Abstract
Seismic interpretation is commonly used to comprehend the structural nature of a field. However, due to its resolution, high uncertainty can lie in identifying essential geological properties such as fault throw, geometry, displacement interval, and strike direction. Geology exists in three-dimensional space, relying only on conventional logs during drilling may lead to improper steering decisions when attempting to understand these complexities. This paper presents real-time mapping of Oblique fault and water zones using advanced real-time 1D/3D Ultra-deep Azimuthal Resistivity (UDAR) inversions as a solution.
The target Well is an oil producer aiming for the optimum zone in a ~45 feet thick carbonate (reservoir A). Geophysical evaluation prognosed drilling a down-dip structure and encountering 4 major faults ranging within 10 - 45 feet displacement. Offset well analysis highlighted uncertainty regarding the reservoir fluids, with possible water influx from a flooded reservoir B below the target reservoir. Geosteering and Geomapping technology was therefore selected, and pre-well scenarios were generated to address the above risk. A comprehensive Logging While Drilling (LWD) suite was deployed to drill 6″ Horizontal section and facilitate comprehensive real-time formation evaluation: 4 ¾″ UDAR; high-resolution Laterolog resistivity imaging, gamma ray, propagation resistivity, density/porosity. Steering relied upon an intelligent push-the-bit rotary steerable system for precise and fast real-time placement.
Drilling started with 1D UDAR inversions confirming the wells position in the lower part of Reservoir A. As drilling progressed, a sudden change was detected by both near and far-field measurements. Conventional log correlation confirmed this abrupt reading was typical of a fault signature. The real-time 1D/3D inversion confirmed the fault down throw showing an approximate 60 feet displacement stratigraphically, penetrating formation Z_Dense above target zone. A decision was made in a timely manner to steer down into the optimum zone of Reservoir A. Simultaneously, water-flooded properties were mapped below trajectory within the lower part of the reservoir, hence the well was maintained close to the top boundary. Three faults were mapped in the drain, one major and two minor. The integration of UDAR 3D inversion with borehole imaging was processed to provide a full 3D reservoir view. The first major fault was confirmed to be oblique, with a strike towards NNW-SSE as aligned with the field's structural tendency. Combining far and near-field LWD measurements increased confidence in the interpretation.
Real-time UDAR 1D / 3D inversion coupled with borehole imaging provided major support for accurate real-time steering, which leads to increasing reservoir contact, maximizing well life, and reducing well time and cost. For long-term benefits, this valuable insight had enabled refinement of geological models and improvement of reservoir field-scale strategies.
