Acid Effect

The acid effect is the change in a pulsed neutron capture log measurement that occurs when an acid treatment dissolves carbonate minerals in the near-wellbore region, creating new porosity and altering the hydrogen index and neutron capture cross-section (sigma) of the formation. In carbonate formations where calcium and magnesium are major mineral constituents, acid treatments create wormholes and vugs that replace solid rock with open pore space. This change in mineralogy and porosity shifts the formation's neutron capture cross-section sigma, the ratio of counts recorded at far versus near detectors, and the apparent bulk hydrogen content, all of which are the quantities measured by pulsed neutron capture tools run through production casing. Running a pulsed neutron log before and after an acid treatment allows the engineer to evaluate whether the acid penetrated the intended zones, how deeply it altered the formation, and whether new porosity was created in the target interval or diverted into an unwanted zone.

Key Takeaways

The pulsed neutron capture (PNC) log works by emitting a burst of high-energy neutrons from a downhole source and measuring the rate at which those neutrons are captured by formation nuclei as a function of time after the burst. The neutron capture cross-section sigma (measured in capture units, cu) varies by mineral: chlorine in formation brine has a very high sigma (~2,000 barns); calcium carbonate has a moderate sigma (~7 barns); quartz has a low sigma (~4 barns); hydrocarbon has a very low sigma (~20 barns total with no chlorine). When acid dissolves carbonate and the resulting voids fill with formation brine, the sigma of the treated zone increases because chlorine-bearing brine is replacing zero-sigma void space (pore space that was previously absent). If the voids fill with oil or gas rather than water, the sigma decreases. The direction and magnitude of the sigma change after acidizing identifies which fluid fills the acid-created porosity.
The acid effect is most clearly visible on the ratio of near to far detector count rates (the ratio curve) and on the sigma log itself. In a clean carbonate that is fully water-saturated after acidizing, the acid effect appears as an increase in sigma and an increase in the near/far ratio relative to the pre-acid log, because the new brine-filled pore space has higher thermal neutron absorption than the dissolved mineral. In a gas-bearing carbonate, acid-created porosity filled with gas shows as a decrease in sigma (gas has very low neutron capture cross-section) and a decrease in the near/far ratio. The magnitude of the shift is proportional to the volume of rock dissolved: a 5% increase in porosity might shift sigma by 1 to 2 cu in a water-saturated carbonate, which is detectable but requires careful baseline comparison to the pre-acid log.
One of the most valuable applications of the acid effect log is evaluating acid diversion. When a multi-zone well is acidized with a single bullhead treatment (acid pumped down the tubing with no mechanical diversion), the acid preferentially enters the highest-permeability zone and may bypass tighter zones entirely. A pre-and-post-acid comparison of the PNC log shows which zones received acid and which did not: zones that acidized show the acid effect (sigma shift) while untreated zones show no change from the pre-acid log. If the log shows that only one of three intended intervals received acid, the engineer can redesign the next treatment with improved diversion (foam, viscous slugs, ball sealers, or mechanical isolation) to reach the bypassed zones.
Temperature and borehole conditions affect the reliability of the acid effect measurement. The PNC tool must be run at comparable conditions (same casing size, same mud in the annulus, same fluid column in the tubing) in both the pre-acid and post-acid surveys for the comparison to be meaningful. Changes in the casing fluid or mud weight between the two runs introduce a sigma change unrelated to the acidizing. Similarly, if the formation fluid redistribution after the acid treatment (oil moving into the treated zone from an adjacent interval) changes the saturation between the two runs, the sigma shift may reflect fluid movement rather than acid effect. Best practice is to run the post-acid survey within 24 to 48 hours of the treatment before significant fluid redistribution occurs.
In carbonate formations with interbedded tight zones, the acid effect log distinguishes between zones that were effectively stimulated and zones that were bypassed. This post-treatment evaluation guides completion decisions: if a zone shows no acid effect and the pre-acid log indicates it has remaining hydrocarbon saturation, it is a candidate for re-stimulation or mechanical perforation cleaning. If a zone shows a large acid effect but is still not producing at expected rates, the problem may be near-wellbore skin from iron precipitates or emulsions rather than lack of acid penetration, redirecting the remediation strategy toward a different type of fluid treatment. The acid effect log converts an otherwise qualitative acid job result (did the acid enter the formation?) into a quantitative zone-by-zone assessment.

Reading the Acid Effect on a Before-and-After PNC Log

Running a PNC log requires the well to be cased: the tool measures through the production casing, which is why it is used to evaluate formations that have already been completed and perforated. The pre-acid PNC log is run before the acid treatment to establish the baseline sigma and count rate profiles for each perforation interval. The tool is run at a consistent logging speed (typically 200 to 300 metres per hour) with the same tool and configuration that will be used for the post-acid run.

After the acid treatment and flowback of spent acid, the post-acid PNC log is run through the same intervals at the same logging conditions. The two sigma logs are overlaid for comparison. Intervals where the post-acid sigma is higher than the pre-acid sigma (in a water-saturated carbonate) received acid and had new water-filled porosity created. Intervals where the logs overlay exactly were not contacted by the acid. A depth shift of the comparison features (sigma peaks and troughs) would indicate that the formation has moved relative to the casing, which should not occur and would indicate a depth reference error that must be corrected before interpretation.

The near/far ratio comparison provides additional confirmation of the acid effect. The ratio is less sensitive to borehole conditions than sigma because both near and far detectors are affected similarly by borehole fluid changes, while only the far detector responds to deep formation changes. A divergence between near and far ratios in the post-acid log indicates an acid effect extending beyond the immediate borehole environment, which is a sign that wormholes have penetrated into the formation matrix rather than just cleaning the immediate perforation tunnels.

Fast Facts

Pulsed neutron capture logging through casing became commercially available in the early 1960s with the development of electronic neutron sources that could be pulsed reliably at downhole temperatures and pressures. Schlumberger's TDT (thermal decay time) tool, introduced in the late 1960s, was the first widely used PNC tool for water saturation monitoring in producing wells. The acid effect as a diagnostic tool for evaluating acid treatment success was recognized by the 1970s as PNC logging became routine in carbonate fields across North America and the Middle East. In Alberta, the Devonian carbonate fields of the Nisku, Leduc, Wabamun, and Cooking Lake formations have used PNC acid effect logs since the 1980s to optimize acid stimulation programs. Modern PNC tools from Schlumberger (Reservoir Monitor Tool, RMT), Halliburton (Thermal Multigate Decay, TMD), and Baker Hughes (Pulsed Neutron Capture, PNC) all record sigma and ratio curves at multiple spacings, allowing more detailed radial depth of investigation comparison than the single-spacing tools of the early generation.

Acid Effect in Shale-Carbonate Sequences

In formations that alternate between carbonate and shale layers, the acid effect log must be interpreted carefully because the neutron response of the shale is very different from the carbonate. Shale has high bound water (and therefore high hydrogen index) and moderate sigma due to its clay mineral content. Acid does not significantly alter shale, so the shale sigma does not change between the pre-acid and post-acid logs. Only the carbonate intervals that received acid show a shift. This differential response is actually helpful for identifying the acid-treatment depth resolution: the log shows clearly which carbonate beds were contacted and which were not, with the shale beds serving as stable reference intervals whose sigma should not change.

In the Swan Hills reef complex of central Alberta, Devonian Beaverhill Lake carbonates are interbedded with organic-rich argillaceous limestones. Acid treatments in these wells can show a complex acid effect: the clean reef carbonate intervals show sigma shifts from wormhole-created brine-filled porosity, while the argillaceous layers show little response. Post-acid PNC comparison logs in these wells have been used to identify which reef zones were stimulated and to target un-stimulated argillaceous intervals for future selective perforation and restimulation jobs.

The acid effect is also called the post-acid sigma shift or acid-induced porosity change in PNC log interpretation literature. Related terms include pulsed neutron log (a through-casing logging tool that measures the thermal neutron capture cross-section (sigma) of the formation; the primary tool for detecting the acid effect in cased-hole carbonate wells), sigma (the thermal neutron capture cross-section of a formation, measured in capture units (cu); the quantity that changes when acid dissolves carbonate minerals and new fluid-filled porosity is created; higher sigma indicates more chlorine-bearing brine; lower sigma indicates hydrocarbons or gas), acidizing (the treatment whose near-wellbore effect on formation porosity and mineralogy is measured by the acid effect PNC log comparison; the acid effect is the diagnostic tool used to evaluate how well the acidizing achieved its objectives), wormhole (the dissolution channel created by acid preferentially entering the most permeable flow path in a carbonate matrix; the physical structure whose creation manifests as the acid effect on the PNC log), and diversion (the use of chemical or mechanical methods to distribute acid across multiple zones rather than letting all acid enter the highest-permeability zone; the acid effect log evaluates whether diversion was effective by identifying which zones showed a post-acid sigma change).

How an Acid Effect Log Revealed That an Entire Reef Zone Was Bypassed in a Nisku Well

An operator was producing an aging Devonian Nisku reef well in the Brazeau River area of west-central Alberta. The well had been producing for 12 years and the productivity index had declined from 0.8 cubic metres per day per kPa at initial completion to 0.18 cubic metres per day per kPa currently, suggesting formation damage or partial reservoir depletion. A pre-acid PNC log was run through the 14-metre perforated Nisku interval and showed three distinct carbonate units (upper Nisku A, middle Nisku B, lower Nisku C) with sigma values of 18, 22, and 16 cu respectively, indicating water saturation in the lower two and possible remaining oil saturation in the upper unit.

A 25-cubic-metre 15% HCl treatment was bullheaded into the well through the existing perforations. No diversion was used because the formation was considered a single reservoir unit. Post-treatment, the well's productivity index improved modestly to 0.26 cubic metres per day per kPa, less than expected from the acid volume pumped. A post-acid PNC log was run 18 hours after the treatment flowback.

The sigma comparison showed a clear acid effect in the lower Nisku C interval: sigma shifted from 16 to 21 cu over the lower 4 metres of the perforated interval, indicating acid-created porosity had filled with formation brine. The middle Nisku B showed a smaller sigma shift (22 to 24 cu, only 2 cu increase). The upper Nisku A, which pre-acid logs suggested contained remaining oil saturation and was the primary target for stimulation, showed no sigma change at all: the acid had not reached the upper zone.