Lessons Learnt from Nine Years of Immiscible Gas Injection Performance and Sector Modeling Study of Two Pilots in a Heterogeneous Carbonate Reservoir

Abstract

The subject reservoir is a heterogeneous carbonate formation in a giant field located offshore Abu Dhabi. Five gas injection pilots were initiated in late 2001 in the Eastern, Central and Western parts of the field both as secondary and tertiary recovery methods to evaluate the benefit of gas injection for pressure support and for recovery improvement. With less than 10% of HCPV gas injection, the pilots to date have provided valuable insight on production performance and pressure support, gravity override, swelling effect and flow assurance issues (such as asphaltene deposition) in the field. Using a 3D compositional model, a sector modeling study was carried out for comprehensive evaluation of the pilot performance to date and to predict definitive results within reasonable time frame (3-5 years) which will have ramifications on long-term full field development decisions. Additionally, the objectives of simulation efforts were to evaluate different recovery processes (gas/water/WAG) and assess key reservoir uncertainty (volumetric sweep) due to reservoir heterogeneity (high permeability streaks). Initially, the sector model was history matched with nine years of pilot performance while both reservoir heterogeneity and well spacing sensitivities were tested in the model. The history matched sector model was utilized to predict performance under different operating conditions using both gas, water and water alternating gas (WAG) injection methods. This paper describes the pilot performance, field observations and results of a sector model study including history match, sensitivity and predictions under different injection scenarios on two of the pilots. Based on the performance and surveillance data gathered on the two pilots and sector modeling study, it was established that both pilots have met their objectives and can be concluded. Through the integration of field observations and sector modeling work, the study provided valuable insight on optimum recovery processes, well spacing and well completion requirements for long-term field development.

Infill drilling and well placement assessment for a multi-layered heterogeneous reservoir

“…Water flooding is the most economical secondary recovery mechanism in most reservoirs where water injection is feasible. However, injected water always follows the easiest path with high permeability streaks leaving a lot of oil un-swept between the producer and the injectors (Konwar et al 2011). In such situations, infill drilling with closer spacing is an attractive option to enhance reservoir access that accelerates oil production with more strings and at the same time; it has a chance to improve the oil recovery by increasing water swept volume.…”

A Case Study on Miscible and Immiscible Gas Injection Pilots in Middle East Carbonate Reservoir in an Offshore Environment

“…These pilots demonstrated the impact of gravity segregation on gas breakthrough and the importance of well placement in a thick pay zone. Asphaltene deposition near the wellbore was also observed in these pilots, highlighting potential flow assurance issues of gas injection (Konwar et al, 2011). Following this, two immiscible water-alternating-gas (WAG) injection pilots were performed onshore Abu Dhabi, resulting in improvements in sweep efficiency and recovery, justifying inclusion of this strategy in the field development plan (Al-Shamsi et al, 2012).…”