Application of Numerical Simulation Method in Geological Reserve Calculation of Tight Sandstone Gas Reservoirs

To address the significant errors in geological reserve calculations caused by inaccurate average gas formation volume factor (FVF) in the volumetric method—particularly in tight sandstone gas reservoirs characterized by substantial pressure variation and multiple gas-bearing layers, this study applies a numerical simulation approach using pressure-dependent FVF across a three-dimensional pressure field. Taking a representative gas reservoir as a case study, an integrated geological model is constructed. The initial numerical simulation model is established based on the relationships between formation pressure and depth, FVF and pressure, as well as saturation parameters, to represent the reservoir’s original state. Geological reserves within well-controlled areas are then calculated using saturation fields and FVF fields that vary with pressure. Subsequently, automatic layer-by-layer production allocation and cumulative gas output statistics are achieved by matching single-well production histories. These are compared with the initially estimated well-controlled geological reserves to correct unreasonable results and derive a more realistic lower bound of reserves. The results demonstrate that the numerical simulation method, which accounts for the relationship between pressure and FVF, yields more accurate reserve estimates than the traditional volumetric method based on a constant average FVF. This approach effectively avoids the overestimation or underestimation of reserves due to misassigned FVF in the volumetric method. The methodology provides valuable insights and reference for the calculation of geological reserves in similar tight gas reservoirs.