USE OF NMR TECHNOLOGY TO UNRAVEL GAS-BELOW-FWL AND CHARGE HISTORY ISSUES IN A ROTLIEGEND GAS FIELD, NORTHERN NETHERLANDS

摘要

Accurate determination of fluid saturations in many Rotliegend gas fields in the Northern Netherlands is hampered by subtle variations in clay type and morphology in combination with the possible presence of residual gas. In recent years it was investigated to what extent current generations of NMR logging tools could provide extra information on these issues and reduce uncertainties. Schlumberger's CMR-plus tool was successfully run on drill-pipe in B-mode EPM in one of our wells drilled with OBM. Significant information has been obtained from this log: 1) The presence of gas below the FWL, tentatively evaluated from the conventional logs, was independently confirmed by the CMR via its low apparent porosity (because of the low hydrogen index of gas) in comparison with the bulk density-derived porosity. 2) Because of an apparent large discrepancy between the CMR-derived permeability (based on a core-calibrated Coates equation) and the core-derived permeability (based on porosity-permeability relationships) below the FWL, it is concluded that the residual gas is present in the clay-associated micro-porosity of the rock. The standard translation of NMR T2-distribution into a pore size distribution therefore breaks down, because a fluid with a high intrinsic T1 and T2 will show up in the FFI part of the T2-spectrum, whereas on the basis of pore size it should show up in the BVI part. This also explains why the gas has not been flushed or dissolved by oil filtrate, as was the case in a nearby well in a different field where no residual gas is thought to be present. 3) Analysis of the gas saturation above the FWL, both in this well and the neighbouring well with core, using a combination of NMR, resistivity-derived saturation and capillary pressure curves, reveals that the residual gas present in the micro-porosity exists up to the top of the reservoir, indicating that the early gas charge has leaked away entirely before being followed by a secondary gas charge leading to a secondary drainage saturation profile. The residual gas is not in direct contact with the secondary gas, and its amount was quantified by the difference between the resistivity-derived and the capillary pressure curve-derived saturations. 4) The uncertainty of the position of the FWL in this well, because of limited pressure information and possible depletion from the neighbouring well, was resolved through this analysis.