A single downhole pressure reading tells you the pressure at one depth, but says nothing about what fluids are stacked in the wellbore above and below that point. A pressure gradient survey answers that by moving a gauge down the well and stopping to record pressure at a series of depths, so that instead of one number you get pressure as a function of depth. Because the rate at which pressure increases with depth depends on the density of the fluid the gauge is passing through, that column of readings reveals where one fluid gives way to another, gas to oil, oil to water, and where a fluid level sits. It is a practical field survey that turns a string of pressure stops into a map of the fluids in the well.
Pressure Gradient Survey in one line: A bottomhole pressure gradient survey steps a pressure gauge down the wellbore, pausing to record the pressure at a series of depths so that pressure can be plotted against depth. The slope of that plot, the pressure gradient, reflects the density of the fluid at each interval, so a change in slope marks a change in fluid such as a gas-liquid or oil-water interface. The survey is used to locate fluid contacts and fluid levels and to correct measured pressures to a common reference depth.
The mechanics of a gradient survey are straightforward in concept. A pressure gauge, run on wireline or slickline, is lowered into the well and stopped at a chosen depth, held there long enough for the reading to stabilize, and the pressure and depth are recorded. Then the gauge is lowered to the next depth and the process repeats, working down the well in steps that may be closer together across intervals of interest and further apart elsewhere. The result is a table of pressure at each stop depth, a traverse of the well from which a plot of pressure versus depth can be drawn. Modern memory gauges make this easy, recording pressure continuously and timestamping each stop so the stops can be picked out afterward.
The information lives in the slope of that plot, which is the pressure gradient, the increase in pressure per unit of depth. That gradient is set by the density of the fluid the gauge is sitting in, because the pressure at any depth is largely the weight of the fluid column above it. A dense fluid like water produces a steep gradient, pressure rising quickly with depth; a light fluid like gas produces a shallow gradient, pressure rising slowly; oil falls in between. So by computing the slope between adjacent stops, the survey effectively measures the density of the fluid in each interval, translating a pressure profile into a fluid-density profile down the well.
This is why the stops matter and why they are placed with intent. Where the gauge is passing through a single uniform fluid, the gradient is constant and a few widely spaced stops capture it. But where a fluid contact is expected, the interesting information is exactly the depth at which the gradient changes, so stops are clustered closely across that region to pin the change down. A survey with too few stops, or stops in the wrong places, can straddle a contact and blur it; a well-designed survey concentrates its readings where the fluids are expected to change and so resolves those changes sharply.
The headline product of a gradient survey is the location of fluid contacts. When the plot of pressure versus depth shows two straight segments meeting at a kink, the kink marks the depth where one fluid gives way to another, because the gradient above and below it reflects two different densities. A steep-to-shallow break going up the well marks a gas-liquid interface; a shallow-to-steep break marks an oil-water contact. Reading the depth of that intersection gives the contact depth directly from pressure, independent of any other logging method, which is valuable both as a standalone answer and as a cross-check on contacts inferred from open-hole logs.
The same principle locates a fluid level, the top of a liquid column standing in the well. Above the liquid level the wellbore holds gas at a shallow gradient, and below it the liquid produces a much steeper gradient, so the depth where the survey's gradient steepens sharply is the fluid level. This is a common use in wells on artificial lift, where the height of the liquid column above the pump, and therefore the pump's submergence, matters for how the well is operated. A gradient survey provides that liquid level as a hard number derived from pressure rather than an inference, which is why it is a standard diagnostic when a lifted well is not behaving.
Beyond contacts and levels, the gradients themselves are useful data. The density implied by each segment's slope characterizes the fluid in that interval, confirming whether the liquid below a contact is oil or water, flagging emulsion or unexpected fluid, and providing the density values needed for other calculations. In a flowing survey, where the well is producing while the gauge is stepped, the gradients also carry information about the flowing fluid mixture and how it changes up the well. In every case the survey converts a set of pressure stops into a quantitative description of what fluids occupy the wellbore and at what depths they change, which is information a single-point pressure can never supply.
One of the most important routine uses of a gradient survey is producing the fluid gradients needed to correct pressures to a common reference depth. A pressure measured at whatever depth a gauge happens to sit is not directly comparable to a pressure measured at a different depth, because the two differ by the weight of the fluid column between them. To compare pressures fairly, or to refer them to a reservoir datum, you have to add or subtract the hydrostatic pressure of the fluid over the depth difference, and that requires knowing the fluid's density. The gradient survey measures exactly that density in situ, so its output feeds directly into the datum correction that makes well-to-well and year-to-year pressure comparisons valid.
In field operations, gradient surveys are workhorse diagnostics precisely because they are relatively simple and answer concrete questions. Where is the liquid level in this idle well? Has the oil-water contact risen since the last survey, signaling water encroachment? Is the pump properly submerged, or is it pumping off? Is the well loading up with liquid it cannot lift? Each of these is answered by stepping a gauge down and reading the gradient, and because the survey needs only a gauge and a wireline unit, it is a comparatively cheap way to look inside a well. The results guide decisions about lift settings, workovers, and how a well is choked and produced.
A monitoring platform such as Merobix complements periodic gradient surveys rather than replacing them, and the two work best together. A permanent downhole gauge trends the pressure at one depth continuously, giving the day-to-day picture, while an occasional gradient survey provides the full fluid profile and the density gradients the point gauge cannot. When the survey results, contact depths, fluid levels, gradients, are logged alongside the continuous trend and the recorded gauge depths, an operator can correct the trended pressure to a datum using measured gradients and can watch how the continuously monitored pressure relates to the fluid picture the survey revealed. The survey establishes the fluid structure of the well, and the monitoring keeps watch on how it evolves between surveys.
The survey records pressure at a series of depths, and the slope of the pressure-versus-depth plot, the gradient, reflects the density of the fluid at each interval. A denser fluid gives a steeper gradient and a lighter fluid a shallower one, so where two fluids meet the plot shows a kink between two different slopes. The depth of that kink is the fluid contact, read directly from pressure independent of other logs.
A single reading gives the pressure at one depth and reveals nothing about the fluids stacked above or below it. A gradient survey stops the gauge at many depths, producing pressure as a function of depth, whose slope reveals fluid density in each interval and therefore the depths of fluid contacts and levels. The survey turns one number into a full fluid profile of the wellbore.
Correcting a measured pressure to a common reference depth requires adding or subtracting the hydrostatic pressure of the fluid column over the depth difference, and that calculation needs the fluid's density. A gradient survey measures that density directly in the well from the slope of its pressure readings. Those measured gradients feed straight into the datum correction that makes pressures from different depths and different wells comparable.
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