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Introduction . Underground storage is the process which effectively balances a variable demand market with a nearly constant supply of energy provided by the pipeline system.Underground natural gas storage acts as the swing capacity due to the seasonal variations in demand. Storage reservoirs are the ware houses to give a ready supply of gas that can serve a market with high peak demands in cold winter..
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3. Introduction Underground storage is the process which effectively balances a variable demand market with a nearly constant supply of energy provided by the pipeline system.
Underground natural gas storage acts as the swing capacity due to the seasonal variations in demand.
Storage reservoirs are the ware houses to give a ready supply of gas that can serve a market with high peak demands in cold winter.
4. Underground storage process principle The natural gas is injected into underground storage reservoirs when market demand falls below the supply.
Natural gas is injected into underground storage reservoirs during the second and third quarters when supply exceeds demand. This injected gas is withdrawn from these reservoirs during the fourth and first quarters when space heating demand peaks and demand exceeds supply. Storage reservoirs are high-deliverability and often closer to the end user compared to the reservoirs from which the natural gas is originally produced.
5. It is also used for a variety of secondary purposes, including: 1-Balancing the flow in pipeline systems. This is performed by mainline transmission pipeline companies to maintain operational integrity of the pipelines, by ensuring that the pipeline pressures are kept within design parameters.
2-Maintaining contractual balance. Shippers use stored gas to maintain the volume they deliver to the pipeline system and the volume they withdraw. Without access to such storage facilities, any imbalance situation would result in a hefty penalty.
6. Secondary factors…..continue 3-Leveling production over periods of fluctuating demand. Producers use storage to store any gas that is not immediately marketable, typically over the summer when demand is low and deliver it when in the winter months when the demand is high.
4-Market speculation. Producers and marketers use gas storage as a speculative tool, storing gas when they believe that prices will increase in the future and then selling it when it does reach those levels.
5-Insuring against any unforeseen accidents. Gas storage can be used as an insurance that may affect either production or delivery of natural gas. These may include natural factors such as hurricanes, or malfunction of production or distribution systems.
7. 6-Meeting regulatory obligations. Gas storage ensures to some extent the reliability of gas supply to the consumer at the lowest cost, as required by the regulatory body. This is why the regulatory body is monitors storage inventory levels.
7-Reducing price volatility. Gas storage ensures commodity liquidity at the market centers. This helps contain natural gas price volatility and uncertainty.
8-Offsetting changes in natural gas demands. Gas storage facilities are gaining more importance due changes in natural gas demands. First, traditional supplies that once met the winter peak demand are now unable to keep pace. Second, there is a growing summer peak demand on natural gas, due to electric generation via gas fired power plants.
Secondary factors…..continue
8. Factors that are conducted in natural gas production technology 1-Practices for more efficient use of the storage reservoir
2-Assurance that the injected gas remained in the reservoir.
3-Ways to handle new problems as they arose.
Types of underground storage reservoirs:
1- Depleted gas reservoirs
2-Depleted oil reservoirs
3-Aquifer storage and
4-Salt caverns.
9. Basic storage parameters Working gas:
This is the gas that is available to produce and sell during the withdrawal period and inject during the fill period.
The amount of working gas available in a given storage reservoir is a function of the reservoir quality, number of wells and degree of stimulation, gas withdrawal schedule and field operating parameters.
Base gas (cushion gas):
This is the volume of gas required to maintain adequate pressure to ensure the deliverability of the working gas.
10. Base gas is rarely, if ever, produced. The amount of base gas required is dependent upon the reservoir quality, number of wells and degree of stimulation, gas withdrawal schedule and field operating parameters. Since base gas is rarely produced, the ratio of base gas to working gas has a significant impact on the economics of greenfield gas storage (i.e., the base gas can be a large upfront capital cost that is not recovered until the storage field is blowndown). Basic storage parameters……cont
11. Deliverability:
The amount of gas that a storage reservoir is capable of producing to sales. Early in the withdrawal cycle, deliverability is usually limited by storage field surface gathering facilities. Later in the withdrawal cycle, deliverability can be limited by the amount of working gas…as working gas diminishes, total reservoir deliverability declines.
Cycling:
The number of times the working gas volumes are injected /withdrawn in a year. Cycling is typically low for depleted oil and gas reservoirs (usually=1) and can be high for salt cavern storage (as high as five times). Basic storage parameters……cont
12. Natural Gas Storage Reservoirs There are three major types of “reservoirs” common to the underground storage of natural gas:
1) depleted reservoir storage,
2) aquifer storage and
3) salt cavern storage. Geographically, the East Coast region is characterized by depleted reservoir and aquifer storage, the Gulf Coast has a mix of depleted reservoir and salt cavern storage and the West Coast has primarily depleted reservoir storage.
14. The most common type of underground gas storage occurs in shallow, high-deliverability depleted oil and gas reservoirs. Although the requirements vary, typically these type reservoirs require 50% base gas (i.e., equal amounts of base gas and working gas).
Advantages of depleted reservoirs include:
These types of reservoirs have several advantages:
·1-Typically, they are near existing regional pipeline infrastructure.
Depleted Reservoir Storage
15. 2-The fields already have a number of useable wells and field gathering facilities (i.e., reduces the cost of conversion to gas storage).
3-The geology is well known. These fields have previously trapped hydrocarbons which minimizes the risk of reservoir “leaks” (see aquifers).
Depleted Reservoir Storage…..cont
16. Disadvantages associated with depleted reservoirs:
1-Because of the nature of the reservoir producing mechanisms , working gas volumes are usually cycled only once per season (extremely high deliverability storage reservoirs are the exception).
2-Often, these reservoirs are old and require a substantial amount of well maintenance and monitoring to ensure working gas is not being lost via wellbore leaks into other permeable reservoirs. Depleted Reservoir Storage…cont
17. Aquifer storage consists of injecting natural gas into underground formations that are initially filled with water (i.e., aquifers). The gas is injected at the top of the water formation and displaces the water down structure. These types of reservoirs account for only 10% to 15% of total U.S. storage deliverability and exist mainly in the Midwest due to the lack of depleted oil and gas reservoirs. Aquifer Storage
18. Advantages of aquifer reservoirs include:
· 1-Typically, close to end user market.
· 2-High deliverability from the combination of high quality reservoirs, plus water drive during the withdrawal cycle.
· 3-The high deliverability increases the ability to cycle the working gas volumes more than once per season.
Aquifer Storage……continue
19. Disadvantages include:
1- A high level of geological risk. These reservoirs have not previously trapped hydrocarbons and, as a result, there is a degree of uncertainty concerning the ability to contain injected base and working gas. The risk for substantial reservoir leaks exists.
2- Because these reservoirs produce via water drive, water production is often experienced during the withdrawal cycle, increasing operating costs. Aquifer Storage……continue
20. 3- Due to the water drive mechanism during the withdrawal cycle, the base gas requirements are high (80%).
A large percentage of base gas is not recoverable after site abandonment . This high base gas requirement likely limits the number of new aquifer storage projects (increases the initial capital cost).
4-Conduct pump test to evaluate reservoir in-situ permeability and evidence of water movement through cap rock.
21. The steps in the procedure for locating and initiating an aquifer storage zone are:
1-study state of the geological records
2-look for outcrops in quarries , mines , river banks ,etc
3-Review water well data , especially deeper ones for villages or commercial plants.
4- seek any records for dry holes in search for oil. Possibly make seismic or gravity studies. Aquifer Storage……continue
22. The character of underground structure and rock layers desired for an underground aquifer site are:
1-there is a structure under which gas may accumulate.
2-there is a container , a porous bed of rock into and out of which fluids may flow through wells.
3-there is a water-filled cap rock ,which prevents the stored fluid from rising vertically due to buoyant forces and also be shaped to prevent lateral movement.
.
Aquifer Storage……continue
23. 4-The anticline or inverted saucer type of structure is common.
5-there is enough overburden to allow storage at pressures much above atmospheric pressure.
6-There is water to confine the stored fluid from all directions. Aquifer Storage……continue
24. Salt Cavern Storage Salt cavern storage sites are solution-mined cavities in existing salt domes/structures. These shallow cavities are filled with injected natural gas and act as high pressure storage vessels.
Salt caverns have the highest deliverability per storage site, even though the vast majority of working gas resides in depleted oil and gas reservoirs.
Advantages include:
1-Low base gas requirements of 25%, which can approach 0% in emergencies.
25. 2-Ultra-high deliverability (much higher than depleted
reservoir and aquifer storage).
3-Operational flexibility as these reservoirs can cycle working gas four to five times a year. Their location in the Gulf Coast allows daily production and nightly injection to help meet peaking natural gas demand during the summer air conditioning season.
4-Salt caverns provide excellent seals (i.e., the salt cavern walls are essentially impermeable barriers) and the risk of reservoir gas leaks is small. Salt Cavern Storage…continue
26. Disadvantages include:
1-Typically, located in the south, far away from the winter heating market.
2-Costly initial startup (disposal of the saturated salt water generated during the solution mining process can be costly and environmentally problematic). Salt Cavern Storage…continue
27. Types of underground natural gas storage facilities
28. Regional Breakdown Of Natural Gas Storage in the USA
30. The primary objectives in designing and operating storage reservoir 1-the first objective is to know the storage capacity for the gas as a function of time (Verification of inventory).
2-the second objective is having a monitoring system to verify where the gas resides and ensure that losses are not occurring (Retention against migration).
3-the third objective is the ability to develop and maintain a specified gas deliverable rate (Assurance of deliverability).
32. The possible losses of gas during the storage The possible losses of gas during the storage are:
1-through the imperfect cementing at casing shoe or opposite any leak in casing joints or cementing tool
2-gas displacing water through a saddle and separating from the gas bubble.
3-gas loss vertically through imperfections in cap rock or due to low local threshold pressure areas.
33. 4-Gas migrating past spill points in storage zone.
5-piping and compression station seepage and valve losses.
6-Gas penetrating cap rock on aquifers The possible losses of gas during the storage
34. The possible losses of gas during the storage The key concepts are those of Net and Gross pay.
Gross pay is always > Net pay. This can also be described by the Net -to - Gross ratio which is always less than or equal to one.
The spill plane is the maximum level to which this particular reservoir can filled before the next anticline starts to be filled.The key concepts are those of Net and Gross pay.
Gross pay is always > Net pay. This can also be described by the Net -to - Gross ratio which is always less than or equal to one.
The spill plane is the maximum level to which this particular reservoir can filled before the next anticline starts to be filled.
35. Energy is a vital part of practically all industrial concerns. Without it, most companies would cease to function. This energy can originate from many basic sources including:
1-Gas 5-Water power
2-Oil 6-Wind power
3-Coal 7-Solar power
4-Nuclear power 8-Geothermal power
Electricity is missing from this list because electricity is a secondary source of power. All of the above sources of fuel can be converted into electricity. Introduction
36. Nuclear power is so regulated and so financially demanding that it is practical only for the production of power in very large amounts. It is not viable for the ordinary medium-sized industrial organization.
Water power is widely used for the production of electricity, but it is no longer used to any extent as a source of industrial power by itself.
Wind power and solar power have had some small development in special situations, but they are not practical for the average fuel user.
Geothermal power is severely limited by its geographic occurrence. This leaves coal, oil, and gas as the primary sources of fuel used by industry. Introduction
37. Fuel Storage Any fuel requires storage to some degree. Coal is delivered from the mine to the point of usage by rail or truck. Since these deliveries are not continuous, coal must be stored to accommodate usage between deliveries. This can be done in bins for moderate amounts or in open stock piles for larger amounts. The coal piles are stable, are not unduly affected by the weather, and the storage effort is reasonably economical.
38. Fuel oil (and other liquid fuels) is similar to coal in some of these aspects. Much of the oil is delivered by truck or rail. This requires that storage be available to accommodate usage between deliveries.
Fuel oil can be delivered to larger industrial users by pipeline. Even in this case, however, the pipeline is not dedicated to delivering oil alone. Usually the pipeline transports batches of different products. These hydrocarbon products may vary from gasoline on one end of the spectrum to heavy fuel oil on the other. Because of this, the shipments of oil by pipeline are periodic, and storage must be provided for the periods of usage between shipments. This storage may take the form of atmospheric pressure tanks.
39. Natural gas is the only one of these commonly used fuels that does not require storage by the user.
Natural gas is delivered to the customer by pipelines. These pipelines are dedicated to the customers they serve, and they transport only the one product. The user always has a supply of natural gas available for use by simply turning on a valve. Unfortunately, this is not the end of the storage story.
40. The transmission pipelines that carry the natural gas to the customer have some limit on their capacity. The pipelines also have a cost associated with them. As long as the loads that the pipeline serves are constant throughout the year, the pipelines can be economically designed to handle the service. When a large portion of the customer loads are temperature sensitive, however, a problem arises. Many residential, commercial, and industrial customers use gas for environmental heating. This is heating to counteract the low ambient temperatures that occur in winter. The colder the atmospheric temperature becomes, the greater is the amount of gas required to heat the customer structures.
41. The coldest days of the winter only occur a few days each year. If a large portion of a pipeline's load is heat sensitive, economically designing facilities to supply this load on a year-round basis can be difficult. If the limitation on service during the very cold periods is due to pipeline capacity limitations, gas storage located near the consumption area is necessary. If the limitation on service is due to a supply limitation, the gas storage location is not as critical.
42. Natural gas is the most difficult to store; because it is a gas, it must be contained in a container that is leak-proof under pressure. Neither coal nor oil has this restriction. Because of its lower heating value per unit volume at atmospheric pressure, gas requires either much larger atmospheric pressure storage vessels or pressurized storage vessels.
43. The underground storage of natural gas has evolved as the preferred means of supplementing natural gas supplies for temperature-sensitive loads on very cold days. In most cases a former producing field is used and transformed into a storage facility. In some cases storage facilities have been developed in aquifers that did not previously hold gas. In either case the reservoir and the supporting equipment must be tailored to the needs of the pipeline company.
44. Historical Weather Data For heat-sensitive gas usage, accurate load forecasting requires accurate weather information. At the present time, accurate long-range weather forecasting is not feasible. It is possible, however, to determine what temperature pattern should be expected for a normal winter period. This normal pattern can serve as a basis for a load forecasting procedure.
45. The procedure used by the weather bureau is to take the high temperature and low temperature for each day and average these to yield a daily average temperature for that location. These daily average temperatures are then averaged for each day of the year for some long period, typically 30 years, to yield a normal temperature for that date.
47. Load Forecasting
48. The vast majority of natural gas storage owners are price insensitive.
Regulated gas utilities inject gas to meet anticipated winter demand and pass the cost through to their customers. Energy marketers attempt to capture any arbitrage between current gas prices and future gas prices
49. Regional storage patterns confirm that storage owners are price insensitive. East Coast storage, which accounts for approximately 55% of total storage and is controlled primarily by the regulated utilities, achieves consistent pre-winter working gas levels (approximately 1,700 bcf). Gulf Coast working gas storage levels, controlled primarily by energy marketers, are highly variable at the beginning of winter…dependent on the existence/absence of price arbitrage
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114. Limitations and assumptions of
116.
which represents underground withdrawal volume.
This term describes the expansion of the oil and liberated free gas.
117. which concerns the expansion of the gas cap gas.
EfjW = Boi (I + m) Here, the rock and connate water expansion is considered.
Ap rblSTB
(8-35)
118. which concerns the expansion of the gas cap gas.
Here , the rock and connate water expansion is considered.
Then, the general material balance Equation may be written as:
……………….(1)
119. For the reservoir with no initial gas cap, negligible water influx, and negligible compressibilities of connate water and rock, equation 8-36 reduces to:
120.
121. Another case of interest is a reservoir with an initial gas cap but with negligible water encroachment. Because of the initial gas cap, the connate water and rock compressibilities may be neglected with little error incurred. Equation (1) becomes:
123. For the case of a water drive reservoir with no gas cap and negligible connate water and rock compressibilities, Equation (1) becomes:
In this case, F/E0 should plot as a linear function of We / E0. The y-intercept will be N. A suitable function describing the water influx is normally used in the analysis.
