for us, so applying the same drilling parameters we used at the previous rock type here will cause a sudden jump in the rate of penetration.
At other times, a drilling break could signify that we just drilled through the caprock (usually shale) and now drilling through the reservoir rock (sandstone or limestone for example). This is a good thing as we have finally gotten to the payzone. But this is not always the case, a drilling break is not always good news. It is also possible that the new zone we just drilled into is an overpressured zone, containing fluids at a pressure higher than the hydrostatic pressure of the drilling mud column. When we drill into zones with significantly higher pore pressure than what we anticipated (overpressured), we will also get a drilling break. So a drilling break could signify one of two things, a change in the type of rock or drilling into a rock with higher pore pressures. Other times, it could be drilling into a more porous rock that is also overpressured at the same time. This is why it is very necessary to stop drilling when we encounter a drilling break, and conduct a flow check to find out if the well is flowing or not. The result of the flowcheck will now help us in deciding if to quickly deploy a blowout preventer (drilling into an overpressured zone) or conduct some form of formation evaluation to tell the hydrocarbon potential of this zone (drilling from caprock into reservoir), or even to continue drilling (simply a lithology change from hard to soft or fractured rock).
What to do when we encounter a drilling break
We stop drilling and check to see if the well flows. Any flow from the well at this point especially when drilling overbalanced gives us an indication that the zone contains overpressured fluid which may be hydrocarbons. A blowout preventer may be deployed to control the rush of fluid to the surface and prevent a potential catastrophe. At some other times, even when we stop drilling and check for flow, the result of the flow test may be difficult to interpret especially if we are drilling overbalanced. Now, it is important to conduct a bottoms-up circulation here. A bottoms-up circulation is circulating bottoms up or circulating everything (both drilling mud and cuttings) from the bottom of the well (where we just encountered a drilling break) up to the surface. As soon as the mud and cutting get to the surface, then the wellsite geologist can conduct tests on the cuttings to check if this is a reservoir rock while the mud logger will go through the mud circulated bottoms up to check for oil or gas shows.
Conducting a flow check during a drilling break has helped several drillers discover small reservoirs that would have ordinarily been bypassed while drilling towards total depth. It is really important to always check for flow or circulate bottoms-up when we encounter a drilling break; that way any flow, if any, from that zone can be controlled. Drilling overbalanced could make it impossible for hydrocarbon fluids in porous formations to flow into the well when we stop drilling to check for flow. This is why; even when we do not immediately see signs of flow after a drilling break, still circulate bottoms up to get the mud and cuttings at the bottom of the well to the surface. Substantial volumes of hydrocarbons which we never expected to find could be discovered this way and this will offset the total cost in drilling the well.