AGR experts Gudmund Olsen, Engineering Manager in Oslo, and Marthe Åsnes Birkeland, Reservoir Engineer, discuss why multidiscipline interaction and local skills development was critical in proving the analytical performance of various scenarios for more advanced Chemical Enhanced Oil Recovery in Sudanese field and enabling future sustainability of the much-required Sudanese petroleum industry.
Following secession 2011, Sudan’s hydrocarbon production reduced from approximately 450 000 barrels per day to 100 000 barrels per day, rendering the Sudanese production status from a substantial oil exporter to a nation just self-sufficient in petroleum. Sudan is currently producing about 250 000 barrels per day.
As part of Norway’s commitment to the implementation of the Comprehensive Peace Agreement 2005, the Oil for Development (OfD) programme provides foreign assistance focusing on petroleum policy including legal and institutional framework, resource and HSE management.
The EOR activity in Sudan was administrated by the Norwegian Petroleum Directorate (NPD) on behalf of OfD. International Petroleum Associates Norway (IPAN) were contracted to be technical advisors of OfD (NPD) and AGR were sub-contracted to IPAN to perform the technical studies and training, concentrating on best methodology for the Enhanced Oil Recovery of Heglig oilfield.
Heglig oilfield, situated within NW-SE trending Muglad Basin (SW Sudan) is part of the Central African Rift System. Three phases of rifting occurred between Cretaceous and Tertiary, resulting in deposition of a long stretch of sediments in this basin. Commercial hydrocarbons are sourced from the Aradeiba Main, Bentiu 1, Bentiu 2 and Bentiu 3. Heglig boasts much of Sudan's proven oil reserves.
Heglig oilfield was first developed in 1996 by Arakis Energy (now part of Talisman Energy), today operated by the Greater Nile Petroleum Operating Company, GNPOC, reportedly peaked production in 2006. Heglig oilfield is connected to Khartoum and Port Sudan via the Greater Nile Oil Pipeline.
Various scenarios of EOR strategies
Based on sector model screening, it was decided that the EOR actions to be incorporated to the full field model were: Horizontal producers in Aradeiba F/Bentiu 1 and Polymer Injection in Aradeiba Main and Aradeiba F/Bentiu 1.
Table 1 Overview and results from EOR measures applied to the full field model of Heglig. Combined case: Horizontal producers and Polymer injection.
Reference Case- pertains to full field model with existing drilling and production plans for oilfield without any EOR actions.
Horizontal Producer Wells
New infill wells within reservoirs - Aradeiba F/Bentiu 1 – require a specific length to obtain an adequate initial productivity and sufficient reservoir contact. Simulations including inflow control device (ICD) completion giving a small increase to the overall recovery. Horizontal wells will also reduce the water conning from the underlying aquifer. The risks pertaining to horizontal drilling was a combination of more directional well placement and associated expenses in comparison to vertical wells, requiring more thorough planning.
This method is expected to enhance oil recovery by improving the unfavorable mobility ratio for water displacement. The EOR potential is dependent on the polymer viscosity; 12 cp based on lab data is considered a highly optimistic case, whilst 1cp is considered to be the most realistic case due to polymer break down and degradation.
Polymers are sensitive to high temperature and high salt concentration. The temperature and connate water, which proved to be fresh, should not be problematic for the shallowest reservoirs of Heglig reservoirs.
Surfactants were also considered as a combination with polymer injection. The aim of surfactants is to reduce the residual oil saturation in water swept zones, thus the premise for surfactant injection is high residual oil saturation. The residual oil saturation in the water swept zones of Heglig is unknown. A program for in-situ measurement is recommended.
Risks with chemical injection include back production in the aspects of facilities, water handling, and polymer degradation during injection that may decrease the desired viscosity.
A Combination of Producer Wells and Polymer Injection
All cases were constrained by a stepwise decline in total field liquid rate which was similar in all simulation cases and based on the reference case. In the current plan for Heglig in 2014, two new water injectors in Aradeiba Main were planned for pressure maintenance.
Figure 1: Simulation Results – EOR Potential and Production Curves
Recovery factor in Heglig 1/1- 2014: 22%
As illustrated in Figure 1, the applied EOR actions generate extra oil compared to the Reference case. Evidently horizontal wells increase the cumulative production instantly after application, whilst it is some time until the effect of the polymer injection is seen in the recovered volumes.
The assignments resulted in recommendation for further planning of the selected Pilot(s). Recommended work prior to the Front-End Engineering Design (FEED) phase evolved from the study:
G & G
- Re-process seismic to improve frequency content
- Interpret shallow events to see if they can explain features in depth
- Implement G&G and Petrophysical work in field static model for a detailed geological analysis of pilot area(s)
- G&G input to history matching
Well planning. Multidisciplinary effort
- Take fluid samples analysis for viscosity and asphaltene content
- Analysis of final results from core experiments and update simulation model
Training for much required sustainability
- Detailed geological evaluation of selected area
- Well design & borehole stability study
- Risk and contingency planningPilot design and detailed planning
- Design pilot facilities (front-end engineering design)
- Impact assessments: Production facilities & Environmental
- Plan infrastructure and logistics
The OfD project of Heglig oilfield EOR study was challenging on many levels due to restriction of available complete data set and history of the field. The multidiscipline interaction was critical to the overall study, integrating knowledge and information: investigating the three main scenarios, from Reference case of current plan, to action of infill horizontal wells, to more advanced Chemical Enhanced Oil Recovery by polymer injection, and combination of the two.
The Heglig oilfield study also conveyed the importance of providing knowledge transfer in the form of training the Sudanese geoscientists and engineers to enable future sustainability of the much-required Sudanese petroleum industry.
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