The well is a hole drilled in the earth for the purpose of finding or producing crude oil or natural gas; or providing services related to the production of crude oil or natural gas. Also, an oil well can be described as a pipeline reaching from the top of the ground to the oil producing formation. Through this pipe, oil and gas are brought to the surface. Wells are normally drilled with a drilling rig in stages, starting with a surface hole drilled to reach a depth anywhere from 60 to 400 meters.
The drillers then pull out the drill string and insert steel pipe, called surface casing, which is cemented in place to keep the wall from caving in. The casing – tubular steel pipe connected by threads and couplings-lines the total length of the well bore to ensure safe control of production and to prevent water entering the wellbore and to keep the rock formations from “sloughing” into the wellbore. The second step is the installation of the production tubing. Tubing is a steel pipe smaller in diameter than the production casing. It is lowered into the casing and held in place by packers which also isolate the production layers of rock.
The tubing hangs from a surface installation called the wellhead. The wellhead includes valves, chokes and pressure gages and makes it possible to regulate production from the well. The third step is to perforate the well. The casing prevents the hole from collapsing, but it also prevents the oil or gas from entering the wellbore. Therefore, holes are made through the casing and into the formation. This is usually accomplished with an explosive device that is lowered into the well on an electrical wireline to the required depth. This device, a collection of explosive charges, is called a perforating gun.
Producing oil and gas from the well. Gas generally flows to the wellbore under its own pressure. As a result, most gas wells are equipped only with chokes and valves to control the flow through the wellhead into a pipeline. When the wellhead pressure is less than the pipeline pressure, a compressor is installed to boost the low-pressure gas into the pipeline.
The production of crude oil is more complicated. Crude oil has larger molecules and moves through rocks less easily. The percentage of the oil in the reservoir that can be produced naturally, called the recovery factor, is determined by a large number of elements. These include the density of the oil, the viscosity, the porosity and permeability of the rock, the pressure in the oil reservoir and the pressure of other fluids such as gas and water in the reservoir.
Pumping. While some oil wells contain enough pressure to push oil to the surface, most oil wells drilled today require pumping. This is also known as artificial lift. If a well requires it, a pump is lowered down the tubing to the bottom of the well on a string of steel rods, referred to as the rod string. The rod string conveys power to the pump either by rotating or moving up and down, depending on the type of pump employed. Submersible pumps3are used on some wells.
Well stimulation. In many oil and gas wells, one additional step is required- stimulating the formation by physical or chemical means so that the hydrocarbons can move more easily to the wellbore through the pores or fractures in the reservoir. This is usually done before installing a pump or when the pump is removed for maintenance.
One form of stimulation- acidizing is the injection of acids under pressure into the rock formation through the production tubing and perforations. This creates channels beyond the perforations for oil and gas to flow back to the well. Fracturing or fracing is another common method of stimulation. A fluid such as water or an oil product is pumped down the hole under sufficient pressure to create cracks (fractures) in the formation.
Proppant- a hard substance such as sand, ceramics or resin-coated material - in injected with the fluid. As the fluid disperses, the material remains to prop open the fracture.
In producing gas and oil, efficient performance of the producing wells has more and more importance. A variety of tests must be made to determine the performance of an oil or gas well. This procedure is called testing. There are a large number of types of well tests and each is needed to obtain certain information about the well.
Various personnel make the many well tests, some of which are routine and some of which are complicated. Depending upon the type of test to be performed, the standard lease producing equipment may be all that is necessary for the test. In other tests, specially designed apparatus may be necessary. In any event, it is very important that the test be done accurately since well test data presents the true history of a well and the reservoir in which it is completed.
Potential test: The most frequently conducted well test is the potential test, which is a measurement of the largest amount of oil and gas, produced by a well in a 24-hour period under certain fixed conditions. The produced oil is measured in an automatically controlled production and test unit. It also can be measured by wireline measurement in the lease tank. Produced gas is measured at the same time with equipment such as an orifice meter or an orifice well tester. The major items of equipment needed for a test of this type are usually available as standard equipment at the lease tank farm.
The potential test is normally made on each newly completed well and often during its production life. The information obtained from this test is required by the state regulatory group, which assigns a producing allowable, which must be followed by the operator of the well. It is necessary to make the tests from time to time and producing allowables are adjusted according to the results of the tests. Very often these tests are performed by the producer to help in establishing proper production practices.
Bottom-hole pressure test: This test is a measure of the reservoir pressure of the well at a specific depth or at midpoint of the producing interval. The purpose of this test is to measure the pressure in the zone in which the well is completed. In making of this test, a specially designed pressure gage is lowered into the well by means of a wire line. The pressure at the selected depth is recorded by the gage. After that gas is pulled to the surface and is taken from the well. Regular bottom-hole tests will provide valuable information about the decline or depletion of the zone in which the well has been producing.
Productivity tests. Productivity tests are made on both oil and gas wells, and include both the potential test and the bottom-hole pressure test. The purpose is to determine the effects of different flow rates on the pressure within the producing zone. In this way, it is possible to establish some certain physical characteristics of the reservoir and to calculate maximum potential rate of flow. This test mitigates risk of damaging the well, which might occur if the well were produced at its maximum possible flow rate.
Special tests: Two types of special tests are fluid level determination and bottom hole determination. The first is required for wells, which will not flow and must be made to produce by pumping or artificial lift. The bottom-hole determination is normally made along with the bottom-hole pressure test and is made to determine the temperature of the well at the bottom of the hole.
It is necessary to lower a specially designed recording manometer into the well on a wire line.
The temperature tests are used by the engineer in solving problems about the nature of oil or gas that the well produces. It is also useful in locating leaks in the pipe above the producing zone. Other special tests are performed with flow rate indicators and radio active tracers.
Separation of oil and gas
Well fluids must be separated into oil, gas and water and each of them must be measured. In the early days of the oil industry, separators were not used. The production from wells was discharged directly into storage tanks. Although this resulted in separation of the liquids and gases, the practice was both wasteful and dangerous. The separators were developed to reduce such waste and the danger of fire and explosion.
Petroleum mixtures are often complex and difficult to separate efficiently. The equipment used to separate the liquids from the gases is referred to as a separator. The simplest form of an oil and gas separator is a small tank in which the force of gravity is used to separate the oil and gas1 . Oil, being heavy compared to the gas, falls to the bottom of the tank from which it goes into storage tanks. Gas, being lighter, rises to the top of the tank and goes from there into a gas-gathering system.
In addition to using the force of gravity, modern separators make use of other forces to get the best possible separation of oil and gas. The way in which each of those forces is used can be better understood by following the flow of a mixture of oil and gas through a separator (see below picture).
Vertical Separator: The mixture of oil and gas enters inlet, where it given a swirling motion by a spiral inlet baffle in the separator space or chamber. At this point there are two forces tending to separate the oil from gas. The first is the effect of gravity; the second is the centrifugal action, which causes the heavy oil particles to collect on the walls of the separator. Gas, which still contains some oil rises through chamber and then enters the swirl cylinder and oil drains through tubes to the bottom of separator. The gas then passes through another chamber and leaves the separator through gas outlet.
Oil leaves separator at the oil outlet. The oil is regulated by a float and control valve, so liquid covers the drain tubes and the oil outlet.
Horizontal separator: Separators of horizontal type are also common; and, although of different design, they have the same uses as the vertical separator. There are single tube and double tube separators. Horizontal separators of the two tube design are often used. The unit is made if two horizontal tubes mounted one above the other. The tubes are jointed by flow channels near the ends of the tubes. The mixed stream of oil and gas enters at one end of the upper tube. The liquids fall through the first connecting flow pipe into the liquid reservoir, which occupies the lower portion of the bottom tube. Oil, separated from gas, goes to stock tanks. Gas leaves the separator through the gas outlet.
Stage separator: Under certain conditions it is often desirable to use more than one stage of separation in order to obtain more complete recovery of liquids. For instance, three-stage separation system operates as follows: the first stage operates at the highest pressure and the second and third at lower pressures.
Low temperature separator: Low-temperature separation is a method of separation sometimes used to handle the production of high-pressure gas wells that produce some light liquids. The liquid separation is made possible by cooling the gas stream before separation.
After gas has been separated from the oil and the oil has been treated to remove water and sediment (if present), the oil goes to stock tanks which are commonly referred to as the tank battery. The tanks in a tank farm will vary in number and size, depending upon the daily production of the lease and the frequency of pipeline runs. The introduction of automatic custody transfer units and their acceptance by pipelines and producers has reduced storage requirements. The total storage capacity of a tank farm is usually 3 to 7 days’ production; that is, 3 to 7 times the maximum daily production or allowable of the wells connected to the tank farm. There are usually two or more tanks in a battery, so that while oil is being shipped from one tank the other tank can be filling.
Most tanks are made of either bolted steel or welded steel. Stock tanks usually have a bottom drain outlet for draining off basic sediments and water. In some areas tanks must be cleaned frequently due to collection of paraffin and basic sediments, which can be removed through the drain outlet. Therefore tanks are equipped with cleanout plates. Cleanout plates can be removed so that a workman can enter the tank.
The point where the pipeline company connects to lease stock tanks is usually one half meter above the bottom of the tank. The space below the pipeline outlets provides room for the collection of basic sediments and water. The pipeline outlet valve is sealed and closed with a metal seal when the tank is being filled and similarly locked in the open position when the tank is being emptied. Oil enters the tank at the top at the inlet opening. Usually a valve is on the inlet line so that it may be closed to prevent oil from entering the tank after the tank is full and ready for delivery. Where oil storage is controlled manually the tank is fitted with a thief or gage hatch in the tank roof so the amount of oil in the tank can be determined with a steel measuring line. The thief hatch is large enough so that a device which is called a “thief” can be lowered into the tank and samples of oil obtained to determine the basic sediments and water content in the oil and its API gravity. This operation is called “thiefing” a tank. The temperature of the oil in the tank is determined while thiefing the tank. .
When storage is done automatically, devices called liquid level controllers signal when tanks are filled and valves open and close according to a prearranged schedule.