Answer:
Polymer‑Dispersed System (PDS) is an advanced two‑component conformance control technology designed to achieve true sweep correction in heterogeneous and mature oil reservoirs. Unlike conventional polymer flooding or bulk gel treatments, PDS does not simply increase water viscosity – it autonomously blocks high‑permeability water‑swept zones (thief zones) while preserving oil‑bearing pathways, thereby forcing injected water into previously unswept, oil‑rich rock.
🔬 Core mechanism: Separate injection + in‑situ flocculation
PDS consists of two components injected separately:
- A high‑molecular‑weight polymer – acts as a flocculant and bridging agent.
- Micron‑sized dispersed solid particles – serve as the structural building blocks of the barrier.
Why separate injection?
The polymer solution and particle suspension are injected independently (as alternating slugs or through separate lines). This prevents any premature viscosity increase or surface reaction, ensuring both fluids remain low‑viscosity and deeply injectable – even into fractured or low‑permeability formations.
What happens downhole?
As the two fronts advance through the reservoir, they naturally converge in high‑permeability, water‑saturated zones. There, the polymer triggers flocculation – the dispersed particles aggregate into stable, gel‑like structures. This reaction occurs only in the presence of excess water. In oil‑saturated rock, no reaction takes place, making PDS intrinsically water‑selective.
The resulting flocculated network forms a durable, flexible, low‑permeability barrier that drastically reduces relative permeability to water while leaving permeability to oil virtually unchanged. This property is known as Disproportionate Permeability Reduction (DPR) – the foundation of true sweep correction.
🧭 True sweep correction vs. mobility control
Traditional polymer flooding is a mobility control method: it thickens the injected water, improving the water‑oil mobility ratio. However, in heterogeneous reservoirs, the thickened fluid still follows the path of least resistance – it flows through thief zones and bypasses oil‑rich, lower‑permeability layers. Sweep efficiency remains poor.
PDS delivers true sweep correction by physically plugging the thief zones. The barrier forces injected water and natural drive fluids to divert into unswept oil zones, increasing sweep efficiency at both the macroscopic and microscopic scales. The result is more oil, less water circulation, and lower lifting costs.
| Feature | Polymer Flooding (Mobility Control) | PDS (True Sweep Correction) |
|---|---|---|
| Primary goal | Increase water viscosity | Block high‑perm water zones |
| Mechanism | Viscous fingering reduction | Disproportionate Permeability Reduction (DPR) |
| Selectivity | None – affects all zones | Water‑selective (only where water cut is high) |
| Effect in thief zones | Still flows through them | Physically seals them |
| Sweep improvement | Partial | Dramatic, proven |
⚙️ Key differences from traditional methods
| Traditional method | Limitation | How PDS overcomes it |
|---|---|---|
| Polymer flooding | Increases viscosity but does not block thief zones – water still channels. | PDS physically blocks high‑perm zones via particle aggregation. |
| Bulk gels (e.g., Cr³⁺‑polyacrylamide) | Unpredictable gelation; can damage oil zones. | PDS is water‑selective – flocculation only in high water cut zones. |
| Pre‑crosslinked particle gels | Poor injectivity; risk of surface plugging. | Separate injection ensures deep, low‑viscosity placement. |
| Mechanical packers / bridge plugs | Expensive, requires well intervention, near‑wellbore only. | Rigless, deep‑penetrating chemical treatment. |