Full-Scale Immiscible WAG Optimization and Management in Yarakta Project (East Siberia)

Abstract

Abstract The paper describes the optimization (NPV maximization) of East Siberian Yaraktinskoe field immiscible WAG project after 5 years of its successful operation. The project currently comprises 74 WAG injection wells, 290 mmscf/day of provided gas injection, 2 compressors with injection to WAG/gas cycling/gas plant. The project execution has revealed operational issues with lack of gas injection after water injection in 6 wells and gas breakthroughs with decrease of liquid/oil rates by 40% in 6 wells. Analysis of the issues has led to necessity to set and control target average reservoir pressure for all immiscible WAG patterns. In addition, it was required to set target gas/water cycle duration considering ESP performance in gas breakthrough conditions. Based on fine-scale simulation of typal WAG patterns with VFP-tables, patterns performances were calculated for various target average reservoir pressures and target gas/water injection cycle durations. We analyzed the sensitivity of the performances to geological and operational parameters. The obtained results were used to calculate NPVs in various target settings. The calculations provided maximal NPV or NPV per injected gas (gas efficiency) option settings selection for each pattern. The procedures provided selection of WAG strategy (max gas efficiency without extra compressors), target average reservoir pressure range and target duration of gas/water injection cycles for each WAG pattern. The targets are used for regular calculations of gas and water injection target rates for WAG patterns. The targets and simulation results provide compilation of time-dependent WAG efficiency rating for injection wells to allocate available gas in accordance with it. The rating was used to make optimized full-scale WAG plan with injection schedule for all injection wells with corresponding injection rates. The plan implies increase of gas to water injection duration up to 1:1 month and up to 260-290 bar (initial reservoir pressure and bubble point pressure is 255 bar, minimum miscibility pressure is 400 bar) as target average reservoir pressure range. Pressure fall-off test interpretation procedure with appropriate accuracy was selected after validation in various synthetic cases and comparison with other procedures. WAG efficiency curves (criteria) were updated to monitor WAG performance relative to them. WAG efficiency analysis workflow was created. The WAG optimization has enabled to increase the WAG project NPV by 72% and increase the compressors’ operation NPV by 17%. The authors present novel procedure of full-scale immiscible WAG management. It considers target average reservoir pressure and duration of gas/water injection cycles as key operational targets. The procedure includes consideration of WAG strategies, generation of gas allocation rating for WAG wells relative to gas refinement/cycling options. Surveillance implies monitoring of reservoir pressure, gas efficiency, returned gas and WAG extra oil relative to target ranges obtained from the sensitivity analysis. WAG efficiency analysis workflow provides optimization adjustments.