Sleipner gas field in the North Sea is the world's first industrial scale CO 2 injection project designed specifically to reduce the emission of greenhouse gas. Here CO 2 separated from natural gas produced at Sleipner gas field is injected into the Utsira sand, which is a major saline aquifer in the North Sea basin. From time-lapse (4D) seismic data, CO 2 plume is observed as a number of bright sub-horizontal reflections within the reservoir. Correlation of log data with the seismic data indicates that CO 2 accumulates within a series of interbedded sandstones and mudstones beneath a thick cap rock of mudstone. Nine reflective horizons have been mapped within the reservoir on the six seismic surveys from 1999 to 2008. Comparison with the baseline seismic survey of 1994 (pre-injection) provides clear impression of the migration of CO 2 plume. In this paper, we attempt to model CO 2 distribution quantitatively within the reservoir by applying a pressure-dependent differential effective medium (PDEM) theory using 4D seismic data. Preand post-injection acoustic impedances are calculated by inverting post-stack seismic data of 1994 and 2001 using a model-based inversion technique. A 3D CO 2 saturation volume is estimated using PDEM theory from inverted acoustic impedance of the year 2001 taking the reference of that from the results of pre-injection data of the year 1994. Since the gas distribution type is seldom known, we estimate the saturation distribution using both a homogeneous and a patchy distribution pattern in our rock physics model. We estimate saturation for homogeneous

Fractal dimension analysis was carried out for optimal designing of 2-D gravity survey network and to determine an optimum range of gridding interval to generate least aliased Bouguer anomaly maps. As a test case, this method has been successfully applied to the Jabera-Damoh region of the Vindhyan Basin, which is considered as a potential hydrocarbon bearing area. In particular, we aim to delineate accurately the lateral extent of a possible hydrocarbon bearing structure. To achieve this aim, fractal dimension of survey network was computed using 2-D distributions of observation points in the planning phase of the survey so that the optimum station spacing for gravity survey can be obtained. A range of optimum gridding interval for the gravity data set was suggested using the box-counting method of fractal dimension determination. Bouguer anomaly maps of the region are prepared utilizing the optimum gridding interval. For the first time, these anomaly maps clearly outline the gravity evidence of an anomalous rifted structure, which is bounded by parallel faults on either side. This structure is interpreted as a favorable basin for the occurrence of hydrocarbons. Another finding of this study has been the delineation of an apparently small ridge-like structure running east-west, dividing this basin in two parts. A subsurface geological model along a profile across the Jabera structure has also been presented.

While in constant head aquifer, groundwater level before pumping and after recovery attain a constant value, it is quite challenging to estimate the actual discharge value in an atoll aquifer where the head fluctuates with tides. Lens shaped atoll aquifers are fragile in nature and pumping causes the water table to fall which could further aggravate sea water intrusion. Thus the objective of the study is to propose an alternative method and demonstrate the importance of tide-induced head fluctuations in analysing the aquifer parameter in atoll environment. The solution to a one-dimensional unsteady groundwater flow model in an atoll aquifer with sinusoidal fluctuations of the tide shows that the tidal response method (TRM) is reliable and safe in comparison to conventional methods of aquifer characterization. The aquifer parameter is described by the ratio of transmissivity to storage coefficient, known as hydraulic diffusivity. The observations of the tide and groundwater levels using loggers at 16 wells over the Kalpeni Atoll, Lakshadweep, India show that with distance from the coast, tidal efficiency decreases roughly exponentially and time lag increases linearly. The calculated hydraulic diffusivity (T/S) values using TRM range from 764 to 1,090,000 m 2 /day. In parallel, pumping tests are conducted in six shallow dug wells to validate the TRM data. Draw down and recovery data during pumping test are influenced by the tide-induced head fluctuations. The effect complicates the interpretation of the drawdown data. After subtracting the net tidal effects from the draw down data, the calculated parameter values are observed to be closer to the estimated parameter values from TRM.

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In situ combustion is one of the approaches used for secondary recovery of heavy oil in which time-lapse seismic data might be used to track the production process. We have analyzed three time-lapse seismic surveys carried out at a time interval of one year during pre- and postcombustion phases of in situ combustion at the Balol field in India. Interpretation of these land time-lapse data using standard seismic amplitude differences in terms of reservoir changes during production and injection can be erroneous. The Balol data, in particular, lacked calibration and had poor repeatability. We have addressed these issues by demonstrating the applicability of independent prestack inversion of baseline and monitor surveys to derive elastic attributes that help to produce clearer images of fluid movement. Independent estimates of wavelets from the baseline and two monitor surveys were used in prestack inversion to estimate acoustic impedance, shear impedance, and Poisson’s ratio. The results show unexpected movement of the thermal front away from the injection wells toward the north and northwest of the injection wells, farther from the production wells. These results have been confirmed by well production data from the field north and northwest of Balol.