Estimating fractions of reinjected water return to produced fluids using natural tracers

建立了集总参数油藏模型，用于估计回注水返回采出流体的比例。该模型主要由产层和回注层组成，采用连续方程对流体的化学成分和质量进行计算。这些层可以代表生产层和回注层以及生产井和回注井的组合。通过文献调查，发现氯离子（Cl）、水的稳定同位素等化学成分是合适的天然示踪剂。特别是Cl，由于其在储层中的不反应性和保守性，可以成为一种很好的示踪剂。利用该模型估算了回注水返回采出液的比例。估计方法采用在线分析算法，在测量的每个时刻估计分数的最优值。对日本三个不同地热区的实测数据进行了分析。各个领域在开发模型时所考虑的化学成分方面表现出不同的特征。估计的分数随时间而变化，这意味着回注计划的变化，如重新定位回注井。分数随时间的变化也表明，回注量的变化并不直接取决于回注方案，而是取决于储层和/或生产条件的变化。对模型的进一步改进包括：1)储层体积估算；2)多口井馏分同时估算

Lumped parameter reservoir models were developed for estimating fractions of reinjected water return to produced fluids. The model basically consists of production zone and reinjection zone, and employs continuity equations to chemical constituents and mass of fluid. These zones can represent a combination of production and reinejection zones as well as production and reinejection wells. The chemical constituents such as chloride (Cl), stable isotope of water were found to be appropriate natural tracers through literature survey. In particular, Cl can be an excellent tracer because of its nonreactivity and conservativity in reservoirs. Fractions of reinjected water return to produced fluids were estimated with this model.Estimating methods employ on-line analysis algorithm and estimate optimum values of fractions at each moment when measurements were made. Field data from three different geothermal areas in Japan were analyzed for the fractions. Respective field showed different characteristics in terms of chemical constituents that were considered in developing the model. Estimated fractions vary with time which implies changes in reinjection program such as relocation of reinjection wells. Changes in fractions with time also suggests magnitude of reinjection return varies not directly by reinjection program but due to changes in reservoirs and/or production conditions.Further improvement of the models includes1) estimating reservoir volume2) simultaneous estimation of fractions in multiple well