Horstmann, Mathias (Schlumberger) | Sun, Keli (Schlumberger) | Berger, Peter (OMV) | Olsen, Per Atle (Statoil) | Omeragic, Dzevat (Schlumberger) | Crary, Steve (Schlumberger) | Griffiths, Roger (Schlumberger) | Abubakar, Aria (Schlumberger)
Detection and quantification of formation resistivity anisotropy is important in many reservoirs, particularly those with laminations thinner than the axial resolution of standard measurements, when the properties of the individual layers cannot be resolved. Resistivity anisotropy evaluation is just as much important for background resistivity estimation in the overburden shales and underlying sequences where controlledsource electromagnetic (CSEM) surveys are acquired to reduce exploration risks. Formation dip determination is vital in almost all reservoirs.
Electromagnetic methods to determine the formation resistivity perpendicular to layering (Rv) and parallel to layering (Rh), in addition to the layering dip and azimuth, became available with triaxial induction measurements deployed on wireline (WL). Around the same time, deep directional electromagnetic (EM) measurements deployed on logging-while-drilling (LWD) tools ushered in the well placement revolution of remote detection of layers of differing conductivity.
With thousands of wells successfully positioned using the LWD directional EM technology, an alternate application for evaluation of resistivity anisotropy and formation dip is showing promise. Directional EM measurements are the LWD equivalent of wireline triaxial induction, also providing anisotropy measurements in vertical and deviated wells with enhanced sensitivity to formation dip decoupled from the anisotropy effect. Further, the LWD tool occupies a large portion of the borehole, displacing the mud and ensuring that the tool remains well centered, both of which reduce borehole effect in water-base mud. Finally, the availability of LWD directional EM while drilling provides the opportunity for thin-bed analysis and early decision making in difficult reservoirs.
The resistivity anisotropy and dip interpretation is based on a multistep inversion workflow optimized to take advantage of measurement sensitivities. In addition to resistivity anisotropy and formation dip evaluation in vertical and deviated wells, the technique provides quality control indicators based on inversion postprocessing.
Measurements and interpretation were successfully qualified in the Norwegian Barents Sea. Here the possibility of borehole integrity issues or its degradation over time increases the risk of having limited or no logging runs on WL. Also, planned riser-less pilot holes may allow data acquisition only while drilling. In these situations LWD resistivity anisotropy data must provide the kernel for interpretation.
The acquired resistivity anisotropy can be used as input for building CSEM models, reduce resistivity anisotropy uncertainties and support defining constraints for the 3D deep EM inversion-based interpretation. Field testing has provided good opportunities to enhance the measurements and validated the service. With implementation of improvements, an agreement with WL triaxial induction, in a range of different environments, is demonstrated. Several field test examples have proven that resistivity anisotropy and dip evaluation from LWD directional EM measurements is a viable alternative to the traditional WL service.