228 Orton — On the Origin of the 'Rock Pressure 



with the escape of gas from a high pressure well are among 

 the most startling in the whole range of mining engineering 

 operations. 



There is generally a rough relation between the depth of 

 the well and its rock pressure. The deeper the well, the 

 greater the pressure to be expected, but this relation is by no 

 means constant. Wells of the same depth below the surface 

 may vary widely in rock pressure. 



It is the original rock pressure which is needed in these in- 

 quiries, and this can be obtained at but one date in the devel- 

 opment of a field, and that is at the very opening. Reduction 

 of pressure speedily follows exploitation, and goes forward 

 more or less rapidly as the development proceeds. When a 

 field is locked in, however, after having been opened, there is a 

 tendency to regain the original pressure. The pressure falls 

 through considerable areas at the same time when drainage of 

 gas is going on from any point within them. The rock pres- 

 sure is a factor of great importance in every gas field. By it, 

 the distance to which the gas can be sent to market, the size of 

 the tubing employed and also the size and strength of the pipe 

 lines are all to be determined. 



To what is this rock pressure due? Three answers have 

 been proposed, viz : (1) it is due to the weight of the rock that 

 overlies the reservoir ; (2) it is due to the expansive nature of 

 the # gas itself ; (3) it is due to the pressure of the salt water 

 column which holds joint occupation with it of the porous 

 rock. The facts derived from the new fields demonstrate the 

 truth of the last answer, so far at least as Trenton limestone 

 gas is concerned. 



A column of salt water, one square inch in section, one foot 

 in height, and having a specific gravity of 1*1 weighs (and will 

 consequently exert a pressure of) about 476 pounds avoirdu- 

 pois. 



Knowing as we do the height to which the salt water rises 

 in the Ohio and Indiana gas fields, when the porous rock con- 

 taining it is reached by the drill, viz : 600 feet above tide, we 

 find in this figure an element that we can employ in every 

 subdivision of the field. If the gas is found at sea-level, it 

 will have upon it a weight of 600 X '476 lbs In other words, 

 its rock pressure should be 286 lbs. If the gas is found above 

 sea-level, its rock pressure should be reduced in proportion to 

 the reduction of the water column; if found below sea- level, 

 its pressure should be correspondingly increased. 



The facts derived from the development of the new fields 

 furnish data by which the validity of the explanation above 

 given can be tested. 



At Muncie, Indiana, the gas was found in the first wells at 



