Application effect of free gas identification by cross-well acoustic background value in coal seam

Free gas in coalbed plays a key role for the high production of coalbed methane reservoirs, and is still difficult to be identified efficiently so far. Difficulties and challenges mainly exist in two ways:how to identify the free gas in coalbed; the general background values and overlapping methods are not enough to prove the existence of free gas. This paper constructed the cross-well acoustic background value, combined with gas saturation and production data to identify free gas. The research shows that using cross-well acoustic background value method to identify free gas can exclude the impact of coal quality and coal seam water content. This method was applied to identify free gas in coal seam No.3, Z block, wells with free gas accounted for 50% of the total analyzed wells. Through interactive tests of cross-well acoustic background value, gas saturation and production data, consistency can be achieved over 80%.