Physical Basis for a Cased-Well Quantitative Gas-Saturation Analysis Method
- F. Inanc (Baker Hughes Incorporated) | W. A. Gilchrist (Lone Star Petrophysics) | R. Ansari (BP America Inc.) | D. Chace (Baker Hughes Incorporated)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- December 2014
- Document Type
- Journal Paper
- 598 - 617
- 2014. Society of Petrophysicists & Well Log Analysts
- 1 in the last 30 days
- 160 since 2007
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Saturation monitoring has traditionally used either pulsed-neutron-capture (PNC) logs for saline water environments or carbon-oxygen (C/O) logs for freshwater cases. Other pulsed-neutron measurements have historically been used as qualitative gas indicators. Recent developments have made it possible to produce a quantitative gas-saturation analysis. The physical principles, modeling, and the interpretation methodology of this new measurement form the subject of this paper.
The physics for this new approach are based on neutron-induced photon transport. Neutron and photon transport are influenced by a number of factors including borehole contents, completion, formation mineralogy, porosity, formation fluids, and shale. Reducing the impact of parameters not related to formation gas saturation is important. One method to reduce unwanted interferences is to base the measurement on the photons generated from the neutron inelastic-scattering interactions.
Incorporating modeling into the process can be used to account for measurement effects introduced by the fluids, completion and mineralogy. If sufficient information exists, it is possible to model the system and predict minimum and maximum expected measurement values in gas and liquid-filled formations. The Monte Carlo code MCNP models include full tool and completion geometry, borehole fluids, and formation minerals and fluids.
Modeling has been used to produce a large database of gas-measurement responses for a range of commonly encountered completions, formations and fluids, and these form the basis of the interpretation method. In addition, special models can be run for nonstandard completion geometries or other unusual conditions.
IntroductionReservoir-performance monitoring has traditionally been based on measurement techniques using high-energy neutrons. This situation is especially true for cased holes because fast neutrons can penetrate through casing material quite easily and interrogate the formation beyond the casing and cement layer. Over the years, various measurement techniques have been developed using fast-neutron beams. Most techniques employ pulsed-neutron generators emitting neutrons with enough energy to induce inelastic scattering interactions in the target zones. The capture of the thermalized neutrons plays a role as well.
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