Application of Injection Fall-Off Analysis in Polymer flooding
- Paul van den Hoek (Shell) | Hassan Mahani (Shell Intl. E&P Co.) | Tibi Sorop (Shell) | David Brooks (AAR Energy) | Marcel Zwaan (Shell Intl. E&P Co.) | Subrata Sen (Shell India Markets Private Ltd) | Khalfan Shuaili (PDO) | Faisal Saadi (PDO)
- Document ID
- Society of Petroleum Engineers
- SPE Europec/EAGE Annual Conference, 4-7 June, Copenhagen, Denmark
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 5.4.1 Waterflooding, 1.2.2 Geomechanics, 4.3.4 Scale, 5.7.2 Recovery Factors, 3 Production and Well Operations, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control
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Polymers exhibit non-Newtonian rheological behavior, such as in-situ shear-thinning and shear-thickening effects. This has a significant impact on pressure decline signature as exhibited during Pressure Fall-Off (PFO) tests. Therefore, applying a different PFO interpretation method, compared to conventional approaches for Newtonian fluids is required.
This paper presents a simple and practical methodology to infer the in-situ polymer rheology from PFO tests performed during polymer injection. This is based on a combination of numerical flow simulations and analytical pressure transient calculations, resulting in generic type curves that are used to compute consistency index and flow behavior index, in addition to the usual reservoir parameters (kh, faulting, etc.) and parameters relating to (possible) induced fracturing during injection (fracture length and height). The tools and workflows are illustrated by a number of field examples of polymer PFO, which will also demonstrate how the polymer bank can be located from the data.
Sweep efficiency is largely determined by the mobility contrast between the displaced and the displacing fluid. Polymer flooding provides a better mobility control in reservoirs containing viscous oil compared to water flooding, thereby increasing oil recovery. Under matrix injection conditions, polymer injectivity is expected to be lower than that of water. However, it is currently accepted (see for instance Seright et al 2009, Khodaverdian et al 2009, van den Hoek et al 2009), that injecting at economical rates is expected to generate induced fractures. Hence, maintaining good polymer injectivity requires managing both the sweep and the growth of induced fractures, while reducing the risk of out-of-zone injection.
Pressure Fall-Off (PFO) testing is accepted as one of the simplest and cheapest surveillance techniques for water injectors. The method consists of analyzing the pressure transient signal produced when shutting the well. Recently the PFO interpretation methodology was extended to characterize induced fracture properties such as length, skin, conductivity, etc (Van den Hoek 2005). In Shell the use of this methodology has become common practice for waterfloods and part of well and reservoir management. Especially when repeated periodically in the same well, PFO testing provides valuable information on injection performance, in-situ mobility changes, induced fracture growth, as well as on formation properties, boundaries, etc (Hustedt and Snippe 2010).
In order to expand PFO test interpretation to polymer flooding, several challenges need to be overcome. The most important is the non-Newtonian nature of the polymer flow, which is reflected in its viscosity dependence on the flow, leading to either or both shear thinning (at low shear rates - typically in the reservoir) and shear thickening (at very high shear rates). For a shear-thinning polymer, the effective viscosity can be generally approximated by a power-law:
µ = K? n-1 , where µ is
the effective viscosity, K is the consistency index, ? is the shear rate and n the flow behaviour (power-law) index. The shearrate dependency of polymer viscosity can have a significant impact on pressure decline signature as exhibited during PFO tests. For a water flood (which is a Newtonian fluid) the index n=1.
In the past, a number of studies on the PTA effects related to non-Newtonian shear-thinning fluids were published (e.g., Ikoku and Ramey 1979, Odeh and Yang 1979, Ikoku 1979, Vongvuthipornchai and Rhagavan 1987, Katime-Meindl and Tiab 2001). However, none of these works resulted in a simple, straightforward methodology for polymer PFO test intrerpretation, particularly under induced fracturing conditions.
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