Block-X Oilfield in Bohai Bay (China) is a typical offshore heavy oil reservoir, which has an oil viscosity of 24-452 mPa·s, an average porosity of 31% and stronger permeability heterogeneity (average 2000 mD). Before polymer flooding, over 10 years water flooding achieved a recovery degree of 13.5% and water saturation reached more than 60%. Some results and lessons involving a successful scale application of associative polymer flooding in Block-X Oilfield were presented in this work.
From September 2003 to May 2005, the first single well polymer flooding pilot (1 injector and 4 producers) in Block-X reservoir was conducted using an industrial associative polymer (AP-P4) to verify the injectivity and offshore application feasibility. Based on the lessons and optimizations from single well pilot, from November 2005 to June 2011, the well-group associative polymer flooding trials (10 injectors and 35 producers) were further performed to identify the long-term stability, dynamic response characteristics and oil displacement effect. Afterwards, the larger-scale polymer floodings were gradually expanded and implemented (now 24 injectors and 105 producers).
The single well pilot, well-group trial and larger-scale application exhibited anticipated dynamic response characteristics and remarkable oil displacement effect. As of June 2019, the associative polymer (AP-P4) has been accumulatively injected for 0.31 PV (equivalently 99,183 tons of polymer consumption), approximately 4.8 million m3 of crude oil has been increased accumulatively, and more than 7.1% oil recovery increment has been achieved. Average single well injection pressure increased from 7.2 to 8.9 MPa and the increase amplitude is 24%. Associative polymer presentsed a multi-layer mobilization characteristic during polymer flooding and lower produced polymer concentration (< 250 mg/L) could be monitored. There were three different dynamic characteristics in producing wells. First one is that some individual wells with higer initial water cut (> 80%) showed significant response of oil production increase but water cut decrease. Typically, in J16 central producing well, oil production increased from initial 71 to maximum 126.9 m3/d, water cut decreased from initial 97% to minimum 40% and mostly kept a relatively stable value around 60% (or 70%) for more than 8 (or 6) years. In B-2# central producing well, although significantly decreasing liquid production, oil production enhanced from initial 44 to maximum 118 m3/d and water cut decreased from initial 81% to minimum 34%. Second one is that some wells with moderate initial water cut (50%~70%) not presented remarkable water cut decrease but the climb of water cut was controlled and tended to be stable for a long time. And third one is that some newly drilling wells with lower initial water cut (25%~30%) showed lower polymer flooding water cut than continuous water flooding using numerical simulation.
The results from single well pilot, well-group test and larger-scale application demonstrate that associative polymer flooding is suitable for offshore chemical EOR, even for the offshore heavy oil reservoirs with significant heterogeneity. Furthermore, the successful scale application also provides some lessons for chemical EOR in onshore and offshore heterogeneous reservoirs.