Pulsed Neutron Tool Research Articles

Time-Lapse Pulsed-Neutron Logs for Carbon Capture and Sequestration: Practical Learnings and Key Insights

Pulsed-neutron logs are a staple of time-lapse monitoring programs for saline aquifer carbon capture and sequestration (CCS) projects and are unsurprisingly the most frequently run wireline logs in both injection and monitoring wells. While the emphasis imposed by government regulators and the focus of operators to date has been on the verification of CO2 containment, it is envisioned that a savvy interpretation of the multiple independent measurements should be able to unlock much greater value for the project than merely detecting the location of stored CO2. Recently introduced capabilities for novel measurements and improved environmental compensation should further increase the repeatability, interpretability, and value of these logs. We reviewed more than 30 time-lapse runs of pulsed-neutron logs acquired over a period of 15 years on three mature CCS projects using both previous- and new-generation pulsed-neutron tools, including measurements of formation sigma, hydrogen index, and fast neutron cross section. Special attention in processing is required when changes occur to the wellbore environment between runs, although this is mitigated by the improved environmental compensation scheme of the newer tool. We performed both standalone estimates of CO2 saturation from single-physics time-lapse measurements and simultaneous interpretation of multiple independent time-lapse measurements and studied the results side-by-side with openhole log interpretation, core analysis, and well test results from the evaluation phase. The apparent changes in saturation were framed within the context of the injection history and important events in the life of the wells. A first finding is that differences in apparent CO2 saturation between the various independent measurement physics of the pulsed-neutron tool are often reconcilable and may carry additional information about the state of the well or reservoir. With respect to verification of containment, depending on the well configuration, it may be possible to differentiate between CO2 in the formation and CO2 in the annulus. The interpreted CO2 saturation itself can have different significance depending on the timing of acquisition and the type of well. Measured at the right time, it is a direct in-situ measurement of formation CO2 storage efficiency. In other cases, the interpretation reveals formation dryout in the near-wellbore region of injection wells, a condition that may presage loss of injectivity. We now understand that it is important for operators to plan the timing and frequency of pulsed-neutron runs according to what they want to measure and not based solely on regulatory obligations. In a CCS project, time-lapse pulsed-neutron logs should be thought of as much more than simple indicators of the presence and migration of CO2. They give important information about migration pathways. They can also help to quantify essential uncertainties on reservoir performance that are difficult to ascertain during evaluation. For example, storage efficiency is difficult to quantify with openhole logs since the formation is initially at zero CO2 saturation. Yet it is one of the keys to determining the ultimate storage capacity of any reservoir. Time-lapse pulsed-neutron logs provide an abundance of information that, when properly history matched, can greatly improve our models of CCS reservoirs to better navigate both the economic and operational risks associated with these projects.

New Generation of Pulsed-Neutron Multidetector Comparison in a Challenging Multistack Clastic Reservoir: A Case Study in Brown Field, Malaysia

Running pulsed-neutron logs in Malaysia has previously been plagued by results with high uncertainties, especially in brown fields with complex multistacked clastic reservoirs. Together with a wide range of porosities and permeabilities, the acquired logs quite often tended to yield inconclusive results. In addition, the relatively fresh aquifer water (where salinity varies from 5,000 to 40,000 ppm) makes reservoir fluid typing and distinguishing between oil and water even more challenging. As a result, the inconsistencies and uncertainties of the results tend to leave more questions than answers. Confidence in using pulsed-neutron logging, especially to validate fluid contacts for updating static and dynamic reservoir models, deteriorated within the various study teams. Due to this fact, the petrophysics team took the initiative to conduct a three-tool log off in one of their wells with the objective of making a detailed comparison of three pulsed-neutron tools in Malaysia’s market today. The main criteria selected for comparisons were the consistency of the data, repeatability, and statistical variations. With recent advancements in pulsed-neutron (multidetector) tool technology, newer tools are being equipped with more efficient scintillation crystals, improving the repeatability of the measurements as well as the number of gamma ray (GR) count rates associated with the neutron interactions. In addition, the newer tools now have up to five detectors per tool, with the farthest detector supposedly being able to “see” deeper into the formation, albeit at a lower resolution. With these new features in mind, the log off was conducted in a single well with a relatively simple completion string (single tubing, single casing), logged during shut-in conditions only, and the logs were acquired directly one after the other (back to back) to avoid bias to any particular tool. Both sigma and spectroscopy measurements were acquired to compare the capabilities of each tool. Due to the relatively freshwater salinity, the carbon-oxygen (C/O) ratio from the spectroscopy measurements is used to identify the remaining oil located in the reservoirs, while the sigma measurements determine the gas-oil or gas-water contact, if present. This paper will illustrate the steps taken by Petronas Carigali Sdn Bhd (PCSB) to compare the raw data and interpreted results from the three pulsed-neutron tools. Consequently, a comparison from all the tools was made to the current understanding of the reservoir assessed. The points from these comparisons will then show which tools are favored over the rest.

Multidetector Pulsed-Neutron Tool Application in Low-Porosity Reservoir–A Case Study in Mutiara Field, Indonesia

In mature fields, pulsed-neutron logging is commonly used to solve for the remaining saturation behind the casing. For years, sigma-based saturation has been used to calculate gas saturation behind casing; however, the high dependency of sigma-to-water salinity of the formation, especially the low-dynamic range at porosity near 12 p.u., has proven to be challenging in low-porosity gas rock. A new measurement from the third detector from a multidetector pulsed-neutron tool (MDPNT) is proposed to provide a better estimation of the gas saturation in a low-porosity reservoir. Two sets of independently measured sigma and the third detector were taken in a casedhole well, with a dual-tubing system of a long string and short string. For the third-detector measurement, the measurement was based on the ratio of the slow capture gate and inelastic gate component from the decay curve created by the long detector. This ratio can be used to detect gas in a tight reservoir with a minimum salinity and lithology effect. This data will then be used to calculate the gas saturation from the third detector, and the result is compared to sigma-based gas saturation. At an interval where the porosity is above 12 p.u., the sigma-based gas saturation and MDPNT-based gas saturation are very much in agreement. However, in a low-porosity reservoir near 12 p.u. or below, the sigma-based measurement starts to show its limitation. Meanwhile, the MDPNT-based gas saturation clearly shows the remaining gas saturation where sigma-based measurements failed to detect it. The subsequent decision was made based on the log analysis result, and perforation was done at a potential interval based on the MDPNT result. The results from the production test confirm the MDPNT-based gas saturation with 700-Mscf/d gas production added. This study showcases a new technology to solve a low-porosity gas reservoir issue where a sigma-based measurement underestimates the remaining gas saturation. Using two different measurements in the same well, the results from the MDPNT measurement demonstrated a better result compared to the sigma-based measurement in low-porosity rock

Self-Compensated Pulsed-Neutron Spectroscopy Measurements

Formation elemental composition and mineralogy measurements, including organic carbon from recently developed spectroscopy tools, provide critical information for formation evaluation in both conventional and unconventional reservoirs. These measurements can be obtained under conditions by using a slim pulsed-neutron tool with two spectroscopy detectors. One primary limitation is that users must manually provide offsets for the elements (silicon (Si), calcium (Ca), and iron (Fe)) present in casing and cement before performing the oxide closure computation to obtain elemental concentrations. This process is time consuming, and the results could be inaccurate and subjective, especially without any local reference. Another limitation is that the formation element signals are smaller in cased hole than in open hole. This increases the noise in the oxide closure-derived environmental yield-to-weight normalization factor (FY2W), which is propagated to all the elemental weight fractions. A self-compensated spectroscopy algorithm was developed to overcome these two limitations. The key breakthrough is the use of raw measurements with very high precision from the two spectroscopy detectors to predict FY2Ws instead of using the oxide closure or inelastic capture (INCP) closure methods. The capture FY2W is mainly determined by the borehole and formation sigma. It can be characterized by using multiple measured apparent sigma values in different timing gates from multiple detectors, which have different sensitivities to borehole and formation sigma. The inelastic FY2W is mainly determined by the borehole and formation geometry and hydrogen index. It can be characterized by using count rate ratios in both burst-on (inelastic) and burst-off (capture) timing gates from multiple detectors. This method reduces the noise in the FY2Ws by an order of magnitude, which improves the precision of all the final elemental weight fractions. Two independent sets of apparent elemental weight fractions can be calculated from the two spectroscopy detectors. The measured elements from the detector with shorter spacing are more sensitive to the borehole environment, including the casing and cement, whereas the ones from the detector farther away are more sensitive to the formation. This enables self-compensation for casing and cement effects. The new processing can be done without user intervention and results in a more accurate, more precise, and less subjective elemental composition and mineralogy. More than 1,600 laboratory measurements in different conditions were used to characterize the algorithm. Several log examples demonstrate the excellent performance of the new compensated spectroscopy measurements.

Technical advances in pulsed-neutron interpretation for cased-hole logging: Physics, interpretation, and log examples

Recently developed multidetector pulsed-neutron tools (MDPNTs — a term describing a pulsed-neutron tool with at least three detectors) can provide three-phase formation fluid analysis in cased wells. These tools are 43 mm (1 11/16 in.) or 54 mm (2 1/8 in.) in diameter and can be logged in or below most tubing sizes. We reviewed traditional oil- and water-saturation techniques as well as indirect gas-saturation techniques, and we compared them with recently developed direct gas-saturation techniques, now available from MDPNTs. A log example developed the data verification and interpretation process. The interpretation process was divided into two parts: First, we verified the log data quality and second, we applied a newly developed gas model to the log data providing gas saturation without any reliance on the previously determined oil and water saturation.

Integration of well-based subsurface monitoring technologies: Lessons learned at SECARB study, Cranfield, MS

An array of closely spaced wells at Cranfield, Mississippi, USA provided a field laboratory to test wireline-based techniques for measurement of substitution of carbon dioxide (CO2) for brine. Characterization of this moderate porosity–moderate permeability sandstone reservoir in the lower Tuscaloosa Formation was conducted using openhole logs. Cased hole monitoring was conducted over the first year of injection using wireline tools including crosswell seismic, sonic, pulsed neutron, and resistivity logs. The wells were also instrumented with casing- and tubing-deployed instruments.The most quantitative wireline measurements were made using time-lapse pulsed neutron and crosswell seismic which documented evolution of the CO2 plume. Theoretically, interpretation of fluid flow would be optimized by collection of as many types of data as possible, realistically in this setting interference among different measurements limited the amount of data collection possible. Complex well completions interfered with resistivity and sonic log quality. Changes in well bore fluids from brine to CO2 can affect measurements of the pulsed neutron tool and additional processing may be required. Data collection with large diameter tools required displacing near well CO2 with heavy brine, which perturbed the near well saturation and geochemistry.These observations provide pragmatic information for future tests to suggest (1) the need to optimize tools to maximize value and avoid interference and (2) suggest avenues of new tool development to avoid interference in CO2 injection settings.

Observations From a Tight-Gas-Reservoir Stimulation

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 117570, "Observations From Tight-Gas-Reservoir Stimulations in the Rocky Mountain Region," by Erik Borchardt, SPE, Schlumberger; Jessica Cavens and Craig Wieland, SPE, EnCana Oil and Gas; and Brian Pluemer, SPE, Don Graham, Abdunnaser Erwemi, Alexei Kazantsev, SPE, and Joel Le Calvez, SPE, Schlumberger, originally prepared for the 2008 SPE Tight Gas Completions Conference, San Antonio, Texas, 9-11 June. The paper has not been peer reviewed. Unconventional tight gas reservoirs are made economical through effective stimulation techniques. Hydraulic-fracture mapping combined with an in-depth knowledge of reservoir geology and geomechanics can provide a better understanding of the effectiveness of reservoir stimulation. Massive hydraulic fractures from two wells in the Rocky Mountain region were mapped in real time with a 3D stimulation-viewer soft-ware package. One well used techniques standard to the area, while some experimental fracture techniques were tested on the other. Regional Background The area of study included 3,000 vertical ft of fluvial and marine sands in the Rocky Mountain region of the US. The formation contains natural fractures, laterally restricted lenticular sandstones, and tight brittle sands. The main natural-gas reservoir interval is from 5,000 to 8,500 ft. The productive sands have an average porosity of approximately 7%, and permeability is in the µd range. The first well was drilled in this region in 1955. In the late 1990s, the discovery of increased production resulting from fracturing lenticular sands made this a profitable area. Wells currently are drilled on 10-acre density. Much of the area uses directional drilling from well pads to reduce surface damage. Pad Layout On the study pad, there are four S-shaped wells (Fig. 1). The middle well (Well A) was used as the monitoring well, and the wells directly north (Well B) and directly east (Well C) were stimulated. Wells B and C are approximately 1,200 ft from the monitor well in the vertical section that was stimulated. Well C was considered the control well, while some experimental fracture techniques were used in Well B. Offset to the south of the monitor well (Well A), a series of openhole (OH) logs was run to help understand formation characteristics. A pulsed-neutron tool was run in all four wells so that geological modeling from OH logging on Well D could be projected to all wells on the pad by means of a neural network. The two treatment wells (Wells B and C) had 11 and 12 stages stimulated, respectively. Perforations were designed as 10 sets of perforations per fracture stage. The fracture stages ranged from 180 to 350 ft of net pay.

Field-Test Results of a Three-Phase Holdup Measurement in Horizontal Wells With a Pulsed-Neutron Source

Summary A new method of obtaining three-phase holdup (oil/water/gas) information in horizontal wells has been field tested. The method uses the data available from a pulsed-neutron tool. This measurement uses the inelastic near/far countrate ratio in addition to the near and far carbon/oxygen (C/O) ratios. The inelastic near/far countrate ratio response is dominated by gas in the borehole, whereas the near and far C/O ratios are used to separate the water and oil contributions of the borehole and formation. By using a linear programming approach, this information is combined to produce quantitative three-phase holdup answers. Calibration ofthe measurement is achieved by the combined use of laboratory measurements and Monte Carlo modeling predictions. To obtain independent verification of the three-phase holdup answer, a local probe tool was run in combination with the pulsedneutron tool. The local probe tool is sensitive to two phases (water and hydrocarbon) and, hence, can be used for a partial verification of results. Field-test results show good agreement between the two independent measurements under various conditions of gas, oil, and water holdup in the horizontal borehole.

Measurement of Oil and Water Flow Rates in a Horizontal Well With Chemical Markers and a Pulsed-Neutron Tool

Summary A new approach to obtaining oil and water flow rates in producing horizontal wells has been developed with a pulsed-neutron tool (PNT). This approach uses separate measurements of oil and water velocities in combination with separate holdup measurements to obtain the flow rates. The velocity measurement uses water-soluble and oil-soluble chemical markers, both of which are insoluble in the other fluid phase for the measurement. The markers are injected into the borehole by a logging tool at one location and detected by a PNT at a second location. The transit time between injection and detection of the marker gives a measurement of the fluid velocity. Because the markers are soluble in only one phase, the velocity of each phase can be measured separately. This measurement has been made under both laboratory and field conditions to measure velocities from 10 to 500 ft/min at horizontal and several degrees deviation from horizontal. The results of these tests show good linearity and repeatability of the measurement. The holdup measurement is performed with the inelastic data from a PNT. With these data, it is possible to obtain quantitatively the holdup of all three phases by combining information from the inelastic near/far (N/F) ratio with the near and far carbon/oxygen (C/O) ratios. This approach to the holdup measurement has been demonstrated by use of a combination of laboratory data, Monte Carlo modeling, and field data. The results of this study have demonstrated that the root-mean-square (RMS) accuracy of this measurement is about 6% on each of the three phases.

Quantitative Monitoring of Gasflooding in Oil-Bearing Reservoirs by Use of a Pulsed Neutron Tool

Summary We developed a new method to monitor gasflooding of oil reservoirs. The method is based on computing two porosities: the base true porosity determined before gasflooding and monitor-apparent neutron porosity determined after gasflooding. The base and monitor porosities provide determination of a hydrogen index of the reservoir fluid after the flooding. The hydrogen index is then used to determine saturation of the flood agent after flooding and, consequently, saturation of water and oil after flooding. An example of quantitative analysis of base and monitor logs illustrates monitoring of CO2 flooding by use of the described method.

A new approach to the interpretation of nuclear borehole logs

Abstract When interpreting nuclear borehole logs it is usually assumed for simplicity that the tool signal and a formation property, e.g. its density, are uniquely related. However, while the tool signal will usually depend primarily on one formation property, it will also be affected by other formation properties. To take this multi-parameter dependence of all the nuclear tools into account, a new approach to nuclear log interpretation has been developed. In this approach four nuclear tools are considered: the gamma density tool, the neutron porosity tool, the pulsed neutron tool and the natural gamma tool. The responses of these four tools depend on formation density (ρ), porosity (φ), neutron absorption (σ a ), and geological type (including shale content). For each of the four tools a physical model has been developed. The signals measured with the four tools and the four models represent together four equations with four unknowns: ρ, φ, σ a and geological type. These four equations may be solved by iteration and a consistent set of formation properties obtained. The interpretation approach has been implemented on a personal computer. Examples of results of such interpretations are presented.

Water-Injection Profiles in Eola Field Determined by Changes in Thermal- Neutron-Capture Cross Section

Summary In waterflood operations it is desirable to inject uniformly into the targeted hydrocarbon-bearing sections of a reservoir. Wireline injection profiles have traditionally been used to determine where fluid enters perforations. Profile logs are inadequate, however, where segregation of flow into long, multiple zones is accomplished through a single tubing string and tubing injection mandrels with packers separating the zones. A technique has been proposed and tested successfully for detecting through tubing which intervals are actually being invaded by injection fluids. Logging the cased well with the Thermal Multigate Decay™ (TMD) pulsed-neutron tool while successively injecting fresh water, salt water, and borated salt water delineated invasion through the resultant increase in the thermal- neutron-capture cross section in the flushed intervals. This technique allowed a qualitative foot-by-foot analysis of the injection profile despite the presence of tubing, packers, and flow regulators. It also provided the data needed to improve injection practices and to enhance the recovery of hydrocarbons.

Cased Hole Logging Technique To Evaluate Reservoirs In New And Old Wells

Abstract Evaluating reservoirs by conventional wireline fogging m open hole has become an accepted and necessary technique over the years. In geologically uncertain areas, open hole logs nm in a particular well provide data not only for the well being evaluated but also for future offsetting wells. One Obvious drawback with these logs is that rig time must be expended to condition the hole to log, to do actual logging, and then to recondition the hole to cement. he open hole data lose some of their usefulness once the well has been cased. Conventional open hole logs cannot be rerun to re-evaluate reservoir performance once casing has being set. To monitor or re-evaluate these cased reservoirs, the pulsed-newton capture and inelastic GST* Gamma Spectrometry Tool has been successfully utilized. This paper will discuss how utilizing this cased reservoir monitoring and re-evaluation device can lead to innovative interpretation techniques that permit the replacement of open hole logs with cased hole evaluations in heavy oil EOR projects. The resulting optimization of rig time and the potential improvement of cementations in new wells resulting in part from this technique will be discussed. Examples of Canadian logs in conventional heavy oil reservoirs, in EOR heavy oil reservoirs, and in conventional right oil reservoirs will be presented and discussed. Introduction Present reservoir evaluations utilizing data obtained prior to running production casing allow accurate estimates of hydrocarbon potential. These evaluations are critical in geologically uncertain areas. Data obtained from cores, logs run in open hole, and data from drillstem tests prior to the casing being set have led to critical decisions, such as whether or not to run the casing; where to complete the well; how to complete the well; and what production mechanisms are needed. Running open hole logs has become arguably a main form of reservoir evaluation. Cores and tests may or may not be taken. This form of reservoir evaluation requires hole conditions that permit the open hole logging tools to get down to potential hydrocarbon horizons. Open hole logging is an economical form of evaluation in deeper wells or in wells that are in non-development areas. In shallow development areas, the time dedicated to open hole logging as compared to the total rig time La drill the hole becomes a substantial percentage of the total rig costs. Using the cased hole GST tool for primary formation evaluation on closely spaced new wells, such as those in heavy oil reservoirs, has proved to be more economical. The open hole log data lose some of their usefulness once casing has been set. Reservoir performance has to be monitored by logs run in casing. Oil companies have been uncomfortable with some of the cased hole log evaluation techniques because the raw data have been difficult to relate to raw open hole logging data. The need for evaluating reservoir conditions and performance has been diminished by this uneasiness. The pulsed-neutron capture and inelastic GST tool has been successfully utilized to monitor reservoir performance in western Canada.