Technology Policy Study Of Gas-cap Low-permeability Viscous Oil Reservoir Development

Jiangsu oilfield Wei8 fault block is a low-permeability and viscous oil reservoir with small gas cap and bottom water,located in Chian structure of Gaoyou sag northern slope.It is characterized by multiple pay zones,thin formations,poor physical properties and strong heterogeneity.The production-injection contrast is severe during production,resulting in low water-flooding efficiency.It is necessary and urgent to study the appropriate development technology to improve the oilfield development and therefore increase the oil recovery factor.Based on the development characters of gas-cap,low-permeability and viscous oil reservoir,various disciplinary technologies and theories,including reservoir engineering,fluid flow mechanics,reservoir description,tertiary oil recovery technology and horizontal well technology,are used to analyze the reservoir water-flood law and the remaining oil distribution,and study the exploitation policy.of development mode,strata series and production-injection well pattern.Then,the adjustment plan is made in view of the contradictions and problems during water injection development.The water flood development law study shows that the water injection utilization is lower,the reserves is produced unevenly,there are more low efficiency flow areas and the finial oil recovery is lower than other similar reservoirs.The remaining oil study shows that the potential areas in the upper strata series include W8-3,W8-8,W8-29,and W8-12well zones,and the potential areas in the lower strata series include W8-29,W8-12,W8-30 and W8-13 well zones.The development mode study shows that the highest oil recovery can be obtained by injecting gas in gas cap to produce oil first and followed by gas production,then is to inject water in flanks to produce oil first and ollowed by gas production;active water flooding is better than conventional water flooding.Development strata series and production-injection well pattern study shows that production is better in formations with max-min permeability ratio lower than 5;for vertical wells pattern,the optimum well-line space ratio of straight composite pattern is 1.0?1.2,while it is 0.8?1.0 for the staggered pattern;for vertical wells and horizontal wells pattern,the optimum well-line space ratio for straight composite pattern is 0.8?1.0 and the penetration ratio is 0.5?0.7,and the optimum well-line ratio for the staggered pattern is 0.6?0.8 and the penetration ratios is 0.3?0.5;high oil recovery factor can be obtained by drilling horizontal wells in relatively low-permeability and high-viscosity layers,and the length of horizontal section should be controlled within 300m.In the new plan,the development strategy is adjusted to produce the oil first,followed by gas production with water injection in oil column to maintain the reservoir energy.The reservoir is subdivided into three strata series.Infill drilling is carried out near oil-water transition zone.Continuous tubing is applied to produce the remaining oil near the gas cap.The total number of producer and injector in the new plan is 14.The oil recovery factor will increase by 5.63%,resulting in increased oil reserve of 14.95 × 104 tons.This adjustment will lay a foundation for the future COZ injection to increase the oil recovery further.