Carbonate reservoirs are often characterized by karst features occurrence, usually related to a significant permeability enhancement in presence of low porosity and low permeability matrix type sediments. The distribution of such karst features is generally highly heterogeneous and difficult to predict, making the reservoir management challenging. In Zubair Field (Iraq), there are numerous evidences of karst events within the Upper interval of Mishrif Formation. The production behavior of Upper Mishrif is therefore very heterogeneous, moving from wells with relatively low flow capacity, as expected from petrophysical interpretation, to wells with a very high flow capacity, hence related to karst enhanced permeability. The integration of petrophysical interpretation, well test and multi-rate production logging allowed to preliminary highlight the improved permeability intervals associated to karst. In addition, accurate image log analysis on the same wells investigated a possible relationship between vug densities and production data, to be extended also to wells lacking the latter data. This process allowed to define a karst flag in more than 60 wells. Then, correlations between karst features and different seismic and geological attributes were identified. The most meaningful parameters were used as input data for a Neural Net Process, leading to the definition of a probability 3D Volume of karst occurrence. The final outcomes of the workflow are karst probability maps, used as a driver for the definition of new wells targets and associated trajectories. The recent drilled wells, with optimized paths according to these prediction-maps, have demonstrated the reliability of this approach intercepting the desired karst intervals. This study represents a valuable opportunity in terms of understanding of the reservoir behavior and impact on the ongoing intensive drilling campaign and related field performance.
During the last years, oil Majors have been struggling trying to make the unconventional business profitable. Indeed, the strategy to build an unconventional portfolio by means of merges and acquisitions is not giving enough return of investment. This is mainly due to internal processes, which contrary to Independents, are customized on a very different business model.In addition, it is becoming clear that unconventional resources cannot be considered and developed as "statistical" ones. Nowadays, several publications are stating that only a small percentage of fractured wells is generating positive return. Even though unconventional reservoirs are considered more complex than conventional ones, less efforts are unjustifiably applied for their understanding. Hence, there is a need to switch from a "drill baby drill" to a "more from less" approach. This implies to address several issues such as: a better understanding of shale gas production mechanism at nano-scale, sweet spots identification, proper fracture placement and treatment, realistic full field simulation of fractured wells. This paper describes how seismic-reservoir integration, advanced production analysis, accurate nanoscale and 3D full field simulations may address the above issues and help oil Companies to be more efficient in developing their unconventional portfolio. This new approach, based on placing and fraccing wells only where needed, is already providing interesting results in mature plays like the Barnett Shale and will be even more crucial for sustainable unconventional developments outside US.
We discuss anomalous gas production data observed downdip of oil production in a thick oil reservoir. At the onset of production, the wells under consideration exhibited a sharp rise of the GOR with large disparity between estimated gas in solution and actual gas production. This was followed by a declining GOR period. After declining, the GOR rose and then fell off again in one of the wells. Such a behavior leaves little doubt that free gas exists. The reservoir pressure is approx. 20 bars above the bubble point, so these observations contrast with the general assumption that gas below the gas/oil contact must be in solution in the oil. The fact that free gas comes in peaks suggests the presence of large bubbles suspended in the oil at variable distances from wells. In order to understand how long a gas bubble can live in undersaturated oil and the conditions under which it can be mobilized, we analyze the system in terms of gravity-capillary-viscous force equilibria and investigate the kinetics of gas diffusion into the oil. We show that bubbles that are sufficiently large in width (hundreds of meters) and sufficiently small in thickness (a few meters) can move horizontally but not vertically (meaning that they remain suspended in the oil and are producible) and take hundreds of thousands of years to dissolve into the oil. The presence of these bubbles is compatible with the available geochemical data, which provide evidences that the source rock of gas is more mature than that of oil and probably still in the generation and expulsion phase. This unusual non-equilibrium situation was reproduced by means of 3D simulation approach, to confirm the conceptual model adopted.
Zubair is a giant oil field located in the South of Iraq. The production started in 1951 and current oil production is around 450 kbopd achieved through 150 wells completed in two main formations: Mishrif (carbonate) and 3rd Pay (sandstone). The scope of this paper is to show how an integrated methodology based on core analysis, open-hole and cased-hole logs unlocked the underneath potential of a sand layer (L1) with an anomalous resistivity. Multiple wells, indeed, show resistivity curves in the L1 interval with surprising low values with respect to the average of other levels of the same sandstone reservoir. Therefore, fit-for-purpose open-hole (OH) and cased-hole (CH) log acquisitions have been integrated with information from cores and dynamic data (i.e. production logging) in order to better understand the phenomena behind the low resistivity scenario. As a consequence, several perforation extensions have been performed with L1 as the main target, providing an overall improvement of hydrocarbon deliverability without any increase in water production. In details, routine and special core analyses in L1 samples delineate the typical setting of a fine-grained low resistivity pay sandstone, able to host a large quantity of irreducible water. However, such behavior is not always present among L1 cores. Therefore, a methodology aimed at characterizing this sandstone behavior was mandatory. Nuclear magnetic resonance logging, commonly used to identify low resistivity pays, was not a suitable option due to bad-hole problems. Hence, an approach based on a detailed integration of OH resistivity and CH pulsed neutron logging (PNL) is used to recognize and characterize such low resistivity pay. This method mainly relies on the fact that formation water is very conductive and strongly affects the resistivity, while its effects on PNL measurements are not so pronounced. Such intuition is confirmed by multi-rate PLT interpretations that dynamically describe the L1 sandstone with fair productivity index and high reservoir pressure, together with a significant dry production contribution. In conclusion, a clear geological trend of L1 resistivity behavior is revealed and associated to the decreasing cementation of the matrix and its coarsening in the same direction. The integrated OH/CH methodology allows characterizing low resistivity intervals as pay zones. Such achievement represents an important milestone for the perforation strategy of new and existing wells in Zubair. As a natural consequence, the overall field production has been enhanced by widely applying the new technique without any increase in water-cut.
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