Modeling Gas Migration, Distribution, And Saturation In A Structurally And Petrologically Evolving Tight Gas Reservoir
Authors: Davis John S. , Belien Isolde , Liu Xiaoli , Dougherty Susan 
Published: March 2015

Early prediction of the distribution and saturation state of natural gas in tight gas reservoirs results in improved development and production strategy, improved recoverable volumes estimates, and decreased costs associated with development and production. Three-dimensional basin modeling coupled with three-dimensional invasion percolation-based migration modeling of the NW German Basin shows promise for prediction of gas distributions and saturations in a tight gas reservoir with a complex history.

In the NW German Basin tight gas accumulations are found in conventional traps in sub-Zechstein Carboniferous Westphalian C & D reservoirs (porosity &lt; 10%, permeability near 50 microdarcies). Top seal is provided by the Zechstein evaporites. Westphalian A and B coals are the primary source for gas. The complex structural history includes folding at the end of the Variscan Orogeny, extension-related subsidence culminating in the Late Jurassic to Early Cretaceous, and Late Cretaceous contractional inversion. The reservoir received multiple charges as the thick coal interval matured.

The basin model is initialized at the base of the Namurian, 330 million years ago. Changes in the three-dimensional structural configuration, porosity, and permeability are modeled in 5 million year time steps to present. The basin model outputs are imported to the migration software, which is run in a dynamic mode. Dynamic mode allows investigation of the impact of trap structural reconfigurations and multiple charges on the final distribution and saturations of gas in the traps.

The outcome of the basin and migration modeling shows reasonable fidelity with known drilling results in the Carboniferous tight gas reservoir. Accumulations are predicted where they are known to exist, and there is a good match between predicted areas of moderate (40–60%) and high (&gt;60%) gas saturations and the wells that penetrate those areas and have similar saturations. While the results are promising, more test cases with good calibration will be required before full confidence in the methodology is gained.

Introduction

Tight gas sandstone reservoirs have been exploited for decades, but it has only been in the last 10 to 15 years that this type of gas accumulation has been investigated in detail. Out of those investigations came the concept of basin-centered-gas (e.g., Law, 2002), which profoundly influenced both exploration and production concepts for tight gas sandstone. Basin-centered gas generally assumes local vertical gas migration from source to reservoir with best producing accumulations centered in the deeper portions of basins. In contrast, some tight gas sandstone fields have been shown to comprise conventional traps with conventional up-dip gas migration to fill them, for example the Jonah Field in Wyoming, USA (Shanley et al., 2004). The contrasting models predict contrasting production characteristics; basin-centered gas predicts gas 'everywhere' with best production toward the basin center, whereas the conventional model predicts gas accumulations in conventional traps with best production near the crests of those traps.