Abstract
Waterflood has been long considered as a cost-effective way to significantly add worldwide reserves. Oil displacement by injected water is a main mechanism of waterflood; however, common factors including vertical heterogeneity, interwell connectivities, and lack of injection and production control lead to less-than-expected flood performance. The intensive data monitoring and interpretation are a key to understand ongoing flood performance. The case study of Sirikit oil field is a good example of waterflood in complex multilayer thin-bedded reservoirs.
The use of diagnostic plots and analytical tools were applied to investigate ongoing waterflood in different levels (area, reservoir, and well). The sweep efficiency plots were used to compare the waterflood efficiency among 13 different areas in Sirikit field. The impacts of key parameters including injection duration, pore volume injection, reservoir depositional environment, level of reservoir depletion when waterflood started, drive mechanism, and flood pattern on incremental recovery factor were scrutinized. In a reservoir level, the estimated recovery factor from production data, reservoir and fluid properties, and Modified-Y plot were compared to give more prudent numbers. The capacitance-resistance model (CRM) was selected to understand the travelling directions of injected water from production and injection data. In well level analysis, this study expands the vertical heterogeneity measurement from the use of core data to injection logging data via the Lorenz plots and the Dykstra-Parson coefficients to capture conformance effects. The vertical heterogeneity maps were constructed from calculated coefficients and well locations to shred some lights on the distribution of reservoir complexity. In addition, Hall's plot was selected to diagnose injection problems.
From analysis, the depositional environment plays an important role in sweep efficiency as areas with clastic braided reservoirs yield higher recovery gain from waterflood than areas with fluvio-deltaic reservoirs. The plots of recovery gain versus pore volume injection highlighted the areas with relatively poorer flood performance, which require further mitigation plans. The results suggest the sooner waterflood started, the higher recovery gain is anticipated. Waterflood in solution gas drive reservoir tends to yield higher incremental recovery gain from waterflood than gas cap drive reservoir. The recovery factor calculated from analytical methods confirms the numbers estimated from a decline curve analysis method. CRM indicates that most injected water (70% to 100%) flows toward producers, which shows good interwell connectivities. The well analysis suggests the spatial distribution of moderate to high vertical heterogeneity. Hall's plots indicate that the 70% of total injectors have experienced injection problems mainly from plugging.
These insights are critically beneficial for future waterflood screening and evaluation, reservoir management, well conformance control, and injector treatment. This quick analysis will assist not only the waterflood performance improvement but also the future EOR decisions.
