4. to Market! Production and Reservoir Engineering

8/6/2019 4. to Market! Production and Reservoir Engineering

1/16

4 - To Market!Production andReservoir Engineering

It is the responsibility of the Production Departmentto bring the hydrocarbon from the reservoir to the surfaceand prepare it for sale to the end user. It is the responsibil-ity of the Reservoir Engineering Department to optimizethe recovery of the resource.

Ideally, the reservoir will contain sufficient energy tocause the hydrocarbons to flow freely to the surface. Thevolumetric gas reservoir in Figure 4.1 is the simplestexample. The volumetric gas reservoir is analogous to theair tank used in diving. In the air tank, air is stored atpressure and is bled through the mouth piece through aregulator. W hen all of the air is gone, the tank is depleted.The volumetric gas reservoir is the same except the tankis replaced by the sedimentary rock formation, which hasholes in it similar to a sponge. The pressure in the reservoiris several times that of the atmospheric pressure. There-fore, the gas flows from the high pressure in the reservoirto the low pressure at the surface. The regulator used onthe air tank is similar to the production unit used at thesurface on the gas well. Typically, about 85 percent of the

99


2/16


3/16

To Market! Production and Reservoir Engineering 101gas in the volumetric reservoir is produced to thesurface.

All other reservoir types are m ore complex and diffi-cult to visualize. The volumetric oil reservoir correspond-ing to the volumetric gas reservoir is known as a solutiongas drive reservoir and is illustrated in Figure 4.2. All oilreservoirs contain gas in varying quantity. The namefollows from the fact that gas is infinitely soluble in oil. Itis similar to the soft drink that contains carbon dioxide.When the top is removed from the soft drink, the liquidoften will flow out the top, especially if shaken. So it iswith the solution gas drive reservoir. When the reservoiris opened up, the gas in solution with the oil (similar tothe carbon dioxide in the soft drink) will expand and causethe o il and gas to flow to the surface. This type reservoiris the least efficient of all, with only about 10 to 15 percentof the oil in place being recovered.

In many cases, a vast aquifer or water-bearing zoneunderlies the o il or gas reservoir. Water is basically incom -pressible. However, when the aquifer is very large in com-parison to the hydrocarbon interval, the potential expansionof the aquifer is significant as a source of energy to drivethe hydrocarbon to the producing well. This is called awater-drive reservoir and is illustrated in Figure 4.3.

There are also water-drive gas reservoirs. The water-drive gas reservoir is less efficient than the volumetric gasreservoir in recovering the resource, while the opposite istrue for the oil reservoir. As little as 50 percent of the gasin place can be recovered from the water-driven gas res-ervoir, while as much as 85 percent of the oil in place canbe recovered from the water-driven oil reservoir.


4/16


5/16


6/16


7/16

To Market! Production and Reservoir Engineering 105

Figure 4.4. Schematic of a typical producing well bore.


8/16

106 Oil-An Overview of the Petroleum Industry

figure 4.5. Beam pumping unit.

closed and the ball freely moves off seat on the travelingvalve and fluid flows into the space above the pump.On the upstroke, the ball on the traveling valve is seatedand the upward motion lifts the fluid to the surface andopens the standing valve. When the standing valve isopened, fluid will enter the tubing between the two valves.


9/16

To Market! Production and Reservoir Engineering 107The cycle is repeated several times per minute and thefluid is pumped to the surface.

Beam pump units generally are limited to onshoreoperations. Offshore operations generally must rely onother forms of artificial lift. O ne form that is common tooffshore operations is the gas-lift system, illustrated inFigure 4.6. In the gas-lift system, natural gas is injectedat high pressure into the tubing annulus, which is isolatedfrom the producing formation by the packer. A series ofvalves in the tubing string permit the gas to enter thetubing at different depths in the tubing. The gas is used tolift the oil to the surface.

One of the primary duties of the reservoir engineer isto optimize the recovery from the reservoir. There arenumerous avenues for him or her to pursue. One of themore com mon is termed secondary recovery (see Figure4.7). When the original reservoir energy is exhausted inan oil reservoir, energy can often can be added to the res-ervoir. One way in which energy is added to the reservoiris by injecting water in specifically determined locationsto drive the remaining oil to a producing well. This typeproject is known as a waterflood. For example, the res-ervoir engineer might determine that the most oil can berecovered by injecting water in the wells on the outeredges of the reservoir and driving the remaining oil towardproducing wells in the middle of the reservoir. This isknown as a peripheral waterflood. There are a varietyof waterflood patterns that are commonly used, such asthe five spot and line drive, to name but two.

In heavy oil reservoirs, where the oil is more like tar,steam often is used in secondary recovery operations.Since the oil in these areas is very thick, it is difficult to


10/16


11/16


12/16


13/16

To Market! Production and Reservoir Engineering 111

Figure 4.8. LNG tanker.In many instances, it is impractical to use pipelines to

transport natural gas. For example, the operator may havea giant discovery in Africa and no market. Or considerJapan, for another example. Japan has considerable needfor natural gas and very small resources within distancessuitable for pipelines. In those instances, the natural gasmust be reduced to liquid form and transported in tankers(Figure4.8).When the tanker reaches its destination, spe-cialized receiving stations are required to return the naturalgas to its original state and deliver it to the consumer.These are referred to as Liquefied Natural Gas or LNGprojects. Currently, the competitive cost of LNG is about$4 per mcf (1,000 standard cubic feet) and the marketprice of natural gas at the wellhead is around $10 per mcf.So long as the market price of natural gas remains abovethe $4 per mcf mark, LNG will remain competitive andbecome more available.


14/16

112 Oil-An Overview of the Petroleum Industry

Figure 4.9. Oil tanker Mae& Pride. (Courtesy of MaerskTankers.)

Oil is easier to understand than natural gas. Domesti-cally, oil is transported by both pipeline and tank truck. Ifthe volume of oil produced is substantial, the pipeline isthe more cost-effective option. If the daily volume of oilproduced is small, the tank truck is more efficient. In m ostexporting countries, the o il has to be transported by pipe-line to the sea port terminal. There, it is loaded on hugeoil tankers such as the Muersk Pride shown in F igure 4.9.These large vessels transport millions of gallons of crudeoil to refineries around the world. There it is processedand d istributed.

On the refining side, the crude oil is processed orrefined by basically cooking it (see Figure 4.10).Differ-


15/16


16/16

114 Oil-An Overview of the Petroleum Industryent products are evolved at different temperatures. Thelighter products cook off first and easiest, while the heavierones cook off last. Typically, a little less than one half thebarrel of oil is converted into gasoline, which occurs inabout the middle of the cooking cycle. The pa rt that cooksoff easier usually makes such things as plastics. The partthat is harder to cook off is used to make diesel, heatingoil, and heavier hydrocarbons, such as greases.

So there you have it! That is the oil industry in a nutshell. Any one of these topics could be the subject for anentire book or even many books. But, I hope this overviewwill give the interested reader a good insight into thebasics of the industry. Thanks for reading it!

Documents

4. to Market! Production and Reservoir Engineering