There are many different steps to drilling a hole, and each one requires the successful coordination of many different rig systems, rig workers, operator personnel, and many service companies. Before we get into the details of each step, let’s get an overview of the entire process. When a well is spudded (drillers’ notation for starting the actual drilling process and is an important legal term impacting leasing and contracting), the first step is start drilling at the bottom of the conductor casing.
This section of borehole is called the surface hole and is critical to the success of the well. This section of borehole will be drilled to a minimum depth beyond the lowest potable water aquifer by some distance as set by regulation. In addition, the surface casing depth can also be deeper than the minimum as the surface casing string set in this borehole will be the anchor for the well control equipment. The pressures anticipated for a potential well control problem must be factored into the decision for the depth of this casing setting point. The strength of the rocks to resist fracturing due to pressures induced by a well control incident are a driving factor. Upon reaching the planned surface borehole depth, the rig crew retrieves the drilling equipment and runs the surface casing. Once the surface casing is in place, the annulus (the ring shaped section of borehole between the casing and borehole wall) is filled with cement. It is critical that this cement be circulated from the bottom of the casing (called the “casing shoe”) to the surface. This ensures that the fresh water aquifers encountered are covered and hydraulically sealed from any contamination from future operations. If the cement doesn’t reach the surface, remediation efforts are commenced immediately before any further activity can take place. Once the surface casing is cemented in place, the wellhead and blowout prevention equipment (BOPE) are installed. These pieces of equipment are designed to allow drillers to control the flow of fluids from the well and close it in if necessary.
Only after the wellhead and blowout preventer are installed can the next drilling operations begin. A drill string, made up of the drill bit, the bottom hole assembly (BHA – the hollow equipment needed for drilling operations – more on this later), followed by sections (called “joints”) of drill pipe (which connects the bit and BHA back to the surface allowing for continuous fluid path), is lowered through the BOPE, wellhead, and surface casing to the bottom of that string.
Once the drill bit is resting at the bottom of the borehole, the entire drill string is rotated by the rig’s rotation system (a kelly system or top drive). This rotation turns the entire string and consequently, the drill bit. In some cases, additional bit rotation may be provided by a downhole mud motor (as part of the BHA0), which uses the flow of the drilling mud at the bottom of the hole to turn the bit.
As the drill string turns, drilling mud is pumped down the inside of the drill string. The mud travels to the bottom of the drill string through the center, out of nozzles set in the drill bit, and returns to the surface through the annulus of the drill string. During this journey, the mud picks up the rock chips (called drill cuttings) made by the made by the drill bit and transports the cuttings to the surface where they can be removed from the mud.
On the surface, drilling mud is circulated through a filtering process that screens out the produced rock chips and other contaminants in progressively smaller sizes so that it can be sent back down the drill string. This is called “solids control” and is critical to the success of the drilling operation. You certainly don’t want to reintroduce the rocks you just drilled back into the borehole. The first piece of solids control equipment is called a shale shaker. It is a vibrating screen that allows the drilling fluid to pour through it with the appropriately sized mesh screening out cuttings larger than that mesh size. The vibrations keep those cuttings from clogging (called “blinding”) the screen and vibrates them off of the screen into a pit. Due to the limit of mesh sizing, other solid control systems are used to cut out smaller and smaller cuttings. In some cases, cuttings on the order of 10 microns can be pulled out of a drilling fluid (using a machine called a “centrifuge”).
As the hole gets deeper, the drill string is lowered to keep the bit in contact with the bottom of the borehole. When the top of the drill string approaches the rig floor, rotation is stopped so that the next joint of drill pipe can be screwed into the top of the drill string. This is called making a “connection.” If one is using a drilling rig with a top drive rotation system, one can connect two or three joints at a time – called a “stand.” Once the string has been lengthened, drilling can continue. Over and over, this continues deepening the borehole.
Occasionally, the drillers will need to pull all of the drill string out of the borehole. This is called “tripping out”. A “round trip” is pulling all of the drill string out of the borehole and returning back to the bottom of the borehole. This opens the borehole allowing drillers to lower tools or instruments down the hole, replace the drill bit if it is worn out, or run casing.
Casing serves many purposes and is essential to the integrity of a well. As noted earlier, casing is lowered into the hole and cemented into place. Typically, a single well will make use of several sizes of casing, which nest inside each other decreasing in diameter as depth increases.
Once the casing is installed, drillers use a smaller bit that is smaller than the inside diameter of the installed casing but is larger than the next planned outside diameter of casing. Drilling continues until the target depth is reached with the borehole telescoping down in size depending upon the number of casing strings run and the starting casing size. The larger one starts, for a given number of casing strings, the larger the final casing string can be. However, the volume of casing, drilling fluid, cement, and cuttings removed goes up by the power of three of the diameter with consequent higher expenses. It pays to predict this judiciously.
Don’t worry if you didn’t catch all the details in this overview – there’s a lot to learn about drilling.
We’ll be going into each step of the drilling process in detail in the following sections.
Images: “Man in Air” by Michael Black