Abstract
Two challenges confront the development of Bohai heavy oil resources: first is the high development costs, which requires an economic limit production of 100,000 m3 for a single well; in addition, the unfavorable geological conditions, including thin payzones and deep burial, cause serious heat losses and unsatisfactory oil recovery under large well spacings. Therefore, this study aims to solve such technical issues as "efficient production with fewer wells" and "high recovery with large well spacings".
A recovery strategy is developed: improved recovery efficiency during the Cyclic Steam-N2-CO2 Stimulation (CSS) stage, and effective conversion from CSS to steam flooding (SF), and uniform heating the reservoir during the SF stage. This attempts to address the inefficient heating in thin pay zones, lack of experiences on SF-after-CSS with large well spacings, and difficult control of temperature field under irregular well patterns.
Learnings are summarized as following: (1) A new technique, weak gel-gas-chemical agent assisted CSS, is proposed. The co-injection results in relatively higher cycle oil production and smaller steam-oil ratio after multiple cycles. The oil recovery is improved from 23.0% to 32.8%, and return on investment (ROI) from 1:1 to 9:1. (2) The threshold pressure gradient is dynamically characterized for offshore wells with large-well spacings. Quantitative identification for the timing for CSS-to-SF is established on the basis of reservoir temperatures. The threshold well spacing is increased from 80 m to 200-300 m, and the corresponding oil recovery boosted from 32.8% to 43.5%. (3) On the basis of the three-level architecture characterization of meandering river sand body, a method is proposed to adjust the temperature field in the reservoir during the SF stage. It improves the aerial sweep between large-spaced wells from 72% to 87% and the oil recovery from 43.5% to 47.0%. (4) The aforementioned techniques have been applied in NB35 reservoir and obtained success, which achieved an incremental oil production of 38.5×104 t and a recovery degree of 32% in the pilot test area.
The study proved SF as a feasible follow-up process of CSS in offshore heavy oil reservoirs with deep burial, thin pay zones, and large well spacings. It provides great support for the efficient development of Bohai offshore oilfield, as well as a new direction for similar reservoirs in the world.