Cased-hole Logs Research Articles

Accelerator Technology for Well Logging: Advances, Challenges, and Opportunities

Accelerators have been integral to subsurface probing for decades. Tools with deuterium-tritium (D-T) generators and scintillators utilizing gamma rays from thermal neutron capture, inelastic scattering, and activation are routine in cased-hole logging tools for reservoir and well monitoring to locate and quantify remaining hydrocarbons prior to initiating secondary or tertiary production. X-ray and neutron generators field-tested to, respectively, replace 137Cs and americium-beryllium (Am-Be) source tools that measure two bulk parameters, formation density and neutron porosity critical for initial characterization of formations, have yielded mixed results. D-T generator-based spectroscopy tools with advanced scintillators that can record both inelastic and capture n-gamma spectra, faster and with much better energy resolution, to provide a more complete mineralogy appear poised to replace Am-Be–based mineralogy tools. In view of their ability to measure both bulk and spectral parameters, accelerator-based nuclear methods appear attractive to extract additional geological information needed to transition to a low-carbon energy future. The paper discusses the current state of application of accelerator-based subsurface probing techniques, notes their potential for nonpetroleum applications, and concludes by briefly exploring technology advances that could significantly advance the state of the art.

Bayesian Framework Helps History Matching of Permian EOR Field With Data Uncertainty

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 200807, “Closing the Loop on a History Match for a Permian EOR Field With Relative Permeability Data Uncertainty,” by Usman Aslam, Emerson, and Jorge Burgos, SPE, and Craig Williams, Occidental Petroleum, et al. The paper has not been peer reviewed. _ Reservoir production forecasts are inherently uncertain because of the lack of quality data available to build predictive reservoir models. Traditionally, a best estimate for relative permeability data is assumed during the history-matching process despite significant uncertainty. Performing sensitivities around the best-estimate relative permeability case will cover only part of the uncertainty space. In the complete paper, the authors present an application of a Bayesian framework for uncertainty assessment and efficient history matching of a Permian carbon-dioxide (CO2) enhanced oil recovery field for reliable production forecasting. Field Details Regional and Structural Geology. The Central Basin Platform (CBP) is a positive tectonic feature that separates the Delaware and Midland sub-basins of the Permian. The study field is located on the northeastern end of the CBP, with the Permian (Guadalupian) -aged reservoir composed of San Andres Formation dolostone. The total thickness of the unit is approximately 1,500 ft, with the main reservoir within the middle 600 ft. Structural Framework. The area of interest features 270 wells that have a total depth in the San Andres formation or deeper. Of these, 234 have digital open- or cased-hole logs that were used to correlate formation tops. After reviewing all well logs, it became clear that, historically, the zones were primarily identifiable from porosity picks, which are more subjective than markers identified with gamma ray methods. A new sequence stratigraphic framework was developed based on core descriptions and outcrop analogs. This correlation framework was then extrapolated to well logs. After a grid of north/south and west/east cross sections was correlated across the field and a series of loop ties was made for quality-control purposes, structure and isopach maps were created for each zone. Reservoir Description. At the time of discovery, natural gas was trapped at the structural high point of the study field. Above the gas/oil interface is the gas cap. Below the gas was an oil accumulation, which extended to the producing oil/water contact (POWC). The POWC was defined by early drilling as the maximum depth where water-free oil was produced. The base of the transition zone is the top of the residual oil zone (ROZ); this reservoir interval extends to the free-water level and is an interval believed to have been waterflooded naturally. The ROZ is the target for CO2 injection. Grid Construction and Property Modeling. A full-field detailed 3D reservoir characterization was modeled at 50-ft × 50-ft grid increments and an approximate 2-ft average layer thickness. The reservoir model consists of 114,222,528 cells. A total of 235 wells in the unit area was correlated and used to constrain the structural framework of the model. Quantitative porosity log suites were available in 233 of the wells in the model area, which were used to create the full-field porosity property in the 3D model. The logs were normalized to the core data and upscaled.

Annular Creep Barrier Evaluation and Qualification Using Ultrasonic Measurements

Summary It is well-known that formations that exhibit active creep behavior under downhole conditions, such as reactive shales and mobile salts, can form annular barriers across uncemented or poorly cemented annular sections behind casing strings. Such creep barriers can simplify well abandonments, particularly in high-cost offshore environments. Evaluation and qualification of creep barriers in the field, however, have proven challenging and labor-intensive when casing is perforated, and annular rock material is pressure-tested to verify its sealing ability. This work seeks to eliminate the need for pressure testing by allowing the barrier to be qualified using only casedhole log measurements. Sophisticated rock mechanical laboratory experiments under realistic downhole conditions were conducted to investigate the formation of creep barriers by North Sea Lark shale. The experiments evaluated barrier formation while varying annular fluid chemistry and temperature. Measurement parameters included creep rate, pressure transmission across newly formed barriers, pressure breakthrough through the newly formed barriers as well as ultrasonic responses by the shale. It was found that the Lark/Horda shale has a distinct anisotropic ultrasonic wave velocity profile that uniquely characterizes it. This can be used to identify its presence in an annular space when contacting the casing. A main conclusion is that a Lark shale barrier can be qualified through casedhole sonic and ultrasonic logging alone without the need for pressure testing if (1) the magnitude of the wave propagation velocity of the shale behind casing can be confirmed (2077 m/s for Lark shale); (2) the characteristic velocity anisotropy profile, unique to the shale (~10.1% for Lark shale), can be verified; (3) good contact with/bonding to the casing is observed; and optionally (4) anisotropy in the time behavior of the shale contacting the pipe is observed when the barrier is stimulated artificially. If these conditions are met, then our experiments show that the barrier will have excellent hydraulic sealing ability, with a permeability of a few microdarcies at most and a breakthrough pressure that approaches the minimum horizontal effective stress value. Additional findings are that shale heating will accelerate barrier formation but may damage the shale formation in the process. Extraordinary fast annular closure and barrier formation with evident shale rehealing was observed by using a concentrated KCl solution as pore fluid, showing the merits of barrier stimulation by chemical means. This result can be explained by considering the effect of solutes on shale hydration forces.

Ultrasonic Measurements Evaluate and Qualify Annular Creep Barriers

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208782, “Annular Creep Barrier Evaluation and Qualification Using Ultrasonic Measurements,” by Eric van Oort, SPE, The University of Texas at Austin, and Akshay Thombare, SPE, and Munir Aldin, SPE, Metarock Laboratories, et al. The paper has not been peer reviewed. Creep barriers can simplify well abandonments, particularly in high-cost offshore environments. Evaluation and qualification of creep barriers in the field, however, have proven challenging and labor-intensive when casing is perforated and annular rock material is pressure-tested to verify its sealing ability. This work seeks to eliminate the need for pressure testing by allowing the barrier to be qualified using only cased-hole log measurements. Creep Barrier Formation and Stimulation Plastically deforming shale and salt or evaporite formations under certain conditions can move into uncemented or poorly cemented annular spaces, filling them and contacting the casing to form an effective pressure-tight barrier. Supporting evidence overwhelmingly points toward creep as the fundamental rock-mechanical mechanism behind this type of barrier formation. Creep barriers have, in fact, become commonplace considerations in the plugging and abandonment of offshore wells in the UK and the Norwegian sectors of the North Sea, leading to simplified and more-cost-effective rigless offshore well-plugging operations. The latter is accomplished by eliminating the need for casing cutting and pulling or casing milling followed by setting openhole abandonment plugs. Materials that present alternatives to ordinary Portland cement (OPC) for well plugging-and-abandonment barrier construction include the use of shale or salt as a barrier (SAAB). Comparison of category scores for SAAB with other materials reveals that SAAB offers a well-rounded performance envelope with no weaknesses in any categories, whereas the other materials, including cement, show weaknesses in one or more categories. A main advantage of SAAB is that it offers the ability to “replace” the caprock, the desired goal in most well abandonments. Moreover, for many materials including OPC, long-term (i.e., hundreds or thousands of years and beyond) sealing performance is unknown for well abandonment. The only category in which SAAB technology appears to lag is in its technology readiness level score. This, then, provides motivation for further research.

Technology Focus: Mature Fields and Well Revitalization (January 2022)

The highlight of this year’s feature on mature fields and well revitalization has been the increasing use of artificial intelligence (AI) and machine learning (ML) for analysis of mature assets. Many paper proposals studied using AI/ML techniques cover a wide range of topics including optimization of production operations, faster reservoir characterization of mature fields, and improvement of ongoing waterfloods. In fact, increased use of AI/ML has been a recurring theme in many of the areas covered in the Technology Focus sections of 2021’s JPT issues. There has also been a focus on implementing novel technologies such as the use of autonomous inflow-control devices in horizontal wells to suppress ingress of unwanted fluids in thin-oil-rim reservoirs, using integrated-asset models for debottlenecking, and the use of cased-hole logging in old wells to look for missed opportunities. After serving for 5 years, my time for writing this column is coming to an end. It has been a highly rewarding exercise going through the paper proposals, experiencing the hard work put in by industry professionals to arrest inevitably declining production from mature assets by putting together ideas in diverse fields such as reservoir management, production optimization, drilling, and formation evaluation to produce the extra barrel from mature brownfields. Out of the 314 paper proposals studied in these 5 years, 137 (44%) dealt with waterflood improvement, reservoir characterization, and simulation, and approximately 78 papers (25%) dealt with well revitalization involving workover drilling, completion, and stimulation. Sustaining production from mature brownfields challenges the best from each discipline, and significant success stories have come from a genuine collaboration of ideas across disciplines from subsurface to engineering. Recommended additional reading at OnePetro: www.onepetro.org. SPE 206195 - Data-Driven Optimization of Intermittent Gas Production in Mature Fields Assisted by Deep Learning and a Population-Based Global Optimizer by Javier Fatou Gómez, TNO, et al. SPE 205546 - Role of Geomechanics in Identification of Possible Mechanisms for Nonproductive Time and Improving Drilling Operations in a Mature Field in Offshore Sarawak, Malaysia by Avirup Chatterjee, Baker Hughes, et al. SPE 206303 - Combining Capacitance Resistance Model With Geological Data for Large Reservoirs by Davud Davudov, Resermine, et al.

Research on remaining oil evaluation method based on cased hole logging technology

In the later stage of the production and development of HB Oilfield, the oil reservoir fluid type is complex. With the development of multi-layer system. The reservoir variables between different types of oil and gas reservoirs are large, and the relationship between oil and water is complex. Most main oil layers have high water drives, and the actual application problem of complex in the formation water mineralization, the surplus oil distribution is complicated. It is highly distinguished by low-facing and low mineral layers in low pores. The property is characterized by reservoir fluid analysis evaluation. During the interpretation process, the influence of the wellbore, invasive belt is eliminated, and the accuracy of the capture of the cross-section curve is guaranteed, and related parameters such as reservoir, oil-containing saturation degree can be accurately and reliably. The accuracy of oil-water layer recognition has been increased to 80%, and the coincidence rate of saturation interpretation has been greatly improved.

Water-Shutoff Technique Extends Productive Life Cycle of Cretaceous Sandstone

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 199070, “Water-Shutoff Technique Extends the Productive Life Cycle of Cretaceous U Sandstone: The Iro Field Case in Ecuador,” by Luis Roberto Bailón, SPE, Ney Holger Orellana, SPE, and Santiago Villegas, Repsol, et al., prepared for the 2020 SPE Latin American and Caribbean Petroleum Engineering Conference, originally scheduled to be held in Bogota, Colombia, 17-19 March. The paper has not been peer reviewed. The water-shutoff technique is used in some wells of the U reservoir in the Iro field of the Oriente Basin in Ecuador as a remediation plan to restore production after an early water breakthrough. The production historical data, workovers, and sand-body correlation of wells are compared to understand reservoir behavior, shale-baffle-sealing continuity, the existence of different sand units, and the effect on production. Introduction The Iro field is in the south of Block 16. Production began in March of 1996. Iro is considered a mature field that produces heavy crude oil. The U sand-stone reservoir at Iro field is constituted by quartz grains subtransparent with fine grain sizes to medium, moderately classified, occasionally clay-like matrix. A thin limestone layer subdivides the U sandstone reservoir into two main stratigraphic units, Upper U and Lower U sandstone. Logging acquisition during the drilling campaign revealed heterogeneous sand-body deposition throughout the field. Depositional features of fluvial channels are developed from the base of the reservoirs, which are overlaid by sand bars. In addition, interbedded shale layers and baffles are present in the U reservoir, in some cases locally. However, the main shale layers are effective seals when they subdivide the Upper and Lower U sandstone units into two or more subunits. A good example is the shale layer that separates channels and bars in the Lower U sandstone unit. This identification was possible after the development of the well-drilling campaign, well correlation, and years of production behavior. Two subunits of the Lower U reservoir, Ui1 and Ui2, were classified as a result of the acquired data. Cased-Hole Logs (Pulsed Neutron) Given the maturity of the fields, during the last 2 years, a logging campaign of pulsed-neutron cased-hole logs has been performed. In the case of the Iro field, pulsed-neutron logs were run in six wells; three of these reached the Lower U reservoir. These three wells have a good correlation between the analog density and neutron curves of the cased-hole and the original openhole curves, providing certainty in the reading of the tool. The logging program obtained data of chemical-element spectra in capture and inelastic modes. This information was processed and analyzed to derive hydrocarbon saturation. In this way, by-passed oil can be identified as well as reservoir zones already drained by production of the same wells or by neigh-boring wells. Data in the Lower U reservoir show fluid movement.

Technology Focus: Formation Evaluation (February 2021)

This marks the first of two Formation Evaluation features for 2021. This topic, along with EOR Operations and Artificial Lift, had such a high number of devoted conference papers, and a high degree of reader interest, that it was expanded and will appear in the August issue as well. The papers chosen by reviewer Rahul Dastidar explore innovative approaches developed by authors of SPE conference papers toward avoiding data bias, working around the absence of openhole data, and reducing the uncertainty of mass-transport complexes. Paper SPE 201358 addresses the problem of bias in inverted transverse-relaxation time (T2) distributions in nuclear magnetic resonance (NMR) measurements. The authors discuss two quality-control approaches that are not meant to replace linearly inverted T2 distributions but which can optimize NMR measurements in real time. The next paper outlines important experience gained in two tight gas fields offshore the UAE for which openhole data were not available for purposes of petrophysical evaluation. The answer, according to the authors of paper SPE 202894, is the use of cased-hole log data. As with the previous paper, the solution is not necessarily offered as a replacement to a conventional approach but as an effective backup solution to a commonly encountered issue. The third paper explores challenges in petrophysical assessment in a wildcat project in Colombia involving mass-transport deposits. The authors of paper SPE 203016 develop a work flow that integrates NMR, multifunction spectroscopy, borehole images, and radial sonic measurements to overcome the uncertainty of non-Archie rock formations. As always, SPE authors continue to innovate in the face of challenges both expected and unexpected - a concept with which all of us can identify as of late. I hope these three papers, as well as the other Technology Focus synopses included in this issue, provide insight and inspiration.

Cased-Hole Standalone Evaluation Succeeds in the Absence of Openhole Data

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 202894, “Cased Hole Standalone Evaluation: Breaking the Barrier To Successfully Evaluate Challenging Deep Carbonate Reservoirs,” by Pradeep Menon and Carey Mills, ADNOC, and Suvodip Dasgupta, SPE, Schlumberger, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually from 9-12 November. The paper has not been peer reviewed. Accurate petrophysical evaluations (formation lithology, porosity, and water saturation) are essential in characterizing potential reservoir zones and estimating resources in place. Typically, these evaluations rely on acquisition of openhole logging measurements; however, this is not always possible. The complete paper outlines two examples from tight gas reservoirs in two separate fields offshore Abu Dhabi in which openhole data could not be acquired and petrophysical analysis was undertaken using cased-hole log data. These evaluations successfully identified gas-saturated porous intervals in each well, one of which was successfully flow-tested. Introduction A growing need exists to increase gas production in the UAE. As a result, specific gas-production targets have been mandated from development of currently undeveloped deep gas carbonate reservoirs such as the Permo-Triassic Khuff formation, the middle Jurassic Araej formation, and the Permian Pre-Khuff Unayzah and Berwarth formations. Recent appraisal wells have aimed at evaluating these reservoirs systematically by acquiring a good suite of openhole logs, cutting conventional cores, and conducting well-testing operations. These well data are combined with an evolving regional understanding to better assess and ultimately develop these complex formations. An accurate petrophysical evaluation requires the petrophysicist to develop a realistic evaluation of formation lithology, porosity, and water saturation. These parameters provide the foundations for further work such as static modeling stands, and they must be robust. The Upper Khuff is composed of dolomite occasionally grading to calcareous dolomite with minor interbeds of claystone and anhydrite. In core and cuttings, the dolomite in the uppermost section exhibits a grainstone texture with poor intercrystalline/intergranular porosity. The Lower Khuff is composed of very hard dolomite in part grading to calcareous dolomite, medium-to-dark grey-brown in places, with occasional very-fine-to-medium grainstone texture and very poor intercrystalline porosity. In this paper, the Upper and Lower Araej members are interpreted to have been deposited in an open, marine- circulation shelfal environment, while the Uweinat member is considered to have been deposited in a more- restricted circulation setting with-in a similar shelfal environment. The Barrier Openhole logging generally is the preference for formation evaluation because it represents the simplest environments and benefits from a comprehensive list of available measurements. The variety of tools and diversity of output data available make openhole log acquisition the gold standard for formation evaluation. However, in certain situations in which openhole logging is not possible because of borehole conditions (re-entry of old cased wells, wellbore instability, over-pressure), no option exists other than acquiring petrophysical data in a cased-hole environment.

Maximizing the Value of Pulsed-Neutron Logs: A Complex Case Study of Gas Pressure Assessment Through Casing

Casedhole logging for formation evaluation and input to determine the redevelopment potential of an oil producer with a challenging production history was conducted. This included an intelligent assessment of formation gas pressure through casing, which was later confirmed by perforating. The target reservoirs are Triassic sandstones drilled as a gas exploration prospect. Based on openhole log data, the prospect appeared to be oil bearing. The well has been producing oil for several years and is now a candidate for a gas cap blowdown. The presence of heterogeneous layers with varied rock quality and producibility indexes coupled to complexity in fluids distribution and zonal isolation issues complicates the development process and ability to optimize recovery from any contributing level. Recently, a new-generation casedhole formation evaluation tool that provides multiple independent formation property measurements was deployed to enhance knowledge of the formation parameters while describing the current gas and oil volumes. Sigma, neutron porosity, fast-neutron cross section (FNXS), and elemental concentrations, including total organic carbon from inelastic and capture spectroscopy, were simultaneously recorded. Because the well is highly deviated in the zones of interest, the tool was efficiently conveyed on wireline using tractor technology. The evaluation techniques used to study this rich set of data reveal several pieces of information that are essential to the petrophysicists and geologists, and to the reservoir and production engineers. A multimineral solver analysis guided by the prior knowledge of the rocks using cores from offset wells was conducted to quantify the porosity and gas-oil contact levels while giving access to detailed knowledge of matrix and rock composition for refining the reservoir models. Additionally, a novel method to determine gas pressure at the current time from the casedhole log measurements was applied to support reservoir management. The highly sensitive sigma, neutron porosity, and FNXS gas properties can be parameterized as a function of pressure and temperature if the formation and fluid properties are known. This is a well-established principle that can finally be applied independently and directly to multiple measurements. The computations are done independently and checked against each other for consistency and to support optimal parameter setting, in an iterative manner. This is particularly important in this scenario where the complexity of the wellbore environment and history of the well could have complicated the ability to achieve enough precision on the estimated pressures when coming from a single method or if modeling is required. The log results, with validation, and implications for the well redevelopment are presented together with a general discussion on the methodology and applicability based on this well experience. The importance of meticulous job preparation, prejob modeling, and data quality control is also highlighted. The information is key to defining the future well development management strategy and clarify the role that casedhole logging can play as part of the complete well evaluation process.

Technology Focus: Mature Fields and Well Revitalization (January 2020)

Technology Focus Production decline with field maturing is a natural phenomenon, but the efforts to mitigate this decline and, in some cases, even to reverse the trend are truly remarkable. Achieving this feat requires not only a deeper understanding of the subsurface process involved but also a total synergy between the various disciplines engaged in maximizing production from brownfields. This message is apparent in the 90 papers reviewed this year covering various challenging aspects of the topic of mature fields and well revitalization. The papers reviewed cover a wide range of topics, including the use of novel drilling fluids to target undepleted hydrocarbon layers below the depleted, highly exploited mature hydrocarbon-bearing layers and using novel logging tools to detect thin low-resistivity/low-contrast plays for adding new oil to mature brown-fields. In the current period of industry downturn, creating and executing opportunities to develop mature brownfields has become more economically challenging. Papers describe technical, commercial, and operational aspects that help in achieving an established economic cut-off for project sanction. Several papers describe the use of tools to monitor the status of development using casedhole logs and other resistivity-independent saturation-estimation techniques to locate bypassed oil. Studies have been conducted using classical reservoir engineering tools to supplement findings of detailed reservoir-simulation studies to locate infill wells. Mature fields generally are believed to be fields having reached their economic limit after primary and secondary efforts. An increasing number of papers reviewed this year, however, also address the issue of refracturing in tight unconventional plays, adding to significant production. The surge in unconventional completions over the past decade has created ample candidates for hydraulic refracturing, and many opera-tors have found refracturing to be a viable alternative to drilling new wells in such formations. Recommended additional reading at OnePetro: www.onepetro.org. SPE 193207 Near-Balanced Nitrified Foam Drilling: A New Frontier for the Drilling of Depleted and Fractured Carbonates—A Study on the Design, Execution, and Results From Multiple Wells by Ali Khalid, Weatherford, et al. SPE 194581 Revival of an Offshore Gas Field: Case Study of a Fractured Basement Reservoir by Arun Kumar, HOEC, et al. SPE 196602 Refracturing the Appalachian Basin: An Economic Analysis by Kade Kiselica, Marietta College, et al.

Comparing mechanical and ultrasonic behaviour of a brittle and a ductile shale: Relevance to prediction of borehole stability and verification of shale barriers

Borehole collapse during drilling operations in shale formations is a well-known and costly problem within the petroleum industry. Recently it has become evident that shales may also form sealing barriers around the casing, reducing the need for cement jobs on new wells, and reducing costs for plugging and abandonment of old wells. The forming of such barriers involves large deformations of shale through creep and plastic processes. Hence, it is important to be able to characterize to what extent shales may fail in a brittle or ductile manner, in both cases causing possible hole instabilities during drilling, and in the case of ductile shales, enabling permanent sealing barriers. Triaxial failure tests, creep tests and tests tailored to follow the failure envelope under simulated borehole conditions have been performed with two soft shales. One shale fails in a more brittle manner than the other and fails to form a sealing barrier in the laboratory. The more ductile shale has been proved to form barriers both in the laboratory and in the field. By comparing their behavior, it is seen that the ductile shale exhibits normally consolidated behaviour, while the more brittle shale is overconsolidated. This points to the stress history and possibly cementation as keys in determining the failure mode. In addition, porosity, clay content, ultrasonic velocities, unconfined strength and friction angle may be used as indicators of brittle or ductile post-failure behaviour. Ultrasonic velocity and in particular attenuation measurements are shown to be sensitive to the failure initiation process, although stress sensitivity is much lower in the ductile than in the brittle case. The experiments provide values for anisotropic velocities as well as P-wave impedances that are necessary for open as well as cased hole log interpretation, in particular for barrier verification and possibly for monitoring of barrier formation.

An Operations Perspective on Injectivity and Capacity

Injectivity and capacity are two of the most important criteria of a CO2 storage site’s screening and selection process. A geologic formation can be characterized directly to quantify injectivity and capacity via the permeability-thickness and porosity- thickness products, respectively. After a project starts, the importance of each to the project is slightly different. Maintaining injection rate is one primary challenge during active injection operations, while the long-term storage capacity is one primary challenge to the storage project. For specific projects, the daily injection rate of CO2 and the ultimate storage quantity desired are based on the capture and compression facility, and the CO2 source is the driver to determine the mandatory qualifications of a site. Injectivity is defined as the ratio of the injection rate to the pressure change between the well and the storage zone, which is proportional to the permeability-thickness product. The initial start-up and subsequent start-ups of CO2 injection following short and extended shut-in periods are ideal times for multirate CO2 injection. These injection tests performed periodically can show injectivity behavior over longer periods of time so that the trends of the injection interval response can be analyzed and projected for longer terms when a stimulation or another sub-interval perforation may be required for injection. Because rate and pressure are generally direct measurements, injectivity-time, rate-pressure-time, and rate-pressure relationships can be used to diagnose trends in injection operations that are not apparent from injection rate alone. Capacity is defined as a quantity (mass or volume) of CO2 that can be injected and stored in a specific geologic formation from a specific injection project; it is proportional to porosity-thickness product. While permeability and capillary pressure are not part of the capacity calculation, both properties determine the accessibility of the porosity to injected CO2, hence the part of the pore space that can be used for storage. Furthermore, local geologic heterogeneity and architecture will strongly influence petrophysical flow properties and determine the plume size and shape. Storage efficiency has been used to define the fraction of the pore volume (or bulk volume) in which CO2 can be stored. It includes geologic (e.g. ratio of net to gross thickness), macroscopic (e.g. gravity), and microscopic (e.g. saturation) terms. It is relatively simple to quantify capacity if the location and distribution of CO2 is not required; however, if approximate dimensions of CO2 plume is necessary, it is very difficult to quantify confidently capacity. Prior to CO2 injection, statistical methods, numerical simulation, and a limited number of analytical methods are available. During injection, the most direct measurement is cased hole logging (e.g. neutron capture logs), which only measures very near wellbore CO2 saturation above a threshold saturation (e.g. 5%). Once the CO2 plume area, thickness, and saturation are adequate to make a detectable seismic contrast, a seismic survey may be helpful, but it detects only the thicker and higher CO2 saturation parts of the plume. Real-time analyses of injectivity and capacity during CO2 injection can provide operators with information necessary to plan for contingencies that increase certainty and meet the CO2 project goals. During site screening and selection, project risk can be reduced, but only after actual CO2 injection can these properties be more convincingly estimated. Injection operation results and observations, from the US Department of Energy sponsored Illinois Basin Decatur Project at Decatur, Illinois, USA, are used to demonstrate changes in injectivity and capacity through time and their influence on operations.

Effects of near surface lithology on velocity modelling and time–depth relationships in the Cooper–Eromanga–Lake Eyre Basin

Structural closures on the western flank of the Patchawarra Trough in the Cooper–Eromanga Basin are truly low relief; drilling opportunities regularly target hydrocarbon columns of similar magnitude to the uncertainty of depth prediction. Improving the accuracy and precision of depth prediction will reduce risk for drilling opportunities, and improve drilling success rates. A detailed study of the near surface geology (surface to ~500 m depth) of the western flank of the Patchawarra Trough has been undertaken to better understand the effect of observed geological variations of the near surface on depth prediction at deeper target levels. The stratigraphic interval investigated includes the top of the Eromanga Basin and the entire Lake Eyre Basin, which is sparingly studied and routinely overlooked in the statics and velocity modelling process. This study analysed recently acquired cased-hole sonic logs in conjunction with gamma logs and mudlog data to map out the observed geological variations, and construct a 3D velocity model of the near surface. Variations of layer thickness and seismic velocity were input into Monte Carlo simulations to investigate sensitivities of each formation on two-way travel time and depth prediction. This investigation has found that velocity variations of the Weathered Winton Formation, and thickness variations of the Namba Clastics have the greatest impact on imaging of structures at depth. Independently, these have the potential to completely conceal or create structures in the time domain. Continued efforts in improved understanding of the near surface will subsequently lead to enhanced imaging of structures, which can then be used in the mapping of structural closures in petroleum exploration and development.

Technology Update: Tool Enables Complete Cased-Hole Formation Evaluation, Reservoir Saturation Modeling

Technology Update For more than 40 years, the industry has used pulsed neutron logging to determine hydrocarbon and water saturations behind casing for reservoir management. Multiphase saturation measurements over time are useful for tracking reservoir depletion, planning workover and enhanced recovery strategies, and diagnosing production problems such as water influx and injection fluid breakthrough. Cased-hole logs also serve as a contingency when openhole logs cannot be run or are not considered for reservoir characterization. Although the cased-hole measurement suite has been greatly improved over many tool generations, the intrinsic physical measurements remained unchanged, which meant that operators could not obtain a complete picture of the rock and fluids behind casing. Input from openhole logs was required from a porosity or bulk density measurement for combination with the neutron porosity. Absent this input, primary formation evaluation in cased wellbores can be ambiguous. An additional challenge with cased-hole logging is correctly compensating for the effects of borehole fluids and the presence of completion hardware. Next-Generation Logs To meet the need for accurate surveillance in cased holes, Schlumberger recently introduced the Pulsar multifunction spectroscopy system. The system builds on innovative technologies originated by the company to provide the first complete cased-hole formation evaluation and reservoir saturation monitoring capability with openhole-equivalent measurements. This next generation in pulsed neutron logging integrates multiple detectors and a high-output pulsed neutron generator (PNG) to significantly improve measurement precision, data acquisition accuracy, and logging speed. The measurements are complemented by powerful algorithms that compensate for variation in the borehole fluids and completion in delivering robust, representative answers in complex conditions. The PNG and four detectors are housed in a 1.72-in.-outside-diameter (OD) tool that is designed for through-tubing access and logging through most completion restrictions. The detector adjacent to the PNG is the compact neutron monitor, which is primarily sensitive to fast neutrons to provide accurate and precise output measurement. There are three scintillation gamma-ray detectors for near, far, and deep detection. The near and far detectors use cerium-doped lanthanum bromide (LaBr3:Ce) scintillators, and the deep detector, farthest spaced from the PNG, has an yttrium aluminum perovskite scintillator. The three gamma-ray detectors are coupled to high-temperature-rated photomultiplier tubes, and their pulses are counted with specialized electronics matched to the high rate and resolution of the LaBr3:Ce scintillators.

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.

Multidisciplinary approach in determining the best zone to land a Haynesville horizontal well

The Upper Jurassic Haynesville Shale Formation in the East Texas Basin, North America, is a black, organic-rich, calcareous mudstone that lies below the Bossier Shale and above the Cotton Valley Limestone. The reservoir quality was controlled by primary depositional process and secondary diagenesis. The best lithology type in terms of drilling, completion, and production is the organic-rich, silty calcareous mudstone with low clay content. The studied well traversed the entire Haynesville Formation with the objective to assess the production potential of this formation. We acquired open-hole, triple-combo logging while drilling (LWD) logs, two types of production logs, and regional microseismic data. Postdrill log modeling confirmed that the horizontal well stayed on the planned trajectory, based on the log correlation between the horizontal and offset wells. Log modeling demonstrated that the LWD resistivity logs in horizontal wells are subject to resistivity anisotropy and polarization horn effects. We integrated the production log results with the open-hole, log formation evaluation and mineralogy-based brittleness index, to understand the static reservoir properties and dynamic inflow performance. The open-hole and cased-hole logs consistently showed that the “rabbit ears” interval at the base of the Upper Haynesville and the top interval of the Upper Haynesville were excellent lateral well-landing zones for best production. We combined microseismic data with production log data and found that the effective stimulated rock volume by hydraulic fracturing was mostly concentrated in the top interval of the Upper Haynesville and the “rabbit ears” interval at the base of the Upper Haynesville. The study demonstrated that a multidisciplinary approach was necessary in determining the best zone to land Haynesville horizontal wells.

Pressure transient testing for assessment of wellbore integrity in the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project

This paper presents an overview of a field program undertaken to assess wellbore integrity in a 54-year old vertical well, conducted as a component of the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project. The well investigated had been drilled to a depth of 1471m (4824ft), cased with 139.7mm (5.5in.) production casing and cemented with a 1-to-1 blend of Portland cement and pozzolan. Cased-hole logging indicated that the top of the cemented annulus was located approximately 500m (1640ft) above total depth of the well. A built-for-purpose tool was used to conduct pressure transient (vertical interference) testing to assess the hydraulic properties of the cemented annulus within the Watrous Formation, which serves as the regional seal in the vicinity of the Weyburn and Midale fields. A modified version of an existing tool was used to mill through casing then drill small cement core samples. This paper presents an overview of the entire testing program, and a detailed account of the analysis methodology used to estimate the equivalent permeability of the cemented annulus from a vertical interference test. The results of this analysis suggest that vertical permeability of the cemented annulus is very low, falling somewhere in the 7–80 microdarcy range. This result is consistent with cased-hole logging results; more specifically, the combined interpretation of sonic and ultrasonic logs, which suggest effective cement emplacement, and the paucity of casing defects identified by an electromagnetic log, which suggests the cement sheath has effectively shielded the casing from exposure to native formation fluids. The results presented here demonstrate that a well constructed using standard practices and materials of the 1950s can serve as an effective barrier against CO2 migration, although similar work should be undertaken in more wells to confirm that this holds across a broad spectrum of conditions.

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.

Pressure-Transient Testing and Production Logging To Monitor Performance of the Mars Deepwater Field

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 115591, "Case- History Review of the Application of Pressure-Transient Testing and Production Logging in Monitoring the Performance of the Mars Deepwater Gulf of Mexico Field," by Jeffrey Weiland, Shell, and Mehdi Azari, SPE, S. Suparman, SPE, Phil Fox, SPE, and Daniel Dorffer, SPE, Halliburton, prepared for the 2008 SPE Annual Technical Conference and Ex hibition, Denver, 21-24 September. The paper has not been peer reviewed. Proper producing-well characterization is necessary for asset management. Whether the reservoir has partial aquifer influx or is waterflooded, the flow profile of produced/injected water and the reservoir pressure, permeability, fluid saturations, and formation compaction are useful to evaluate the overall sweep efficiency properly. This information is more crucial in a field with multilayered reservoirs having different permeabilities, pressures, and structures. Multirate/multi-zone (MRMZ) production logging and pressure-transient testing help obtain necessary information. Introduction Many of the Gulf of Mexico Mars field wells produce from multiple reservoir layers, making MRMZ production-logging and well-testing surveys valuable methods to estimate the layer flow rates, pressures, permeabilities, and skin factors and the distance to and type of boundaries. A surveillance logging program has been conducted since 2004, with initial baseline surveys to monitor and optimize production performance and to track waterflood progress. This approach allows reservoir characterization without requiring zonal isolation. It also helps to evaluate where the waterflood has influenced pressure maintenance. Monitoring the waterflood with cased-hole logs is critical to improving the performance and increasing the overall recovery. The ability to acquire carbon/oxygen data for monitoring sea-water injection and sigma data for reservoir-fluid changes, combined with MRMZ production logging and testing, is essential in reducing unnecessary risks of well intervention. Objectives Production logging in the Mars field has facilitated reservoir management by identifying and reducing excessive water production. Production logging was designed to obtain one or more of the following reservoir and wellbore objectives.Characterize production or injection along the productive intervals of the wells.Determine the source of the excessive water production for further remedial work.Determine crossflow between the layers at shut-in and at low flow-rate conditions.Verify if all perforations are effective.Determine the productivity index for each layer and for the entire perforated interval.Evaluate individual-layer pressures and flow rates for each fluid phase.Check for bypassed hydrocarbon zones with pulsed-neutron and carbon/oxygen logging.Monitor stimulation or completion effectiveness.Confirm the length and sizes of downhole tubulars used.