Active vocabulary

Date: 2015-10-07; view: 468.

Unit 1. Primary and secondary production

Chapter 6.

C. Discussion

Topics:

1) Running tubing string and coiled tubing.

2) Installing the Christmas tree.

LEARNING OBJECTIVES:

Having worked through this chapter the student will be able to:

• describe primary production;

• list and describe secondary recovery methods;

• describe the three major types of enhanced oil recovery operations;

• state the causes of formation kicks and describe the primary means of detecting a kick;

• describe blowouts;

• describe well control procedures.

Once an oil or gas reservoir is discovered and assessed, production engineers begin the task of maximizing the amount of oil or gas that can ultimately be recovered from it. Oil and gas are contained in the pore spaces of reservoir rock. Some rocks may allow the oil and gas to move freely making it easier to recover. Other reservoirs do not part with the oil and gas easily and require special techniques to move the oil or gas from the pore spaces to a producing well. Even with today's advanced technology, in some reservoirs more than two-thirds of the oil in the reservoir rocks may not be recoverable.

Trapped crude oil is under pressure; if it was not trapped by imper­meable rock it would have continued to migrate upward until it es­caped at the surface of Earth. When a well bore is drilled into this pressured accumulation of oil, the oil expands into the low-pressure sink created by the well bore in communication with Earth's surface. As the well fills up with fluid, however, a back pressure is exerted on the reservoir, and the flow of additional fluid into the well bore would soon stop, if no other conditions were involved. Most crude oils, however, contain a significant amount of natural gas in solution, and this gas is kept in solution by the high pressure in the reservoir. The gas comes out of solution when the low pressure in the well bore is encountered, and the gas, once liberated, immediately begins to ex­pand. This results in oil flowing up to Earth's surface. Depending on reservoir conditions, this "natural flow" may continue for many years.

Nevertheless, as fluid withdrawal continues from the reservoir, the pressure within the reservoir gradually decreases, and the amount of gas in solution decreases. As a result, the flow rate of fluid into the well bore decreases, and less gas is liberated. The fluid may not reach the surface, so a pump (artificial lift) must be installed in the well bore to continue producing the crude oil. In the U.S., above-ground pumping units are often called "horsehead" pumps because of their unique shape and movement.

Most wells produce in a predictable pattern called a decline curve. Production will increase for a short period, then peak and follow a long, slow decline. The shape of this decline curve, how high the production peaks, and the length of the decline are all driven by reservoir conditions. Some wells may stop producing in economic quantities in only a few years.

When a large part of the crude oil cannot be recovered simply by allowing the original reservoir pressure to fur­nish the driving energy, a method for supplying extra energy must be found. Usually, an injected fluid such as water or gas replaces the produced oil and thus maintains the reservoir pressure. When this procedure substantially increases the potential production of oil, it is called sec­ondary recovery. When gas alone is used as the injected fluid, it is usually put into the top of the reservoir, where gas normally collects to form a gas cap. The effective use of this type of secondary recovery is restricted to reservoirs thathave a high flow potential so that gravity allows the oil to drain to the lower part of the formation, where it may be produced. In such a situation gas injection can be a very effective recovery method. When gravity segregation of thegas and oil cannot occur, however, or when little vertical relief exists in the formation, only a small increase of the natural pressure decline production is realized.

Another even more widely used secondary recovery method is water flooding. Water, after treatment to re­move certain impurities, is injected through some of the wells. In moving through the formation, it pushes oil to­ward the remaining production wells. The wells to be used for water injection are usually spaced evenly among the producing wells so that the oil has the shortest possible paths to follow. Water is an effective injection fluid and is not dependent upon gravity segregation. A water flooding project often increases the oil recovery to at least twice that obtained by pressure decline methods. The major exception is oil reservoirs that are connected to an active subsurface water source and are thus subjected to water flooding without injection.

Although water flooding greatly increases the recovery of oil from a particular reservoir, it leaves one-fourth to one-third of the oil behind, and several other methods have been tried in efforts to achieve even better oil recovery.