Abstract: Applying NMR Spin-Echo Logging to Shaly Sand Formation Evaluation: Case Studies of Rocky Mountain Region Gas Wells
Richard K. Ladtkow, Brian J. Stambaugh, Duncan Mardon, George R. Coates
Conventional wireline logs often fail to provide a reliable evaluation of reservoir quality and producibility in shaly sand formations, particularly in the shaly gas sand reservoirs of the Rocky Mountains which are characterized by low porosity (i.e., <15 pu), low permeability (0.1 to 1 md), very fine grain size, and high irreducible water saturations. A new-generation nuclear magnetic resonance (NMR) logging tool, the Magnetic Resonance Imaging Log (MRIL(FOOTNOTE®)), enhances log interpretation in such situations by providing lithology-insensitive measurements of effective porosity and the volume fractions of free (i.e., mobile) and capillary-bound (immobile) fluid volumes. This paper presents three examples from Rocky Mountain region wells in which MRIL data were crucial for resolving ambiguous or otherwise misleading interpretations from conventional nuclear porosity and resistivity log data alone.
In one well in the Almond Formation in WY, neutron and density logs show "cross-over" indicating gas-bearing beds are present at several locations in the logged interval. The MRIL shows all these beds have low effective porosity and free fluid volume (FFI) and thus are non-reservoir Conversely, the MRIL identifies one bed which has high FFI but shows no gas effect on the nuclear logs. Well testing confirmed that this bed was the dominant contributor to total gas production of the well.
In two wells in the Frontier Group, WY, conventional log responses are nearly identical -- the: nuclear logs show gas effect while deep resistivity is nearly constant at ~10 ohmm and shows little contrast with readings in nearby shales -- but only one well produces gas at commercial rates. In the producing well, the MRIL shows the Frontier interval has significant (6 to 8 pu) free-fluid volume while in the other well FFI averages approximately ~1 pu, consistent with production. In addition, permeabilities computed from MRIL porosity and free/bound fluid volume ratio show excellent agreement with core: data and reveal that the permeability-thickness of this thick, low resistivity zone is comparable to that of an overlying thin, but prolific, gas-producing sand.
FOOTNOTE ®. Mark of NUMAR Corporation
AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada