Enhanced oil recovery lives or dies on water quality. The winning designs strip solids to near‑zero at massive scale—then catch the last fines just before the pumps.
Oil fields are awash with water—and increasingly required to put it back underground. Global produced water volumes exceed 70 billion barrels per year (≈15,000 million m³/yr), yet only ~16% of this water is reinjected; ∼19 billion barrels were simply discharged in 2008 (jpt.spe.org) (jpt.spe.org).
Rules are tightening fast. The North Sea now effectively mandates zero‑discharge, targeting ≈95% uptime for reinjection systems (jpt.spe.org). At the same time, EOR (enhanced oil recovery) schemes demand extraordinary volumes—the Petronas Angsi CEOR project offshore Malaysia is designed to inject ≈160,000 BPD (barrels per day; ≈25,400 m³/d) of water (jpt.spe.org).
Indonesia frames injection of produced/surface water as waste “disposal” into a formation. Current Indonesian regulations (KLHK Permen 5/2021) require that any wastewater injected to formation meet general ambient water‑quality standards (roughly Class 2 quality) and undergo technical review (id.scribd.com). An SPEAPOG (2011) example cites an Indonesian produced water volume of ~8,000 BPD (onepetro.org).
The physics are unforgiving: even “trace” suspended particles in injection water can slash reservoir permeability. Modeling shows that fine particles (<10–50 μm) can cut permeability by 40–70%, spiking injection pressures; one analytical model predicted porosity loss of 3–4% and permeability loss of 188–236 mD (millidarcies) in a representative sandpack (www.mdpi.com). Operators therefore polish injection water to very low solids levels, often targeting turbidity <1 NTU (Nephelometric Turbidity Unit) or SDI≤3 (Silt Density Index, a fouling propensity index where lower is better).
Treatment train architecture and flow
A typical EOR injection‑water treatment train handles tens of thousands of barrels per day and removes solids in stages: coarse devices first, then bulk removal, then fine polishing just before the pumps. Coarse removal starts with hydrocyclones (desanders; centrifugal separators) and strainers. Hydrocyclones cheaply remove most particles >50–100 μm, and special “fine” hydrocyclones can polish down to ~10 μm (onepetro.org) (onepetro.org).
Basket or wire‑wound strainers take the very coarse particles (>250 μm) and are backwashed regularly (petrowiki.spe.org). Many designers standardize on a strainer at the headworks, and some deploy an automatic screen for continuous debris removal where coarse loading is persistent.
Downstream of these primaries, bulk solids are removed by either large granular‑media filters or membrane filters. A fine polishing filter—typically a cartridge or microfiltration vessel—sits directly before the high‑pressure injection pumps to catch residual fines (1–10 μm) that could raise wear rates or impair the formation (onepetro.org). (Fig. 1: Representative multi‑stage solids filtration in water‑injection EOR: coarse strainer → desander/hydrocyclone → granular‑media or membrane filter → cartridge fine filter. petrowiki.spe.org; onepetro.org.)
Primary solids removal with media filters
Conventional large‑flow polishing almost always uses multimedia (sand) filters. These vessels stack graded layers of coarse and fine media; well‑designed beds trap particles throughout the top layers. Typical designs remove >25 μm solids, with many claiming 5–10 μm removal (onepetro.org) (onepetro.org). Designers commonly specify sand media as the workhorse bed.
Filter throughput is keyed to cross‑sectional area and loading rate. Conventional down‑flow units are rated ~2–5 gal/min‑ft² (≈12–31 L/min·m²) with solids capacity ~0.5–1.5 lb/ft² before backwash; high‑rate filters can run ~7–15 gal/min‑ft² (~45–98 L/min·m²) and carry 1–4 lb/ft² loads before backwash (onepetro.org) (onepetro.org). (Roughly, 10 gal/min‑ft² is ≈17,000 BPD per 50 ft² of filter area.) A top layer of anthracite is often used in dual‑media configurations for depth loading.
Upflow configurations tolerate heavier solids but must run slower (6–8 gal/min‑ft²) to prevent media loss (onepetro.org). Multiple parallel filters are standard for high availability and to allow backwash cycling. Limitations remain: sub‑5 μm particles are not reliably removed, and backwash water demand is material.
Primary solids removal with membranes (MF/UF)
Membrane systems push removal to finer scales and smaller footprints. Ultrafiltration (UF; ~0.1–0.5 μm pores) and nanofiltration (NF; ~0.01–0.1 μm) can achieve near‑complete fine solids removal, while also reducing oil, bacteria, and some dissolved species. Sensitivity to fouling and higher CAPEX/footprint keeps them reserved for cases needing extremely low solids or constrained space (jpt.spe.org). As pretreatment, ultrafiltration is increasingly modeled for seawater or produced‑water duties.
In a polymer EOR example, ultrafiltration and nanofiltration were modeled for seawater pretreatment: “water treatment can be simple if good quality source water [] is available. However, if the water quality does not meet EOR specifications, then special equipment (e.g. NF/RO) is needed” (link.springer.com). Where hardness or TDS is high, designers may escalate to nano‑filtration or even seawater RO within an integrated membrane train. Some projects combine media backwash ahead of membranes to reduce load, then UF cartridges to achieve final clarity; new ceramic membrane filters with automated backwash offer ≫5 μm polishing without frequent cartridge changes (jpt.spe.org). For larger integrated solutions, packaged membrane systems consolidate RO, NF, and UF on modular skids.
Performance targets and monitoring metrics
A well‑designed primary filter train should reduce turbidity and suspended solids to below a few ppm, with many injection guidelines targeting SDI <3 or turbidity <1 NTU. Cartridge filters are often rated “absolute” (e.g., 2 μm absolute) to guarantee cut size (onepetro.org). Systems must deliver near‑continuous service: surveys show >50% of EOR operators now reuse produced water (often seawater‑blended) for injection (jpt.spe.org).
In offshore/high‑pressure applications, every 100 mg/L of fines in feedstock can cause injector plugging; industry practice errs on the side of over‑filtering. Engineers size for worst‑case solids loads (storms, surges) and trigger maintenance via pressure drop and particle counters.
Polishing filters before injection pumps
The final “polishing” stage sits immediately upstream of the multistage high‑pressure injection pumps. These pleated cartridge or microfiltration vessels are typically rated 1–2 μm and are “capable of removing solids particles 2 μm or larger” (onepetro.org). Multiple elements—often 10–20 or more in parallel—are used so that individual cartridges can be swapped as differential pressure rises. Change‑out occurs when ΔP reaches ~25 psi (onepetro.org).
These fine filters protect pumping hardware from abrasion and ensure essentially all colloidal/fine granular matter is removed before water enters the formation. In offshore systems, the housings are heavy‑duty because injection pressures can reach 3000–6000 psi. Typical cartridge media are polyester or cellulose blends with glass‑fiber reinforcement for strength; one vendor notes pleated glass‑fiber cartridges can sustain differential pressures up to 4 bar (60 psi), with 2.5 bar (35 psi) optimal change‑out (www.kwfiltration.com). Many operators specify an upstream bank of cartridge filters as the final barrier.
By the time water reaches the pumps, residual solids should be near‑zero. One study of automating membrane de‑aeration noted that “anywhere else you can save weight and space in the overall package will be an enabler, like in an ancillary system like the de‑aeration unit” (jpt.spe.org), a reminder that right‑sizing filtration—especially fine filters—matters where platform space is scarce.
Capacities, removal efficiencies, and uptime outcomes
At 10 gal/min‑ft², a single 4×10 ft (~40 ft²) sand filter can treat roughly 20,000–30,000 BPD (≈3200–4800 m³/d) continuously (onepetro.org). An EOR project needing 100,000 BPD would require several parallel 40–100 ft² filters (plus backups for backwash). A set of 40‑inch 2 μm pleated cartridges (e.g., 6 in parallel) might handle a few thousand m³/d; large injector trains may use tens to hundreds of cartridges in duplex or triplex arrays.
Media filters typically remove 90–99% of particles larger than their cut size. If raw seawater has ~50 mg/L TSS (total suspended solids), a sand filter can bring it below 5–10 mg/L (petrowiki.spe.org). After polishing cartridges, residual turbidity can be <0.1 NTU (a few ppm TSS). A North Sea operator employing dual‑media and ultrafilter polishing achieved >95% pump availability (versus <80% historically) (jpt.spe.org).
Trends are toward more robust and automated solids removal. As volumes and scrutiny rise, systems adopt high‑rate or membrane pretreatment to lighten the load on final filters (onepetro.org) (jpt.spe.org). Self‑cleaning ceramic membranes and automated UF modules are gaining traction, promising minimal manual change‑outs. In all cases, design remains data‑driven, with filters sized for worst‑case loads and operations managed via differential pressure and particle counts.
Source notes and further reading
Background, regulatory context, and technology trends draw on Oil & Gas Facilities/JPT surveys and notices by Boysen et al. (jpt.spe.org) (jpt.spe.org) and Boysen (2015) on water management and membrane de‑aeration (jpt.spe.org) (jpt.spe.org). Modeling of fines‑induced permeability loss is summarized by Nesic et al. (www.mdpi.com). UF/RO for chemical EOR injection is discussed by Kouider et al. (link.springer.com).
Detailed filtration guidelines—including hydrocyclones, media filters, and cartridge filters—are in PetroWiki/OnePetro’s “Surface Water Treatment for Injection” (2025) and related chapters (onepetro.org) (onepetro.org) (onepetro.org) and in broader “Water Treating in O&G Operations” notes on hydrocyclones and cartridges (onepetro.org) (onepetro.org). Indonesian regulatory reference: KLHK Permen 5/2021 (id.scribd.com); example local produced water volume via SPEAPOG (2011) (~8,000 BPD) (onepetro.org). New polishing technologies include ceramic UF modules (jpt.spe.org).
