Transmissibility Corrections and Grid Control for Shale Gas Numerical Production Forecasts

Abstract

In the context of shale gas production, the very low effective permeability of the formation leads to flowing conditions that are essentially transient. Even after months or years of production, the pressure drop remains mainly localized around the hydraulic fractures. Using an unstructured grid, finite-volume simulator, we show that the non-linear nature of the pressure field around horizontal wells with multiple hydraulic fractures can have a non-negligible impact on shale gas production forecasts. We first show a very simple synthetic production example with a purely linear PVT (Pressure Volume Temperature). In this case, standard (linear) transmissibility derivations overestimate the forecast after 10 years by 5%, compared to the analytical solution. We propose a new approach for transmissibility derivations, based on numerical integrations of source point solutions. Resulting transmissibility values account for the strong non-linearity of the pressure field in the vicinity of the fractures and for fracture interferences. As a consequence, forecasts are significantly improved. With a real gas PVT, non-linear effects become even more critical in the vicinity of the well. While analytical solutions only partially account for these effects, numerical simulations are more accurate, provided that the grid is fine enough. In order to reduce the computational cost, long-term simulations are usually performed on a coarser grid, with coarse transmissibility corrections obtained from near-well upscaling techniques. We show that even if near-well numerical upscaling is extremely robust for conventional problems, the choice of an optimal simulation grid size becomes essential for shale gas. A recently proposed automatic adjustment of the grid to the considered problem (including permeability and time resolution) is tested. © 2012, IFP Energies nouvelles.