Jiankun Yang

The increasing of deepwater oil field developments brings a growing need for thermal management to prevent hydrate and wax formations in the subsea production system, due to the low environment temperature and long-distance transportation. Pipeline insulation coating is a typical strategy for thermal management. In a subsea production system, pressure, temperature, flowrate, and length of each flowline vary, leading to different thermal performances of the transported fluid. Therefore, the insulation coating should be carefully designed from the overall perspective to minimize the total material volume, thus reducing the cost. In this paper, an optimization model for the insulation material volume of a rigid subsea flowline system is proposed. Then, the best insulation thickness of each subsea flowline can be determined under given flow parameters and temperature requirements. The factor that defines the temperature drop from the riser base to the top termination is introduced and analyzed. There is a proper temperature drop factor associated with the insulation material volume for subsea flowlines, as well as a proper insulation capacity for the risers. This optimization model can define the subsea system insulation and provide reliable results for cost estimation.

The increasing of deepwater oil field developments brings a growing need for thermal management to prevent hydrate and wax formations in the subsea production system, due to the low environment temperature and long distance transportation. Pipeline insulation coating is a typical strategy for thermal management. In a subsea production system, pressure, temperature, flow rate and length of each flowline vary, leading to a different thermal performance of the fluid inside. Therefore, the insulation coating should be carefully designed from the overall perspective to minimize the total material volume, thus reducing the cost. In this paper, an optimization model for the insulation material volume of a wellhead-manifold-FPSO production system is proposed. Then, the best insulation thickness of each sub-sea flowline can be determined under given flow parameters and temperature requirements. The factor that defines the temperature drop from riser base to the top termination is introduced and analyzed. There is a proper temperature drop factor associated with the insulation material volume for subsea flowlines, as well as a proper insulation capacity for the risers. This optimization model can define the subsea system insulation and provide reliable results for cost estimation.

As the tendency of the offshore oil industry is going deeper and further, the subsea pipeline is exposed under tougher condition combining lower temperature with higher hydrostatic pressure. The severe condition creates a challenge towards flow assurance, which often results in a high cost solution. Reducing the cost while providing a qualified insulation performance is of great significance to deepwater development. For ultra-deepwater beyond 1500m, single-wall pipe usually fails to meet the flow assurance requirements or requires a huge amount of insulation material. Pipe-in-pipe configuration can provide a good insulation performance but comes with a high cost associated. Sandwich pipe is a new concept composed of two concentric steel pipes separated by a cementitious composite annulus that provides a combination of high structural strength with thermal insulation. It is reported to be a promising alternative for both flexible and rigid conventional pipes in applications for long distance pipelines. In order to further investigate its feasibility in deep waters, a subsea production system with depth at 2200m was used as a case study for a comprehensive evaluation of insulation performance of the sandwich pipe, including both steady-state and shut-in working conditions. For a comparative study, scenarios using single-wall pipe (SW), pipe-in-pipe (PIP) and flexible pipe (FP) were also considered separately. The results showed that (i) sandwich-pipe performs better in steady-state but worse in between shut-in and the restart period (ii) sandwich-pipe with larger diameter performs better than it with smaller diameter. The reasons for the sandwich pipe behavior were discussed and suggestions to improve the performance are presented.

QHD 29-2 block is a newly discovered offshore oilfield of China with an average water depth of 27.6m, covering approximately 2300 km2. Waterflood is not suitable due to the characteristics of complex lithology, strong heterogeneity, small pores and throats, and low water injectivity. In addition, a wide range of CO2 content (24-90%) in the production gas that presents a difficulty for the mixed gas handling. Thus, near-miscible flooding of the production gas re-injection is proposed for the development of this reservoir. It is concluded that the loss of displacement efficiency can be compensated by the mobility improvement with the pressure decrease. Thus, oil recovery is comparative with that of miscible flooding. Also, the economic and operational costs declined to some extent because smaller gas volumes and lower compression are needed with the reduction of pressure. Slim tube test, slim tube simulation, and selected empirical formulas are used together to determine the near-miscible pressure interval under the condition of different CO2 contents (40%, 55%, 70%, 85%, 100%). On this basis, a new empirical formula was established to determine the lower and upper boundaries of near-miscible flooding. Two new points are determined as minimum near-miscible pressure (MNMP) and minimum miscible pressure (MMP), respectively. A higher MMP can be expected by this process which is consistent with the argument that some successful application of CO2 miscible flooding are actually partial miscible flooding or near-miscible flooding. Results showed that the ratio of MNMP to the newly defined MMP is about 0.8 to 0.86. In addition, for the wells QHD 29-2E-4 and QHD 29-2E-5, the lower limit of the CO2 content in the production gas to achieve near-miscible flooding are 34% and 64%, respectively. Moreover, the relative error of the new empirical correlation is less than 10% for the prediction of the pressure interval of near-miscible flooding in the targeted wells. To the best of our knowledge, this is the first time to study the potential of implementing near-miscible flooding by the production gas re-injection with varying CO2 content in an offshore reservoir of China. The results in the paper can provide a new screening criteria for both enhancing oil recovery and reducing greenhouse gas emissions for similar offshore reservoirs.

The development of a subsea field implies a complex design procedure with very high costs involved. The combination of low oil price, harsh environment, very deep waters and high drilling costs has emphasized the need for new ideas to reduce both CAPEX and OPEX. The paper considers different subsea layout scenarios of a typical deep water field. Each scenario is optimized considering pipeline length, equipment cost and flow assurance. The software QUE$TOR is then applied to estimate the cost of each optimized scenario. Two of them are selected for further studies, FPSO connected to six slots subsea manifolds, and subsea separator connected to eight slots cluster manifolds with tie-back to an existing platform. The latter representing a hybrid solution based on the subsea-to-shore concept. Cost and technical feasibility are considered in order to evaluate the two scenarios.