Chan diagnostic plot — reading water production mechanisms

● Production Engineering · May 17, 2026 · 11 min read

Every mature oil field eventually produces water. The question is not whether, but how — and where it's coming from. Chan's 1995 diagnostic plot turns water-oil ratio time series into a mechanism identification tool. Read it correctly and the workover plan writes itself.

The water problem

An oil well produces stable for years, then water cut starts climbing. Two months later it's at 50%. Six months later, 80%. Production drops, lifting costs rise, and a decision has to be made: workover, chemical treatment, abandonment, or wait.

The decision depends entirely on where the water is coming from. Water rising from below the perforations (coning) needs a different response than water moving along a high-permeability streak (channeling). Treating one as the other wastes money and accelerates field abandonment.

K.S. Chan published the diagnostic in 1995 using log-log plots of WOR and its derivative to distinguish water production mechanisms based on signature shapes. The technique is simple, free, and built from data that production engineers already have.

The two curves: WOR and WOR'

Chan plots two quantities versus time on log-log axes:

Water-oil ratio WOR = q_w / q_o

Simple ratio of water rate to oil rate in stock tank barrels.

The derivative is the rate of change of WOR with respect to log time:

WOR derivative WOR' = d(WOR) / d(log t) = t × d(WOR) / dt

Numerically: WOR'_n ≈ t_n × (WOR_n+1 − WOR_n-1) / (t_n+1 − t_n-1)

Both are plotted on the same log-log axes versus time. The shapes of the two curves — not their absolute values — diagnose the mechanism.

Figure 1Why the derivative matters. WOR alone (amber solid) shows a transition near breakthrough but the exact onset is ambiguous. WOR′ (green dashed) peaks at the transition — turning a soft inflection into a sharp diagnostic event. The mechanism is identified by reading both shapes after the peak, not just the WOR magnitude.

Chan's insight: WOR alone is ambiguous. WOR plus its derivative is mechanism-specific. The pair fingerprints what is happening downhole.

Figure 2Four canonical Chan signatures on log-log axes. Coning rises gradually (concave-up shape from radial inflow geometry). Channeling rises steeply along a near-straight line — high-permeability streak short-circuits between injector and producer. Near-wellbore problems show a flat WOR — water enters at high cut from the start. Multilayer with crossflow falls in between. Each pattern points to a different remediation strategy.

The four characteristic signatures

Chan identified four distinct mechanism signatures, each with characteristic slopes for WOR and WOR' on the log-log plot:

1. Water coning

Water cone rising vertically through the oil column toward the perforations. Common in reservoirs with strong bottom-water support and limited vertical permeability barriers.

Curve	Slope on log-log	Pattern
WOR	~0.3 – 0.5	Gradual upward sweep, smooth
WOR'	Near constant	Flat / horizontal, no spike

The flat WOR' signature is the key — coning produces a steady, predictable rise in water cut. The well "warns" you it's coning by giving smooth, continuous WOR growth.

2. Water channeling (high-permeability streak)

Water moving laterally through a thief zone or fracture, often from an injector or active aquifer. The water finds a fast path and breaks through suddenly.

Curve	Slope on log-log	Pattern
WOR	~1.0 – 2.0	Steep climb after breakthrough
WOR'	Sharp upward jump	Spike at breakthrough, then flat or declining

The WOR' spike at breakthrough is diagnostic. Channeling shows up suddenly because the high-perm streak was always there — water just took time to traverse it.

3. Near-wellbore problems (channel behind pipe, casing leak)

Cement failure, casing corrosion, or perforation issues allowing water from an adjacent zone to enter the wellbore. Often from a depleted zone above or below the producing interval.

Curve	Slope on log-log	Pattern
WOR	Very steep, > 2.0	Near-vertical climb, sometimes step-change
WOR'	Extreme spike	Spike often 10× or more above baseline

The extreme WOR' spike — often 10-100× normal — is the smoking gun for mechanical near-wellbore problems. Reservoir-level mechanisms rarely produce derivative spikes this sharp.

4. Multilayer channeling with crossflow

Common in stacked reservoirs where water enters from one perforated zone and flows behind pipe to another. Combines features of channeling and near-wellbore problems.

Curve	Slope on log-log	Pattern
WOR	Stepped or erratic	Multiple inflection points
WOR'	Multiple spikes	Spike sequence as different layers cone or channel

Interpreting the plot in practice

The Chan plot is read in three stages:

Identify the breakthrough point. Where does WOR begin rising from near-baseline? That marks t = 0 for the diagnostic.
Estimate post-breakthrough WOR slope. Use the steady-state portion after initial transient. Slope tells you the mechanism class.
Look for WOR' spikes. Sharp upward jumps in WOR' indicate either breakthrough events or mechanical changes. Flat WOR' suggests steady mechanism (coning).

A worked example

Consider an oil well with the following monthly production history (last 6 months):

Month	q_o (bopd)	q_w (bwpd)	WOR
t = 0	450	45	0.10
t = 30 d	420	95	0.23
t = 60 d	380	160	0.42
t = 90 d	340	270	0.79
t = 120 d	290	410	1.41
t = 150 d	240	650	2.71
t = 180 d	180	1200	6.67

Plotting WOR vs time on log-log axes, the curve has an apparent slope of about 1.4 between months 1 and 6 — well above coning range, in classic channeling territory. WOR' shows a sharp uptick around month 4 (t = 120 d), consistent with a breakthrough event.

Interpretation: Water channeling, likely along a high-permeability streak. A breakthrough event occurred around month 4. Workover recommendation: chemical conformance treatment (gel or polymer) to seal the high-perm pathway, not bottom-water control.

Why the distinction matters operationally

Coning: reduce drawdown (smaller pump), gas injection, or shut in lower perforations. Workover unlikely to help — water still rises eventually.

Channeling: conformance treatment (gel, polymer, foam), selective perforation isolation, or recompletion. Workover often saves the well.

Near-wellbore: casing repair, squeeze cement, perf re-isolation. Mechanical fix, not reservoir fix.

Same WOR=2, three different cures. Wrong call = wasted money.

Slopes summary table

WOR slope	WOR' pattern	Mechanism	Action
0.3 – 0.5	Flat	Bottom-water coning	Reduce drawdown
1.0 – 2.0	Spike then flat	Channeling	Conformance treatment
> 2.0	Extreme spike	Near-wellbore problem	Mechanical repair
Stepped	Multiple spikes	Multilayer w/ crossflow	Selective isolation

Common pitfalls

1. Reading raw daily data. Daily WOR is noisy. Average to weekly or monthly before plotting. Otherwise random spikes get misread as breakthrough events.

2. Mistaking choke/rate changes for mechanism signals. A change in drawdown alters water cut without changing the underlying mechanism. Annotate the plot with operational changes (choke setting, pump speed, downtime) to avoid confusion.

3. Using too short a window. Mechanism signatures need time to develop. Less than 3 months of post-breakthrough data is unreliable.

4. Forgetting that mechanisms can change. A well that started with coning can transition to channeling years later (after pressure depletion changes flow paths). Refit periodically — don't trust an old diagnosis.

5. Treating Chan as the only evidence. Combine Chan with: production log surveys, water analysis (chemistry can fingerprint water source), pressure response in nearby wells. Chan is a screening tool, not the final answer.

When Chan doesn't work

Chan's framework assumes the well's productivity changes are dominated by water mechanism physics. It struggles when:

Intermittent production: Wells with frequent shut-ins distort the time-based derivative
Multiple producing zones: Composite signal can mask individual mechanism behavior
Heavy oil with WOR < 0.1: Numerical instability in WOR' calculation near zero
Workover history during the window: Any change in completion resets the diagnostic — refit from event
Active injection: Injection rate changes affect water response independently of reservoir behavior

Automated screening at field scale

Chan diagnostic is well-suited to automation. For a field with hundreds of wells, a surveillance dashboard can:

Compute WOR and WOR' from raw production data
Fit log-log slopes over rolling windows
Classify each well by dominant mechanism
Confidence-score the classification based on fit quality, data sufficiency, and spike detection
Flag low-confidence cases for engineering review

This converts a manual interpretation that takes 10 minutes per well into a screening pass that runs every night. Field reviews then focus on the wells the classifier flagged — not the wells already running smoothly.

Three takeaways

Always plot WOR and WOR' together. Either curve alone is ambiguous; the pair is diagnostic.
Slopes matter more than levels. A WOR of 2 means nothing in isolation; a WOR climbing along a slope of 1.5 means channeling.
Confirm with a second method. Use water chemistry, PLT, or pressure transients before committing to a treatment plan. Chan is a screening tool.

References & further reading:
Chan, K. S. (1995). Water Control Diagnostic Plots. SPE 30775.
Yortsos, Y. C., Choi, Y., Yang, Z., Shah, P. C. (1999). Analysis and Interpretation of Water/Oil Ratio in Waterfloods. SPE Journal, 4(4), 413–424.
Seright, R. S. (2010). Disproportionate Permeability Reduction with Pore-Filling Gels. SPE Journal, 15(2), 376–389.
Bailey, B., Crabtree, M., Tyrie, J., et al. (2000). Water Control. Oilfield Review, Schlumberger, Spring 2000.
Sydansk, R. D., Romero-Zerón, L. (2011). Reservoir Conformance Improvement. SPE.

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