PDO 4D gravity inversion is aimed at detecting lateral steam chest movement within a highly fractured carbonate oil reservoir. The PDO field uses Thermally Assisted Gas-Oil Gravity Drainage (TA-GOGD) method to produce the low API oil. The shallow reservoir rests at about 300 meters below the surface.
We use regularized least-squares inversion code to invert for reservoir 4D density changes between base and repeat. Base gravity survey is carried out in 2013 and two repeat surveys in 2015 and 2016. Two methods of gravity surveying were deployed in 2013, relative and absolute gravity surveys, to acquire a total of 302 points per each method. Only relative surveys were acquired for both 2015 and 2016 repeat surveys. Relative surveys used a CG5 gravimeters, while the absolute gravity survey used the A10 Micro-g LaCoste specialized instrument to measure absolute gravity at each point. In this study the calculated 4D gravity signal is based on the difference between the 2013 Absolute base survey and the Relative repeats in 2015 and 2016. Gravity survey data is known to suffer from different types of noise, the highest of which is the gravity overprint caused by shallow water aquifer water level changes on the reservoir gravity signal, and secondly, any changes in surface elevation. Using water level monitoring wells we estimated shallow aquifer signal and removed from the observed gravity data values prior to inversion. Surface elevation changes are tracked using InSAR satellite data. Also, observed gravity data is pre-conditioned by removing outliers and de-bias the data to improve inversion stability.
After the removal of the shallow aquifer gravity signal, the observed 4D gravity anomaly is found to vary between 21µGal to −35 µGal between 2016-2013, which is relatively comparable with an estimated forward 4D gravity signal calculated from history matched reservoir models for the same periods of time (2013-2015 & 2013-2016). The Least Squares inversion is carried out assuming a single reservoir layer of a certain (heated) thickness, inverting the 4D gravity signal to estimate the density changes at the reservoir for the periods 2013-2015 and 2013-2016. The inversion results, with the shallow aquifer compensated, do not show dramatic difference with the non compensated inversion. Assuming a thickness of the steamed/heated zone 100m, the estimated change in density varies between −29 kg/m3 to 12 kg/m3. We find the largest negative anomaly to correspond with the steam injection area. We conclude from this work that 4D gravity signal is detectable, which is the result from reservoir density changes caused by steam injection. A steam chest outline can be estimated by applying a density cut-off. Also, we conclude from this study that surface elevation changes error contribution in this field is negligible, and a single elevation survey combined with time-lapse, InSAR satellite elevation changes is sufficient for both base and repeat survey.