About the Author

Norman J. Hyne is Professor of Petroleum Geology at the University of Tulsa in Tulsa, Oklahoma. He is also president of NJH Energy, a company that owns and manages the operation of oil wells. Norman is well known for his "Basic Petroleum Geology for Non-Geologists" short course that he has taught through out the world. He is the author of "Non-Technical Guide to Petroleum Geology, Exploration, Drilling and Production" and "Dictionary of Petroleum Exploration, Drilling and Production" published by PennWell Books. Norman has also edited four books on the petroleum geology of the mid-continent.

Finding and Producing Oil and Gas

Contents:

Chapter 1 - Gas and Oil

Chapter 2 - Petroleum Geology

Chapter 3 - Petroleum Exploration - The Prospect

Chapter 4 - Land - Leasing and Site Preparation

Chapter 5 - Drilling the Well

Chapter 6 - Testing the Well

Chapter 7 - Completing the Well

Chapter 8 - Producing Gas and Oil

Chapter 8

Completing a Well

After the tests have been run, the well is either plugged and abandoned or completed. To plug and abandon a well (P&A ), cement is poured down the well to prevent any salt water from flowing up the well and polluting fresh waters which occur in the near surface rocks. A metal plate is then welded to the top of the well.

Setting pipe means to complete a well. Large diameter steel pipe called casing is screwed together to form a casing string that is run into the well. Wet cement called slurry is then pumped between the casing and sides of the well (Figure 81) by a service company during a cement job. The casing stabilizes the well, preventing the sides from caving in, and prevents water from other formations from flowing into the well.

Figure 8-1 Casing in a well

There usually is a casing program as the well is being drilled. The well is drilled, cased, drilled deeper and then cased again. The well is first drilled down to a certain depth with a large diameter bit and then the drillstring is run out of the well. Large diameter casing, called surface casing, is cemented into the well. The well is then drilled down to the drilling target and tested. A string of smaller diameter casing, called production or oil string casing, is then run through the surface casing and into the well and cemented (Figure 82a). Sometimes, an intermediate diameter casing string called protection or intermediate casing is run between the surface and production casing (Figure 82b). Each casing string runs back up the well to the surface and each deeper casing string is smaller in diameter.

Figure 8-2 Casing programs

The bottom of the well is then completed either open hole or perforated. In an open hole completion, casing is run and cemented down to the top of the producing zone and the bottom of the well is left open. Horizontal drain holes are often completed open hole. In a perforated completion, casing is run and cemented through the producing zone. Holes, called perforations, are then shot into the casing at the level of the producing zone by a perforating gun with shaped explosive charges (Figure 83).

Figure 8-3 A perforated completion

Relatively small diameter, steel pipe called tubing is then run down the center of the well (Figure 83). The produced fluids (salt water, gas and oil) are brought up the well through the tubing string. This is to prevent the produced fluids from coming in contact with the casing and corroding the casing. Because the casing has been cemented to the sides of the well, it is very difficult, if not impossible, to repair the casing. The tubing, which is being suspended in the well, is relatively easy to remove and repair during a workover.

The steel casting on the surface above the well is called the wellhead. If this is a gas well, all gas wells flow to the surface by themselves. They don't have to be pumped. There are some oil wells in which the oil has enough pressure to flow all the way up the tubing to the surface. These flowing oil wells usually occur only early during the development of an oil field.

To complete a gas or flowing oil well, a series of pipes, valves and gauges, called a Christmas tree, is bolted to the wellhead (Figure 84). On most oil wells, however, the oil does not have enough pressure to flow all the way up the tubing.

Figure 8-4 A Christmas tree on a gas well

In artificial lift, the oil is pumped up the tubing to the surface. A common artificial lift technique is a beam pumper or sucker rod pump (Figure 85).

Figure 8-5 A beam pumper on an oil well

An electric motor or gas engine on the ground causes a steel walking beam to pivot up and down. Attached to the opposite end of the walking beam is a long, small diameter steel rod called a sucker rod string. Sucker rods come in twenty five foot lengths that are screwed together to form the sucker rod string. The sucker rod string runs all the way down the well through the tubing to the downhole pump on the bottom of the tubing. The walking beam causes the sucker rod string to rise and fall. This activates the downhole pump which lifts the oil up the tubing to the surface.

Most oil wells also produce salt water and natural gas. The salt water, called oilfield brine, shares the pores of the reservoir with the oil. The natural gas which is dissolved in the subsurface oil, bubbles out of the oil on the surface. The produced fluids flow though a plastic or steel flowline from the wellhead to a long, metal tank, called a separator (Figure 8-5). The crude oil is separated by gravity from the oilfield brine and gas in the separator. The crude oil then flows though a flowline to large, metal tanks, stock tanks , where it is stored.

When the stock tanks are filled with oil, either a service company picks up the oil with a tank truck and takes it to the refinery or the oil is transferred to a pipeline and taken to the refinery. In both cases, the service or pipeline company gives the operator of the well a run ticket telling how much oil was taken, the degrees API gravity and temperature of the oil. The operator is then paid from the run ticket. A sample of the oil is centrifuged to measure the basic sediment and water (BS&W) content of the oil. This is a measure of the impurities of the oil. The refinery or pipeline usually will accept oil with a maximum of about 1% BS&W.

The gas from a gas well is always sold to a pipeline that takes the gas to a market to be burned for heat. Most natural gas coming from wells contains impurities that must be removed in the field in a process called gas conditioning before the pipeline will accept it. The gas conditioning equipment is installed in the field next to the gas wells. Water is taken out in a process called dehydration and corrosive gasses such as carbon dioxide and hydrogen sulfide by sweetening .

Some wells produce wet gas containing a valuable liquid hydrocarbon called condensate. Wet gas is almost never put in a pipeline. The valuable liquid hydrocarbons are removed in the field by a gas processing plant. The wet gas is cooled to remove the condensate, butane, propane and ethane which are called natural gas liquids (NGLs). The NGLs are then sold to a refinery and the dry gas (methane) is put in the pipeline.

In some wells there are two or more producing zones. The production from the zones can be mixed (commingled ) and brought up the well through a single tubing string. Production from two producing zones, however, is commonly kept separate by a dual completion in the well (Figure 87). Two packers are used to isolate the production which is brought up two tubing strings.

Figure 8-7 A dual completion

On an offshore field, the wellhead, separation and treatment equipment must go on a production platform (Figure 8-8 ). Most offshore platforms are fixed and have legs called the steel jacket that are pinned to the ocean bottom. On top of the steel jacket is one or more decks. The field is developed from the production platform using deviation drilling so that the field is serviced from one platform. Up to 62 deviated wells can be drilled from one platform although the number is usually much smaller. The produced fluids from the wells flows onto the platform where it is separated and treated. The oil and/or oil then goes ashore by submarine pipeline. At least one derrick is left on the offshore platform for well workovers. Equipment for offshore workovers and well stimulation is skid mounted and brought out to the offshore platform on a crane barge. It is lifted onto the platform by the crane.

Figure 8-8 An offshore production platform

The deepest water production by a fixed production platform is 1,350 feet of water. In very deep waters, floating production platforms (tension leg platforms and spars) or anchored ships (FPSO vessels) are used. Some wellheads are located in over 5,000 feet of water and the production is brought to a production platform in shallower water by flowlines.

Coming next: Chapter Nine:  Producing Gas & Oil

Copyright © 1996 - 2007
 by Lewis G. Mosburg, Jr.

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