Page 2 - Transmissibility Corrections and Grid Control for Shale Gas Numerical Simulation

VA - DF: Transmissibility Corrections and Grid Control for Shale Gas Numerical Simulation p 1/23
Transmissibility Corrections and
Grid Control for Shale Gas
Numerical Simulation
Vincent Artus - Dorian Fructus 0B
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. 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.
Introduction
Data history matching and production forecasting in shale gas context raises new challenges
linked to the low-permeability, fully transient context. Recently, a complete workflow has been
proposed and tested [1, 2]. As described in the cited papers, the workflow starts with the
simplest methods, such as diagnostic plots and straight line analysis, and progressively
includes more sophisticated models in order to account for increasing physical complexity. The
last two main steps, and most advanced options of the workflow are:
- A transient, analytical model for horizontal wells with multiple hydraulic fractures. This model
accounts for interferences between hydraulic fractures and allows for various fracture flow
models (infinite conductivity, finite conductivity or uniform flux). However, it does not account
for the non-linearity of gas PVT properties. Also, it cannot account for advance effects such as
desorption, multiphase flow, or unconsolidation…
- A numerical model including a large range of physical complexity, such as non-linear PVT,
complex fractures geometry, heterogeneity, non-consolidation and desorption. Although this
model is the one that should ultimately be used for forecasting, it potentially involves so many
unknowns that it should be used at the end of the analysis process only, i.e. once the main
unknowns have been significantly constrained by diagnostic tools and analytical interpretation.