A technique is shown for estimating pressure and saturation changes from 4D seismic without the need for rock and fluid physics. This is a further refinement and development of a previously published method. Here, a multi-attribute approach is taken, in which a cycle of calibration, training, and principal component analysis produces an optimal combination of attributes. The methodology is applied to a Jurassic reservoir in the UK North Sea with encouraging results. It has highlighted saturation changes previously hidden by the effect of pore pressure on the rock frame, even where AVO methods have failed, and can be validated against flow rates for water production.
4D or time-lapse seismic methods have been proven by Shell and many other companies to be a valuable tool for reservoir monitoring and management1–4. In this paper, we use case studies to describe best practise. The main example is a mature field in the Northern North Sea, where continuous drilling of infill wells for two years has, so far, confirmed.the interpretation of remaining oil distribution from the 4D seismic data. In mature fields, where infill wells and sidetracks are targetted at small remaining volumes (where partial water flooding may have occurred), it becomes very important to achieve a good integration of 3D and 4D seismic data with the static and dynamic model. This is achieved qualitatively using new visualisation tools, and quantitatively using simple modelling methods such as 4D wedge modelling or more complex methods such as the generation of full field synthetic seismic. The updating of the reservoir simulator is performed in greater detail with the objective that the prediction remains accurate further into the future. This allows more reliable well targetting years after the acquisition of a 4D monitor survey, thereby reducing the necessary frequency of monitor survey acquisition. Impacting the business Infill targets. Shell has demonstrated the value of 4D both in relatively young fields (e.g. Draugen, Norway1), and in more mature fields (e.g. UK Central3 and Northern North Sea4). In a young setting infill well targets can be quite sizeable, whereas in a mature setting, remaining oil targets are generally smaller in volume, and can only be accessed economically using sidetracks or wells with multiple targets. Within many of these fields development is phased, with the most productive high permeability (‘primary’) levels targetted by early wells, and the less productive lower permeability (‘secondary’) levels being targetted by sidetracking of early primary wells which have watered out. This phasing is particularly advantageous for 4D driven development. Due to field maturity, remaining oil pockets at the primary-level are often small and would present uneconomic targets on their own, even for sidetracks. However, while infill wells and sidetracks are planned for an untapped secondary-level target they can often be designed to pass through these small primary-level oil pockets, allowing their production and further improving the ultimate recovery. Most recent production wells drilled now achieve this dual primary-level / secondary-levelpurpose. Re-perforations. Re-perforation is also used for optimising well production as a result of 4D observations. In the example shown in Figure 1 the well was largely watered out, producing approximately 2000 bbls of oil per day. The 4D seismic indicated that the water cut was coming from the middle and toe of the well (beyond a small fault), whereas the heel of the well still lay in unswept oil. As a result of these observations, the middle and toe of this well were packed off, and new perforations were made at the heel of the well. Oil production increased to approximately 5000 bbls per day and the cumulative production increase has exceeded 500 000 bbls - a substantial return for a simple packer/re-perforation job. Although one can also use production logging to find out such information, the 4D can sometimes be much cheaper per well than production logging. 4D also gives a approximate value of the vertical stand-off of the water from the well, and therefore the likelihood of water coning after re-perforation. Injector placement. Most 4D-driven targets are remaining oil pockets, however the improved understanding of fluid flow and connectivity offered by 4D can also be used to improve injector placement or the optimal scheduling of injector well drilling. In the Northern North Sea, we observe not only 4D hardening due to water sweep, but also 4D softening as a result of reservoir inflation. This is due to poorly connected water injectors pressuring up isolated parts of the reservoir (Figures 2 and 3). Infill targets. Shell has demonstrated the value of 4D both in relatively young fields (e.g. Draugen, Norway1), and in more mature fields (e.g. UK Central3 and Northern North Sea4). In a young setting infill well targets can be quite sizeable, whereas in a mature setting, remaining oil targets are generally smaller in volume, and can only be accessed economically using sidetracks or wells with multiple targets. Within many of these fields development is phased, with the most productive high permeability (‘primary’) levels targetted by early wells, and the less productive lower permeability (‘secondary’) levels being targetted by sidetracking of early primary wells which have watered out. This phasing is particularly advantageous for 4D driven development. Due to field maturity, remaining oil pockets at the primary-level are often small and would present uneconomic targets on their own, even for sidetracks. However, while infill wells and sidetracks are planned for an untapped secondary-level target they can often be designed to pass through these small primary-level oil pockets, allowing their production and further improving the ultimate recovery. Most recent production wells drilled now achieve this dual primary-level / secondary-levelpurpose. Re-perforations. Re-perforation is also used for optimising well production as a result of 4D observations. In the example shown in Figure 1 the well was largely watered out, producing approximately 2000 bbls of oil per day. The 4D seismic indicated that the water cut was coming from the middle and toe of the well (beyond a small fault), whereas the heel of the well still lay in unswept oil. As a result of these observations, the middle and toe of this well were packed off, and new perforations were made at the heel of the well. Oil production increased to approximately 5000 bbls per day and the cumulative production increase has exceeded 500 000 bbls - a substantial return for a simple packer/re-perforation job. Although one can also use production logging to find out such information, the 4D can sometimes be much cheaper per well than production logging. 4D also gives a approximate value of the vertical stand-off of the water from the well, and therefore the likelihood of water coning after re-perforation. Injector placement. Most 4D-driven targets are remaining oil pockets, however the improved understanding of fluid flow and connectivity offered by 4D can also be used to improve injector placement or the optimal scheduling of injector well drilling. In the Northern North Sea, we observe not only 4D hardening due to water sweep, but also 4D softening as a result of reservoir inflation. This is due to poorly connected water injectors pressuring up isolated parts of the reservoir (Figures 2 and 3).
In the central North Sea ‘Gannet‐A’ field, a 50 ft oil rim is overlain by a gas cap of variable thickness. Oil is produced from horizontal wells which initially produced dry oil, but as the field became more mature, a significant water cut was seen in several wells. A dedicated 4D seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves. By forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations (using 2D and 3D wedge models), and comparing the results with real seismic data, we are able to decipher the contact movements across the field. It is shown that, in one part of the field, the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap. This oil‐rim displacement produces a very different 4D seismic response from the case of a static gas–oil contact and rising oil–water contact (normal production). As a result of these observations, we are able to optimize field production by both re‐perforation of existing wells and by drilling sidetracks into the displaced rim: a brown‐field development opportunity that might otherwise be missed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.