Holistic assessment of project economics and subsurface characterization provides a framework to handle challenging reservoirs. Capturing ranked uncertainties based on their impact on the project and meticulous working towards de-risking the project is key for the success of the entire project. Committing increased production from the field is dependent on proper evaluation of the reservoir. This paper reviews characterization of a tight reservoir deposited in the intra-shelf Bab basin during lower Aptian time. Initial stage reservoir characterization is critical in formulating reservoir development plan and estimating a realistic assessment of rates and volumes for the field. The target formation is a low-permeability (average permeability 0.5 mD) heterogeneous carbonate reservoir sitting directly above and adjacent to a producing carbonate reservoir. It is essential to understand communication between the zones. The pilot well is drilled with 225 ft of conventional core and quad-combo logs. Advanced logs such as resistivity image, cross-dipole acoustic, nuclear magnetic resonance, vertical interference test (VIT), formation pressure (including pressure transient data), and fluid samples were acquired. The main objectives of the evaluation program were to determine the formation pressure, collect representative oil sample(s), conduct vertical interference tests between the sub-zones and collect appropriate data for geomechanical and rock-physics characterization. Thorough pre-job planning and cross-discipline cooperation during the operation provided high fidelity log data and interpretation of the data into a coherent result. This included integration of image data with vertical interference tests from the wireline formation tester (WFT) where barriers were confirmed. In addition, NMR permeability was matched and calibrated using pretest mobility measurements and formation pressure data was combined with full waveform advanced acoustic processing to explain the communication between the upper target zone and the lower producing reservoir. Advanced acoustic analysis helped to fully characterize the target formations with stoneley permeability, azimuthal anisotropy, and presence of fractures. This paper demonstrates the importance of multi-disciplinary team effort in characterization of challenging reservoirs. It highlights the importance of holistic planning before the execution phase, and keeping a focus on the larger goal while executing individual aspect of a complicated project. Formation evaluation measurements have evolved over decades and occasionally it benefits the industry to provide a review of how the latest logging measurements fit together in an integrated manner, for successful evaluation of a challenging reservoir.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThere is a common mis-understanding about the transition zone in which both water and oil should be mobile. This paper will illustrate that in the transition zone only oil is movable above the Free Water Level (FWL) at normal gravity conditions. Putting wells on production in the transition zone produce dry oil unless the drawdown pressure disturbs the normal gravity conditions.To avoid any confusion among geoscientists and reservoir engineers, we introduce new term called the "Wedge Zone", where the water saturation increases down flank towards the oil-water contact as a result of gradual increase in the micropores with bounded water due to diagenesis and cementation. The intensity of the diagenesis (cementation) is related to the timing of oil migration and entrapment.Four wells that had been drilled in an onshore field in the Middle East in the Wedge Zone and have producing dry oil for the last two years. As result of repositioning the new wells based on the Wedge Zone concept, an extra amount of recoverable oil will be added.The Modeling techniques over this zone have been revised resulting in changing the concept of modeling the transition Zone to that modeling of a Wedge Zone. As a consequence, the model prediction of sweep efficiency is also impacted.
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