Water Flood Fronts Monitoring with Ultra-Deep 1D and 3D Resistivity Inversions, A Pilot Case Study in a Mature Carbonate Reservoir, Offshore Abu Dhabi, UAE

Abstract

Abstract The concept of drilling multilateral wells has been applied offshore UAE since the 1990’s, in order to increase the productivity of low permeable sublayers. The subject, mature Carbonate Offshore field is one of the pioneers to the application of Ultra-deep Azimuthal Resistivity technology (UDAR). The lateral drains target different layers with different petrophysical properties within the Upper Jurassic sequence. Efficient geosteering and the innovative (UDAR) technology applications are key components to secure the fluid fronts mapping in a heterogeneous, water-flooded zones. This case study sheds the light on the vital steps for preparing, executing, and evaluating multilateral well geosteering and geomapping, and how this can be applied to well placement within water-flooded formations. The plan was to drill an 8 ½" mother hole for data gathering, followed by batch drilling a set of two six inch horizontal, lower and upper drains with approximately 4000 ft MD of lateral length, each. A four and three-quarters OD (UDAR) tool has been deployed. Advanced processing with a real time 1D/3D inversions algorithm maximized the tool’s capability to provide measurements with a high depth of detection to map formation and fluid boundaries. Pre-well Geosteering modelling was generated to demonstrate the expected the tool responses and simulate that in 1D inversion. Real time monitoring was conducted using 1D/3D inversions. In the execution stage a four and three-quarters inch triple combo tool string, equipped with high resolution borehole imaging capability was combined with the (UDAR) LWD assembly. The lower drain targeted reservoir B. During drilling, the UDAR inversions identified the sudden unexpected rising of the conductive zone below, interpreted to be a shallowing water contact. Associated with this conductive zone, several high angle features were identified and mapped real-time with 1D UDAR inversions and borehole imaging measurements. These sharp boundaries appeared to influence the water movement. The complex nature of the flooded zone necessitated the application of 3D EM inversion to better understand the relationship between the high angle dislocations and the water. Analysis indicated these dislocations have an orientation running NW-SE. An encountered, operational problem was encountered while opening the upper drain window, resulted in the requirement to cement and re-drill the lower drain section with a different azimuth. During the re-drilling phase, the 1D Inversion mapping confirmed the same dislocations of the water flood zone, in a self-validating practice. The improved reservoir understanding from the lower lateral was applied in the upper lateral horizontal section to enable successful placement and comprehensive fluid reservoir mapping. Successful placement of these laterals brought increased reservoir understanding. Identifying the complex nature of the oil-water contact identification, which is also complicated by faulting. Well placement and production optimization are improved by this increased understanding.