Attenuation Resistivity: Definition, LWD, and Invasion Profile

Attenuation resistivity (Att-R) is a measurement of formation resistivity derived from the amplitude ratio of an electromagnetic wave propagating between a transmitter and two receiver coils in a logging-while-drilling (LWD) propagation resistivity tool. Specifically, the tool measures how much the wave's amplitude is reduced as it travels through the formation: the greater the attenuation, the more conductive (lower resistivity) the formation. Because the physics governing amplitude decay differ from those governing phase shift, attenuation resistivity provides a deeper depth of investigation and a different sensitivity to invasion than the companion phase-shift resistivity measurement made by the same transmitter-receiver array. Together, the two measurements form the foundation of real-time formation evaluation during directional and horizontal drilling campaigns worldwide.

Key Takeaways

• Attenuation resistivity is computed as Att = 20 x log₁₀(A₁/A₂), expressed in decibels (dB), where A₁ and A₂ are the amplitudes recorded at the near and far receivers respectively.
• For the same transmitter-receiver spacing and operating frequency, attenuation resistivity reads deeper into the formation than phase-shift resistivity: at 2 MHz, Att-R investigates roughly 50 cm (20 in) versus approximately 25 cm (10 in) for PS-R; at 400 kHz, Att-R extends to roughly 75 cm (30 in) versus 50 cm (20 in) for PS-R.
• A difference between Att-R and PS-R values is the primary indicator of mud-filtrate invasion: when Att-R exceeds PS-R (resistive invasion), oil-based or low-salinity filtrate has displaced connate brine in the flushed zone; when PS-R exceeds Att-R (conductive invasion), water-based mud filtrate has invaded an oil-bearing zone.
• Propagation resistivity tools operate at two or more frequencies (commonly 400 kHz and 2 MHz) and multiple transmitter-receiver spacings (typically 16, 22, 28, and 34 in / 0.41, 0.56, 0.71, and 0.86 m), yielding up to eight simultaneous resistivity curves for invasion profiling and anisotropy detection.
• Attenuation resistivity is a standard real-time geosteering input: the deeper-reading curves detect approaching bed boundaries before the bit crosses them, enabling the driller to steer within a thin reservoir layer.

How Attenuation Resistivity Works

A propagation resistivity tool consists of one or more transmitter coils and at least two receiver coils mounted axially along a sub in the LWD bottomhole assembly. The transmitter generates a continuous electromagnetic wave at a fixed frequency (400 kHz, 2 MHz, or both simultaneously depending on tool design). As the wave propagates outward through the borehole fluid and into the formation, it is attenuated in amplitude and shifted in phase by the formation's resistivity and dielectric permittivity. The two receiver coils, separated by a short spacing (typically 6 in / 15 cm), record the wave independently. The ratio of the two received amplitudes is converted to the attenuation measurement in decibels, and the difference in the phases of the two received signals yields the phase-shift measurement in degrees. Each quantity is then converted to an apparent resistivity using theoretical or empirical transform curves derived from forward modelling of a homogeneous, isotropic formation.

The depth of investigation of any propagation resistivity measurement is controlled primarily by the transmitter-to-receiver spacing and the operating frequency. Lower frequencies penetrate deeper because the electromagnetic skin depth in a conductive medium increases as frequency decreases. Attenuation resistivity consistently reads deeper than phase-shift resistivity at the same frequency and spacing because the amplitude of the wave decays more gradually with radial distance than the phase velocity changes. In practical terms this means that in an invaded formation, Att-R is more representative of the uninvaded zone (true formation resistivity, R_t) while PS-R is more influenced by the flushed zone (R_xo). When invasion is shallow and the mud contrast with the formation is modest, both curves converge near R_t. When invasion is deep or the resistivity contrast is large, the two curves separate, and the magnitude and direction of that separation tell the log analyst about the invasion profile and, ultimately, about the moveable fluid content of the reservoir.

Correction for borehole effects, tool eccentricity, and formation anisotropy is applied either through look-up charts (historically) or through real-time inversion software that simultaneously fits all available attenuation and phase-shift curves across multiple spacings and frequencies. Modern proprietary inversion platforms (such as Halliburton's StrataXaminer, Schlumberger's QuickSilver, and Baker Hughes's DISTORT) can resolve a three-layer invasion model in real time at the surface while drilling, delivering R_t, R_xo, invasion radius (r_i), and formation anisotropy (R_h/R_v) with a measurement update every 30 cm (1 ft) of depth.

Invasion Diagnosis: Reading the Att-R and PS-R Separation

The relative position of the attenuation and phase-shift curves is one of the most diagnostic patterns in LWD log interpretation. Because Att-R samples deeper than PS-R, in an uninvaded formation both curves should overlay. Any separation signals the presence of a resistivity contrast between the flushed zone and the virgin formation, i.e., invasion.

Resistive invasion (Att-R > PS-R) is by far the more common pattern in productive reservoirs drilled with water-based mud (WBM). Fresh or moderately saline WBM filtrate displaces saline connate brine in the near-wellbore region, making the flushed zone more resistive than the deeper, undisturbed formation (which may still contain conductive brine at irreducible water saturation mixed with hydrocarbons). The shallow PS-R reads this high-resistivity invaded zone while the deeper Att-R reads the lower true-formation resistivity. Hence Att-R < PS-R in this scenario, which is why it is sometimes described as "PS over ATT" pattern and labeled resistive invasion despite the inverted curve order.

Conductive invasion (PS-R > Att-R) occurs when the filtrate is more conductive than the formation fluid it displaces, most commonly when a saline WBM invades a hydrocarbon-bearing zone. Here the flushed zone is saltier (lower resistivity) than the hydrocarbon-saturated uninvaded formation, so the shallow-reading PS-R is suppressed below the deeper-reading Att-R. This "ATT over PS" pattern in an apparently clean sand is a classical hydrocarbon indicator and is treated as a direct fluid contact flag during real-time drilling decisions.

The quantitative invasion radius and the resistivity contrast can be estimated by simultaneous inversion of multiple Att-R and PS-R curves from different spacings. If the shortest-spacing curves separate substantially and the longest-spacing curves converge, invasion is shallow (less than 30 cm / 12 in); if all spacings remain separated, invasion is deep and R_t may be difficult to determine without deeper-reading tools such as an array induction log acquired on a wireline run.

International Jurisdictions and Regional Practice

Canada (Western Canada Sedimentary Basin). In Alberta and British Columbia, propagation resistivity LWD tools are standard equipment on virtually all horizontal Montney, Duvernay, and Cardium wells. The Alberta Energy Regulator (AER) requires that formation evaluation logs be submitted digitally in LAS format; attenuation and phase-shift curves at multiple spacings and frequencies are typically included in the mandatory log package. Because Montney formations are tight (matrix permeability 0.001 to 0.1 mD) and often drilled with oil-based mud, the Att-R/PS-R separation pattern is routinely used to confirm OBM invasion and validate R_t before hydraulic fracture design. Canadian well licensing under the AER Directive 050 mandates full LWD log submission within 60 days of rig release.

United States (Permian Basin, Eagle Ford, Bakken). The US horizontal drilling boom drove widespread adoption of multi-frequency, multi-spacing propagation resistivity tools across all major unconventional plays. In the Permian Delaware and Midland basins, attenuation resistivity is a primary geosteering input for landing lateral sections in the Wolfcamp, Bone Spring, and Spraberry benches. The US Bureau of Land Management (BLM) and state regulators such as the Texas Railroad Commission (RRC) and North Dakota Industrial Commission (NDIC) accept LWD logs as the primary petrophysical record; LAS files are typically attached to the well completion report within 60 days of spud.

Norway and the North Sea. The Norwegian Petroleum Directorate (NPD, now Sodir) mandates submission of all LWD log data to the DISKOS national data repository. High-angle and horizontal wells in the Troll, Johan Sverdrup, and Edvard Grieg fields routinely use propagation resistivity as the primary formation evaluation tool during drilling. The cold, high-salinity formation waters in Jurassic sandstone reservoirs of the North Sea present a strong resistivity contrast with hydrocarbon columns, making Att-R and PS-R separation highly diagnostic. Norwegian operators including Equinor, Aker BP, and ConocoPhillips Norway publish standard LWD log formats compliant with POSC/Energistics WITSML data standards.

Middle East (Saudi Arabia, UAE, Kuwait, Iraq). Carbonate reservoirs of the Arabian Platform, including the Arab-D, Mishrif, and Shuaiba formations, are drilled extensively with horizontal and maximum- reservoir-contact (MRC) wells. Saudi Aramco's In-Kingdom drilling programs use multi-frequency propagation resistivity as one of the primary real-time reservoir navigation sensors. Carbonate sequences often exhibit strong vertical resistivity anisotropy (fracture-controlled permeability versus tight matrix), so the separation between Att-R and PS-R from tilted or azimuthal transmitter-receiver antenna geometries is exploited to resolve fracture corridors and stylolite zones. Regional well data submission standards are governed by national oil company specifications rather than a pan-regional regulatory framework.

Australia (Browse, Carnarvon, and Cooper Basins). The National Offshore Petroleum Titles Administrator (NOPTA) oversees data submission requirements for Australian offshore wells. LWD propagation resistivity data, including both attenuation and phase-shift curves, are included in the mandatory well completion report package. In the Carnarvon Basin (North West Shelf), gas-bearing Triassic and Jurassic sandstones drilled by Woodside, Chevron, and Shell use LWD resistivity for real-time water saturation determination and to detect gas-water contacts in thin interbedded sequences.