Reservoir Baffling Seen by Disequilibrium of DFA Fluid Gradients and by Wireline Pressure Transients and DSTs

摘要

In this reservoir study, three adjacent structures have been subject to largely the same fluid charge withrnan original oil charge and a very recent light hydrocarbon charge (all indicated by petreolum systemsrnmodeling and organic geochemical analyses). In addition, all wells have an oil-water contact (OWC) andrntwo have a gas-oil contact (GOC), while the GOC in the third well is not far away. Thus, allrndepth-dependent in-reservoir fluid geodynamic processes are visible within each well; no extrapolationrnaway from the wells is required. Moreover, the ~30 meter oil columns in evidently connected sands arernamenable to simple dynamic modeling thereby aiding in understanding. All three structures are found tornhave very different reservoir realizations, particualry regarding the dispositions of asphaltenes in spite ofrnnominally similar initiation conditions after the secondary charge. That is, different reservoir fluidrngeodynamic processes took place producing different constraints on production in each of the structures.rnOne well showed a light oil underlain by a tar mat. A second well showed large, disequilibrium gradientsrnof gas-oil ratio (GOR) and asphaltene content without phase separated asphaltenes in core. The third wellrnshowed a tar mat on a shale break in the oil column not far form the GOC. There were no asphaltenes inrncore above the shale break and tar mat. However, below the shale break there was asphaltene throughoutrnthe core while retaining permeability, but no tar mat on the OWC. These differences were not ascribedrnto any paleo-OWC nor are they related to differing biodegradation. In addition, these differences are notrnconsistent with any depressurization expalanation.rnTo resolve the differences, a variety of data streams were used including downhole fluid analysisrncoupled with thermodynamic analysis from the Flory-Huggins-Zuo Equation of State (FHZ EoS) and itsrnreliance on the Yen-Mullins model of asphaltenes. In addition, well test results provide key productionrninformation and are corroborated by vertical intereference testing on wireline. Petroleum system modelingrnand organic geochemical analysis of oil and core extract samples have also been very useful. Therndifferences seen in the different wells relate to several important parameters such as the different extentrnof baffling which impacts both production rates as well as equilibration rates of reservoir fluids. Anotherrnkey parameter is the extent of density stacking versus a lateral fluid front for the secondary lightrnhydrocarbon charge. Proximity to the charge point enhances the lateral nature of the charge yielding rapid asphaltene instability precluding tar mat formation in sections of the well. This information is very usefulrnwhen projecting reservoir properties away form wellbore.