Study Investigates Formation Damage Induced by Water Reinjection in Unconsolidated Sands
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In recent years, many studies have focused on investigating formation damage caused by produced-water reinjection (PWRI). Nevertheless, many questions about this subject remain unanswered, particularly with respect to the occurrence of this phenomenon in unconsolidated sands. This paper describes a coreflooding program performed with sandpacks at different permeabilities, water qualities, and injection conditions.
The authors performed a complete experimental laboratory study using suspensions containing solid particles, mono-sized oil droplets, or both. Several coreflooding experiments using highly permeable sandpacks were performed over a long duration, during which significant volumes, sometimes reaching 100 L, have been injected. Also, permeability evolution has been monitored along three sections of each sandpack in order to better understand the dynamic of associated formation damage.
A schematic of the experimental setup used to carry out the coreflooding experiments is shown in Fig. 1. The suspensions containing solid particles or oil droplets were previously prepared in a 70-L reservoir tank. The tank is made from glass to facilitate suspension stirring and to prevent the aggregation of solid particles within it. The tank’s volume allows an injection over days and nights without interruption. The injection of suspension is ensured by a pump equipped with two low-diameter section pistons to ensure a proper injection of suspension without sedimentation of solid particles.
Injection of Emulsions With Only Oil Droplets. In investigating the permeability decline generated by the injection of an emulsion with 100 mg/L of oil and 2-µm mean droplet size, the authors found that the formation damage generated by the deposition of oil droplets took place over the entire length of the sandpack. Also, results showed that oil deposition took place first at the entry section, followed by the second and third sections. The oil-deposition dynamic suggests the propagation as a front. Indeed, the oil droplets deposit along the first section until the permeability decline reaches a minimum. Once the permeability decline is stabilized within the first section, the same phenomenon is reproduced successively in the second and then third sections.
These results indicate that the permeability reduction of sandpacks depends on the droplet size rather than on oil concentration even if the kinetics of the permeability decline depend on the oil concentration. For the same mean droplet size of 2 µm, the permeability decreases more slowly for an emulsion of 50 than for 200 mg/L, for example. However, whatever the oil concentration, the final permeability decline stabilizes at the same level of approximately 74% of the virgin permeability (26% of permeability loss).
The mean droplet size has its own effect. For the same oil concentration of 100 mg/L, the emulsion prepared with oil droplets of a mean size of approximately 6 µm generated a stabilized final permeability decline of approximately 33% (67% of permeability loss) in contrast to the one prepared with 1-µm mean droplet size, which generated a stabilized final permeability decline of approximately 86% (14% of permeability loss).
Injection of Suspensions With Only Solid Particles. To investigate the difference between the effect of oil droplets and that of solid particles in terms of formation damage, another set of coreflood tests was performed by injecting in the same conditions with suspensions prepared with solid particles only. Compared with the injection of oily suspensions, during which the oil deposition and the permeability decline took place along the entire sandpack, the formation damage with only solid particles took place only within the first section of the sandpack because it generated almost a complete loss of permeability. No permeability variation was observed within the second and third sections. In addition, formation damage with solid particles is more aggressive than that involving oil droplets. For comparison, the permeability in this example was almost completely lost, while, with oily emulsions, it was limited to between 30 and 65%, depending on the droplet size.
At the beginning of injection, solid particles invaded the sandpack progressively and deposited preferentially within the first 2 cm. After this internal invasion or damage phase, solid particles deposited at the surface of the sandpack and built an external filter cake on the sandface. In addition, during the external-cake-buildup phase, the permeability declined much faster when the filter cake filled the open space between the sandface and the piston. This phenomenon reproduces what should occur during the sanding of the lower completion of a water injector that functions as an alarm to clean up the well. The normalized effluent-concentration evolution shows that all the injected solid particles were retained inside the sandpack.
The permeability-decline kinetic depends on injected-solid-particle concentration and size. The internal-damage intensity accelerates with solid-particle concentrations, because the higher the solids concentration is, the faster the permeability declines.
The effect of the flow rate on permeability decline also was investigated. When the flow rate is increased from 20 to 40 cm3, the permeability decreases more slowly with the higher flow rate because the associated high flow velocity tends to push the solid particles away from the sandpack entry face and delays the external-filtercake buildup.
Injection of Emulsions With a Mixture of Oil Droplets and Solid Particles. The injection of this mixture generates more, and faster, formation damage and, thus, permeability decline. The permeability of the sandpack decreases sharply and reaches very low values close to zero rapidly. The formation damage took place mainly in the first section of the sandpack because of not only the effect of internal damage but also the rapid buildup of an external filter cake.
Test results suggest that PWRI in wells results in a preferential deposition of solid particles (with some oil) at the near-wellbore region while the injected oil migrates deeply into the reservoir. However, under different experimental conditions, solid particles could be transported with the oil more deeply in the sandpack, inducing higher formation damage in the second and third sections.
The present experimental work was performed using soft sandpacks with a permeability of a few darcies, in which significant volumes of water have been injected in order to allow understanding of the formation-damage process that can take place around injection wells. In addition, permeability evolution was monitored at different locations of the sandpacks in order to identify formation-damage propagation during PWRI. The work should help in building or refining the mechanisms and theory used in PWRI modeling and prediction. Some guidelines and best practices in terms of water-quality specification also could be extracted. Experimental findings include the following:
- When present alone in produced waters, oil in suspension would generate only internal formation damage on the entire length of the sandpacks and no filter cake built up on sandfaces. The permeability-decline velocity depends on the oil concentration in the injected water only at the beginning of injection. However, permeability decline stabilizes at a certain level thereafter, which depends on oil-droplet mean size rather than oil concentration. Injection at high flow rate does not allow for recovery of the permeability already lost or reduction of permeability decline.
- Formation damage caused by solid particles present alone in water—as in seawater, for example—is different from formation damage induced by oily water and is more aggressive. Permeability-decline velocity depends on both the concentration and the size (or jamming ratio), but, whatever their concentration and particle size, solid particles deposit only in the first 1 or 2 cm of the sandpack entrance and have a tendency not to propagate deeply within sandpacks despite their high permeability. This work showed that high injection-flow rates would be very favorable to the prevention and delay of formation damage induced by solid-particle deposition.
- Formation damage induced by a water containing a mixture of oil and solid particles in suspension results from the addition of the formation damage induced by each component alone. This formation damage is much more aggressive than that induced by solid particles only.
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Study Investigates Formation Damage Induced by Water Reinjection in Unconsolidated Sands
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