Imaging Injected Water flood Fronts between Wells in a Complex Carbonate Reservoir: Designing Completions to Optimize Image Resolution

Abstract

In the paper, we will briefly review the pilot design and demonstrate the utility of applying the EM imaging to the pilot. We will also show the benefit of the optimized casing material on the resolution of the crosswell EM resistivity images and describe the methods employed for monitoring the fluid flow and show preliminary results of the modeling process. This crosswell EM technique which has been successfully employed and proven in other geographical areas is being implemented first time in UAE. The EMI technology is being deployed in southern part of a complex carbonate reservoir in the middle-east where an uneven flood front advance has been observed in different reservoir units. It has been observed that water front has advanced much faster in the highly permeable upper reservoir units as compared to lower reservoir units. In order to understand the horizontal and vertical fluid flow behavior, an inverted 5-spot water injection pilot pattern is being implemented. The pilot will address the issues of the uneven sweep efficiency, bypassed oil and effectiveness of stylolites across different units. The pilot results and observed data will be used in the simulation to design an optimum development scheme for the lower reservoir units in the southern part of the field. The current dynamic simulations predicted that the injected water will reach producers after 7 to 10 years. However, the decision on field developments have to be taken early enough to avoid the slumping of water from upper to lower units and loss of reserves in the lower units. Early imaging of the injected water from the injection well into the reservoir is paramount in assessing the success of the pilot and future field development issues. It is anticipated that this tomographic Cross-well Electro-magnetic (EM) resistivity technique will provide sufficient imaging information to track the water flood movement between wells. The most favorable conditions to acquire reliable formation resistivity distribution information, EMI require at least one kilometer distance or separation between wells. Prior to the field deployment, simulations were run to confirm the applicability of the technique and define the parameters for the survey with objectives; 1) to check the sensitivity of EM technique to the reservoir conditions and injected fluids, and 2) to carry out actual EM tool simulation and check the quality of tool response. The study concluded; 1) cross-well EM resistivity technique is well suited for tracking the water front in the current reservoir conditions, 2) the injected fluids create enough resistivity contrast to be easily picked up by the technique, and 3) the flood front progress can be captured by conducting the surveys in a time-lapse mode. As part of this project, lab tests were conducted to choose a material that would limit the attenuation at high frequency as much as possible at source and receiver locations. Introduction Peripheral water flooding as pressure maintenance method was commenced within few years of the discovery of the giant complex carbonate reservoir in the middle-east region. Although initially quite successful, the field has experienced uneven distribution of water flood front, vertical and lateral sweep due to reservoir complexity. Detailed simulation modeling was performed to delineate optimum strategy for maximizing reserves in the lower two oil bearing units of reservoir thereby controlling the process at a more local level (Ref. 1). The benefits of this process are more efficient and faster recovery. The potential drawbacks are greater costs and higher local pressures which could induce uneven flows.