Exploration and Production
Today’s oil and natural gas well has several features to minimize the risks associated with drilling into hydrocarbon bearing rock formations. This includes steel conductor casing strings (which includes the conductor casing, surface casing, intermediate casing, and long string (or production casing)) placed down the hole to keep minerals flowing into the wellbore and not anywhere else. When an oil or gas well is drilled, each string is locked into place with cement. Hence, underground sources of ground water are at least “triple protected” from oil and gas infiltration due to these several strings of steel casing that are cemented into the earth.
How an Oil or Gas Well is Drilled
Before a Well is Drilled
Before a well is even drilled, an oil and natural gas producer expends time, money and effort to find where oil and gas may be located. Geologists are hired who will measure seismic waves to search for geological structures that may form oil or natural gas reservoirs. The “classic” method includes measuring and observing seismic waves that provide information about the geological structures under the ground. This may be done by placing several geophones on the surface or by running specialized trucks that create seismic waves.
These waves will bounce against subsurface rock formations and show where “voids” are located within them. These voids can contain three things – oil, natural gas or water. The key is to find and drill on the top of one of these voids as it will offer the most potential for an oil and natural gas well.
The well is created by drilling a hole 5 to 50 inches in diameter into the earth with a drilling rig. Most drilling rigs operating in Ohio are called rotary rigs, meaning the drill itself rotates a drill string with a bit attached. After the hole is drilled, sections of steel pipe (called casing), slightly smaller in diameter than the wellbore, are placed in the hole. Cement is placed between the outside of the casing and the wellbore. The casing provides structural integrity to the newly drilled wellbore, in addition to isolating different rock formations and producing zones from each other and the surface.
With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper using a smaller drill bit. This string will be cased with a smaller sized casing. Modern wells often have two to five sets of subsequently smaller hole sizes drilled inside one another, each cemented into place with casing.
The drill bit, aided by the weight of thick-walled pipes called “drill collars” above it, cuts into the rock. Drilling fluid (or drilling mud) is pumped down the inside of the drill pipe and exits at the drill bit. Drilling mud is a complex mixture of fluids, solids and chemicals that must be carefully tailored to provide the correct physical and chemical characteristics required to safely drill the well. Drilling mud serves several functions, including cooling the bit, lifting rock cuttings to the surface, preventing the rocks near the wellbore from destabilizing and overcoming the pressure of fluids inside the rock so that these fluids do not enter the wellbore.
The generated rock (or cuttings) are swept up by the drilling fluid as it circulates back to surface outside the drill pipe. The fluid then goes through “shakers” which strain the cuttings from the good fluid which is returned to the drilling pit.
The pipe or drill string to which the bit is attached is gradually lengthened as the well gets deeper by screwing in additional 30-foot sections or “joints” of pipe under the topdrive (or “kelly”) that pushes the drill pipe into the hole at the surface. This process is called making a connection. Usually, joints are combined into three joints, which equal one stand.
This process is all facilitated by a drilling rig which contains all necessary equipment to circulate the drilling fluid, hoist and turn the pipe, control downhole, remove cuttings from the drilling fluid, and generate on-site power for these operations.
After drilling the well to total depth, it must be “completed”. Completion is the process in which the well is enabled to produce oil or gas.
In a cased-hole completion, small holes called perforations are made in the portion of the casing which passed through the production zone. This is done to provide a path for the oil or natural gas to flow from the surrounding rock into the production casing.
After a flow path is made, the well is usually hydraulically fractured, which is used primarily to stimulate the reservoir rock to optimally produce oil or natural gas into the wellbore. Finally, the area above the producing oil or natural gas section of the well is cemented off inside the casing and connected to the surface via a smaller diameter pipe called tubing. This arrangement provides a barrier to leaks of hydrocarbons as well as allowing damaged sections to be replaced.
In many wells, the natural pressure of the subsurface oil and natural gas reservoir is high enough for the oil or gas to flow to the surface. However, this is not always the case, especially in depleted fields where the pressures have been lowered by other producing wells, or in low permeability oil reservoirs. Installing a smaller diameter tubing may be enough to help the production, but artificial lift methods may also be needed. Common solutions include downhole pumps, gas lifts, or commonly surface pump jacks.
The production stage is the most important stage of a well’s life. By this time, the oil rigs and workover/service rigs used to drill and complete the well have moved off the wellbore. The top of the well is usually outfitted with a collection of valves called a christmas tree. These valves regulate pressures, control flows, and allow access to the wellbore in case further completion work is needed. From the outlet valve of the production tree, the flow can be connected to a distribution network of pipelines and tanks to supply the product to refineries, natural gas compressor stations, or oil tanks.
As long as the pressure in the reservoir remains high enough, the production tree is all that is required to produce the well. If the pressure depletes and it is considered economically viable, an artificial lift method mentioned above in the completions section may need to be employed.
When a well no longer is producing economic capacities of oil or natural gas or the well is a dry hole, the well must be plugged. When a well is plugged, casing and other equipment is removed. Then, cement plugs are placed in the wellbore to prevent the migration of fluids from different rock formations. The surface area is then reclaimed and returned close to its original state.