US energy firms added 15 billion cubic feet of natural gas to storage in the week ended August 21, below market expectations of 20 bcf and marking a smaller increase than both the 17 bcf injection recorded during the same period last year and the five-year average of 33 bcf. The latest build also slowed slightly from the 16 bcf increase reported for the week ending August 14. Total inventories rose to 3.184 trillion cubic feet, leaving stockpiles around 0.9% below levels seen a year earlier but still approximately 5.5% above the five-year average for this time of year.