Discrete Natural Fracture Uncertainty Modelling for Produced Water Mitigation: Chuandongbei Gas Project, Sichuan, China

The Chuandongbei project is estimated to contain a resource base of 5 TCF (1400 × 108 m3) of gas in five Triassic carbonate reservoirs. It is expected to produce at a capacity of 740 MMscf/d (2100 × 104 m3/d) for several years. There is evidence indicating that natural fractures are present in the reservoir: well test deliverabilities are substantially higher than expected from matrix permeability alone, and natural fractures are observed in cores and image logs. These potentially conductive fractures pose the risk of aquifer water breakthrough to wells. We evaluated the impact of natural fractures on water production using an Experimental Design approach. Traditionally, fractured reservoirs are represented as dual-porosity models that simplify the matrix-fracture interaction. In this work, we instead represent faults and natural fractures explicitly as unstructured Discrete Fracture Models, which are then flow-simulated using the INTERSECT† simulator. We considered uncertain parameters associated with the fractures (density, size, and permeability) as well as matrix permeability and gas-in-place. We also evaluated the trade-off between two different well patterns, designed to mitigate reservoir compartmentalization and water encroachment respectively. There was a concern that natural fractures would cause early breakthrough from encroaching aquifer water. However, our results showed that fractures can be beneficial to gas production in some situations but detrimental in others. This study highlights the non-intuitive consequences of natural fractures in a gas field that has an aquifer. It also points to the importance of reducing geological uncertainty (e.g., fracture characteristics and matrix properties) for effective mitigation of water production. Finally, our probabilistic approach illustrates a framework to optimize the well pattern in the face of this uncertainty.