Borehole Image and Production Logging Drives Well Interventions in Horizontal Gas-oil Gravity Drainage Wells in a Giant Middle Eastern Carbonate Field

Abstract

Some recent insights are presented into a fractured carbonate oil-rim development in Oman that is produced through both gas-oil gravity drainage (GOGD) and waterflood. Resistivity borehole image (BHI) data are used to characterize the matrix and fractures of the open-hole reservoir sections of the horizontal GOGD producers. Cores in vertical wells were used to calibrate image logs for matrix and fractures confirming, for instance, nodular fabrics seen in BHI. The BHI data are combined with mainly recently acquired production logging tool (PLT) dynamic data in GOGD producer wells to understand the dynamic behavior of matrix and fractures. Six case histories are used to show how this understanding has been used to relocate wells to reduce gas cuts, and through well interventions, isolate water influxes, and attempt to isolate gas influxes. The BHIs show a conductive fracture population that was divided into three main types based on increasing apparent aperture. The distribution of fractures partly reflects a mechanical stratigraphy. The wackestone to packstone coarsening cycles carry increased numbers of fractures with reduced fracture spacing and wider apertures in the packstone reservoir sublayers. Attempting gas shutoff in these layer-bound fractured intervals has not yet been successful. A general absence of fractures in the commonly underlying nodular wackestone layers of the GOGD system is observed. Well trajectories targeted specifically into these nodular wackestones, particularly in the lower Natih C2, have avoided fractures associated with overlying packstones with successfully reduced gas cuts allowing continuous instead of cyclical GOGD production. In addition to the layer-bound fractures, through-going fracture networks impact individual wells. Both aquifer and water injection influxes through such fractures have been successfully shut off, suggesting that the fracture networks are relatively isolated. These shutoffs have allowed previously noncontributing matrix to produce oil.