A pressure survey is conducted after the first year of production in a gas well and consequently annually. The survey is required to review the decreasing trend of the pressure in a gas reservoir in order to estimate the amount of gas that remains. Since the pressure in a reservoir decreases in a linear manner to the decrease volume (each 1% of gas removed should in principle cause a decrease of 1% in the reservoir pressure), it is a good instrument to gauge the amount of reserves remaining after a number of years of production. After 5 years if you have taken out 1/3 of the gas from a 15 year field the usual pressure will have dropped equally by 1/3 of the original pressure, so that after 5 years a competent operator would know how much gas he has left in his field. If you have an initial pressure of 2000 PSI and after 5 years you have 1500 PSI the operator should conclude that he has 15 years worth of gas left. The survey helps the operator to have a better understanding of the well performance