Neutron Borehole Logging

Basic Concept

Neutron borehole logging is an active-source method that measures the responses of subsurface materials to neutron radiation emitted by a controlled radioactive source. The emitted neutrons collide with the constituent atoms of the matter surrounding the borehole and subsequently undergo a series of physical reactions. In these atomic-scale interactions, the emitted neutrons pass through three energy levels as they are slowed, scattered, and eventually absorbed.

The evolution of neutron logging has rendered a variety of logging tools that can be used to detect neutrons within different energy phases. Most neutron-logging tools are primarily sensitive to hydrogen content and can be used to estimate formation porosity. Each methodology has its own set of advantages and limitations. However, the major limitation that pervades all tools is their radioactive contents. Though special permits may be required, neutron logs have proven valuable for numerous environmental investigations.

Theory

Neutron logging is an active-source method that exploits principles of particle physics to deduce information about the formations surrounding boreholes. A radioactive chemical source (e.g., americium-beryllium or plutonium-beryllium) housed within the tool continuously emits neutron radiation, which is essentially a stream of free neutrons. Initially, the emitted neutrons are “fast”, as they have a kinetic energy that, though dependent on the source type, is typically greater than 4 Megaelectronvolts (MeV) (Collier, 1993; Mussett and Khan, 2000).

Neutrons penetrating the formation strike stationary atomic nuclei resulting in either elastic or inelastic collisions, which, respectively, decrease or deflect the energy of the neutron. In billiard-like elastic collisions, neutrons transfer part of their energy to nuclei, and due to momentum conservation, the magnitude of energy loss depends on the particle sizes. Because their masses are comparable, neutrons lose the most energy to hydrogen nuclei, and total neutron-energy loss within a formation can be attributed to its hydrogen content (Mussett and Khan, 2000).

Neutrons pass through different energy states as they interact with formation matter. Epithermal neutrons range between 0.1 and 100 electronvolts (eV) and continue undergoing elastic collisions. Thermal neutrons, which near 0.025 eV and are in thermal equilibrium with the formation. Thermal neutrons are highly susceptible to become absorbed by (i.e., inelastically collide with) heavy nuclei, which emit capture-gamma photons to compensate for absorption. Additionally, inelastic collisions involving non-thermal neutrons, gamma rays and secondary, slowed neutrons are emitted (Collier, 1993).

There are neutron-logging tools that can measure gamma photons, thermal neutrons, and/or epithermal neutrons. The neutron-gamma method counts gamma photons that are assumed to only be associated with thermal-neutron absorption. However, higher-energy neutrons can be captured by certain subsurface elements (e.g., chlorine, boron), which also produce capture-gamma photons. Thus, the neutron-gamma method cannot explicitly relate gamma counts to hydrogen content, as measured gamma counts may be higher than if only thermal neutrons were absorbed (Collier, 1993)

Today, the neutron-gamma log is obsolete and has been replaced by neutron-neutron logs, which tend to produce more reliable estimates of hydrogen content. Neutron-neutron tools count thermal and/or epithermal neutrons. Because thermal neutrons are highly susceptible to being captured by formation atoms, thermal-neutron measurements can be affected by lithology and water chemistry. However, epithermal-neutron measurements are less likely to be affected by extraneous factors and, therefore, are mostly related to hydrogen concentration.

Formations with high-hydrogen concentrations cause neutrons to slow more readily and travel shorter distances from the radioactive source before they are captured. Thus, measurements of high-neutron counts indicate low-hydrogen concentrations (i.e., neutron counts are inversely proportional to hydrogen content). If all hydrogen is assumed to be contained within the pore fluids (i.e., water, hydrocarbons), neutron counts can indicate porosity in saturated formations. Otherwise, in unsaturated zones, neutron counts represent volumetric water content.

Applications

Several factors (e.g., minerology, borehole size, borehole fluid, casing, etc.) can impact hydrogen content and, thus, affect formation-porosity estimates. As such, neutron logs may require multiple corrections, the most commonly applied of which are borehole size and shale effect. Correcting the shale effect, which, because shale has high immobile water content, increases porosity estimates, can be performed by the analyst using other logs (e.g., gamma, bNMR). Additional corrections are often conducted or described by the manufacturer.

Additionally, calibration, which is lithology-dependent, is essential for producing accurate porosity values. Neutron tools may not have undergone a manufacturer calibration, which typically involves running the tool in a thermally stable test well composed of known earth materials. These tools often have conversion charts specific to various subsurface environments. Unscaled tools that output neutron counts per second (CPS) can be calibrated to porosity units by the analyst using various methods (e.g., two-point method, high-porosity/low-porosity).

Neutron logs are most commonly collected using either a sidewall- or compensated tool. Compensated neutron logs (CNL) use two thermal and/or two epithermal detectors to measure the neutron cloud surrounding the radioactive source. Most tools are run decentralized, and sidewall tools often employ a mechanical arm to ensure contact with the borehole wall and collect ancillary caliper data. Additionally, the depth of investigation (DOI) of most compensated-neutron tools is approximately twice that of sidewall tools.

The depth of investigation is a function of several factors (e.g., source strength, source-to-detector spacing, hydrogen content). Generally, the depth of investigation increases with the source strength and the distance between the source and detector(s). If all else is equal, the hydrogen content of the formation controls the depth to which a tool is sensitive. Because of the energy loss and absorption mechanisms that occur, an increase in hydrogen concentration decreases the depth of investigation (Collier, 1993)

The major disadvantage of the borehole neutron logging method is that the tools contain radioactive source material. Special permits and/or licenses may be required for use, and some field sites may prohibit deploying the method in attempts to avoid any possible groundwater contamination. Additionally, because of the intricacy of neutron logs, knowledge of the tool design, tool theory, borehole construction, and the subsurface environment is important for accurate data interpretation.

Because neutron-porosity values depend on lithology and involve an inherent degree of interpretation, neutron logs are often acquired in conjunction or combined with gamma-gamma density, sonic, or drilling logs. With such, neutron data can allow for the estimation of parameters such as saturation, shale content, gas content, wet and dry density, void ratio, and lithology (U.S. Bureau of Reclamation, 2001). Additionally, neutron logging has added value to numerous geophysical studies, some of which include the following:

Determination of porosity in saturated formations
Determination of moisture content in the vadose zone
Delineation of porous formations
Detection of perched water tables
Identification of geologic layer boundaries

Examples/Case studies

Bhuyan, K. and Passey, Q.R., 1994, Clay Estimation From Gr And Neutron -Density Porosity Logs, in Proceedings, SPWLA 35th Annual Logging Symposium: Tulsa, Oklahoma, Society of Petrophysicists and Well-Log Analysts, 15 p.

Abstract: Clay estimations from gamma-ray logs and neutron-density porosity logs are commonly used techniques. However, failure to recognize the difference between shale and clay leads to confusion when comparing weight percent clay estimated from well logs with weight percent clay estimated by x-ray diffraction analysis. The principal error in clay estimation stems from the assumption that shales are composed of 100 percent clay. Our study shows that shales are commonly composed of 50 to 70 percent clay, 25 to 45 percent silt- and clay-sized quartz, and 5 percent other minerals that include feldspars and carbonates. The non-clay minerals in the shale do not commonly affect the total gamma-ray count or the neutron-density log separation. The estimation of weight percent clay can, therefore, be corrected by multiplying the GM (gamma ray index) by a factor (C), which is weight percent clay of average shale adjacent to the zone of interest. Typically this factor ranges from 50 to 70. This approach will significantly improve the weight-percent clay estimations from the gamma-ray log, but will not correct for the inaccuracies associated with the distribution of heavy minerals. We developed empirical relationships that will account for errors associated with clay content of shale as well as distribution of heavy minerals. A similar approach is applicable to clay estimation from neutron and density porosity logs. Neutron porosity is unaffected by the common non-clay minerals such as quartz, feldspar, carbonates and pyrite, which are present in shales. Therefore, the estimation of volume percent clay can be corrected by multiplying the conventional clay-estimation equation by the factor C as discussed above. In the neutron-density crossplot, Schlumberger''s (1989) ''Clay Point'' is redefined as a ''Wet Shale Point''. The area between the clean sand line and the Wet Shale Point is rescaled from 0 to 60 volume percent dry clay, and a new ''Clay Point'' is established to represent 100 percent wet clay. A neutron-density crossplot shows the locations of the Wet Shale Point, the new Wet Clay Point, the Dry Clay Point, and the Dry Shale Point.

Collett, T.S., 1998, Well Log Characterization of Sediment Porosities in Gas-Hydrate-Bearing Reservoirs, in Proceedings, SPE Annual Technical Conference and Exhibition: New Orleans, Louisiana, Society of Petroleum Engineers, p. 765-776, doi:10.2118/49298-MS.

Abstract: With growing interest in natural gas hydrates, it is becoming increasingly important to be able to determine the volume of gas hydrate and included gas within natural gas hydrate accumulations. Gas volumes that may be attributed to gas hydrates are dependent on a number of reservoir parameters, one of the most difficult reservoir parameters to determine is porosity. Well logs often serve as a source of porosity data; however, well-log calculations within gas-hydrate-bearing intervals are subject to error. The well-logging devices that show the greatest promise of yielding gas hydrate reservoir porosities are the gamma-gamma density and neutron porosity logs. Well log response modeling has revealed that under most conditions, the bulk-density of a water-bearing sedimentary section is almost identical to the bulk-density of a gas-hydrate-bearing sedimentary section as measured by a gamma-gamma density logging tool. At relatively high porosities (O>40%) and gas-hydrate saturations (Sh>50%), however, the downhole log derived bulk-density porosities need to be corrected for the presence of gas hydrate. A neutron well-log response computer simulator, SNUPAR, has been used to calculate nuclear transport and capture parameters for various gas-hydrate-bearing reservoirs. The calculated thermal neutron capture cross section of various hypothetical gas-hydrate-bearing reservoirs indicates that methane hydrate has little effect on neutron porosity measurements within "normal" reservoir conditions (O<40%) and low gas-hydrate saturations (Sh<50%). Within this study, density porosity and neutron porosity nomographs have been developed with which it is possible to correct for the effect of high gas-hydrate saturations on the log derived porosities. In the field verification phase of this study, downhole density and neutron porosity log data (in some cases corrected for the presence of gas hydrate) have yielded accurate porosities for gas-hydrate-bearing reservoirs on the North Slope of Alaska.

Flaum, C. and Cedex, M., 1983, Dual Detector Neutron Logging In Air Filled Boreholes, in Proceedings, SPWLA 24th Annual Logging Symposium: Calgary, Alberta, Society of Petrophysicists and Well-Log Analysts, 21 p.

Abstract: Air drilled boreholes have for a long time presented problems in formation evaluation. Many logging tools exhibit every different behaviour in an air environment than in liquid. Neutron porosity logging has been a particularly bothersome problem, as the extremely successful density/neutron combination could not be utilized in air filled wells, without resorting to the sequential suite using the single detector SNP tool. The failure of standard Dual Detector Neutron to give adequate porosity response can be traced to the properties of thermal neutron distribution and previous analysis methods. With the upcoming introduction of the new CNT-C tool, which includes two thermal and two epithermal detectors, a whole new dimension of neutron logging is accessible. In particular, through a case study, it has been demonstrated that the two epithermal detectors can be used to obtain a reliable porosity measurement, minimally affected by environment. A simple analysis technique will be described. Several field examples will be demonstrated. Extension to thermal detection and its feasibility will be discussed.

Galford, J.E., Flaum, C., Gilchrist Jr., W.A., and Duckett, S.W., 1989, Enhanced Resolution Processing of Compensated Neutron Logs: Society of Petroleum Engineers Formation Evaluation, v. 4, no. 2, p. 131-137, doi:10.2118/15541-PA.

Abstract: Compensated neutron logging (CNL(SM)) uses a two-detector system that was developed to reduce borehole effects. The ratio of counting rates from the detectors provides the basic tool response from which a porosity index is obtained. Each detector in this system has a different vertical resolution because of its spacing. A new method of processing the counting rates has been developed to enhance the vertical resolution capabilities of the neutron porosity index by exploiting the better vertical resolution of the near detector. Because no additional or new measurements are required, data from older wells can easily be re-evaluated. Results from the new method have been compared with microspherically focused logs (MicroSFL(SM)) and electromagnetic propagation logs (EPT(SM)). They show repeatable thin-bed resolution on the order of 1 ft [0.3 m] for data sampled at 6-in. [15-cm] intervals; the typical vertical resolution from ratio processing is approximately 2 ft [0.6 m]. The statistical precision of the high-resolution processing is superior to that of the standard ratio method. An additional parameter, obtained with the new processing method, provides information about borehole effects. This parameter can be used for qualitative indications of gas when invasion is not deep and environmental effects are not large. The new method has been applied successfully in carbonate and laminated sand formations. Studies show that thin beds can be detected in high-porosity formations where normal processing has significant statistical variations resulting from reduced counting rates.

Gilchrist, W.A., 2009, Compensated Neutron Log Response Issues: Petrophysics, v. 50, no. 5, p. 416-426.

Abstract: Compensated neutron (CN) logs have been around for many years, and have been used successfully for many petrophysical applications. Although the basic design and general response characteristics of these instruments have remained relatively constant over the years, service companies have remained relatively constant over the years, service companies have refined the response characterizations and have updated their hardware, providing evolutionary improvements. Unfortunately, the general understanding among petrophysicists of how CN instruments work and how best to make use of the log data appears to have lagged or perhaps even worsened in recent years. This paper discusses several obstacles to understanding and interpreting neutron logs and proposes some techniques for avoiding problems. Although this paper is not an exhaustive treatment of CN logging issues, some of the more important ones are discussed. Some alternative ways of looking at the data are presented.

Halker, A., Kuznir, N.J., Mellor, D.W., and Whitworth, K.R., 1982, The synthesis of fracture/strength logs using borehole geophysics: a new geotechnical service: Quarterly Journal of Engineering Geology and Hydrogeology, v. 15, no. 1, p. 15-28, doi:10.1144/GSL.QJEG.1982.015.01.04.

Abstract: In the coal industry, the present energy situation and economic climate have increased the need for rapidly acquired, cost effective, reliable assessment of the strengths of rock which may be encountered in shafts, drifts and access tunnels, this need having hitherto been met by conventional sampling and testing procedures. This paper describes a new technique based on the processing of down-hole geophysical logs to provide a continuous, accurate record of these properties speedily and at relatively low cost. The basis of the technique is to include a Neutron-Neutron log in the standard logging package and process it with the caliper log either to generate a Hydrogen Index log or simply a caliper compensated Neutron-Neutron log. The derived log is then split into selected lithology-dependent sub-groups and modified according to a predetermined set of calibration constants. These calibration constants have been determined by the authors for the Coal Measures of Staffordshire but can be readily re-determined, if necessary, to suit other coalfields or prospects as required. The application of the technique has the potential to improve all forms of engineering site investigation where both stability and ‘extractability’ of rock are under consideration, often removing the need to acquire expensive rock core samples and saving considerable labour costs. In the U.K. coal mining industry fracture logs synthesized using this technique are at present being supplied, so that their value, in the planning and location of shafts and tunnels, in the design of shaft insets and tunnel supports, in the selection of tunnelling machines and forecasting drivage rates and in predicting roof, floor and overburden characteristics may be assessed. It is believed that they will have applications in many other mining or site investigation environments.

Kamel, M.H. and Mabrouk, W.M., 2003, Estimation of shale volume using a combination of the three porosity logs: Journal of Petroleum Science and Engineering, v. 40, no., 3-4, p. 145-157, doi:10.1016/S0920-4105(03)00120-7.

Abstract: An equation was developed for evaluating the volume of shale using standard porosity logs such as neutron, density and acoustic logs. The equation is written in terms of several parameters that are readily available from well-log measurements. This equation, which takes into consideration the effect of matrix, fluid and shale parameters, applies reasonably well for many shaly formations independent of the distribution of shales. The results demonstrate the applicability of the equation to well-log interpretation as a procedure for computing shale volume in shaly sand sedimentary sections. Three key advantages of the proposed equation are: (1) it incorporates several parameters that directly or indirectly affect the determination of shale in one equation, (2) it integrates the three porosity tools for a more accurate determination, and (3) it works well in hydrocarbon-bearing formations and where radioactive material other than shale is present. Successful application of the equation to shaly sand reservoirs is illustrated by analyses of samples from the Gulf of Suez.

Tang, H., Killough, J.E., Heidari, Z., and Sun, Z., 2017, A New Technique To Characterize Fracture Density by Use of Neutron Porosity Logs Enhanced by Electrically Transported Contrast Agents: Society of Petroleum Engineers Journal, v. 22, no. 4, p. 1034-1045, doi:10.2118/181509-PA.

Abstract: Fracture-density evaluation has always been challenging for the petroleum industry, although it is a required characteristic for reliable reservoir characterization. Production can be directly controlled by fracture density, especially in tight reservoirs. Previous publications showed that use of high thermal neutron-capture cross-sectional (HTNCC) contrast agents can enhance the sensitivity of neutron logs to the presence of fractures. However, all these studies focus on locating the proppants. In this paper, we introduce a method of injecting electrically transported charged boron carbide (B4C) contrast agents to naturally fractured formations to enhance the propagation of the contrast agents into the secondary-fracture (natural and induced) network by use of an externally applied electric field and to characterize the fracture density in the unpropped region by use of the enhanced neutron porosity logs. We perform numerical simulations to validate the feasibility of the proposed technique. A physical model derived from electrophoretic velocity and material-balance formulations is proposed and solved to simulate the spatial distribution of contrast agents. Furthermore, we simulate neutron porosity logs by solving the neutron-diffusion equation, which allows a fast analysis for the proposed technique. The simulation results confirmed that an external electric field can significantly enhance the transport of charged contrast agents into the secondary-fracture network. Sensitivity analysis revealed that increasing particle f-potential can efficiently decrease the transport time. Furthermore, we applied the introduced technique on synthetic cases with variable secondary-fracture density ranging from 1 to 8%. The relative variation in the simulated neutron porosity before and after applying the electric potential field was up to 50% in a formation with 8% fracture density after applying an electric field for 6 hours. The proposed technique can potentially enable application of neutron porosity logs in fracture characterization, including assessment of secondary-fracture density, if combined with other well logs.

Wetton, J.A. and Elkington, P.A.S., 2012, Processing and Interpretation of Density and Neutron Logs for the Evaluation of Coal Bed Methane Reservoirs, in Proceedings, SPE/EAGE European Unconventional Resources Conference and Exhibition: Vienna, Austria, Society of Petroleum Engineers, 11 p., doi:10.2118/150216-MS.

Abstract: Density and neutron well log processing algorithms designed for conventional oil and gas reservoirs are not optimum for coal bed methane evaluation. In particular the corrections applied to measured electron density values (to derive bulk density) assume a calcium carbonate rock matrix, and quantitative analysis of neutron porosity logs is hindered by low count rates in coal and a lack of published information regarding the sensitivity of the measurement to variations in coal composition. The thinly-bedded nature of many coals is an additional challenge. This paper describes a new log processing method that simultaneously enhances statistical precision and vertical resolution whilst seeking to avoid additional sensitivity to the borehole environment. It then describes a fast nuclear rock properties modelling application developed to study the sensitivity of density, photo-electric cross-section (Pe) and neutron porosity measurements to variations in coal chemistry. The model has been validated using an accurate (but slow) Monte Carlo particle transport code which has been extensively benchmarked in independently characterized test blocks. The findings are applied to high resolution log data acquired in wells drilled for the evaluation of coal bed methane reservoirs. The key parameter used in the transformation of electron to bulk density is investigated and optimum values suggested. The sensitivity of density and neutron porosity measurements to variations in the volumes and chemistry of organic material, mineral matter and moisture is determined, and it is shown that appropriately processed neutron porosity logs have usable sensitivity to such compositional variations. The inclusion of neutron porosity improves our ability to differentiate coal types from logs, and addresses an important source of uncertainty in the reconciliation of log and core density values; in so doing it helps improve estimates of in-situ coal properties and associated quality attributes including gas-in-place.

References

Collier, H.A., 1993, Porosity Tools, in Borehole Geophysical Techniques for Determining Water Quality and Reservoir Parameters of Fresh and Saline Water Aquifers in Texas: Austin, Texas, Texas Water Development Board, v. 1, p. 289-343.

Ellis, D.V. and Singer, J.M., 2007, Neutron Porosity Devices, in Ellis, D.V. and Singer, J.M., eds., Well Logging for Earth Scientists: Dordrecht, The Netherlands, Springer Netherlands, p. 351-382.