4D Seismic Monitors Fluid Changes Over Time in a Carbonate Reservoir

Abstract

Summary Methods that can monitor fluid changes in a carbonate reservoir with time have the potential to help improve oil recovery from Middle East producing fields. Results from an onshore 4D seismic pilot study have successfully demonstrated that if performed properly, fluid changes in a reservoir can produce detectable differences in seismic data collected before and after the fluid change. The 4D seismic results, which show where a fluid change has occurred in the reservoir are produced by subtracting the two seismic images collected at different times. The 4D seismic method is found to be a good match with current production challenges that impact oil recovery, which include water flood override and inverse cone development. The 4D results are providing the reservoir engineer with flood-front information away from well control that can be used to quantify flood front efficiency and locate by-passed reserves. The results from this 4D pilot successfully demonstrated that 4D responses can be observed in carbonate reservoirs where sufficient saturation changes have occurred. The 4D results showed that 3D seismic surveys can be repeated onshore with acceptable levels of background noise, if done in the correct manner. Model predictions from feasibility studies were found to be in agreement with the 4D results. 4D responses were found in the main reservoir layer in agreement with calibration wells. Detailed 4D validation showed the 4D results to be in agreement with available production logging tool (PLT) measurements. The 4D results also suggest that pressure changes may be contributing to some of the 4D responses. Based on the results of this pilot, it has been shown that 4D can monitor saturation changes in a carbonate reservoir. The 4D results are helping evaluate sweep efficiency and identify potential bypassed oil reserves. Introduction In order to evaluate the potential of the 4D seismic technology in carbonate reservoirs a pilot was performed on a giant Middle East oil field. Giant Middle East fields have considerable remaining oil potential, even after many years of production, which could amount to significant additional oil with only incremental increases in recovery. A technology such as 4D seismic that can help identify zones with higher remaining oil potential would be of great value in monitoring recovery. The field used in this pilot is located onshore Abu Dhabi and covers an area that is approximately 12 by 30 kilometers with 180 meters of relief. The approximate depth of the reservoirs is 2500 meters. These Upper Cretaceous carbonate reservoirs were formed in a carbonate ramp depositional environment and are highly layered. The production is from three main reservoir zones with zonal connections from fault juxtaposition. The middle zone is the main reservoir and is the focus of this 4D study. The main reservoir is divided into two parts that will be referred to as the upper and lower reservoir layers. The main reservoir, ranges in thickness from 48–58 meters. The porosity of both the upper and lower layers of the main reservoir ranges from 20 percent in the water leg on the flanks to as high as 35 percent in the oil-bearing crest of the field. The upper layer is composed of grainstone-supported limestone and the lower layer is matrix-supported limestone. Thin dense low porosity sub-layers are also present within the main reservoir zone. Figure 1 illustrates how the 4D seismic method was found to be a good match with current production challenges in the field. Due to differences in rock type and diagenesis, large permeability contrasts exist in the main reservoir zone and create many complex production challenges. The upper reservoir layer has better permeability in the Darcy range and the lower reservoir layer has permeability's in the tens of milliDarcy range. The pore pressure in the main reservoir is maintained by peripheral water injection. Due to the permeability differences, water injected in the lower reservoir layer has a tendency to rise quickly to the upper layer where it overrides the oil in the lower layer. When the water overrides the lower layer it does not sweep the lower layer oil to the production wells. The oil in the lower layer is effectively bypassed, sweeping of oil up-dip to producing wells near the crest of the field occurs only in the upper layer. Vertical wells when used to produce from the lower layer, with time, draw down an inverse water cone around the wellbore and the well eventually ceases to flow oil. In order to access lower layer reserves a program of horizontal infill producers was started. The horizontal wells take a longer time to develop water cones and drain oil from larger areas.