Probe

Key Takeaways

• Wireline formation tester probe design and operation begins with the hydraulic extension of the probe pad against the borehole wall under sufficient force (typically 600 to 1,000 pounds of axial load from the tool's internal hydraulic system) to create a seal between the probe face and the formation, after which the drawdown piston within the tool retracts to create a small-volume low-pressure chamber that draws fluid from the formation through the probe filter screen at a controlled rate: the pressure response during drawdown (falling from the mud hydrostatic to the formation pressure under the withdrawal rate) and the subsequent buildup (recovering to the undisturbed formation pressure after the piston is stopped and the formation fluid stops flowing) is recorded at high temporal resolution and analyzed to give the undisturbed formation pressure (from the buildup asymptote) and the effective formation mobility (kh/mu, from the drawdown and buildup response curves); the probe test is repeated at 5 to 20 foot intervals through the reservoir section to build a formation pressure profile versus depth that is the primary dataset for identifying fluid gradients (gas gradient approximately 0.05 to 0.10 psi/ft, oil gradient approximately 0.30 to 0.40 psi/ft, water gradient approximately 0.43 to 0.50 psi/ft), pressure compartments, and fluid contact depths from the intersection of different gradient lines.
• Dual-probe and multi-probe tool configurations extend the basic single-probe measurement to estimate formation vertical permeability, which is one of the most critical and least-known reservoir parameters governing water coning, gas coning, vertical sweep efficiency, and the feasibility of horizontal well development: a dual-probe formation tester tool places a source probe (through which fluid is withdrawn from the formation) and an observation probe (typically at a distance of 0.5 to 1 meter above or below the source probe) in simultaneous contact with the formation, with the observation probe measuring the pressure response to the source probe drawdown; the time delay between the source drawdown and the appearance of a pressure response at the observation probe, combined with the magnitude of the observation probe response, provides estimates of the vertical permeability and the spherical flow permeability that cannot be measured by a single probe (which measures only the horizontal permeability near the wellbore in a radial flow geometry); the ratio of horizontal to vertical permeability (kv/kh) measured by dual-probe testing is typically 0.01 to 0.10 in layered formations with shale baffles and 0.5 to 1.0 in massive homogeneous sands, with lower ratios indicating more effective barriers to vertical fluid flow that will slow aquifer encroachment and reduce coning risk in production.
• Formation fluid identification using the WFT probe involves withdrawing sufficient formation fluid to characterize its type and properties using optical sensors and gas analyzers within the WFT tool string, providing real-time discrimination between drilling mud filtrate (which contaminating the near-wellbore formation) and undisturbed reservoir fluid: the optical fluid analyzer (OFA) measures the light absorption spectrum of the fluid at multiple wavelengths as it flows through the tool's flowline, detecting the characteristic absorption signatures of crude oil (which absorbs strongly in the near-infrared), gas (lower absorption than oil), and water (which has a different absorption pattern than oil and gas); the contamination fraction (percentage of drilling mud filtrate in the flowing fluid sample) is estimated in real-time from the absorption-based optical density, and the wellsite operator uses this contamination log to decide when the fluid is sufficiently clean to be captured in a sample chamber (typically when contamination falls below 5 to 10 percent by volume); the gas-oil ratio (GOR), API gravity estimate, and H2S content from the downhole fluid analyzer recorded as the contamination decreases provide formation fluid characterization data that guides the decision about whether a more time-consuming full sampling program (using large-volume sample chambers) is warranted at that depth station.
• EM probe sounding in geophysical exploration uses controlled-source electromagnetic methods to measure the variation of electrical resistivity with depth at a surface location by transmitting a time-varying electromagnetic signal from a source (loop or dipole) and recording the diffusion of the electromagnetic field into the subsurface with receiver sensors at the surface or in a borehole: the depth of investigation of an EM sounding is controlled by the frequency of the signal (lower frequency penetrates deeper because the electromagnetic skin depth is inversely proportional to the square root of frequency times conductivity) and by the source-receiver geometry, allowing the operator to probe different depth horizons by using different frequencies or source-receiver separations; magnetotelluric (MT) probing uses naturally occurring EM fields (from lightning and solar wind interaction with the ionosphere at different frequency bands) to probe the Earth's resistivity structure from tens of meters to tens of kilometers depth, making it the primary EM method for mapping deep basin structure and sub-salt resistivity in frontier exploration where surface-towed marine EM is not practical; the distinction between probing (recording vertical resistivity variation at a fixed surface location) and profiling (recording lateral resistivity variation along a surface traverse at a fixed investigation depth) determines whether the EM survey provides depth-section information or plan-view lateral information about the resistivity structure.
• Probe seal quality and borehole condition directly determine the reliability of wireline formation tester measurements, because a probe that cannot form a hydraulic seal with the formation (due to mud cake, borehole washout, rugosity, or formation fractures intersecting the probe face) will allow mud fluid to leak into the probe during drawdown, contaminating the sample and producing a pressure response that reflects the probe hydraulic resistance rather than the formation's true permeability: the probe seal is monitored during the WFT operation by the supercharge pressure test (a rapid low-volume drawdown that checks whether the probe is drawing from the formation or from the borehole fluid leaking through a poor seal), and a probe that fails the supercharge test is re-seated or moved to a new depth station where the borehole condition is better; in highly rugose boreholes (washed-out zones in reactive shales or fractured carbonates), a large packer element (the straddle packer configuration) that bridges the rugosity may be required to establish a reliable seal, though the packer configuration reduces the vertical resolution of the pressure profile and cannot be operated in all borehole geometries.