The Thermal Recovery Methods and Technical Limits of Bohai Offshore Heavy Oil Reservoirs: A Case Study

Abstract

Heavy oil accounts for a large proportion in offshore petroleum reserves. For Bohai offshore oilfield in China, about 85% of the OOIP is the heavy oil. Heavy oil has become an important form to guarantee the offshore oil production. Considering the limited space in offshore oil platform, cold production method is the commonly-used development method. But for some heavy oils with higher viscosity (>350cp), cold production method is less effective, and thermal recovery process will be a better choice. In this paper, we focuses on three different heavy oil reservoirs from Bohai offshore oilfield, including the blocks of NB35-2, LD-1 and LD-2. These three blocks essentially represents the main heavy oil reservoir types of Bohai oilfield (edge-water, bottom-water and thick oillayer). Then through the analogical analysis between onshore reservoirs and offshore reservoirs, the development methods for the three blocks are determined firstly. In this process, we make a survey on the development status of onshore heavy oil reservoirs in China. Then, based on the geological properties of the three blocks, a set of numerical simulation runs are performed to analyze the influence of many sensitive factors (e.g., reservoir depth, thickness, permeability, net-to-gross and water-zone). After that, through the computation of net present value (NPV), we reevaluate the economic limit indexes of thermal recovery process in Bohai offshore heavy oil reservoirs and determine the technical limits. From the analogical results, it is concluded that different heavy oil reservoir will have different thermal recovery method. For the three heavy oil reservoirs, cyclic steam stimulation (CSS) process is a potential EOR method for block LD-1, and steam flooding is a better choice for NB35-2, and LD-2 could adopt the methods of steam flooding process and SAGD process. From the numerical simulation results and NPV results, we found it is not economical to perform a CSS process in LD-1. For the other two blocks, NB35-2 and LD-2, the economical cumulative oil-steam ratios under the corresponding thermal recovery methods are calculated. And the technical limit of thermal recovery process in Bohai offshore heavy oil reservoirs are derived. It shows that a steam flooding process is suitable to the edge-water heavy oil reservoirs in Bohai oilfield whose heterogeneity is relatively weak, formation is relatively thin (10m<thickness <25 m), and water zone energy is low (water/oil ratio<5). And a steam flooding process is suitable to the heavy oil reservoirs in Bohai oilfield whose thickness is higher than 30m, net-to-gross is greater than 0.55, and reservoir depth is less than 1400 m. This work is useful to extend the thermal recovery process in Bohai offshore heavy oil reservoirs. It will play an important role for the effective development of offshore heavy oil reservoirs.