Petroleum: a primer for Kansas, Page 12 of 15
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Figure 22--A drilling rig with its major components and related equipment.
Figure 23--Diagram illustrating the drilling-fluid (drilling-mud) system and the flow of fluids through the system.
Figure 24--A casing is installed in the surface hole to prevent the contamination of freshwater zones and to support the production casing. A) The conductor pipe has been cemented into place. A predetermined amount of casing has been inserted into the well bore below the deepest freshwater zone. Cement is pumped down the inside of the casing until cement flows to the surface through the annulus. B) The cement in the bottom of the casing has been drilled out so that drilling can be resumed.
As drilling operations continue, a geologist constantly examines drill cuttings for signs of oil and gas. Sometimes special equipment known as a mud logger is used to detect the presence of oil or gas in the drill cuttings or drilling fluid. By examining the drill cuttings, the geologist determines the type of rock that the drill bit is penetrating and the geologic formation from which the cuttings are originating.
Today's conventional drill bit utilizes three revolving cones containing teeth or hardened inserts which cut into the rock as the bit is revolved (fig. 25). The teeth or inserts chip off fragments of the rock which are carried to the surface with the drilling fluid. The fragments or chips, while they are representative of the rock being drilled, do not present a clear and total picture of the formation being drilled or the characteristics of the rock being penetrated as to porosity and permeability. For this purpose, a larger sample of the rock is required. Such a sample is acquired by using a diamond coring bit and a core barrel. The diamond core bit is essentially a cylinder with industrial diamonds set into one end. The other end is threaded so that it may be connected to a core barrel, a device which contains equipment for holding the core as it is cut. As the drill stem and coring bit are turned, the diamonds cut the rock and a cylindrical core of the rock is cut. The core passes upward into the core barrel where it is held until the drill string can be extracted from the well bore. At the surface the core is removed from the core barrel where it is examined by the geologist. The core is usually sent to a laboratory for core analysis and testing.
Figure 25--At the top is a conventional rock, or cone, bit. As the bit rotates, the teeth on the cones turn and bite into the rock and chip off fragments. Drilling fluid passes through the bit to cool and lubricate it and to carry the rock chips to the surface. The diamond bit, below, is used in conjunction with a core barrel for cutting a core out of the rock. The bit is hollow so that as it cuts into the rock, a core of rock is cut which passes through the bit and into the core barrel.
Drill-stem testing is accomplished by removing the drill string from the bore hole. The drill bit is removed and a drill-stem test tool with a packer is attached. The test tool, packer, and drill string are inserted back into the bore hole to the desired depth. The packer, which is an expandable device, is set and expanded at the predetermined depth to isolate the zone to be tested. The test tool contains a valve which may be opened and closed to allow formation fluids to enter the test tool and drill string. If there is sufficient fluid and pressure within the zone being tested, the formation fluid (oil, gas, water) may rise to the surface and flow into special test tanks used for that purpose. If gas is present, it is burned at the surface as a flare. By analyzing the rate of flow or the amount of formation fluid recovered in the drill string and the formation pressures recorded, obtaining a good indication of reservoir characteristics such as porosity, permeability, and the nature of the fluids or gas contained therein is possible.
Electrical logs measure the natural electric potential and the effect of induced electricity on the formations. Radioactivity logs measure the natural radioactivity and the effect of induced radioactivity on the formations. Sonic logs measure the velocity of sound waves in the formations. By analyzing these logs, experienced geologists and engineers can determine the depth from the surface to various formations and intervals, formation characteristics such as rock type and porosity, and indications of the presence of oil or gas and quantity.
If the well is to be plugged and abandoned as a dry hole, a cementing company is called to the drill site. The well bore is filled with drilling fluid, which contains additives which give it special properties that prevent its movement from the well bore into the surrounding rock. Cement plugs are required within the well bore at intervals where porosity has been detected to isolate these porosity zones and prevent the movement of formation fluids from one formation to another. The cement is pumped into the well bore through the drill string. The cement is mixed at the surface in special trucks which are equipped with high-volume pumps. The pumps are connected to the drill string which has been inserted into the well bore to a predetermined depth. A quantity of cement is pumped into the well bore through the drill string and displaced out of the bottom of the drill string with drilling fluid. The drill string is then pulled up to the next interval that is to be cemented. This process is repeated until all the required plugs have been set. A cement plug is also set at the base of the surface casing, which remains in the hole, and another plug is set at the surface. In cultivated areas the surface casing is cut off below plow depth. A steel plate is welded at the top of the surface casing. All drilling equipment and materials are removed from the drill site. The pits are allowed to dry up and are backfilled and the site is restored as nearly as possible to its original condition.
If a decision is made to attempt to complete the well as a producer, casing is delivered to the site and a cementing company is called. The well bore is filled with drilling fluid that contains additives to prevent corrosion of the casing and to prevent the movement of the fluid from the well bore into the surrounding rock. The casing is threaded together and inserted into the well bore much in the same manner as the drill string. Casing may be inserted to a total depth of the hole or a cement plug may have been set at a specific depth and the casing set on top of it. Cement is mixed at the surface just as if the well were to be plugged. The cement is then pumped down the casing and displaced out of the bottom with drilling fluid or water. The cement then flows up and around the casing, filling the space between the casing and the well bore to a predetermined height. Special tools are sometimes used with the casing which allow the setting of cement between the outside of the casing and the well bore at specific intervals. This is done to protect the casing and to prevent the movement of formation fluids from one formation to another (fig. 26).
Figure 26--Cementing the production casing in the well. A) This illustrates how the cement is pumped down the casing. The casing shoe makes it easier to insert the casing into the bore hole. The float collar prevents drilling fluid from entering the casing. The bottom plug precedes the cement down the casing, and the top plug follows the cement and precedes the displacement fluid. B) The production casing when the cementing operation is completed.
A well-perforating company is then called to the well site. It is necessary to perforate holes in the casing at the proper position to allow the oil and gas to enter the casing. The perforating company is commonly the same company that has performed the logging of the well. A special perforating tool is inserted into the casing and lowered to the desired position on the end of a cable. The cable contains a number of electrical circuits and is connected to a recording and control truck at the surface. The perforating tool contains a number of shaped charges which are spaced at specific intervals. When the perforating tool has been lowered to the desired position, the shaped charges are fired remotely from the control truck at the surface and jets of high-temperature and velocity gas perforate the casing, the cement, and the surrounding rock for some distance away from the well bore.
A smaller-diameter pipe, called tubing, is then threaded together and inserted into the casing. If it is expected that the oil or gas to be produced will flow to the surface naturally, the tubing is equipped with an expandable packer at the lower end. The tubing is inserted into the casing and the packer is expanded or set at a predetermined point above the perforations. At the surface, a well head is installed which is equipped with valves to control the flow of oil or gas from the well. The well head is known as a "Christmas tree" (fig. 27).
Figure 27--The two types of well heads or "Christmas trees." The well head on the left is for a flowing well and the well head onthe right is for a pumping well.
During well-completion it is sometimes desirable or necessary to treat or stimulate the producing zone in order to improve the permeability of the rock and increase the flow of oil or gas into the casing. This may be accomplished by the use of acid or by the injection of fluid and sand under high pressure in order to fracture the rock. Such a treatment usually improves permeability and facilitates the flow of oil or gas into the casing. At this point, the drilling and completion phase have ended. The well is about to enter the production phase which, hopefully, will continue for many years.
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