ENVIRONMENTAL AND PETROPHYSICAL EFFECTS ON DENSITY AND NEUTRON POROSITY LOGS ACQUIRED IN HIGHLY DEVIATED WELLS

摘要

Conventional interpretation methods of nuclear measurements for vertical wells can produce incorrect calculations of porosity in complex rock formations. The case of logs acquired in highly deviated wells relative to sand beds may yield inaccurate estimates of bed boundary depth and porosity. We describe a systematic sensitivity analysis of petrophysical and environmental effects of neutron and density measurements acquired in highly deviated wells that penetrate sand-shale laminated sands. The study focuses primarily on the effects caused by the angle between the wellbore and formation layering. Raw nuclear tool responses are calculated with Monte Carlo simulations of generic source-sensor configurations via the code MCNP. Our objective is to quantify the effect of complicated formation conditions, such as thin laminations and high deviation angle of the wellbore with respect to the formation, on raw nuclear measurements. Moreover, we quantify the shift in vertical resolution with respect to angle of deviation. Of special interest is the case of synthetic composite rock formations of sand–shale laminations for various formation thicknesses. Simulated neutron and density porosity logs for the case of highly deviated wells are compared against the logs simulated for the case of a vertical well penetrating the same formation. Our study also describes the effect, on the vertical resolution and bed boundary detection, of azimuthal tool position around the perimeter of the wellbore for the case of a deviated well. Finally, we show simulations for the case of a gas-saturated sand bed penetrated by a high-angle well. Results from this study indicate that shoulder beds can have a significant impact on the nuclear response of thin layers penetrated by high-angle wells. Shifts in vertical resolution of 10 and 11 in. were observed in high-angle wells (70 and 85 degrees of deviation from the vertical) for a 30 in. sand bed bounded by shale beds for the case of density; similarly, shifts in vertical resolution of 14 and 16 in. were observed for the case of neutron measurements. In the case of a well deviated 70 degrees from the vertical, azimuthal effects on density measurements can originate biases on bed boundary detection up to 2.4 in., and up to 2.8 in. for neutron measurements. These results strongly suggest that improved interpretation methods are necessary to accurately estimate the porosity of laminated formations penetrated by high-angle wells.