A Novel Approach to Gas Lift Design for 40,000 BPD Subsea Producers

Abstract

Gas lift in Terra Nova production wells serves two purposes. The first is to increase the rate at which reservoir fluids can be produced, especially as water cuts increase with time and reservoir pressure diminishes. The second is to stabilize flow in the wells, flowlines and flexible risers. This paper describes the application of engineering tools to achieve an optimized subsea gas lift design for a 6350 m3/d (40,000 bpd) production rate, taking into account reservoir, wellbore, subsea and topsides constraints. Introduction The Terra Nova development is located on the Canadian Grand Banks, 350 km (220 miles) East-Southeast of St. John's, Newfoundland. The oilfield is being developed with an ice-strengthened, double hulled, floating production-storage-offloading vessel (FPSO) and subsea wells, (Fig. 1). Subsea equipment and trees are located in large ‘glory holes’ to protect this equipment from scouring icebergs. The focus of this paper is the Terra Nova gas lift design. The prime objectives of the Terra Nova gas lift design were to determine gas lift injection rates, valve setting depths and orifice sizes that would ensure maximum well productivity. Combined well and flowline performance was analyzed using a multiphase network hydraulic simulator, which determined the maximum transport capacity of a complex well and flowline system, while ensuring that reservoir simulator, wellbore and facilities constraints were being honoured. The deliverability analysis considered the production rates, water cuts and reservoir pressures provided by reservoir simulations to explore a range of reservoir depletion scenarios, from ‘pessimistic’ to ‘optimistic’. Wellbore and flowline studies were done to ensure that gas lift rates and orifice sizes would result in stable flow in the wells, flowlines and flexible risers. This design work also investigated the potential to simplify well completions by eliminating gas lift unloading valves. Various well startup scenarios were examined. Choke discharge coefficients for the dual-check operating valves were determined in lab testing to ensure accurate modeling. 1.0 Reservoir Description The Terra Nova reservoir (Fig. 2) has been mapped as a number of large blocks, defined by major faults. Within these blocks are many smaller blocks separated by minor faults. The reservoir geological model is represented by six sand sequences and seven shale sequences. Four of the major blocks will be developed in the initial 24 well Terra Nova development. Three reservoir blocks will be developed with pressure support by waterflood. The fourth block will be developed with updip gas injection, with water-alternating-gas (WAG) as a future option. The target reservoir for this study, the North East (N.E.) block, is a syncline, with the centre dipping towards the South. Seismic has identified faults that traverse the N.E. block in an East-West direction. In addition, subseismic faulting is suspected. Water injection wells are located in the valley of the syncline, while producers were drilled updip along the West and East boundaries of the pool. Table 1 summarizes the reservoir properties. 2.0 Well Description Wells in the field were designed as 178 mm (7") monobore completions, meaning that the tubing is the same diameter as the cemented liner through the zones of interest. The monobore design facilitates future well interventions for production logging or zonal isolation. The anticipated high rates required that 178 mm tubing be selected for the producers. 178 mm tubing was also chosen for water and gas injectors to reduce friction pressure drop and maximize injectivity. A typical production well is illustrated in Figure 3. Numerous horizontal producers were originally envisioned, but high well productivities and reservoir fault location uncertainties have led to more inclined than horizontal producers being drilled.

SAGD Gas Lift Completions and Optimization: A Field Case Study at Surmount

“…For this stability assessment, a transient gas lift program was used to model the dynamics on the lift gas side and across the downhole chokes/nozzles. This is the same methodology used for single-point gas lift injection in subsea applications (4,5) . Figure 2 is a typical plot for a given set of conditions.…”

Reservoir Management Challenges of the Terra Nova Offshore Field: Lessons Learned After Five Years of Production

“…The well tests have been analyzed with both conventional welltest software and by a specially developed welltest imaging program 13 . A lot of effort has been spent on integrating the well test results into the reservoir management.…”