(Table 1) Gravity and pressure within the Sleipner area relative to ones at the reference station in 2002 and 2005

At Sleipner, CO2 is being separated from natural gas and injected into an underground saline aquifer for environmental purposes. Uncertainty in the aquifer temperature leads to uncertainty in the in situ density of CO2. In this study, gravity measurements were made over the injection site in 2002 and 2005 on top of 30 concrete benchmarks on the seafloor in order to constrain the in situ CO2 density. The gravity measurements have a repeatability of 4.3 µGal for 2003 and 3.5 µGal for 2005. The resulting time-lapse uncertainty is 5.3 µGal. Unexpected benchmark motions due to local sediment scouring contribute to the uncertainty. Forward gravity models are calculated based on both 3D seismic data and reservoir simulation models. The time-lapse gravity observations best fit a high temperature forward model based on the time-lapse 3D seismics, suggesting that the average in situ CO2 density is about to 530kg/m**3. Uncertainty in determining the average density is estimated to be ±65 kg/m**3 (95% confidence), however, this does not include uncertainties in the modeling. Additional seismic surveys and future gravity measurements will put better constraints on the CO2 density and continue to map out the CO2 flow.在斯莱普纳（Sleipner）气田，二氧化碳被从天然气中分离出来，并被注入地下咸水含水层以实现环保目的。含水层温度的不确定性会导致二氧化碳原位密度（in situ density）的不确定性。本研究于2002年和2005年，针对该注入场址海底的30个混凝土基准点开展重力测量，以约束二氧化碳的原位密度。此次重力测量的重复性在2003年为4.3微伽（µGal），2005年为3.5微伽（µGal），由此得到的时移不确定性（time-lapse uncertainty）为5.3微伽。由局部沉积物冲刷引发的基准点意外位移，进一步增加了测量不确定性。正向重力模型基于三维地震数据（3D seismic data）与油藏模拟模型（reservoir simulation models）构建。基于时移三维地震数据得到的高温正向模型，与本次时移重力观测结果的匹配度最佳，表明二氧化碳的平均原位密度约为530kg/m³。平均密度的测定不确定性经估算为±65 kg/m³（95%置信度），但该结果未包含建模环节的不确定性。后续开展的额外地震勘探与未来的重力测量工作，将进一步约束二氧化碳的密度，并持续刻画二氧化碳的运移分布。