--> Complex Petrophysical Studies to Evaluate the Safety of an Underground Gas Storage in Porous Rocks

AAPG ACE 2018

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Complex Petrophysical Studies to Evaluate the Safety of an Underground Gas Storage in Porous Rocks

Abstract

Underground storage of natural gas plays a decisive role for the reliability of energy supply. In this context the integrity of the reservoir and its seal is essential.

We are currently investigating a former natural gas reservoir as possible pore space underground gas storage with respect to the safety of the structure. The reservoir is situated within the eastern part of the Bavarian Molasse Basin (Germany). To check the stability and tightness of the storage strength, in situ stresses and in situ threshold pressure measurements were carried out on drill core material from an exploration well. The core covers the reservoir and its seal. Our investigations will be completed by cyclic compressibility tests to get insight into the poroelastic behavior and its change with cyclic loading/unloading of the reservoir rock and the seal. The compressibility tests are part of the joint scientific-industrial research storage safety project SUBI.

The seal is a smectite-rich clayey to silty marlstone. Its porosity is in average 2 %. Threshold pressure measurements gave values between 10 and 15 MPa pointing to a brine permeability of 10-7 mD. The reservoir consists of fine grained sandstone and calcareous marl. Its porosity scatters due to lithological differences between 3 and 33 %. Gas permeability varies as a function of porosity between 10-1 to 103 mD.

In situ stresses were investigated in 3D on drill core material from the seal marlstone and the reservoir sandstone with the RACOS® (Rock Anisotropy Characterisation On Samples) method. Additionally, the changes of in situ stresses with increasing pore pressure were determined. The main principal total stress equals the vertical overburden pressure, while the high pore pressure effectiveness in the sandstone leads to a significantly lower effective vertical stress in the reservoir. The main horizontal effective and total stresses are oriented NNW-SSE to N-S as a result of the alpine orogenesis.

Both, seal and reservoir rock are weakest perpendicular to an E-W trending fault zone and show signs of ductile failure.

The combination of stress and strength data let us assume a high stability of the storage system. Even leakage due to a high gas injection pressure is unlikely due to the relatively high threshold pressure in the range between 10 and 15 MPa.

The planned cyclic compressibility tests will prove if loading and unloading of the reservoir and its seal has an impact on the stability and integrity of the storage system.