Comparisons of Various Algorithms for Gas-lift Optimization in a Coupled Surface Network and Reservoir Simulation

Abstract

Gas lift is used for gas-liquid two phase producers to boost the production. Increasing gas liquid ratio GLR reduces the average fluid mixture density in the well connections and therefore reduces the hydrostatic pressure drop and hence reducing the bottomhole pressure resulting in a higher production rate or a longer individual well production period. This technique is also used for deep offshore fields injecting the gas at the toe of the FPSO risers located at the seafloor.But as the lift gas supply is increased further, friction pressure losses in the tubing or the riser become more important and the production rate peaks then starts to decrease. Also high gas flowrates may induce corrosion problems, and the total amount of available gas-lift is limited. Therefore an optimal gas-lift rate exists for a maximum production rate or more generally a maximum production profit.Various gas lift optimization algorithms have been proposed in literature for optimizing gas-lift to individual wells or within a group of wells, very few are suitable for long-term reservoir development studies with gas-lift injection within a network (FPSO riser toe for example).Commercial simulators such as Eclipse, VIP, Nexus, … provide internal optimization tool which are convenient for gas-lift at wells connections. Optimizing gas-lift within a network with additional gas rate inside flowlines or risers becomes much more complicated since the THP limits of all the other wells in the network are affected. Each time a lift gas increment is added or substracted, the whole network must be rebalanced with the appropriate lift gas rate in order to recomputed the change in the field oil production rate. The paper reviews some of the optimization methods available in commercial coupled surface network and reservoir simulators such as Resolve -Gap-Eclipse, Nexus, Avocet, … Several optimization algorithms linked to a surface facility model coupled to an open reservoir simulator prototype have been also applied (Ensemble based method, Steepest-Descent, Gauss-Newton, BFGS, ..) and compared to previous commercial software solutions.Results of the comparisons are presented on a surface network model coupled to a reservoir simulator where the objective is to optimize the gas-lift allocations at riser toes.One of the test cases based on a modified SPE 9 comparative test is fully documented and allows the readers to compare results with their own optimization algorithm. Introduction Problem statementThe optimization problem is to maximize daily hydrocarbon production by optimally selecting lift-gas rates subject to the pressure and rate constraints of surface facilities. The problem has been addressed by several authors in the oil industry. Fang and Lo 1 proposed a linear programming technique to allocate lift-gas rates. Hepguler et al. 2 uses a sequential programming (SQP) optimization algorithm. Wang et al. 3 continued on the Fang and Lo's work. The optimization step is invoked at the Newton iteration level of a reservoir simulator and becomes a part of...

Integrated application of flow pattern map for long-term gas lift optimization: a case study of Well T in Indonesia

“…These approaches have been developing as an answer to increase practicality and long-term use of gas lift systems. The second approach is mainly developed in conjunction to the need for field wide gas lift optimization and scheduling or injection allocation, with sophisticated algorithms taken from the field of nonlinear optimization, machine learning and genetic algorithm (Ranjan et al 2015;Wang and Litvak 2008;Lu and Fleming 2011;Posenato and Rosa 2012;Samier 2010;Salazar-Mendoza 2006;Deng et al 2019).…”