Turning a High-Pressure, High-H2S Oil Carbonate Reservoir Development Viable Through Miscible Gas Injection in the Sultanate of Oman

Abstract

Abstract The development of low porosity, low permeability carbonate oil reservoirs remains a challenge in the industry. Identifying a technically and economically viable development is further complicated when high pressures, high temperatures & high H2S concentrations are encountered at large depths. For such reservoirs, depletion developments are typically the only economically justifiable option but typically result in very low Hydrocarbon Recovery Factors. This paper presents a case study of a complex field located in the south of the Sultanate of Oman that includes a carbonate stringer reservoir encased in the Ara Salt. A viable Miscible Gas Injection (MGI) development scheme has been identified and turned into an economical option for this field. The field X A3EC was discovered in 2011 within the Greater Birba area. Since it was put on production in 2013, the performance of this carbonate stringer has been relatively poor compared to other analogue fields. No feasible technical solution could be identified to develop the field beyond the very low estimated ultimate Recovery Factor (~6%) achievable under depletion. Alternative developments could not be sanctioned mainly because of poor economics resulting from limited expected incremental recovery. Moreover, the drillability of additional wells could not be confirmed following subsurface re-interpretation & dynamic history matching of the available pressure data. In 2017 a study was carried out that incorporated newly collected pressure data, seismic and geological interpretations, as well as static & dynamic modeling. A new approach to history matching was also applied. This study turned further field development from technically unfeasible into an economically attractive & doable project. Moreover, new appraisal and well stimulation plans were identified to address the remaining field uncertainty on the in-place volume and well productivity. The proposed technical solution for further development of Field X A3EC involves a phased implementation of a MGI scheme which will increase the expected Recovery Factor in the target zone by more than 35 %. The drillability of the targeted field area was established by reducing the uncertainty on the reservoir pore pressure through extensive dynamic simulation work supported by newly collected long-term pressure build-up data. Field X A3EC development will feed into and benefit one of the largest projects in the Middle East - the Rabab Harweel Integrated Project (RHIP) that is currently under construction, further adding to the value of this large integrated facility. This paper highlights the challenges faced whilst developing complex contaminated oil carbonate reservoirs and identifies technically feasible solutions for further development of such fields via an integrated approach.