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ABSTRACT Nonmiscible fluid displacement without salt exchange takes place when oil-base mud (OBM) invades connate water-saturated rocks. This is a favorable condition for the estimation of dynamic petrophysical properties, including saturation-dependent capillary pressure. We developed and successfully tested a new method to estimate porosity, fluid saturation, permeability, capillary pressure, and relative permeability of water-bearing sands invaded with OBM from multiple borehole geophysical measurements. The estimation method simulates the process of mud-filtrate invasion to calculate the corresponding radial distribution of water saturation. Porosity, permeability, capillary pressure, and relative permeability are iteratively adjusted in the simulation of invasion until density, photoelectric factor, neutron porosity, and apparent resistivity logs are accurately reproduced with numerical simulations that honor the postinvasion radial distribution of water saturation. Examples of application include oil- and gas-bearing reservoirs that exhibit a complete capillary fluid transition between water at the bottom and hydrocarbon at irreducible water saturation at the top. We show that the estimated dynamic petrophysical properties in the water-bearing portion of the reservoir are in agreement with vertical variations of water saturation above the free water-hydrocarbon contact, thereby validating our estimation method. Additionally, it is shown that the radial distribution of water saturation inferred from apparent resistivity and nuclear logs can be used for fluid-substitution analysis of acoustic compressional and shear logs.
- Europe (1.00)
- North America > United States > Texas (0.93)
- Geology > Geological Subdiscipline (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.69)
- Geology > Mineral > Silicate (0.68)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.33)
- Europe > United Kingdom > North Sea > North Sea > Northern North Sea > South Viking Graben > Block 16/28 > Andrew Field (0.99)
- Europe > United Kingdom > North Sea > North Sea > Northern North Sea > South Viking Graben > Block 16/27a > Andrew Field (0.99)
- Europe > United Kingdom > North Sea > Central North Sea > Northern North Sea > South Viking Graben > Block 16/28 > Andrew Field (0.99)
- Europe > United Kingdom > North Sea > Central North Sea > Northern North Sea > South Viking Graben > Block 16/27a > Andrew Field (0.99)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)
Inversion-based method for interpretation of logging-while-drilling density measurements acquired in high-angle and horizontal wells
Mendoza, Alberto (The University of Texas) | Ijasan, Olabode (The University of Texas) | Torres-Verdín, Carlos (The University of Texas) | Preeg, William E. | Rasmus, John (Schlumberger) | Radtke, R. J. (Schlumberger) | Stockhausen, Edward (Chevron ETC)
ABSTRACT We introduce a sector-based inversion method to improve the petrophysical interpretation of logging-while-drilling density measurements acquired in high-angle and horizontal wells. The central objective is to reduce shoulder-bed effects on the measurements. This approach is possible because of a recently developed technique to accurately and efficiently simulate borehole density measurements. The inversion-based interpretation method consists of first detecting bed boundaries from short-spacing detector or bottom-quadrant compensated density by calculating their variance, representative of the measurement inflection point, within a sliding window. Subsequently, a correlation algorithm calculates dip and azimuth from the density image. Depth shifts that vary azimuthally and depend on relative dip angle, together with the effective penetration length of each sensor, refine previously selected bed boundaries. Next, the inversion method combines sector-based density measurements acquired at all measurement points along the well trajectory to estimate layer-by-layer densities. In the presence of standoff, the method excludes upper sectors most affected by standoff to reduce inaccuracies due to borehole mud. To verify the reliability and applicability of the inversion method, we first use forward simulations to generate synthetic density images for a model constructed from field data. Results indicate that inversion improves the interpretation of azimuthal density data as it consistently reduces shoulder-bed effects. Inversion results obtained from field measurements are appraised by quantifying the corresponding integrated porosity-meter yielded by inversion methods in comparison to standard techniques that use simple cutoffs on field-processed compensated density. Integrated porosity-meter of inverted synthetic density measurements increases by 4.6% with respect to noninverted field measurements. Also, integrated porosity-meter obtained from inversion results that include only bottom sectors improved by 65.4% with respect to that calculated with field-compensated, bottom-quadrant density measurements.
- Geology > Geological Subdiscipline > Stratigraphy (0.68)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.46)
- North America > United States > Texas (0.30)
- North America > United States > Colorado (0.30)
- North America > United States > California (0.29)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.31)
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.48)
ABSTRACT Calculation of mineral and fluid volumetric concentrations from well logs is one of the most important outcomes of formation evaluation. Conventional estimation methods assume linear or quasi-linear relationships between volumetric concentrations of solid/fluid constituents and well logs. Experience shows, however, that the relationship between neutron porosity logs and mineral concentrations is generally nonlinear. More importantly, linear estimation methods do not explicitly account for shoulder-bed and/or invasion effects on well logs, nor do they account for differences in the volume of investigation of the measurements involved in the estimation. The latter deficiencies of linear estimation methods can cause appreciable errors in the calculation of porosity and hydrocarbon pore volume. We investigated three nonlinear inversion methods for assessment of volumetric concentrations of mineral and fluid constituents of rocks from multiple well logs. All three of these methods accounted for the general nonlinear relationship between well logs, mineral concentrations, and fluid saturations. The first method accounted for the combined effects of invasion and shoulder beds on well logs. The second method also accounted for shoulder-bed effects but was intended for cases where mud-filtrate invasion is negligible or radially deep. Finally, the third method was designed specifically for analysis of thick beds where mud-filtrate invasion is either negligible or radially deep. Numerical synthetic examples of application indicated that nonlinear inversion of multiple well logs is a reliable method to quantify complex mineral and fluid compositions in the presence of thin beds and invasion. Comparison of results against those obtained with conventional multimineral estimation methods confirmed the advantage of nonlinear inversion of multiple well logs in quantifying thinly bedded invaded formations with variable and complex lithology, such as those often encountered in carbonate formations.
- Geology > Mineral (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.32)