This paper was prepared for presentation at the 8th Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, U.A.E., 11-14 October 1998.
As a common production aspect of Thamama formation (carbonate reservoir), in both onshore and offshore of Abu Dhabi, unexpected early-water breakthrough through specific high-conductive zones has been observed in several water injection schemes. Observed field data, such as PNC logs, indicated non-occurrence of injected water slumping away from well bores. A concept of "Capillary Force Barrier" was introduced several years ago to tackle this issue, in which the role of capillary force on cross flow in stratified layers is modeled1,2,8. This paper tries to revisit and fine-tune the concept in hysteresis of a moderately-oil wet system. In this attempt, firstly, some synthetic measurements of special core analysis and the interpretations are presented, in which the results are analytically formulated by a published methodology to generate saturation functions consistent with the hysteresis under an assumption of wettability5. An application to numerical reservoir simulation was carried out in a systematic manner, as the Reservoir Rock Type (RRT) scheme of the model was based on primary drainage curves that can be fully linked with the generated saturation functions. It is demonstrated on cross sections how small difference in imbibition capillary pressures can affect the water movement through complicated RRT contrast in moderately oil-wet system. The proposed formulation is a powerful tool for generalizing saturation functions as matrix properties in a consistent manner and open door to systematically incorporating not only hysteresis but also wettability into the numerical reservoir simulation model. Introduction There are many giant carbonate reservoirs in the Middle East areas in which some of Thamama formations are well known water injection schemes are applied to the reservoirs known as oil-wet. The injected water, however, does not slump and instead move through thinner high-conductive zones that has been considered as one of key reasons for unexpected early-water breakthrough to oil producers. In order to explain the phenomena, a concept of "Capillary Force Barrier" was introduced to model the role of negative imbibition capillary pressures in water displacement process for oil-wet system2. Meanwhile, in a general sense, since carbonate rocks are more heterogeneous, it is very difficult to characterize them with considering geologic features so as to systematically apply the above concept to reservoir simulation. On the other hand, numerous papers have described detailed measurements of special core analysis to emphasize importance on some of specific core characters such as capillary pressure, relative permeability, wettability and so on. There seems to be, however, not so much elaboration3,4 about modeling results of the measurements in an integrated manner because of the data availability and poor link with geologic features that is the most important guide to distribute the petrophysical parameters in numerical reservoir simulation models. Therefore, we intend, in this paper, to develop a systematic generation scheme of saturation functions as rock matrix properties for reservoir simulation model. The targets of this work are as follows.Consistent view of specific saturation functions through special core analysis and the analytical formulation.Understanding mechanism of "Capillary Force Barriers" in the formulation.Incorporating wettability into reservoir simulation in a consistent manner. It is worth mentioning that not only the formulation but also consistent RRT schemes are keys for successful reservoir simulation modeling. A concept of RRT contrast will be also highlighted.
The development strategy for a new field is based on the data available from seismic surveys and exploration and appraisal wells. Limitations in this information may still leave large uncertainties in field performance predictions leading to a low confidence in ranking of reservoir recovery methods. Typical development planning based upon single point deterministic reservoir modeling may not be sufficiently flexible to cover all possible outcomes in an optimum manner. This problem may be largely overcome by adopting a more geostatistical approach in which the data uncertainties are reflected within probability distributions for relevant reservoir parameters. In addition to reservoir uncertainties the importance of fully integrating drilling, surface facilities, and economic constraints is sometimes not obvious during the initial reservoir screening exercise. The ranking of reservoir recovery methods and related development strategies may be radically altered by imposition of these constraints. Analysis of key decision points and the risks associated with them may be utilised to assist in the formulation of field development strategies that minimise the impact of uncertainties in the limited data. Many strategies may be tested and levels of associated risk and value quantified. Measures to reduce unacceptable risks may be specified. Ultimately an optimal development strategy may be selected. This paper describes application of the above considerations within an integrated development planning study carried out for an undeveloped field offshore Abu Dhabi.
This paper investigates the effects of perforated zones on the waterflooded performance of layered oil reservoirs with a two-dimensional numerical model. For the purpose of the study, the effects of the location of perforation are studied for a layered reservoir, with high permeability in the middle layers and low permeability in both top and bottom layers.The following characteristics of performance changes, according to the changes in the perforation scheme, are observed.(1) Perforation only of the middle and permeable layers for production well makes but slight difference in the whole performance from the perforation of all layers.(2) Perforation only of the middle and permeable layers for injection well causes less recovery of oil from the top and bottom layers of low permeability, but it is compensated by more oil recovery from high permeability layers, and the performance as a whole changes only slightly.(3) Perforation only of low permeability layers for production well gives higher efficiency although the pressure drawdown for the same oil production is rather large compared with that of other cases.
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