Modelling Non-Matrix Flow And Seals Integrity In Soft Sand Reservoirs
- Atef Onaisi (Total S.A.) | Jalel Ochi (Total) | Marc Mainguy (Total) | Phillipe Antoine Souillard (Total E&P USA, Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE European Formation Damage Conference, 7-10 June, Noordwijk, The Netherlands
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.3.9 Steam Assisted Gravity Drainage, 1.7 Pressure Management, 1.8 Formation Damage, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment, 6.5.2 Water use, produced water discharge and disposal, 4.3.4 Scale, 5.3.2 Multiphase Flow, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 2.4.6 Frac and Pack, 5.5 Reservoir Simulation, 1.2.3 Rock properties, 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.2.2 Geomechanics, 1.6 Drilling Operations
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Most of oil reservoirs operated by Total in West African deep offshore are multi layered highly unconsolidated sands. Burial depths, below the mud line, ranges from 500 to 2500mTVD. Sea water injection is done for pressure maintenance in deviated and horizontal wells equipped with sand control. Polymers injection for EOR and PWRI are also practiced.
In theory, the very high permeability of soft sand reservoirs should allow injecting in matrix mode at high rates with small differential pressure. In reality, declining injectivity over time requires injecting in non-matrix mode, at much higher pressures than anticipated, hence the concerns about the confinement of the injection. The mechanisms of injectivity restoration by increasing the pressure in soft sands and the subsequent risks of cap rock failure have to be understood. Their incorporation in modeling tools is crucial for predicting the operating pressure range and designing the surface and wells facilities accordingly. Standard tensile fracturing models commonly applied to consolidated reservoir rocks do not seem to be adapted to soft materials which behavior is dominated by shear failure and resulting in dilation. This paper will present modeling approaches aimed at predicting the responses of soft sands and of the cap rock to produced water injection taking into account the coupling between geomechanics, flow and formation damage. Field observations are presented and confronted with model predictions.
Fields operated by Total worldwide are producing equal amounts of water and oil in average. In ten years time, the ratio of water to oil will almost double. The re-injection of produced water is economically attractive and an environmentally compliant discharge option. The ideal situation would be to mix produced water with sea water and to inject the all into producing reservoirs to take advantage of reservoir management needs while addressing crucial environmental issues. A significant number of Total's re-injection projects concerns deep offshore developments of complex, multi-layered, relatively shallow and unconsolidated reservoirs, especially in the Guinea Golf.
The main concern with re-injection is injectivity decline with time entailed to the presence of oil and suspended solids in varying proportions in the produced water. Oil and solids contents are not always properly and continuously monitored during injection, which, combined with a poor understanding of downhole plugging mechanisms and subsequent geomechanical response, explain why it is still difficult to make reliable forecasts of injectivity for a whole well life.
Injectivity loss occurs as mixed oil and suspended solids either agglomerate at the face of the injected formation forming an almost impermeable external cake or infiltrate the reservoir causing progressive damage to the permeability of the flooded area in the vicinity of the well. Poor injectivity arising rather from chemical interactions between water and down-hole environment, such as tubing corrosion, reservoir souring, scale precipitation and clay swelling, does exist but is out of the scope of this paper. Sustainable matrix injection of produced water is awaiting for the emergence of reliable ultra-filtration technology. Despite promising results though, ultra-filtration does not deliver yet the desired zero damage water quality.
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