Injection Fall-Off Analysis of Polymer flooding EOR
- Hassan Mahani (Shell Intl E&P Co) | Tibi Sorop (Shell) | Paul van den Hoek (Shell) | David Brooks (Shell Intl E&P Co) | Marcel Zwaan (Shell Intl E&P Co)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Characterisation and Simulation Conference and Exhibition, 9-11 October, Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 2.4.6 Frac and Pack, 5.4.1 Waterflooding, 5.7.2 Recovery Factors, 3 Production and Well Operations, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.2.2 Geomechanics, 4.3.4 Scale, 5.5 Reservoir Simulation, 2.4.3 Sand/Solids Control, 5.6.3 Pressure Transient Testing, 5.6.4 Drillstem/Well Testing
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Polymer solutions, in contrast to water, exhibit non-Newtonian rheological behavior, such as in-situ shear-thinning and shearthickening effects, leading to a varying viscosity distribution in the reservoir. Consequently, a different pressure distribution and a rather slower pressure decline rate are exhibited during Pressure Fall-Off (PFO) test (compared to a waterflooding case).
Therefore, applying a different interpretation method, compared to conventional approaches for Newtonian fluids is required. In this paper we provide a new, simple and practical method to infer the in-situ polymer rheology and the induced fracture dimensions 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, flow behavior index, fracture dimensions, and reservoir parameters (kh, faulting, etc.) from the measured pressure derivative curves.
The novelty of this study are the analysis of interference caused by fracture sizes on the radial stabilization of polymer, as well as the use of realistic polymer rheology, combined with an analysis method that derives polymer rheology parameters based on the pressure derivative curve.
This method can be used for interpretation of PFO tests on existing EOR polymer flooding projects, where monitoring of injection performance and of in-situ effective polymer rheology are key in the success of a project.
In an immiscible displacement process, sweep efficiency is largely determined by the mobility contrast between the displaced and the displacing fluid. Polymer flooding provides a better mobility control in a reservoir containing relatively viscous oil compared to water flooding, thus increasing oil recovery. Due to the higher viscosity of the polymer solution compared to water, under matrix injection mode, polymer injectivity is typically lower than that of water. It is currently accepted, however (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 lead to injection pressures that may quickly reach the formation fracturing pressure and 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.
One of the cheapest surveillance techniques for water injectors is Pressure Fall-Off (PFO) testing (Koning and Niko 1985, Larsen and Bratvold 1990). The method consists of analyzing the pressure transient signal produced when shutting the well.
Especially when repeated periodically in the same well, PFO testing provides valuable information on injection performance and in-situ mobility changes, as well as on formation properties, boundaries etc (Hustedt and Snippe 2010).
Most of the current commercially available Pressure Transient Analysis (PTA) tools provide packages that can be used to properly analyze waterflood PFO tests for matrix injection and for static hydraulic fractures (i.e. propped fractures, such as, for instance, frac-and-packs). Recently the methodology has been extended to characterize induced fracture properties such as length, skin, conductivity etc (Van den Hoek 2005). In Shell the use of this methodology is becoming common practice for waterfloods and part of well and reservoir management.
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