Publications

Lost returns of drilling mud are common while drilling Oligo-Miocene carbonates and can present a serious well control problem. Achieving zonal isolation for production perforation is very important to optimally manage a carbonate reservoir production during its life and can be quite challenging after experiencing severe losses. It is important to study losses during previous drilling campaigns to better understand the causes, predict where the greatest risk lies and to apply the best techniques to achieve zonal isolation. This study of Cepu and other carbonate buildup losses and cementing experience will help Mobil Cepu Ltd to better quantify the risks and plan the zonal isolation strategy for the upcoming 42 well drilling campaign. The main part of this study examines losses experienced in 6 previous wells in the Banyu Urip field plus incorporates knowledge from other wells in the Cepu Block. Losses ranging from seepage to total losses have occurred frequently in these wells. This has caused well control problems, drilling inefficiency and safety issues. Cement quality has been influenced by the presence of losses in the carbonate section. Full returns cementing have been achieved in several wells with losses but a few have not achieved full returns. One strategy is to ensure losses zones are fully cured during drilling with DOB2C (Diesel Oil-Bentonite-two Cement). However, if this cannot be achieved, swell packers can be employed as a backup plan. After study, losses appear to have occurred mostly in the tight rock of the drowning cap of the reservoir which is characterized by fractures and hydrothermal leaching. Losses also occurred in the clastic section concentrated mostly in 50’ sections * Mobil Cepu Ltd. (MCL) above the top carbonate reservoir that are carbonate cemented and brittle. Many sidetracks have been drilled close to wells with catastrophic losses without experiencing losses at the same depth indicating that losses are primarily vertical features rather than horizontal super-permeability zones. This makes losses very difficult to predict. Borehole images and drilling data were utilized to characterize the loss correlation with open fractures. FMI logs show that losses are correlated more to chaotic FMI signatures than to open fractures. The chaotic zones are interpreted to be hydrothermally leached fault systems and cave collapse breccia in those zones. As the result of this study, some recommendations have been made to optimize the future Banyu Urip development drilling, such as: (i) losses are more likely in wells with thick tight drowning cap sequences (ii) to avoid severe well control problems, it is important to case off as much of the gas cap before drilling into losses zones necessitating a 250 ft of safety distance above top carbonate, (iii) attempt to cure losses fully with DOB2C before cementing in early wells and if that fails, use backup plan of swell packers. These recommendations have been implemented in the current Banyu Urip Development drilling program in designing the well, formation evaluation program and completion strategy.