A drilling break, also known as a drilling jump, is a sudden unplanned change in the drilling rate. In this article, we answer three key questions to discover the truth about drilling breaks.
What is a Drilling Break?
When drilling a well, the rate of penetration tells the driller just how fast or how slow the drilling process is going. For example, if we are drilling at 50 feet per hour (15 meters per hour) and suddenly we start drilling a new section at 200 feet per hour (60 meters per hour), this sudden jump or increase in drilling rate is called the drilling break. Another name for a drilling break is a drilling jump.
It is important to note, all things being equal, that the deeper we drill vertically into the earth, the harder the formations are because the overburden pressure from overlying formations increases with depth. Harder formations are more difficult to drill than less compact formations, which is why it is important to take notice when we suddenly begin drilling faster as we go deeper. (Learn more about investigating formations in Seismic Mapping: Technology that has Changed Oil Production Surveys.)
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 can 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 are drilling. The new rock being drilled may even be naturally fractured, which is one possible reason 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 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 are now drilling through the reservoir rock (sandstone or limestone, for example). This is a good thing as it indicates that we have finally gotten to the pay zone.
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 encounter a drilling break.
Thus, a drilling break could signify one of several things: a change in the type of rock, drilling into a rock with higher pore pressures, or drilling into a more porous rock that is also overpressured at the same time. (To learn more about porosity, read The Significance of Porosity to Original Hydrocarbon in Place.) This is why it is necessary to stop drilling when we encounter a drilling break and conduct a flow check to discover if the well is flowing or not. The result of the flow check will help us decide if we should quickly deploy a blowout preventer (drilling into an overpressured zone) or conduct some form of formation evaluation to ascertain 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 Actions Should We Take Upon Encountering a Drilling Break?
First, we stop drilling and check to see if the well flows. Any flow from the well at this point, especially when drilling overbalanced, indicates that the zone contains overpressured fluid that may be hydrocarbons.
A blowout preventer may be deployed to control the rush of fluid to the surface and prevent a potential catastrophe. At 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.
It is important to conduct a bottoms-up circulation at this point. 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 reach the surface then the well site geologist can conduct tests on the cuttings to see if this is a reservoir rock. The mud logger will examine 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 very important to always check for flow or circulate bottoms-up when we encounter a drilling break so that any flow 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 if 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 that we never expected to find could be discovered this way, which will offset the total cost of drilling the well.