Reservoir formations are often very heterogeneous and fluid flow is strongly determined by their permeability structure. Thus, when a scale inhibitor (SI) slug is injected into the formation in a squeeze treatment, fluid placement is an important issue. To design successful squeeze treatments, we wish to control where the fluid package is placed in the near-well reservoir formation. In recent work1, we went "back to basics" on the issue of viscous SI slug placement. That is, we re-derived the analytical expressions that describe placement in linear and radial layered systems for unit mobility and viscous fluids. Although these equations are quite well known, we applied them in a novel manner to describe scale inhibitor placement. We also demonstrated the implications of these equations on how we should analyse placement both in the laboratory and by numerical modelling before we apply a scale inhibitor squeeze. An analysis of viscosified SI applications for linear and radial systems was presented both with and without crossflow between the reservoir layers.
In this previous work, we assumed that the fluid being used to viscosify the SI slug was Newtonian. However, the question has been raised concerning what the effect would be if a non-Newtonian fluid was used instead. We mainly consider the effect of shear thinning although our analysis is generally applicable if the non-Newtonian flow rate/effective viscosity function is known. We address the questions:
Does the shear thinning behaviour result in more placement into the higher or lower permeability layer (in addition to the effect of simple viscosification)?
Can the shear thinning effect be used to design improved squeeze treatment?
Chemical scale inhibitors (SI) have long been applied in downhole "squeeze" treatments to prevent mineral scale formation[2–8]. In a homogeneous reservoir layer, adsorption may be the only retention mechanism governing the SI return from the well. However, reservoir formations are rarely homogeneous but are made up of highly heterogeneous rocks which may have a layered or more complex structure as determined by various sedimentological, structural and diagenetic factors. Here we will consider only layered systems where the various layers have different permeabilities, k (and porosities, F) in the near-well formation. In such systems, SI placement within the formation is an additional aspect of a squeeze treatment that must be considered since this may affect the SI returns.
Scale inhibitors are typically applied as aqueous solutions at concentrations, typically in the range 10,000 - 150,000 ppm. These solutions usually have a viscosity (µ) close to that of an injection brine; i.e. ∼1 cP at 20°C and 0.3 cP at 100°C. Therefore, apart from a slight temperature effect, the injected brine displaces formation water (FW) at unit mobility. Also, for lighter oils, a unit mobility displacement is often involved although viscosity and relative permeability effects may be more important in heavier oils. In unit mobility injection into a heterogeneous layered linear or radial system, as shown schematically in Fig. 1, the fluid placement into layer i is governed solely by the (kh)i product. That is, injecting fluid at a total volumetric flow rate of QT into an N-layer system of the type shown in Fig. 1, then flow into layer i, Q i, is given by: (1) It can easily be shown that this is true for unit mobility displacement in a linear or a radial system with or without crossflow. However, this well established result might foster the belief that linear and radial systems are also very similar under viscous slug injection with and without crossflow and this is not the case.