gas

• The underground storage of natural gas has become increasingly important after World War II
• The demand for natural gas has been higher in the winter, prompted by residential heating
• Types of underground storage facilities


  • Depleted oil or gas reservoirs
  • Aquifers
  • Salt caverns
• Depleted oil or gas reservoirs


  • Can use existing wells, gathering systems, and pipeline connections
  • Usually close to consumption centers
• Aquifers


  • Suitable if the water bearing sedimentary rock formation is overlain with an impermeable cap rock
  • Storage is created by injecting gas and displacing the water
  • Water movement and cap rock quality should be taken into account
• Salt caverns


  • Provide very high withdrawal and injection rates relative to their working gas capacity
  • Base gas requirements are relatively low
  • Cavern construction is more costly than depleted field conversions
• To determine a field's suitability as a natural-gas-storage, its physical characteristics such as porosity, permeability, and retention capability should be examined along with the site preparation costs, deliverability rates and cycling capability
• Total gas storage capacity

  The maximum volume of gas that can be stored in an underground storage facility by design
• Total gas volume in storage


  The volume of storage in the underground facility at a particular time
• Base gas or cushion gas


  • The volume of gas intended as permanent inventory in a storage reservoir to maintain adequate pressure and deliverability rates throughout the withdrawal season
  • Recoverable base gas - the portion that can be withdrawn with current technology
  • Non-recoverable base gas - the portion that cannot be withdrawn with the existing facilities
• Working gas capacity


  The total gas storage capacity minus base gas, i.e., the volume of gas in the reservoir above the level of base gas
• Injection volume


  The volume of gas injected into storage fields during a given period
• Deliverability or deliverability rate, withdrawal rate, withdrawal capacity


  A measure of the amount of gas that can be delivered or withdrawn from a storage facility on a daily basis
• Injection capacity or rate


  The amount of gas that can be injected into a storage facility on a daily basis
• Calculating total gas volume in a depleted gas reservoir


  1. Gi = Gv - Gs

  2. Gs = G - Gv

  3. Gs = (pi/Zi)*(Vp/T) - (pf/Zf)*(Vp/T)
• Gas loss in gas storage is a very serious issue, it happens when the cap rock does not seal well, cement around the wellbore is flawed, or there is a communication between the storage and other reservoirs
• Natural Gas Transportation


  Pipelines and Compressed
• Transport of natural gas over long distances has become very important
• Technologies used to transport natural gas


  • Pipelines (70% of transported gas)
  • Liquefied natural gas (LNG) (30% of transported gas)

  
• Pipelines


  • Cost-effective technology of choice for transport over land
  • Underwater pipelines are quite expensive, as much as ten times the cost of on-land pipelines of same length, and are limited by the underwater terrain they have to traverse
• LNG


  • Technologically proven and safe method of transport
  • Investment cost is quite high for LNG facilities, both for the regasification process at the receiving terminal, and particularly, for the liquefaction process at the shipping terminal
  • Energy consumed for LNG liquefaction and transport is high, amounting to as much as the equivalent of one quarter of the gas
• The de facto choice for natural gas transport, when a pipeline cannot be used, is currently LNG
• Pipelines


  • Interstate pipelines (trunklines) are long-distance and wide-diameter (20–42 in.), and traverse more than one state
  • There are more than 1,400 compressor stations to maintain pressure on this pipeline network
  • Intrastate pipelines operate inside a single state
• Basic pipeline capacity design


  1. Supply sources of natural gas imported into a pipeline (another pipeline, LNG, gas processing plants, and gas gathering systems)

  2. Gas goes through a long-distance trunkline and eventually reaches the consuming markets

  
• The kinetic energy pressure drop was neglected with the assumption that the flow rate is not very high. In a high rate, low pressure line, however, the change in kinetic energy may be significant and should not be neglected.
• The total length of the pipeline is 4,000 km, therefore, the number of compressor stations needed is: 4,000/310 - 1 = 12
• Compression


  Pressure of natural gas flowing through a pipeline decreases along the distance because of friction pressure drop, so compressors are needed to ensure that the natural gas gets to the destination with sufficient pressure along the path and outlet
• Types of compressors used


  • Reciprocating
  • Turbine engines
• Most compressors have natural gas-fired and high-speed reciprocating engines
• Compressors are periodically retrofitted to cope with new emerging technologies, but most of the time, to increase efficiency and safety
• Besides compressors, there are other components in a compressor station, including scrubbers and filters
• Although gas is treated before entering the transportation pipelines, liquid may still condense and accumulate in the pipelines during the transportation process, and particulates may form with the coating materials inside of the pipeline
• Horsepower (hp or HP)


  The work done over a period of time, 1 hp equals 33,000 ft-lb/min, or 746 watts, or 75kg-m/s
• Theoretical hp of the compressor


  Can be calculated by assuming the system to be either isothermal (ΔT = 0) or adiabatic/ isentropic (ΔH = 0)