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An Innovative Approach to Monitor Depletion of Reserves Integrating 3-D Static Models and Time-Lapse Saturation Logs


Tracking depletion volume and pattern is of utmost importance for any mature field but a difficult task to accomplish. Thermal Decay Time or TDT logs are very useful in assessing saturation around the bore-hole provided the stipulated conditions for porosity and water salinity are met. Time lapse TDT campaign in a field over its production life can thus give us important clue on how the water front is advancing as a result of depletion. This paper attempts at integrating the information on oil water interface in wells from time lapse TDT with 3D static model (porosity and saturation volumes) to capture both the depletion pattern and volume. The study was conducted in a sector of a large Middle Cretaceous clastic reservoir in Kuwait having a commercial production history of 76 years. The sector is well isolated through geological features like faults to ensure that there is no fluid movement in and out of the sector. The studied Sandstone reservoir is fluvial braided channel sands of Albian age of Cretaceous system. It is laterally continuous with an average thickness of 100m. Strong bottom water drive and high intergranular porosity of the rock in general results in excellent sweep. The water salinity is very high (150,000 ppm) and hence TDT log are effective in differentiating oil and water in the reservoir. The study estimated the reserves (remaining oil) of the sector in two successive years using the highest known water surfaces generated from TDT data at wells. The difference between the reserves should ideally be the production in that year. The computed volume was found matching well with the actual production. Three cases of volumetric calculations were performed using 0.6, 0.7 & 0.8 as recovery factors and the results were compared with the production from the 137 wells completed in this reservoir. It was observed that the best match was found when we used the accepted recovery factor of 0.7. The study also established the pattern of movement of the water front, i.e. how it has moved over time. The study thus established an innovative way of tracking depletion in a brown-field using time lapse TDT. Further it can be used to estimate the life of field and to plan infill drilling locations in the sector to optimize production and enhance recovery.