Pulsed Neutron Capture Logs Research Articles

An improved method for evaluating fracture density using pulsed neutron capture logging

Fracture density is a critical fracture evaluation parameter for the optimization and production prediction of hydraulic fracturing models. Nonradioactive tracer (NRT) techniques have successfully used a quantitative fracture density method based on neutron-induced gamma rays from formation elements. Furthermore, fracture density can be calculated using the formation capture cross section before and after hydraulic fracturing based on pulsed neutron capture logging. However, the response sensitivity of the macroscopic scattering cross section to the fractures decreases when the fracture density is high due to the neutron self-shielding phenomenon. Therefore, an improved fracture density evaluation method is applied, which combines the macroscopic capture cross section and the neutron self-shielding correction factor. In the new method, the peak area of titanium from the captured gamma spectrum is used to obtain a neutron self-shielding correction factor to improve the sensitivity of fracture density determination. Furthermore, the response of capture cross-section variation to fracture density at various tagged proppant concentrations and formation backgrounds is investigated. The findings indicate that the tagged proppant concentration influences the detection limit of fracture density and the sensitivity of fracture density identification. The accurate calculation range of fracture density using the new method has been extended from 5% to 10% under the condition that the tagged proppant concentration is 0.2%. Meanwhile, whereas water salinity significantly impacts capture cross-section variation, the effects of porosity, lithology, and fluid type on capture cross-section variation are negligible. A simulated fracturing example demonstrates the method’s applicability in various measurement environments. The results show that fracture density and height are consistent with the model settings after correcting for water salinity, and the fracture density calculation error is less than 3%. Therefore, our evaluation method for fracture density corrected for the neutron self-shielding effect improves response sensitivity and fracture density calculation accuracy.

A new diffusion effect correction method in pulsed neutron capture logging

The neutron diffusion effect contributed considerate bias to the sigma estimation in the pulsed neutron capture (PNC) well logging, and thus this effect must be corrected to achieve an accepted sigma prediction. This paper developed a new diffusion effect correction method for a more accurate sigma estimation in the PNC well logging method. Since the diffusion effect with a given source spacing in a PNC tool mainly depends on neutron slowing down and absorption, two parameters, the capture gamma ray counts ratio (RCAP) of the far to near detectors and the gamma ray counts ratio of inelastic scattering to capture (RIC) in the near detector, were used in the new method to effectively correct the diffusion effect. This is because RCAP is sensitive to neutron slowing down ability and RIC is sensitive to thermal neutron absorption ability. A combined expression of RCAP and RIC thus can reflect both the scattering and absorption contribution to the diffusion coefficient, which is subsequently used to correct the diffusion effect. No additional efforts are required to measure RCAP and RIC because they can be simultaneously obtained with the gamma ray decay time spectrum measurement. Computational experiments based on Monte Carlo simulations demonstrated the sigma calculated with the new correction method has shown to be more accurate than the non-corrected apparent sigma in the PNC logging. • A diffusion effect correction method developed for accurate sigma estimation. • Diffusion effect in sigma logging demonstrated by Monte Carlo simulation. • Two self-correction parameters of RCAP and RIC were used to correct the diffusion effect.

Demonstration of Non-Endangerment at the Secarb Anthropogenic Test Site

The Southeast Carbon Sequestration Partnership (SECARB) Anthropogenic Test became the first fully integrated carbon capture, transport, and storage project to successfully demonstrate non-endangerment towards the closure of a Alabama Department of Environmental Management (ADEM) underground injection control (UIC) permit. Despite being a state-issued UIC Class V experimental well permit, this permit contained many elements specific to geologic sequestration of CO2 included in the United States Environmental Protection Agency (EPA) regulations for Class VI wells. This paper will outline the permit structure and the steps taken to demonstrate non-endangerment for the first fully integrated Carbon Capture and Storage (CCS) project on a coal-fired power plant using advanced amines for CO2 capture. The SECARB Anthropogenic Test is a U.S. Department of Energy (DOE), Southern Company, and Electric Power Research Institute (EPRI) funded, Southern States Energy Board (SSEB) managed, project designed to demonstrate deep underground injection and containment of anthropogenic CO2 sourced from a 750 MW (net) coal -fired electric generating unit at Plant Barry, Alabama. From August 2012 to September 2014, a total of 114,104 metric tonnes of CO2 were captured and transported via a 12-mile pipeline from Alabama Power Company’s Plant Barry and successfully injected and stored in the upper Paluxy formation, a Lower Cretaceous sandstone unit, at the Denbury Onshore operated Citronelle Oil Field in Citronelle, Alabama. One CO2 injection well and three monitoring wells were drilled for this project. The initial Class V experimental technology UIC permit application was modified to include some Class VI permit requirements by the EPA to demonstrate protection of underground sources of drinking water (USDWs). These requirements included but were not limited to bottom to top cement coverage, a model-based Area of Review (AoR) determination with periodic updates, and a suite of Monitoring, Verification and Accounting (MVA) methods to track and monitor the CO2 plume migration and containment within the permitted injection zone. The UIC permit required extensive monitoring of the injected CO2 with objectives to create and sustain well integrity, assure safe CO2 injection operations, verify the location and migration of the CO2 plume, and monitor for any CO2 leakage. As such, a suite of technologies were applied and developed to demonstrate non-endangerment of underground sources of drinking water (USDW’s) and to ensure that CO2 does not migrate out of the intended storage zones. This involved several levels of monitoring: surface monitoring, shallow groundwater monitoring, and deep reservoir monitoring via commercially available and experimental MVA methods. Sufficient evidence was provided by the suite of monitoring efforts to indicate successful non-endangerment of USDWs at the Citronelle site. No CO2 release or build-up was detected using groundwater analysis, tracer detection, and soil CO2 flux monitoring to indicate any shallow or surficial leakage of CO2. Additionally, no evidence of gas saturation was observed within or above the confining zone, based on the results of time-lapse pulsed neutron capture logging. Cross-well seismic results show no negative velocity anomalies in or above the confining unit, which implies that there is no detectable leakage out of the injection zone. The associated models simulating the distribution of CO2 through the injected geological layers agreed with observed monitoring data, demonstrating confinement within the injected zone and confirming that the Citronelle Dome structure forms an ideal CO2 storage complex for the confinement of CO2 in the subsurface.

Developing Best Practices for Evaluating Fluid Saturations with Pulsed Neutron Capture Logging Across Multiple Active CO2-EOR Fields

The Midwest Regional Carbon Sequestration Partnership (MRCSP) is leading an extensive study and monitoring effort of large-scale carbon dioxide (CO2) injection and storage during enhanced oil recovery (EOR) in depleted oil fields in the Michigan Niagara Reef Trend. Detailed monitoring of caprock and reservoir zones is vital for understanding oil, gas, and water saturations and potential migration of CO2. Baseline and repeat pulsed neutron capture (PNC) logs were collected as part of the monitoring effort and study for these storage sites. An overview of the development and enhancement of best practices for PNC logging in complex EOR fields is presented.

Significance of Time-Lapsed Hydrocarbon Saturations From Pulsed-Neutron-Capture (PNC) Logs and the Impact of Borehole-Environment Parameters on PNC-Derived Saturations

Summary Reservoir surveillance with pulsed-neutron-capture (PNC) logs has shown its importance historically in providing reservoir hydrocarbon saturations behind the casing. However, there is little in the literature to show the importance of PNC logs for contact tracking and accessing bypassed/moved hydrocarbons. In addition, no one has ever attempted to characterize the various borehole environments and subsequently understanding their impact on the PNC-derived hydrocarbon saturations. This paper is divided into two parts. The first part shows the importance of time-lapse PNC logs in understanding reservoir-fluid movement and chasing migrated hydrocarbons behind the casing. A case study from the UK Central North Sea is presented to highlight the importance of the PNC sigma data. The second part of the paper shows the computation of borehole-environment parameters from simple to complex completions including multiple casing/tubing strings, varying hole sizes, and annuli fill. For the first time, the work presented in this paper shows the importance of having correct environmental parameters and the impact of using incorrect parameters on the PNC-derived hydrocarbon saturations. Not making such detailed checks may result in misinterpretation of the PNC data, leading to incorrect conclusions about fluid dynamics, and risky decisions in adding perforations and water-shut off (WSO) opportunities.

Reconstruction of water encroachment history in 4D in a brown field using a time-lapse pulsed neutron capture log – A case study

The objective of this paper is to illustrate the use of pulsed neutron capture log in reconstruction of water encroachment history in time and space in a brown field. Time-lapse pulsed neutron logs tracked the movement of oil-water contact in wells through time. This data from multiple wells was used to reconstruct the oil-water contact surfaces at different times for the area studied. The study resulted in a remaining oil column height map for the area and also predicted future water movement patterns. The remaining reserves at different times were calculated using a static model and these fluid contact surfaces. Estimated production between these time surfaces as computed from the static model matched well with the recorded actual production giving credence to the study.

Geophysical Monitoring Methods Evaluation for the FutureGen 2.0 Project

A comprehensive monitoring program will be needed in order to assess the effectiveness of carbon sequestration at the FutureGen 2.0 carbon capture and storage (CCS) field-site. Geophysical monitoring methods are sensitive to subsurface changes that result from injection of CO2 and will be used for: (1) tracking the spatial extent of the free phase CO2 plume, (2) monitoring advancement of the pressure front, (3) identifying or mapping areas where induced seismicity occurs, and (4) identifying and mapping regions of increased risk for brine or CO2 leakage from the reservoir. Site-specific suitability and cost effectiveness were evaluated for a number of geophysical monitoring methods including: passive seismic monitoring, reflection seismic imaging, integrated surface deformation, time-lapse gravity, pulsed neutron capture logging, cross-borehole seismic, electrical resistivity tomography, magnetotellurics and controlled source electromagnetics. The results of this evaluation indicate that CO2 injection monitoring using reflection seismic methods would likely be difficult at the FutureGen 2.0 site. Electrical methods also exhibited low sensitivity to the expected CO2 saturation changes and would be affected by metallic infrastructure at the field site. Passive seismic, integrated surface deformation, time-lapse gravity, and pulsed neutron capture monitoring were selected for implementation as part of the FutureGen 2.0 storage site monitoringmore » program.« less

FutureGen 2.0 Monitoring Program: An Overview of the Monitoring Approach and Technologies Selected for Implementation

Abstract The FutureGen 2.0 Project will design and build a first-of-its-kind, near-zero emissions coal-fueled power plant with carbon capture and storage (CCS). To assess storage site performance and meet the regulatory requirements of the Class VI Underground Injection Control (UIC) Program for CO 2 Geologic Sequestration, the FutureGen 2.0 project will implement a suite of monitoring technologies designed to 1) evaluate CO 2 mass balance and 2) detect any unforeseen loss in CO 2 containment. The monitoring program will include direct monitoring of the injection stream and reservoir, and early-leak-detection monitoring directly above the primary confining zone. It will also implement an adaptive monitoring strategy whereby monitoring results are continually evaluated and the monitoring network is modified as required, including the option to drill additional wells in out-years. Wells will be monitored for changes in CO 2 concentration and formation pressure, and other geochemical/isotopic signatures that provide indication of CO 2 or brine leakage. Indirect geophysical monitoring technologies that were selected for implementation include passive seismic, integrated surface deformation, time-lapse gravity, and pulsed neutron capture logging. Near-surface monitoring approaches that have been initiated include surficial aquifer and surface- water monitoring, soil-gas monitoring, atmospheric monitoring, and hyperspectral data acquisition for assessment of vegetation conditions. Initially, only the collection of baseline data sets is planned; the need for additional near- surface monitoring will be continually evaluated throughout the design and operational phases of the project, and selected approaches may be reinstituted if conditions warrant. Given the current conceptual understanding of the subsurface environment, early and appreciable impacts to near-surface environments are not expected.

Quantitative interpretation of pulsed neutron capture logs: Part 2 — Inversion of measurements in thinly bedded formations

We have developed an inversion method to reduce shoulder-bed effects on pulsed neutron capture (PNC) logs for estimating layer-by-layer-capture cross sections Σ. The method is based on a previously developed rapid approximation of PNC logs. Tests performed on synthetic examples that include a variety of lithology, saturating-fluid, and bed-thickness configurations confirm the efficiency, reliability, and stability of the inversion procedure. Inversion consistently improves the vertical resolution and Σ definition of PNC logs across beds thinner than 45 cm. Our fast, iterative algorithm inverts Σ logs in seconds of CPU time and is therefore suitable for joint petrophysical interpretation with other open- and cased-hole logs.

Quantitative interpretation of pulsed neutron capture logs: Part 1 — Fast numerical simulation

Pulsed neutron capture (PNC) logs are commonly used for formation evaluation behind casing and to assess time-lapse variations of hydrocarbon pore volume. Because conventional interpretation methods for Σ logs assume homogeneous formations, errors may arise, especially in thinly bedded formations, when appraising petrophysical properties of hydrocarbon-bearing beds. There exist no quantitative interpretation methods to account for shoulder-bed effects on Σ logs acquired in sand-shale laminated reservoirs. Because of diffusion effects between dissimilar beds, Σ logs acquired in such formations do not obey mixing laws between the Σ responses of pure-sand and pure-shale end members of the sedimentary sequence. We have developed a new numerical method to simulate PNC rapidly and accurately logs. The method makes use of late-time, thermal-neutron flux sensitivity functions (FSFs) to describe the contribution of multilayer formations toward the measured capture cross section. It includes a correction procedure based on 1D neutron diffusion theory that adapts the transport-equation-derived, base-case FSF of a homogeneous formation to simulate the response of vertically heterogeneous formations. Benchmarking exercises indicate that our simulation method yields average differences smaller than two capture units within seconds of computer central processing unit time with respect to PNC logs simulated with rigorous Monte Carlo methods for a wide range of geometrical, petrophysical, and fluid properties.

Best Practices in Testing and Analyzing Multilayer Reservoirs

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 132596, ’Best Practices in Testing and Analyzing Multilayer Reservoirs,’ by Yan Pan, SPE, Chevron Energy Technology Company, and Michael Sullivan and David Belanger, SPE, Tengizchevroil, prepared for the 2010 SPE Western Regional Meeting, Anaheim, California, 27-29 May. The paper has not been peer reviewed. Layered formations are the norm rather than the exception among oil and gas reservoirs. Knowing individual-layer properties is important for development strategies, especially for secondary recovery. A set of best practices was established in the design, execution, and analysis of multilayer pressure-transient tests in the Tengiz oil field in western Kazakhstan. Use of multilayer testing and analysis techniques provided a lower level of uncertainty in layered-reservoir characterization than can be obtained with a simpler commingled-pressure-transient test alone. Introduction Formations in the platform area of Tengiz consist of multiple zones with different reservoir pressures. Therefore, crossflow often occurs during shut-in periods. In some cases, the effect from different layer pressures and skin distributions could result in 100% difference in individual-layer permeability estimations. To reduce uncertainty, the selective-inflow-performance (SIP) production-logging technique is used to measure the bottomhole pressures and flow contributions of individual layers during a commingled-pressure-transient test. Pulsed-neutron-capture (PNC) logs are used to gain information about stimulation effectiveness (skin) in each zone. Then, a robust step-by-step analysis method that uses all data and information is applied to obtain individual-layer properties. The best practices for use of this methodology and the associated uncertainty are detailed in the full-length paper. The challenge in characterizing layered reservoirs is the large number of unknown parameters. The approach described in the paper resolves enough of these unknowns in a systematic fashion for a more direct solution to be obtained, with a resulting lower uncertainty. Tengiz Field The central platform of the Tengiz carbonate reservoir is dominated by matrix porosity. The surrounding rim and flank regions exhibit evidence of fracture networks. Three major zones contribute to oil production. The upper reservoir units have been produced for a longer period of time and, therefore, are more depleted. All Tengiz wells are essentially vertical and are producing single-phase oil with flowing bottomhole pressure above the bubblepoint, which provides good conditions for production-log profiling. Because most wells produce no water, the connate water has low salinity, and irreducible water saturation is approximately 10%, elevated chlorine-concentration levels remained in the near-wellbore region of the formation after acid stimulation, which increased the chance for a PNC log to detect acid effects. Sour-gas injection for miscible flooding began in late 2007 in the central-platform area. Understanding individual-zone formation properties was important in the success of the hydrocarbon-miscible flood.

Formulation of Capillary Force Barriers in Moderately Oil-Wet Systems and Their Application to Reservoir Simulation

SummaryAs a common production aspect of the Thamama formation (a carbonate reservoir) in both onshore and offshore Abu Dhabi fields, unexpected early-water breakthrough through specific high-permeability layers without a clearly impermeable layer underneath has been observed in several water-injection schemes. Observed field data such as pulsed neutron capture (PNC) logs indicate the absence of injected water slumping away from wellbores. The concept of capillary force barriers was introduced a decade ago to resolve this issue, in which the role of capillary pressure forces on crossflow in stratified layers is modeled.1–3This paper tries to revisit and fine-tune the concept of capillary force barrier and model hysteresis expected in a moderately oil-wet system. First, some measurements of special core analysis and related interpretations are presented in which the results are analytically formulated by a published methodology to generate saturation functions consistent with hysteresis using an assumption of wettability.4An application of the formulation to numerical reservoir simulation was carried out in a systematic manner because the reservoir-rock-type (RRT) scheme of the model was based on primary-drainage curves that can be fully linked with the generated saturation functions. It is demonstrated on cross sections how small differences in imbibition capillary pressures can affect the water movement across contrasting RRT boundaries in a moderately oil-wet system.The proposed formulation is an effective tool for generalizing saturation functions related to matrix properties in a consistent manner, and it systematically incorporates hysteresis and wettability into the numerical reservoir-simulation model.

The Use of Pulsed-Neutron Capture Logs for Reservoir Management in the Midway-Sunset Field

Summary Systematic time-lapse pulsed-neutron capture (PNC) logging was conducted during a 2-year steam-foam mechanistic field trial in the steamflooded Monarch reservoir, which is composed of interbedded sandstones, conglomerates, siltstones, and diatomaceous mudstones. Observation wells were drilled with one continuously cored through the reservoir interval and two emptied to facilitate better temperature logging. Uncalibrated PNC log responses in the air-filled wells were normalized to water-filled conditions and were used to generate a series of steam/gas saturation profiles that indicate clearly the dynamics of the fluid and foam in the reservoir during the experiment. Correlations between neutron capture cross sections (sigma), mineralogy, and rock chemistry from core samples were examined for additional use of the PNC logs for steamflood reservoir characterization. Sigma was found to increase with increasing clay/mica content and diatomite content. This finding may result in improved stratigraphic correlations of the more laterally continuous diatomaceous mudstones based on PNC logs. The PNC log data from the field trial, when used in conjunction with core, temperature, and pressure data, were critical in developing a better understanding of foam generation and propagation in the reservoir. Furthermore, they minimized the time and cost required to complete the field trial successfully. They also led to opportunities for vertical expansion of steamflooding in the area.

Differentiation of Hydrocarbon Type in Gulf of Mexico Clastic Reservoirs by Inelastic Pulsed Neutron Capture Data

Summary This paper outlines our results and experiences in hydrocarbon typing Pliocene/Pleistocene turbidite sand reservoirs in two Shell Offshore Inc. Gulf of Mexico fields. While differentiation of hydrocarbon type was important in identifying completion targets for Produce While Drilling (PWD) operations, hydrocarbon typing was extremely difficult. This hydrocarbon typing problem was a result of (1) compositional make-up of both oil and gas, (2) high reservoir pressures encountered (up to 10,000 psi), (3) geological complexity (structural and stratigraphic), (4) the use of oil based mud to drill many wells, and (5) complex mineralogical makeup of the sands. Because openhole data and routine pulsed neutron capture (PNC) data were often inconclusive, secondary curves presented on PNC logs were extensively examined. The PNC ratio of inelastic short to long spaced data (RIN) was found to be an extremely diagnostic gas indicator, and has been used to further refine and calibrate petrophysical evaluations. These refined evaluations were then incorporated into redesigning a new completion strategy for each well examined.

Pulsed Neutron Capture Log Interpretation In Laminated Formations: A Dual-Exponential-Decay Model

Summary Industry standard shaly sand equations for pulsed neutron capture (PNC) logs are premised upon homogeneous formations. Many interlaminated sands and shales, however, may not appear as homogeneous mixtures to PNC logs, despite the fact that the laminations may be very small (3 or 4 in.) and well below the vertical resolution of the PNC device. This can result in serious interpretation errors leading to bypassed pay. Log response equations and interpretation techniques for inhomogeneous formations are derived in this report and compared with a large sampling of observed data in river-dominated delta fringe sands of the Louisiana Gulf Coast. The excellent agreement warrants consideration of the new PNC log equation over the current industry standard equation as the primary method of evaluation in this type sandstone. The response of the natural gamma ray log to shaly sandstones is also reviewed. Possible misapplication of the industry standard PNC equation in deriving gamma ray/shale volume relations for inhomogeneous laminated formations is illustrated. The resulting impact on log interpretation is examined, and more appropriate relations are discussed.

Integration of Production Log and Oxygen Activation Techniques For Diagnosing Water Production Problems

Summary Production logging (PLT) is used to diagnose a variety of production problems and for reservoir evaluation. A primary use of PLT is the identification of water entry into producing wells. If the water flow rate is low, the sensitivity of PLT does not allow adequate characterization. When this occurs, it is possible to use the recent water flow log (WFL) technique, which can locate and evaluate small water movement, both upflow and downflow, inside or outside the casing. This technique is based on the oxygen activation (OA) principle and incorporates the pulsed neutron capture (PNC) logs. One weakness of using the WFL technique is that quantification of flow rates may be difficult when water velocity tends to vary as a result of turbulence effects and changes in cross-sectional area. However, these difficulties can be overcome, and good results are possible in both open and cased hole by the combination of PLT and WFL techniques. To achieve good results, an optimal measurement program should be planned through the use of all available data: previous openhole and cased-hole logs, production data, well completion profile, etc. This paper describes the application of the PLT and OA techniques to some Italian wells in a variety of reservoir contexts. The interpretation of these logs together provided additional information helpful for planning remedial action for water shutoff. Introduction A serious problem encountered during the production life of wells is undesirable water production. The problem has economic, environmental, and managerial ramifications. The subsurface disposal of waters produced from hydrocarbon wells is onerous: increasingly restrictive new environmental regulations impose specific water treatments before produced water can be injected into a reservoir. In producing wells, high water production can cause the well to water out. Consequently, artificial lift may be necessary, which increases operating costs. Therefore, the early monitoring of water entries is essential so that the phenomenon does not become too difficult to control. Water-control techniques are becoming more sophisticated. Besides the traditional mechanical techniques (conventional bridge plug, casing patch, etc.), new through-tubing mechanical techniques (such as expansion bridge plugs and cement plugs) that require the use of coiled tubing are improving. Also, the new chemical treatments for near-wellbore shutoffs (gel, foams, and polymers) are undergoing substantial developments. Application of these new techniques requires thorough understanding of water production phenomena, so well conditions inside and outside the production casing must be carefully investigated. For this purpose, conventional PLT's (flowmeter, fluid density tool, manometer, and thermometer) are generally used to obtain the flow profile in the well. These tools cannot characterize water flow behind casing and have limited applicability in cases of low flow rate. Different methods to detect and locate water movements have been developed. The only method that gives good results is the OA technique, the most recent development of which is the WFL. The PLT and WFL can be used individually or in combination, according to the flow profile to be investigated. Table 1 summarizes the field operational guidelines for the separate or combined use of these techniques for production and water injection wells.

Applications of Pulsed-Neutron-Capture Logs in Ekofisk Area Fields

Summary Phillips Petroleum Co. Norway is engaged in an extensive reservoir-monitoring program in the seven greater Ekofisk area fields. This program includes pressure-transient testing, production log surveys, compaction monitoring, and pulsed-neutron-capture (PNC) log surveys. This paper describes the applications of PNC logging in the six Ekofisk area chalk reservoirs and the analysis techniques associated with those applications. Gas has been injected into the upper portion of the Ekofisk reservoir since 1975, and waterflooding was initiated in 1987. These projects complicate PNC log analysis. Water breakthrough reduces or eliminates the "acid effect, " which can mask any increase in water saturation unless the acid-effect change can be quantified. Gas-saturation changes can introduce systematic error in water-saturation calculations unless taken into account. Phillips Petroleum Co. Norway recognized these complications and developed solutions that can be applied as part of the "standard" analysis techniques. Examples of each application are presented.

Evaluation and Comparison of Residual Oil Saturation Determination Techniques

SummaryThe amount and distribution of residual oil saturation (ROS) are critical parameters for determining whether to apply an EOR process to a reservoir. A brief review of available ROS techniques is presented, indicating advantages, limitations, problems, and possible improvements of each technique. Advantages and disadvantages of each ROS-determination technique are summarized. Screening criteria for determining the best ROS technique under certain wellbore or reservoir conditions are presented.This paper also presents results from comparisons of ROS measurements obtained from the literature as calculated from resistivity logs, pulsed neutron capture (PNC) logs, pressure coring, single-well tracer tests, nuclear magnetism logs (NML), carbon/oxygen (C/O) logs, and electromagnetic propagation tool (EPT) measurements. In this study, the ROS measured by each method is compared with that determined by other methods conducted in the same well. The comparison shows that average values of ROS determined by C/O log, PNC-LIL (log-inject-log), and single-well tracer test do not differ statistically when compared with other methods. The resistivity log tends to give higher than average [2 saturation units (s.u.)] ROS measurements, while pressure coring tends to give lower than average (4 s.u.) ROS values. EPT and NML show deviations of about 8 s.u. of ROS values from other methods, which indicates a statistically significant difference.ROS vertical profiles obtained by two different methods from the same well were compared to eliminate the ROS variation resulting from formation depth. The vertical profiles based on ROS zoning and foot-by-foot measurements were studied to provide more "resolution" for comparisons. The results show that discrepancies in measurement methods are more pronounced when vertical profiles are divided into different zones. This could mean that the discrepancies are much greater for some zones than for others. This approach offers the possibility of studying ROS-method discrepancies as a function of different ROS values.

A New Digital Multiscale Pulsed Neutron Logging System

Summary A new multiscale pulsed neutron capture logging system (PDKSM-100) is discussed. The instrument is a 11116-in. [1.7-cm] -diameter, microprocessor-controlled logging sonde that processes raw data occurring in the short-and long-spaced detectors. The timing spectrum (10 μsec per channel and 100 channels for the decay component) of the detector events is accumulated downhole and transmitted to the surface computer logging system upon command by a pulse code modulation data transmission system. This configuration makes use of significantly increased count rates to improve log repeatability greatly. Unique data reduction techniques are also possible with the high-resolution data. The gamma rays during the burst of 14-MeV neutrons are also processed. This unique feature allows derivation of the ratio of inelastic to capture count rates, which is a function of porosity. All conventional pulsed neutron capture (PNC) responses can be observed with the new pulsed neutron logging (PNL) system. However, many additional aspects of formation evaluation are possible as well. Several of the more unique features of the system are demonstrated in specific log examples.

Abnormal formation pressures and their detection by pulsed neutron capture logs

The present paper highlights the importance of abnormal formation pressure environments in oilfield operations. The detection and quantitative evaluation of overpressured formations is critical to exploration, drilling, and production operations involving hydrocarbon resources. As a result, an interdisciplinary technical team approach is required to optimize the safety, engineering, and financial aspects of operating in such hostile subsurface environments. Pulsed neutron capture (PNC) logging devices can be used for detection and quantitative evaluation of overpressure environments through the drillpipe and to monitor pressure depletion behind casing. Field observations clearly indicate that the Σ-values in shale formations decrease in a regular fashion with depth in normally compacted clastic sequences. Abnormal formation pressures are, however, flagged by divergence from this normal Σ-trend. Several field examples are presented to illustrate applications of PNC logging devices.