Strauss, Jonathan Patrick (Petroleum Development Oman) | Alexander, David Mobey (Petroleum Development Oman) | Al Azri, Nasser Said (Petroleum Development Oman) | Al-Habsi, Mohammed (Petroleum Development Oman) | Al-Musallami, Talal (Petroleum Development Oman) | Koning, Maartje (Petroleum Development Oman) | Eriavbe, Francis Eseoghene (Shell Petroleum Dev Nigeria SPDC) | Mukmin, Mukmin (Petroleum Development Oman) | Al-Jarwani, Riyadh Mohammed (Petroleum Development Oman) | Landman, Anke Jannie (Shell Intl. E&P Co.)
This paper covers EOR development concept screening from a sub-surface perspective. The field in question is a medium sized heavy oil field with complex geology that is located in South Oman. The two front running concepts considered are steam and polymer flood, both of which present their own challenges. Common to both concepts are the difficulty in obtaining adequate conformance in a field that is characterised by high and highly variable permeabilities in a channelised environment and that includes lateral extensive shales that break the system up into vertically distinct sand units. Additional challenges are presented by a permeable regional scale aquifer, an erosive top surface that reduces the equivalent oil column (EOC) in the core of the field leaving thicker columns laterally close to the edge aquifer and the friable nature of the sand that makes sand control necessary. Challenges specific to steam are the relatively high initial pressure, inferred connection to a regional-scale strong aquifer, and relatively high CAPEX associated with the development. Polymer on the other hand represents a relatively untested option for oil with viscosities of greater than 400cP as are present in this field.
Modelling work used to identify risks and the subsequent development potential of these two options is presented. Potential development and maturation solutions for the various options are discussed and concepts are compared.
The structure is a four-way dip closed anticline caused by outward salt withdrawal and dissolution from salt walls outside the field limits (Figure 1). No extensional forces are required to develop this structure. A significant feature of the structure is its ‘bow-tie' shape, which is caused by the embayment in the south-east. This embayment is a result of the salt dissolution and is flanked by faults. The oil bearing reservoir is characterised by thin 10-20m Middle to Lower Gharif Permian age fluvial sandstones, of which the two upper Middle Gharif units (HSGHM4a and HSGHM4c) are the main pay that the developments target and will continue to focus upon (Figure 2). The lowest unit, HSGHLG2, is only oil bearing across the crest of the field and contains a small oil column with bottom water. It only accounts for a very small proportion of the total STOIIP and if it were to be brought on production is subject to extreme water coning behaviour and as such does not represent an attractive target for development. The three oil bearing units are separated by laterally extensive floodplain shales (HSGHLG1 and HSGHM4b) and unconformably overlain by the Nahr Umr Lower Cretaceous shales (Figure 2). The Gharif formation is underlain by the extensive glacial lacustrine ‘Rahab Shale'. Additional non-extensive shales are also present within the units, particularly in the uppermost HSGHM4c unit. The unconformity has resulted in some of the upper units of the reservoir being absent; no Upper Middle or Upper Gharif sands are present in the crest. In the three cores obtained in the field, the sandstones are observed to be very friable. Core permeabilities are typically high with 60% of the population above 1D and an upper limit of 10D that is more a reflection of measurement capability and core integrity than intrinsic permeability. Production performance supports the presence of multi-darcy (>5D) sands and three wireline mini-DSTs give single sand unit horizontal permeability averages of 4 to 8D.
Al Azri, Nasser Said (Petroleum Development Oman) | Al Ajmi, Widad (Petroleum Development Oman) | Kazzaz, Ahmed (Petroleum Development Oman) | Ramalingam, Sankaranarayanan (Petroleum Development Oman) | Morshidi, Abulaziz (Petroleum Development Oman) | De Kruijf, Alexander (Petroleum Development Oman) | Jamal, Esam Abubaker (Petroleum Development Oman) | Al Busaidi, Iman Khamis (Petroleum Development Oman) | Al Mahrouqi, Abdullah (Petroleum Development Oman) | AlKharusi, Badar (Petroleum Development Oman) | Brooks, David (Shell Intl. E&P Co.)
Implementing Enhance Oil Recovery techniques in heavy oil reservoirs with strong bottom water drive has been a challenge in the oil industry. This paper describes an Enhanced Oil Recovery process in which polymer is injected into a clastic reservoir with a strong bottom aquifer drive bearing heavy-oil (250-500 cP). The high reservoir permeability (2-5 Darcy) enables stretching the viscosity limit of a standard polymer application.
The presence of a strong bottom aquifer maintains high reservoir pressure, which could provide a challenge to injectivity. The close proximity of injectors to the oil water contact reduces the efficiency of the polymer flood through water fingering, and polymer loss to the aquifer. To best understand details of the influence of aquifer on the recovery process, test different development scenarios and address key uncertainties, detailed simulation study was conducted. The simulation results showed that the optimum development concept which would help reduce impact of polymer loss to the aquifer would be to utilize the currently existing and future horizontal producers, augmented with additional infill horizontal injectors placed approximately mid-way in the oil column. Optimization of the development was performed using the simulation model where the polymer viscosity, slug size, and injector location were optimized for net present value.
Uncertainty analysis using the simulation model showed that factors such as poor injectivity, poor conformance control and high kv/kh ratio have negative impact on process efficiency. To address and mitigate these key risks and uncertainties a number of activities are underway. These activities include intensive laboratory tests, field injectivity test and a field trial where polymer is injected in newly drilled injectors. The paper discusses study to identify the optimum development concept, key uncertainties and associated risk reduction activities. Finally, this paper discusses the design and the surveillance aspects of the upcoming field trial.