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Petroleum: a primer for Kansas, Page 12 of 15
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Drilling the well

All of the efforts of the petroleum landmen, geologists, and geophysicists, and everyone else involved in leasing and exploration activities up to this point must culminate in a decision by the management of the petroleum company to either drill or not drill a well. If the exploration activities conducted thus far have not found encouraging indications for the accumulation of petroleum in the area being explored, the leases held in the area will likely be dropped. If exploration activities have been encouraging, however, a decision will likely be made to drill an exploratory well. Such a decision may require the commitment of hundreds of thousands or even millions of dollars for the drilling program. In this section we will discuss the activities related to the drilling of an exploratory well. State regulations regarding drilling, development, and production will be discussed in a subsequent section.

Drill-site selection

The selection of the drill site is based largely on the geological evidence indicating the possible accumulation of petroleum. The exploration company will want to drill the well at the most advantageous location for the discovery of oil or gas. Surface conditions, however, must also be taken into consideration when selecting the drill site. There must be a nearly level area of sufficient size on which to erect the drilling rig, excavate reserve pits, and provide storage for all of the materials and equipment that will be required for the drilling program. All of the required legal matters need to have been attended to, such as acquiring a drilling permit, surveying of the drill site, and so on. When all of these matters have been resolved, the work on site preparation will begin.

Drill-site preparation

Once the drill site has been selected and surveyed, a contractor or contractors will move in with equipment to prepare the location. If necessary, the site will be cleared and leveled. A large pit will be constructed to contain water for drilling operations and for the disposal of drill cuttings and other waste. A small drilling rig, referred to as a dry-hole digger, will be used to start the main hole. A large-diameter hole will be drilled to a shallow depth and lined with conductor pipe. Sometimes a large, rectangular cellar is excavated around the main bore hole and lined with wood. A smaller-diameter hole called a "rat hole" is drilled near the main bore hole. The rat hole is lined with pipe and is used for the temporary storage of a piece of drilling equipment called the "kelly." When all of this work has been completed, the drilling contractor Will move in with the large drilling rig and all the equipment required for the drilling of the well.

Rigging up

The components of the drilling rig and all necessary equipment are moved onto the location with large, specially equipped trucks. The substructure of the fig is located and leveled over the main bore hole (fig. 22). The mast or derrick is raised over the substructure and the other equipment such as engines, pumps, and rotating and hoisting equipment are aligned and connected. The drill pipe and drill collars are laid out on racks convenient to the rig floor so that they may be hoisted up when needed and connected to the drill bit or added to the drill string. Water and fuel tanks are filled. Additives for the drilling fluid (drilling mud) are stored on location. When all these matters have been attended to, the drilling contractor is ready to begin drilling operations (spud the well).

Figure 22--A drilling rig with its major components and related equipment.

drawing of drilling rig

Spudding in

"Spudding in," or to "spud" a well, means to begin drilling operations. The drill string, consisting of a drill bit, drill collars, drill pipe, and kelly, is assembled and lowered into the conductor pipe. Drilling fluid, better known as drilling mud, is circulated through the kelly and the drill string by means of pipes and flexible hose connecting the drilling fluid or mud pumps and a swivel device attached to the upper end of the kelly. The swivel device enables drilling mud to be circulated while the kelly and drill string are rotated. The mud pump draws fluid from mud tanks or pits located nearby. The drilling mud passes through the kelly, drill pipe, drill collars, and drill bit. It is returned to the surface by means of the well bore and the conductor pipe where it is directed to a device called a shale shaker. The shale shaker separates the drill cuttings and solids from the drilling mud, which is returned to the mud tanks to be circulated again (fig. 23). As the drill string is rotated in the well bore, the drill bit cuts into the rock. The drilling mud lubricates and cools the drill bit and drill string and carries the drill cuttings to the surface (fig. 23).

Figure 23--Diagram illustrating the drilling-fluid (drilling-mud) system and the flow of fluids through the system.

drilling-mud system

Drilling the surface hole

When a well is spudded in, a large-diameter drill bit is used to drill to a predetermined depth. This is for the purpose of drilling the surface hole. The surface hole is lined with casing. The casing protects aquifers which may contain freshwater, provides a mounting place for the blowout preventer, and serves as the support for the production casing that will be placed in the well bore if the drilling program is successful. The surface hole may be several hundred or several thousand feet deep. When the predetermined depth is reached, the drill string will be removed from the well bore. Steel casing of the proper diameter is inserted. Sufficient cement is pumped down the surface casing to fill the space between the outside of the casing and the well bore all the way to the surface. This is to insure the protection of freshwater aquifers and the security of the surface casing (fig. 24). The casing and the cement are tested under pressure for a period of 12 hours before drilling operations may be resumed. A piece of equipment known as a blowout preventer is attached at the top of the surface casing. This device is required to control the well in the event that abnormal pressures are encountered in the bore hole that cannot be controlled with drilling fluid. If high-pressure gas or liquid blows the drilling fluid out of the well bore, the blowout preventer can be closed to confine the gas and fluids to the well bore.

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.

casing can protect aquifer zones from contamination

Drilling to total depth

After the surface casing has been tested and the blowout preventer installed, drilling operations are resumed. They will continue until the well has been drilled to the total depth decided upon. Usually, the only interruptions to drilling operations will be to remove the drill string from the well bore for the replacement of the drill bit, a procedure known as tripping, and for the testing of formations for possible occurrences of oil or gas, known as drill-stem testing. Other interruptions may be due to problems incurred while drilling, such as the shearing off of the drill string (known as "twisting off'), and the loss of drill-bit parts in the well bore, known as "junk in the hole."

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.

two kinds of drilling bits

Drill-stem testing

If the geologist detects the presence of oil or gas in the drill cuttings, a drill-stem test is frequently performed to evaluate the formation or zone from which the oil show was observed. Drill-stem tests may also be performed when the driller observes a decrease in the time required to drill a foot of rock, known as a "drilling break." Since porous rock may be drilled easier than nonporous or less porous rock, a drilling break indicates the presence of porosity, one of the qualities of reservoir rock. A drill-stem test enables the exploration company to obtain a sample of the fluids and gases contained in the formation or interval being tested as well as pressure information, which is determined by special gauges within the test tool.

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.

Well logging

Drilling operations continue until the predetermined total depth of the well is reached. A logging company is then called to the well site. The drill string is removed from the well bore to allow the insertion of logging tools, which are lowered all the way to the bottom of the hole by means of a special cable. This cable contains numerous electrical circuits. The tools are reeled slowly back to the surface. Specific properties of the formations are measured as the tools are retrieved. Signals detected by the tools are recorded in a recording truck at the surface by means of the electrical circuits contained in the cable.

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.

Completing the well

When drill-stem testing and well-logging operations have been completed and the results have been analyzed, the company management must decide whether to complete the well as a producing well or to plug it as a dry hole. If the evidence indicates that no oil or gas are present, or they are not present in sufficient quantity to allow for the recovery of drilling, completion, and production costs and provide a profit on investment, the well will probably be plugged and abandoned as a dry hole. If, on the other hand, evidence indicates the presence of oil or gas in sufficient quantity to allow the recovery of these costs and provide a profit to the company, an attempt will be made to complete the well as a producer.

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.

cement seals off well

After the cementing of the casing has been completed, the drilling rig, equipment, and materials are removed from the drill site. A smaller rig, known as a workover rig or completion rig, is moved over the well bore. The smaller rig is used for the remaining completion operations.

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.

two types of well heads

If there is not sufficient reservoir pressure to cause the oil and gas to flow naturally, pumping equipment is installed at the lower end of the tubing. Various types of pumping equipment are used including piston pumps, submersible pumps, and jet pumps. Power to operate the pump may be supplied by a gas engine or an electrical motor.

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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Kansas Geological Survey, Education
Placed online April 2001
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