Controlling factors of gas well deliverability in the tight sand gas reservoirs of the upper submember of the second member of the upper Triassic Xujiahe Formation in the Anyue Area, Sichuan Basin

Abstract: The gas reservoir of the upper submember of the second member of the upper Triassic Xujiahe Formation in the Anyue area was recently discovered with the largest reserves in the Sichuan Basin. However, from the trial test and recovery of gas wells after being put into production, a big difference was shown between the tested and the actual well productivity. In view of this, the relationships between single-well productivity and the factors such as geological characteristic, sand fracturing, and liquid producing capacity are analyzed systematically. In conclusion, dominant factors influencing the single-well productivity are as follows. First, the depositional microfacies in underwater branch channels and river mouth bars, as well as the coarse sand bodies are favorable plays formed from paleo-hammock or palaeo-geomorphology in the Leikoupo Formation. Second, high single-well productivity is due to numerous fractures or being at the fracture-developed zones, as shown by the fact that 80 percent of the ultra-low deliverability wells are located in the region where fractures are not well developed. Third, during the fracturing treatment of high-productivity wells, the minimum pumping pressure needed is much lower than low or ultra-low productivity wells, and with a high daily flowing back rate of more than 75%. Forth, the relationship between gas and water is complex and the volume of water produced is not controlled by the structure. As for the well which does not produce water, its productivity and downhole pressure are consistent; while as for a water-cut gas well, the increase of its productivity will result in severe water invasion and sharp decline of its productivity and downhole pressure.