amount of gas usually nitrogen or carbon dioxide in it if the gas to water ratio were decreased abruptly without some compensatory action there could be undesirable dropping of proppant in areas where proppant is not wanted such as the bottom of the well hole this is known as 34 sanding out 34 one way of avoiding this is to hold the gas to water ratios initially constant while the proppant to water ratio being introduced into mixer 28 is decreased when the proppant to water ratio at the mixer 28 is reduced to less than half of the former level then the rate of gas supply from supply 23 through pump 24 is also reduced optionally with a simultaneous increase in the rate of pumping of liquid until the foam collapses if desired the gas can be discontinued completely however it is usually desirable to have a small amount of gas for example 300 standard cubic feet of nitrogen per barrel of water in the water which is supplied to the well this assists the foam in the driving out of the water during the later clean out if desired an alternative method of changing from foam to water or aerated water can be used while still avoiding 34 sanding.