Editor’s note: This is the second in a two-part series where Wayne Nash is exploring Oil Wells 101.
Once the site is prepared and the rig is on location, it gets interesting. While the rig is rigging up, additional supplies such as fuel, mud, chemicals, water, bits, mud motors and sometimes housing must be brought in. Also, additional service companies must be coordinated to arrive when needed. Mud engineers, mud loggers, cementers and directional drillers are part of most modern wells.
When the rig is ready to “spud,” the rig goes “on the clock” and day rate begins. Time is money and the faster the well can be drilled, the cheaper it generally is, but safety cannot be compromised, especially in an area known to have hazards such as high pressure or sour gas. The rig crew members usually like to make good time too: It assures them of the next well in the program. But the operator’s on-site representative must be sure that proper procedures are followed to assure proper completion and production of the well.
As the well is drilled deeper, various casing strings must be run. They will have been arranged ahead of time, and must arrive and be ready to run at the proper to time. As soon as the casing is run, cement must be pumped on each casing string to isolate the formations, making a seal and allowing continued drilling. The cement company should be on location and rigged up when the casing string reaches bottom. Each cement job is slightly different, and the cement company and the operator’s engineers will meet ahead of time to decide on the appropriate formula and quantity of cement to pump.
Drilling then proceeds to the next casing point, adjusting parameters to account for increased depth and formation changes. Unexpected formation changes may call for different mud formulas or different bits. A bit change requires a trip out of the hole, and takes time, so correct bit choices are important.
When the final intermediate casing is set and cemented, the rig will drill to the total depth (TD) chosen by the operator and his engineers. Before the final string of casing, called the long string is run, the hole is open-hole logged. A wireline company with very sophisticated tools will log the hole from bottom to the intermediate shoe. They will determine such things as where the oil is, where the gas is (if any), where the water is and many other parameters. In some areas, hole stability can be a problem, so extra effort is taken by the rig crew to have the hole in near perfect condition to assure successful logging. It may take some time to properly condition the hole to prevent sticking the logging tools.
With logging complete, it is time to run the “long string.” This is usually the final string of casing in a hole, although in competent formations, the well may be completed-open hole, that is-with no casing in the final production zones. It may come all the way back to the surface, or it may just extend up inside the intermediate casing, in which case it is called a liner. In some areas, it is picked up and run as soon as the loggers are out of the hole. In other areas, a bit trip is made to clean up and condition the hole to make sure the casing gets to the bottom.
After running the long string, it is cemented in place. Even though no oil is yet produced, the well is temporarily capped. This completes the job of the drilling rig and it is released to go to the next job.
Next is the completion phase. A smaller, lighter rig more suited to quickly handling the pipe sizes used for completion and production is contracted. While waiting for the completion rig, it is typical to hire a lease crew to build the auxiliary equipment on location, such as separators, heater-treater, flair pits and tank batteries. If a pipeline is close by, gathering lines may be run.
Completion methods are as varied as the wells. Some wells require nothing more than running the production equipment and kick-starting the well. This is very rare, as most of the easy wells have been drilled and completed. Nowadays, more involved completions are the norm. In cased-hole wells, the well must be perforated at the appropriated depth. This is correlated to the open-hole log run by the wireline company earlier. If the well is to produce from a fairly porous and permeable formation, production equipment may be run. Now, many wells are set in formations that are not very permeable, that is they don't want to give up the oil easily. In this case, a hydraulic fracturing operation may be called for. In this case, a high pressure string of pipe is temporarily run in and tied to the liner top.
If the well is going to be fracked, the completion rig is temporarily rigged down and moved off location. The equipment for the frack job is moved in. This usually takes a tremendous amount of very sophisticated, high-horsepower equipment, and general takes up the entire location. It is not uncommon to see a frack spread with over 20,000 available horsepower. Large amounts of water and storage tanks must also be rigged up. A frack job can be expensive, often costing as much as the initial drilling of the well.
After the frack job (if necessary) is complete, the completion rig is rigged back up for “flow-back.” This is where the high-pressure water is flowed back to the surface and properly disposed of. Usually, at first, the pressure is quite high, and mostly water is produced. Eventually, oil or gas, or both, start to come in, depending on the well. There is usually some naturally occurring water that will be produced for the life of the well, but that is what the separator is for.
When the well has stabilized, a decision is made as to what kind of production equipment will be needed. In some areas, there is enough formation pressure for the well to flow on its own. In other areas, a sucker rod pump and pump jack may be installed. Sometimes a well will flow for several years before it needs the help of a pump.
Once the well has stabilized and oil is flowing to the tank battery, you are well on your way to money in the bank.