When a producing well is shut in, the bottomhole pressure climbs back up as the reservoir recovers, and buried in the shape of that recovery is information about the rock and about how much pressure the reservoir has left. The Horner plot is the classic way to extract it. By plotting the shut-in pressure not against ordinary time but against a specially constructed time ratio, the buildup straightens into a line whose slope reveals the reservoir's flow capacity and whose extension points to the reservoir pressure the well would reach given infinite shut-in time. It is one of the foundational tools of well test interpretation, and it turns a curved recovery record into two numbers an engineer can act on.
Horner Plot in one line: A Horner plot is a semilog graph used to interpret a pressure buildup test, with shut-in pressure on the vertical axis and the Horner time ratio on the horizontal axis. During the period of radial flow, the data fall on a straight line whose slope is proportional to the reservoir's permeability-thickness, and extrapolating that line to the point representing infinite shut-in time gives the extrapolated reservoir pressure, called P star. It converts a curved buildup into a straight line from which permeability and reservoir pressure can be read.
The trick that makes a Horner plot work is the choice of what to put on the horizontal axis. Instead of plotting shut-in pressure against the elapsed shut-in time directly, the analyst plots it against the Horner time ratio, a dimensionless quantity that combines how long the well produced before shutting in with how long it has been shut in. That ratio accounts for the fact that the pressure recovery is not just responding to the shut-in but also still feeling the history of the production that preceded it. By folding the production time into the time axis, the Horner ratio corrects for that history in a way ordinary elapsed time cannot.
The payoff of this construction is that the physics of radial flow into the wellbore, which is logarithmic in nature, becomes a straight line when pressure is plotted against the logarithm of the Horner ratio. During the middle portion of the buildup, after the earliest data are distorted by wellbore effects and before any reservoir boundaries are felt, the reservoir is in true radial flow and the data land cleanly on that line. Identifying this correct straight-line portion is the central skill in reading a Horner plot, because everything the plot tells you is read from that line, not from the curved parts at either end.
It is worth being clear about what the Horner plot is and is not. It is a specific analytical technique for interpreting the data from a buildup test, not the test itself. The test is the physical act of shutting the well in and recording the pressure recovery; the Horner plot is one of the ways that recorded data is analyzed afterward to extract reservoir properties. Understanding the buildup test as the measurement and the Horner plot as its interpretation keeps the roles straight, because a good test can be ruined by poor interpretation and a clean Horner analysis depends on a properly conducted test underneath it.
The first thing a Horner plot yields is the reservoir's flow capacity, read from the slope of the straight-line portion. Radial-flow theory relates that semilog slope directly to the product of permeability and net thickness divided by the fluid's viscosity, so once the slope is measured, the permeability-thickness of the reservoir can be calculated. A steep slope means a low flow capacity, a reservoir that recovers pressure slowly because fluid moves through the rock with difficulty; a shallow slope means a high flow capacity, a reservoir that recovers quickly. This makes the slope one of the most important single numbers to come out of a well test, because it characterizes how readily the reservoir will give up fluid.
The second thing the plot yields is the extrapolated reservoir pressure, and this comes from where the straight line points rather than its steepness. If the well had been shut in for infinite time, the Horner time ratio would reach the value one, and the pressure the reservoir would have recovered to at that limit is read by extending the straight line to that point. That extrapolated pressure is called P star, and in a simple, unbounded reservoir it is a good estimate of the reservoir pressure. It is precisely the extrapolation that makes the Horner plot valuable in practice, because it delivers an estimate of stabilized reservoir pressure without keeping the well shut in for the impossibly long time full recovery would require.
Both readings depend entirely on correctly identifying the radial-flow straight line, and this is where interpretation demands judgment. The earliest buildup data curve away from the line because the wellbore itself, not the reservoir, dominates them, and the latest data may bend away as the pressure transient reaches a boundary or another well. Fitting a line to the wrong span, including the distorted early points or the boundary-affected late ones, throws off both the slope and the extrapolation, giving a wrong permeability and a wrong P star. The discipline of the Horner plot is largely the discipline of picking the right portion of the data to trust, which is why complementary diagnostics are used to confirm where true radial flow actually occurs.
The Horner plot is only as good as the pressure data underneath it, and that dependence has grown as the method has been asked to resolve finer features. Reading a clean slope requires a smooth, well-sampled buildup, and extrapolating confidently to P star requires that the straight-line portion be genuine radial flow rather than a coincidental alignment of noisy points. Old mechanical gauges with coarse resolution and drift made this harder than it needed to be, whereas modern high-resolution quartz gauges record the buildup finely and stably enough that the radial-flow line stands out clearly and the extrapolation rests on trustworthy data. Better gauges have quietly made Horner analysis more reliable.
This is where continuous monitoring changes the economics of getting a good buildup. A well with a permanent downhole gauge records the full pressure recovery every time it is shut in, and a platform such as Merobix can capture that entire curve automatically, timestamped and complete, rather than relying on a scheduled survey and a technician to log readings. A complete, densely sampled buildup captured this way is exactly what a Horner plot needs, and because every shut-in becomes usable data, an operator accumulates far more buildup records than a program of dedicated tests would ever produce, giving repeated updates of permeability and P star over the life of the well.
Having the raw buildup preserved in a monitoring system also protects the interpretation itself. Because the Horner reading hinges on selecting the correct straight-line portion, an analyst benefits from the full recovery curve being available to re-examine, rather than a handful of readings jotted down at the wellsite. Stored alongside the well's production history, which sets the production time that goes into the Horner ratio, and the gauge depth needed to refer pressures to a datum, the buildup can be re-analyzed as understanding improves or as newer diagnostic methods are applied. The monitoring layer does not replace the Horner plot; it feeds it clean, complete data and keeps that data available so the analysis can be trusted and revisited.
The Horner time ratio is a dimensionless quantity that combines how long the well produced before being shut in with how long it has been shut in. It is used on the horizontal axis instead of plain elapsed time because it accounts for the production history the recovering pressure is still feeling. Folding that history into the time axis is what makes the radial-flow portion of the buildup fall on a straight line.
The slope of the straight-line, radial-flow portion is proportional to the reservoir's permeability-thickness divided by fluid viscosity, so measuring it lets you calculate the reservoir's flow capacity. A steep slope indicates low permeability and slow pressure recovery, while a shallow slope indicates high permeability and fast recovery. It is one of the most important numbers a well test produces because it describes how readily the reservoir will deliver fluid.
P star is the extrapolated reservoir pressure, found by extending the straight-line portion of the Horner plot to the point representing infinite shut-in time, where the Horner time ratio equals one. In a simple, unbounded reservoir it is a good estimate of the reservoir pressure. The value of the extrapolation is that it estimates stabilized reservoir pressure without keeping the well shut in for the impractically long period full recovery would take.
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