3D UDR-DTB Inversion for Geosteering in Complex Carbonate Reservoir

Abstract Problem Statement/Objectives/Scope A recent appraisal well in a complex carbonate reservoir revealed a resistive reservoir unit mapped by Ultra Deep Resistivity Distance to Boundary (UDR-DTB), Oil samples from this unit showed Gas-Oil-Ratio (GOR) three times higher than observed from other wells in the field. To confirm these findings, a second producer well was planned to map and target the new identified unit to confirm appraisal well findings using UDR-DTB to overcome the associated geosteering challenges. Well placement was complicated by a severely karstified sequence boundary and uncertainty in seismic's velocity model. Furthermore, the presence of karst poses drilling risks due to wellbore instability. This paper discusses the planning, execution, and analysis of a horizontal producer using advanced 3D UDR-DTB geosteering technology. The 360° inversion provides volumetric resistivity distribution around the borehole enabling real-time mapping of the karst structures and target zone beyond from the borehole. Post-well analysis demonstrated how the UDR-DTB 360° inversion can be used to enable better interpretation of seismic data aiding future well planning. Methods, Procedures, Process A pre-drill model was developed using nearby well data and seismic information to assess the feasibility of the 3D UDR-DTB and select optimize tool spacing and Borehole Assembly (BHA) design. The BHA included Near-bit Gamma Ray (NBGR) for early detection of shale or coal-filled karsts from the above Laffan formation. Sonic porosity was chosen for operational risk mitigation. A resistivity image log was recorded for structural and facies assessment. During drilling, real-time 360° tensor data were acquired using 3D UDR tool. Cloud computing was utilized for 2D azimuthal pixel-based inversion. Interpolating the 2D transverse inversion slices generates a 3D resistivity volume. This volume is then filtered to identify geo-bodies, including karst features, fluid distribution and target formation top. The 3D resistivity volume for both laterals filtered on the top of the reservoir is presented in Figure 1. Results, Observations, Conclusions The initial lateral was landed 29 ft TVD below top Mlshrlf to maintain safe offset from the karsts. Inversions mapped a high-resistivity layer 9 ft TVD above the wellbore which was targeted to assess Its properties. However, after drilling 550 ft, severe karstification was encountered, preventing further drilling progress. The 3D Inversion mapped a collapsed body with an 80 ft diameter and 150 ft vertical extent. This deeper insight Into the Laffan-MIshrlf geometry was used to design a sidetrack plan, avoiding major karst features and enabling a safer trajectory. The sidetrack successfully bypassed a deep karst and built inclination to Intersect the high-resistivity target zone. Inversions from the lateral holes showed strong structure depth correlations. Post-drill analysis on the 3D UDR-DTB data confirmed a strong correlation between the karstified zones, and seismic anomalies. Overlaying the UDR resistivity volumes on the seismic cross-section demonstrated that conductive Intervals aligned closely with seismic depressions. This paper highlights the potential that using 3D UDR-DTB with a 360° Inversion can provide valuable information for real-time geosteerlng decision s. Additionally, It enables existing seismic interpretation through ant-tracklng analysis. These findings provide valuable Insights Into the field as a whole and allow for better planning of future wells.