Assessment of Residual Hydrocarbon Saturation with the Combined Quantitative Interpretation of Resistivity and Nuclear Logs
- Zoya Heidari (The University of Texas at Austin) | Carlos Torres-Verdín (The University of Texas at Austin) | Alberto Mendoza (ExxonMobil Exploration Company) | Gong Li Wang (Schlumberger Sugar Land Product Center)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- June 2011
- Document Type
- Journal Paper
- 217 - 237
- 2011. Society of Petrophysics and Well Log Analysts
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- 281 since 2007
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Estimation of residual hydrocarbon saturation remains an outstanding challenge in formation evaluation and core analysis. Standard interpretation methods for nuclear-resistivity logs cannot distinguish between mobile and residual hydrocarbon saturation. In extreme cases, fluid pump out or production testing are the only options to ascertain whether the reservoir’s in-situ hydrocarbon is mobile.
We develop a new method to distinguish mobile from residual hydrocarbon and to quantify residual hydrocarbon saturation. The method combines modeling of resistivity and nuclear logs with the physics of mud-filtrate invasion to quantify the effect of residual hydrocarbon saturation on both nuclear and resistivity logs. This strategy explicitly takes into account the different volumes of investigation of resistivity and nuclear measurements and does not assume that the near-borehole region is flushed to the level of residual hydrocarbon saturation. The method begins with an initial multi-layer petro physical model which is constructed via standard procedures of well-log interpretation and core measurements. Thereafter, we simulate the physics of mud-filtrate invasion and the corresponding resistivity, density, and neutron logs. Initial estimates of residual hydrocarbon saturation and parametric relative permeability are refined until achieving a good agreement between simulated and measured neutron and density logs. Next, we refine initial estimates of water saturation, porosity, and permeability until securing a good match between numerically simulated and measured resistivity logs.
The method of interpretation considers two specific options for implementation: (1) quantification of the influence of residual hydrocarbon saturation on the radial distribution of fluid saturation due to invasion, and (2) appraisal of invasion effects on the vertical distribution of fluid saturation within a flow unit that exhibits both hydrocarbon and water saturation in capillary equilibrium.
Application examples are described for the cases of tight-gas sand reservoirs invaded with water-base mud (WBM) and oil-bearing reservoirs invaded with oil-base mud (OBM). In the case of tight-gas sands, our method explains the marginal productivity of deeply invaded beds that exhibit cross-over between density and neutron logs. For a 15-porosity unit formation, when the residual gas saturation increases by 10 saturation units, the cross-over between neutron and density logs increases by 2.4 porosity units. Interpretation results indicate measurable sensitivity of nuclear logs to residual hydrocarbon saturation in cases of deep WBM invasion due to immiscibility between invaded and in-situ fluids. However, the accuracy of the method decreases with increasing values of both hydrocarbon pore volume and hydrocarbon density. In the case of OBM invasion, reliable estimations of residual hydrocarbon saturation are possible with relative density differences above 15 per-cent between mud filtrate and in-situ hydrocarbon.
|File Size||10 MB||Number of Pages||21|