NATURAL GAS. 
17 
mined the intensity of the resulting pressure in the reservoir ; that is, 
a high degree of compression produced a high rock pressure, and a 
low degree of compression produced a low rock pressure. Typical 
rock pressure decline curves are shown in figures 3 and 4. 
FIG. 3. DECLINE IN BOCK PBESSUBE OF NATURAL GAS WELLS OF THE 
UNITED FUEL GAS CO. IN KERMIT FIELD IN KENTUCKY AND WEST 
VIRGINIA. 
WHY KOCK PRESSURE AND VOLUME MUST DECLINE. 
The rock pressure and volume must decline as gas is removed, 
because in the removal of the deposit of gas we are confronted with 
the following: 
1. — A fixed volume of the reservoir. 
2. — A fixed amount of gas inclose^ in this fixed reservoir. 
3. — A certain rock pressure resulting from the contraction of the 
gas volume into the fixed reservoir. 
Now, if a part of this fixed volume of gas is removed by tapping 
the reservoir from the surface of the earth, the remaining gas volume 
90682 — 18 — Bull. 102 2 
