Saturation Monitoring, Sweep Evaluation and Identification of Bypassed Oil in a Heterogeneous Carbonate Reservoir, Raudhatain Field, North Kuwait
The Mauddud Formation in North Kuwait is a giant carbonate reservoir undergoing pattern sea water flooding for past 9 years in absence of aquifer support. The reservoir has a 48 year production history with reservoir pressure depleted below bubble point with only 4% of inplace oil produced from few wells of high productivity index. In addition to extreme reservoir heterogeneity, there is areal and vertical variation in oil properties. Flood water break-through in producers has varied from 6 months to 3 years. The reservoir pressure has shown uneven increase and is close to initial pressure at structurally favorable areas. In order to optimize production and sweep in the water flood, it is important to identify the layers of premature water breakthrough and monitor subsequent saturation changes.
Pulsed Neutron Decay logs are the conventional logging tools used for analyzing high saline formation water encroachments in Burgan and Zubair reservoirs of the field. With less saline sea water injection, these logs are not sensitive enough to evaluate the encroachments as neutron capture properties of injected water and oil are closer. Mixed formation and injected water pose further challenge in Mauddud Reservoir, but we have found that Pulsed Neutron Scatter logs (C/O) logs in combination capture logs are useful to monitor water movements.
We have carried out an integrated study using C/O log water saturation from 19 wells with production logs, well performance, core data and structure for evaluating sweep, saturation monitoring and thief zone identification. The C/O logs have been quality checked and optimized for different vendors, available tool sizes, number of logging passes, oil/water contribution from PLT, actual volume of produced oil/water and effect of oil gravity. The integrated approach enabled identifying thief zones in perforated and unperforated layers in wells and mapping their areal distribution. Stochastic and deterministic water saturation models biased to petrophysical and structural trends were used to identify swept zones and bypassed oil.
The quantitative results from C/O logs were useful in high porous zones only when an integrated approach eliminated the effects of hold up, filtrate invasion, oil quality and Shale volume. But, our results show that the technique is qualitatively useful for water flood monitoring, planning infill wells and in formulating perforation policy to target bypassed zones while avoiding thief zones.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009