RESERVOIR DYNAMICS-II

Difference between oil and gas field development.

Major difference between oil and gas field development are-  

11.The economics of transporting gas

22.  Market for gas

33. Product specification

44. Efficiency of turning gas into energy.

On a calorific basis approximately 6000scf of gas is equivalent to 1 bbl of oil. The compression of gas costs add to the transportation cost. Transportation of gas is much costlier than oil. For offshore facility there should be at least 0.5 tcf to be economical. If there is already an existing infrastructure than at least 50 bcf gas is required. For this reason a local market is a must for gas production. If there is 5 tcf of recoverable gas than case for building a LN terminal is justified.

Unlike a spot market for oil, for gas there is contract. Unlike oil prices, the gas prices vary from place to place. Thus producer, customer contract is an important thing in gas business. The contract generally has 2 main clauses-

1.       Guarantee minimum quantity of gas for long duration.

2.       Peak production when required.

Unlike oil, the gas has long plateau period of 8-10 years (compared to oil 2-5 years).

Sub surface development of gas reservoir.

Mobility= k/u   (1) it determines which fluid will rush towards well bore.

K=permeability

U= viscosity

Typical viscosity of Gas= 0.01 cp

Oil=0.5 cp

Water=0.3cp

It is for this reason that in a gas well, gas rushes (1) to the well bore with almost zero water cut. Also due to high compressibility of gas it is very uncommon to attempt to support the reservoir pressure by injecting of water and the reservoir is simply depleted with time. In a gas field, the wells are generally located at the crest of the reservoir so that they are far away from rising gas-water contact.

If the mobility greater than 1 then water will preferably move if it is less than 1 then there is stable displacement. The mobility ratio can be altered by changing the viscosity of fluids by EOR.

 The gas which comes out of the well contains water vapor, H2S, nitrogen etc. Water vapor is removed by passing the gas through molecular sieve or passing through glycol. Hydrate inhibition can be obtained by glycol injection.