North Sea Wells Research Articles

Changing the Game: Well Integrity Measurements Acquired on Drillpipe

The industry is continuously challenged to improve the efficiency and safety of operations. This is evident over the last 30 years in the development and improvement of measurements acquired while drilling. However, this has, in general, until now not been applied to well integrity measurements such as casing integrity and cement evaluation, which have traditionally been acquired utilizing wireline deployment. This paper will show the results of a new drillpipe-deployed tool that can be run in parallel with existing well operations. The results from two differing North Sea wells will be compared to traditionally acquired wireline-deployed tools and will demonstrate that these measurements and the resultant interpretation can successfully be acquired on drillpipe. This allows for much improved efficiency of operations and, in fact, the ability to acquire this important data in well conditions and environments where it is difficult or, in some cases, impossible to log with conventional wireline techniques. Two wells were selected with different degrees of difficulty in terms of measurement acquisition and showing different well trajectories and mud types. Both wells were logged with both the new drillpipe-deployed technology and traditional wireline technology, allowing a direct comparison of the techniques and tools and paving the way for acceptance of the new drillpipe-conveyed technology. The new drillpipe-conveyed tool can be run anytime drillpipe is utilized in the well. A radial distribution of ultrasonic transducers arranged on the circumference of a drill collar allows for full azimuthal interpretation of the casing and cement while rotating the drillpipe. Analysis of the acquired data allows for the interpretation of caliper thickness and an evaluation of the material in the annular space behind the casing. In addition, the tool can provide casing collar location in real time and has the ability to orient downhole devices such as whipstocks, perforating guns, and oriented cutters. The two well examples conclusively demonstrate that the tool can be run in parallel with existing operations to minimize rig time and eliminate the need for a dedicated, standalone wireline operation. Also, the cement evaluation interpretation was comparable to the equivalent wireline technology. We will investigate which measurements and applications the new tool can be used for and where there may be further room for improvement.

Occurrence of Mid Jurassic, organic-rich, lacustrine deposits on the southern Utsira High

A detailed biostratigraphic and organic geochemical investigation was undertaken of an organic-rich, lacustrine interval at ∼2622–2639 m in well 16/4–10, in the Fosen Prospect on the Utsira High in the Norwegian North Sea. A Bajocian to Aalenian age is suggested, similar to that for the immediately overlying Sleipner Fm, whereas the underlying strata were effectively barren and could not be dated with confidence. The dark grey claystone of the lacustrine unit is dominated by Botryococcus contributions, as indicated by palynofacies analysis. The kerogen exhibits a typical algaenan signature with a dominant single activation energy value of ∼54.5 kcal/mol for its bulk kinetics. Total organic carbon (TOC) increases from ∼2% at the base to ∼11% at the top, although the hydrogen index (HI) varies throughout (∼400–∼800‰ HC/Corg). The unit is on the verge of oil generation (Tmax ∼446 °C; estimated vitrinite reflectance 0.5–0.6%; spore colour index 5). Overall, it is similar to lacustrine intervals reported in the Møyfrid Discovery (North Sea wells 2/2-1 and 2/2–4) and Kap Stewart (Jameson Land, East Greenland). Freshwater/brackish lacustrine environment indications include a high tetracyclic polyprenoid (TPP) ratio and particularly abundant 4-methyl-24-ethylcholestanes. Specific botryococcal markers are absent, but the abundant 24-ethylcholestanes probably derive from this organism. Algal contributions increase upwards (at the expense of woody higher plants), as do 3-methylhopanes, whereas the pristane/phytane ratio (Pr/Ph) decreases (from ∼4 to 2), which may reflect increasing methanogenic/methylotophic activity. The immediately overlying Sleipner Fm has much lower TOC and HI values and a greater woody higher plant contribution, in what appears to be a proximal marine setting. Beneath the lacustrine facies is ∼10 m of dominantly sandstone, underlain by brown claystone, exhibiting the characteristics of a highly oxic marine environment, with very low TOC and HI values, but high oxygen index (OI) and Pr/Ph. The Fosen lake seems to correspond to one of numerous mini-basins in the area attributed to Zechstein salt withdrawal during the Jurassic. Like the Møyfrid lake, it appears to lie near the periphery of the sub-aerial, central North Sea volcanic dome. It is possible that organic-rich lacustrine facies developed in other, deeper mini-basins and have expelled some oil. Their subtle contributions to accumulations bearing other Jurassic charges may have been overlooked in the absence of obvious biomarker characteristics.

Developing a well-centric flow model - The first step in a risk-based approach to oil and gas well decommissioning

The final stage in the life of a hydrocarbon well is decommissioning, which aims to ensure that the original integrity of the cap rock is restored and all penetrated sub-surface formations with flow potential are isolated with an “eternal perspective” (OIL&GAS_UK, 2018). Well plug and abandonment (P&A) costs constitute a significant proportion of the total decommissioning expenditure. Careful planning and execution is always required, because a failed P&A often calls for costly and complex remedial operations, and can potentially result in significant environmental costs, reputational damage and financial loss. The high cost of well P&A operations and the increasing number of North Sea wells approaching the end of their economic life have resulted in a growing need to identify and develop new approaches to well P&A that are both fit-for-purpose and cost-effective. The development of a tool to support a risk-based analysis of P&A designs has been identified as one of the means for achieving these twin objectives. This paper describes the first stage in this process - the development of a novel modelling framework that can provide a quantitative assessment of the leakage risk associated with each well P&A design scenario over a specified period of time. The framework is built on a commercial, grid-based, numerical, reservoir simulator and can be used to model most North Sea wells scheduled for P&A during the coming years. Preliminary results are presented to illustrate the value that can be derived from the framework. The importance of understanding the ranking of model inputs on the basis of their impact on modelled leakage rates, and hence the significance of any associated uncertainties, is also illustrated using an example which shows cement absolute permeability (k) rather than relative permeability (kr) to be a more critical determiner of hydrocarbon leakage rates for the P&A cases examined. The modelling results also demonstrate the robustness of a grid-based numerical simulation approach to well P&A system modelling.

A new stratigraphic framework for the Miocene – Lower Pliocene deposits offshore Scandinavia: A multiscale approach

Here, we present an updated stratigraphic subdivision of the Oligocene to Pleistocene succession (880–610 m) in the newly proposed type well for the Molo Formation, the industrial 6407/9‐5 well, located on the continental shelf in the eastern Norwegian Sea. Furthermore, new data from the Danish North Sea wells Nora‐1, Vagn‐2, and Tove‐1 are presented. The studied succession in the 6407/9‐5 well is composed of five sedimentary units separated by hiati. The dating of these five units is based on correlation to the stratigraphically more complete Neogene succession in the (Danish) central North Sea area. In this study, a robust stratigraphic framework of these five units, based on a combination of dinoflagellate cysts (dinocyst) stratigraphy and seismic data, is established. The Oligocene succession is referred to the NSO zonation of Van Simaeys et al., Review of Palaeobotany and Palynology, 2005, 134, 105–128, while the Miocene–Pliocene succession is referred to the dinocyst zonation of Dybkjær and Piasecki, Review of Palaeobotany and Palynology, 2010, 161, 1–29. In well 6407/9‐5 the two lowermost units, located below the Molo Formation, comprise a Rupelian (Lower Oligocene) succession up to 803 m, and an Aquitanian/Burdigalian (Lower Miocene) succession from 803 to 787 m, respectively. Both of these units are referred to the Brygge Formation. The Molo Formation is separated from the underlying Brygge Formation by an unconformity. Furthermore, and in contrast to previous studies, our study shows that the Molo Formation (787–703 m) has an unconformity within it. The lower part of the formation (787–724 m) is dated to late Tortonian (Late Miocene), and referred to the Hystrichosphaeropsis obscura dinocyst Zone. The upper part (724–703 m) is dated to Zanclean (Early Pliocene), and referred to the Melitasphaeridium choanophorum dinocyst Zone. The uppermost unit studied (703–670 m) in the well is referred to the Gelasian (Lower Pleistocene) and is included in the Naust Formation. The regional correlation of this stratigraphy with the complete succession in the Danish North Sea reveals that the hiati found in the Miocene succession on the Norwegian Sea shelf are controlled by tectonism, while the internal depositional patterns of the Molo Formation were controlled by eustatic sea‐level changes.

Ammonite occurrences in North Sea cores: implications for Jurassic Arctic–Mediterranean marine seaway connectivity

The paucity of ammonite recovery from North Sea wells has meant that offshore correlations are largely dependent upon microfossil assemblages. While rare, ammonites have been found in a few boreholes during the course of oil exploration activities. The occurrence of ammonites in ten wells in the UK sector of the Viking Graben and the Moray Firth rift arms provides a new basis by which to demonstrate that there was a distinct separation between Arctic and sub-Mediterranean species that lasted from Bajocian to Early Callovian times. Five wells contain ‘Boreal Bathonian' ammonites from the Arctic Realm. Arctocephalites from the Boreal Arcticus Zone (uppermost Bajocian) correlates basinal partly anoxic mudstones in the Beryl Embayment (9/13b) with both bioturbated siltstones in the southern Viking Graben (9/10b), and calcareous mudstones in the East Shetland Basin (211/21). Upper Bajocian Pompeckji Zone Cranocephalites and younger Arcticoceras from Lower to Middle Bathonian Greenlandicus, Ishmae and Cranocephaloide zones are confined to 211/21 demonstrating that the marine transgression began earlier and lasted longer. A Cadoceras from well 3/3-8 dates to the Lower Callovian Koenigi and Calloviense zones during which renewed extensional faulting re-established ammonite migration routes between the Boreal and sub-Mediterranean realms. A Middle Oxfordian (Densiplicatum Zone) Perisphinctes from well 22/5b-8 confirms an episode of northward migration from the sub-Mediterranean into the Boreal Realm. Upper Oxfordian (Regulare to Rosenkantzi zones) Amoeboceras in wells 211/21-1 and 9/13b-19 are close to Upper Bajocian/Lower Bathonian faunas, suggesting an absence of Upper Bathonian to Middle Oxfordian strata as a result of rift-related footwall uplift and erosion. In four wells from Block 15/21 (-4, -11, -12A and -25) Lower Kimmeridgian ammonites have been documented, including Rasenia , Amoebites , Aulacostephanoides and Zenostephanoides , from the Baylei (?), Cymodoce, Mutabilis and Eudoxus zones, the latter (confirmed at well 13/28b-8) dating a widespread regional marine flooding surface in the Inner Moray Firth. Supplementary material: The detailed measurements of dimensions of the ammonites described are available at: https://doi.org/10.6084/m9.figshare.c.5087313

Velocity-attenuation model from check-shot drift trends in North Sea well data

Seismic attenuation distorts phase and narrows bandwidth in seismic surveys. It is also an exploration attribute, as, for example, gas or overpressure, may create attenuation anomalies. Compensating attenuation in imaging requires accurate models. Detailed attenuation models may be obtained using full-waveform inversion (FWI) or attenuation tomography, but their accuracy benefits from reliable starting models and/or constraints. Seismic attenuation and velocity dispersion are necessarily linked for causal linear wave propagation such that higher frequencies travel faster than lower frequencies in an attenuative medium. In publicly released well data from the Norwegian North Sea, we have observed systematic positive linear trends in check-shot drift when comparing (lower frequency) time-depth curves with (higher frequency) integrated sonic transit times. We observe velocity dispersion consistent with layers having constant seismic attenuation. Adapting a previously published method, and assuming an attenuation-dispersion relationship, we use drift gradients, measured over thick stratigraphic units, to estimate interval P-wave attenuation and tentatively interpret its variation in terms of porosity and fluid mobility. Reflectivity modeling predicts a very low attenuation contribution from peg-leg multiples. We use the attenuation values to develop a simple regional relationship between P-wave velocity and attenuation. Observed low drift gradients in some shallower units lead to an arch-shaped model that predicts low attenuation at both low and high velocities. The attenuation estimates were broadly comparable with published effective attenuation values obtained independently nearby. This general methodology for quickly deriving a regional velocity-attenuation relationship could be used anywhere that coincident velocity models are available at seismic and sonic frequencies. Such relationships can be used for fast derivation (from velocities) of starting attenuation models for FWI or tomography, constraining or linking velocity and attenuation in inversion, deriving models for attenuation compensation in time processing, or deriving background trends in screening for attenuation anomalies in exploration.

Real-Time Measurements Improve Managed-Pressure Operations in HP/HT North Sea Well

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 195767, “Using Real-Time Downhole and Along-String Measurements During Drilling and Cementing Operations To Improve Managed-Pressure Operations in a Complex High-Pressure/High-Temperature North Sea Well,” by Tom Brian, Total, and Andy Hawthorn and Duncan Groves, Baker Hughes, prepared for the 2019 SPE Offshore Europe Conference and Exhibition held in Aberdeen, 3–6 September. The paper has not been peer reviewed. This paper describes how new technology was implemented and deployed through a downhole acoustic network through a sequence of runs in complex North Sea wells under managed-pressure conditions. The authors demonstrate how measurements and real-time solutions evolved over the course of the wells to exceed the originally perceived value of top-of-fluid under severe losses. As a result of this deployment and the lessons learned, the acoustic telemetry network will now be used on upcoming, equally challenging wells, and its range of operations will be expanded to include drilling, tripping, and liner-cementing operations. Introduction Several mature fields in the North Sea experience significant challenges relating to high pressures and temperatures, accompanied by the infill drilling challenge of very narrow margins between pore and fracture pressures. To navigate these narrow mud-weight windows, understanding the bottomhole pressure (BHP) is critical. However, in instances of fractured formations above the target zones, severe losses can be encountered during drilling and cementing operations, often leading to the inability to maintain a full mud column at all times and even threatening the ability to reach total depth (TD). To address these issues, the operator investigated an acoustic telemetry system that could provide internal and external pressure measurements, along with other downhole measurements, independently of traditional mud-pulse telemetry in the drilling assembly, to provide real-time distributed pressure data essential to understanding the downhole conditions regardless of circulation, even under severe losses or during tripping and cementing operations. Network Operation The downhole measurement and acoustic telemetry network was deployed on several wells through multiple hole sizes for loss management, liner running, and cementing operations. The initial primary purpose was to monitor the top of the mud at all times, even in significant loss situations. The network consisted of inline tools that picked up and handled like drillpipe. A surface sensor was deployed and attached to the quill of the top drive. This surface receiver then transmitted through a wireless network to a decoding laptop in a safe area. Once there, the data were integrated with other downhole data from the traditional measurement-while-drilling string and with surface data from the rig floor and sensor measurements from the managed-pressure-drilling (MPD) equipment. This process is described more fully in the complete paper. As real-time data were acquired, it became apparent that the data could also be used in real time to help quantify actual downhole pressures. New calculations were designed for simpler visualization of equivalent circulating densities (ECDs) at the shoe, bit, and weak zones in the well at depths beyond the acoustic tools themselves. These data were used to manage the BHP within a 300-psi mud-weight window to enable the well to be delivered ultimately to planned TD.

Resolution limits of fluid overpressures from mineralogy, porosity, and sonic velocity variations in North Sea mudrocks

Pore pressure identification in mudrocks largely relies on indirect methods, including analyses of sonic velocity data. The magnitude of pore pressure changes that can be resolved from such data depends on the impact of fluid pressures on well log responses and on the velocity responses to lithological variations that could be mistaken for indications of pore pressure changes. Analysis of relationships between sonic velocity, mineralogy, and porosity from 11 North Sea wells demonstrated porosity variations of up to 5% from the mean value in mudrocks at any given depth between 1 and 3.5 km (3300–11,500 ft). Comparisons of these wells with seven wells from the Norwegian Sea demonstrated that the sonic velocity and density variations are influenced by the content of silica with a biogenic origin, but that they can otherwise not, in general, be related to mineralogy and pore pressure variations. The sonic velocity deviations from the mean value are of the same magnitude as those that would be caused by porosity changes of ±8% in mudrocks of uniform lithology. Furthermore, the deviations are not significantly reduced when the velocity data are compensated for changes in density (porosity). Detection limits for mechanically compacting mudrocks that are at their maximum effective stress are equivalent to static mud density changes of 0.1–0.15 g/cm3. These numbers apply to mudrocks with similar lithological variability to those analyzed in our data set but should be much higher when significant amounts of silica with a biogenic origin are present. The impact of effective stress changes on the sonic velocities of chemically compacting as well as of mechanically unloading rocks is much less because small or no porosity changes follow from the effective stress changes in such cases. The velocity responses for mechanically unloading and stress-insensitive chemical compaction can only be identified from four-dimensional (time-lapse) seismic data unless the rocks are close to the fracturing limit. These observations imply that reasonably accurate velocity-based pore pressure identification in North Sea mudrocks requires a knowledge of local mudrock heterogeneity and compaction that cannot easily be obtained. As a result, sonic velocity data should be treated with considerable caution in pore pressure evaluation.

Verification of Downhole Choke Technology in a Simulator Using Data from a North Sea Well

Summary The most important contributor to improved oil recovery on mature fields is drilling of infill wells. Managed–pressure drilling (MPD) and continuous–circulation–system (CCS) techniques can be used for improved control of bottomhole pressure when drilling wells in depleted fields with narrow pressure windows, but rig heave is a challenge when drilling from floating drilling units. Rig heave, caused by sea waves, induces downhole pressure oscillations that could exceed the operational pressure window. These oscillations are called “surge and swab,” and occur during tripping in and tripping out of the borehole, as well as during drillpipe connections, while the drillstring is suspended in the slips. Downhole choking was introduced as a method to reduce downhole pressure oscillations induced by the rig heave, and the concept was tested at laboratory scale and using computer simulations (Kvernland et al. 2018). The simulations were performed using a purpose–developed software that uses such input variables as wave height, pump flow, drillpipe movements, rig characteristics, and drilling–fluid properties, along with well design, drillpipe, and bottomhole–assembly (BHA) data, to simulate downhole pressure induced by rig heave. The simulator is designed to model dynamic interactions between the drilling fluid and the drillstring in a rigorous manner, which gives it the ability to accurately predict rapid downhole changes, such as those induced by ocean waves. In this paper, we provide an overview of the surge–and–swab simulator, describing its capabilities and limitations. Data from drilling a North Sea well are then used to validate the simulations performed using the software. The well used as an example in this paper was drilled conventionally from a floating rig. The downhole pressure variations recorded during three different drillpipe connections are compared with simulated downhole pressure. The simulations are performed on the basis of the recorded rig heave as well as the actual drilling–fluid, well–design, and drillpipe data. Results show that there is a good correlation between simulated and actual measured downhole pressure. The surge–and–swab simulation software is then used to simulate the same drillpipe connections using three different techniques and combinations of techniques used for improved downhole pressure control: (1) MPD, (2) MPD combined with CCS, and (3) MPD combined with CCS and a downhole choke. Results show that rig heave–induced downhole pressure variations are reduced to a level that is considered acceptable for drilling a well with a narrow pressure window for the last two cases, whereas use of backpressure MPD alone is not sufficient. The combination of MPD and CCS reduced surge and swab for two out of three drillpipe connections. For the third and deepest connection, the surge–and–swab pressure increased. The largest reduction in significant downhole pressure variations occurs when MPD and CCS are combined with downhole choking. Future work will consist of further developing the surge–and–swab simulator so that it will be possible to use it in well planning and as real–time decision support during drilling operations. The simulator will also be developed to include the possibility of simulating various well completion operations such as running casing and liners. The next hardware development phase consists of designing and building a complete downhole tool for testing in a well.

Early Oligocene dinocysts as a tool for palaeoenvironment reconstruction and stratigraphical framework – a case study from a North Sea well

Abstract. The lower Oligocene (Rupelian) successions are climate record archives of the early icehouse world in the Cenozoic. Even though the number of studies focussing on the generally cold Oligocene is increasing, little is known about climatic variations in the mid-latitudes to high latitudes of the Northern Hemisphere. One of the major obstacles is the lack of stratigraphically complete uppermost Eocene to Oligocene successions in these regions. This study focusses on dinoflagellate cysts (dinocysts) from a thick nearly complete Rupelian succession in the Syracuse Oils Norge A/S well 11/10-1 drilled in 1969 in the Norwegian part of the North Sea basin. The well provides a record of mid-latitude dinocyst assemblages, which yield key biostratigraphical and palaeoenvironmental information. All the analyses were undertaken on ditch cutting samples. The dinocyst assemblages confirm that the well penetrates about 600 m of Rupelian sediments and (as supported by correlation with the Nini-1 well) that the lowermost Rupelian (below the top or the last occurrence of Areosphaeridium diktyoplokum) is expanded. These assemblages also indicate the presence of two hiatuses: the first extends from the Lutetian to the Priabonian (equivalent to the D9nb–D12nb zones), and the second spans the Rupelian–Chattian boundary (equivalent to the D14nb subzone or the NSO-5 zone). Despite the risk of caving, the dinocyst assemblages support the existing sequence stratigraphic framework. The assemblages reflect a clear transition from distal to proximal deposition in the vicinity of the site (across the regional seismic sequences OSS-1 – OSS meaning Oligocene seismic sequence – to OSS-2). The proximal deltaic deposits of the OSS-2 regressive system tract (RST) are characterised by pulses of high sea-surface productivity and pronounced shifts in the dinocyst assemblages, reflecting a highly dynamic environment in a restricted marine to marginal marine setting. The Rupelian succession penetrated by well 11/10-1 yields one new species, Areoligera? barskii sp. nov., which is described here in detail. The cold-water-tolerant dinocyst Svalbardella cooksoniae is present in two intervals in the studied succession. These intervals are related to the early Oligocene cooling maxima (the Oi-1a and the Oi-2 events). Furthermore, these two intervals correlate with two local sequence boundaries, suggesting that they are most probably of glacioeustatic origin. From these observations, I postulate that the early icehouse climate played an important role in the depositional development of the Oligocene succession in the North Sea basin. Even though the Eocene–Oligocene transition interval is not complete (i.e. Lutetian to Priabonian is either missing or condensed), well 11/10-1 merits high-resolution studies of the early icehouse climate for the North Sea region. Although any detailed studies should ideally be undertaken on conventional cores instead of ditch cuttings, no such samples spanning the Eocene–Oligocene transition exist in this area.

Multiple episodes of regional exhumation and inversion identified in the UK Southern North Sea based on integration of palaeothermal and palaeoburial indicators

Abstract We present a consistent synthesis of palaeothermal (apatite fission track analysis (AFTA) and vitrinite reflectance) data from UK Southern North Sea wells with the regional pattern of exhumation defined from sonic velocity data. Cenozoic exhumation across most of the region began in the Paleocene between 63 and 59 Ma. Amounts of removed section are around 1 km across the offshore platform, increasing to 2 km or more on the Sole Pit axis. Neogene exhumation within this area began between 22 and 15 Ma, and led to removal of up to 1 km of section. Along the eastern flank of the Sole Pit axis, sonic data define a pre-Chalk event, and AFTA data from these wells show that exhumation began between 120 and 93 Ma. This timing correlates with events defined from AFTA data in the Sorgenfrei–Tornquist Zone, further east, presumably reflecting a response to regional tectonic stresses. East of the Sole Pit axis, AFTA and sonic velocities suggest that Neogene exhumation dominates, while further east towards the central parts of the North Sea Mesozoic sediments appear to be at maximum burial today except for local effects related to salt movement. The multiple episodes of exhumation and burial defined here have important implications for exploration.

Comparison of Quantitative Analysis of Image Logs for Shale Volume and Net to Gross Calculation of a Thinly Laminated Reservoir between VNG-NERGE and LAGIA-EGYPT

This study focuses on one of the resistivity imaging tools owned by Schlumberger; Full-bore Formation Micro Imager (FMI). The objective of this paper is to analyze quantitative analysis that is carried out focusing on calculating the volume on clay volume and net-to-gross measurement. Clay volume in a thinly bedded interval was estimated based on the gamma ray log and one FMI trace (out of 192 traces).The gamma ray log data resolution is considerably lower than the FMI log to reflect accurate lithology changes in thinly bedded reservoirs. It has been found afterthought some calibrations and corrections on the FMI resistivity log, the new processed log is used for clay volume and net to gross calculation of the reservoir, indicating the potential of this log for analysis of thin beds. A comparison between VNG-NERGE, NORTH SEA WELL, NERWING and LAGIA-8, LAGIA, EGYPT indicates the calculation for shale volume at different intervals using FMI tools.

Characterizing borehole plumbing using full-waveform ultrasonic data: Application to data from a North Sea well

We have analyzed ultrasonic flexural data acquired in a North Sea well using a commercial tool optimized for generating such data, and found how one might separate refractions along successive layers of casings, and from these separated refractions, characterize successive pipes and annuli. From the timing of refracted events, we determined the shape of a pipe, and by examining the amplitudes of these refracted events — a measure of the conductance of a pipe wall to transverse movement — we characterized the material in the annuli around a pipe. Data from two separate depth intervals were analyzed, demonstrating that a well plan might not give a sufficiently accurate description of the well. In the deeper interval in which the inner pipe was supposed to be free, we found that the annulus was most likely filled with sedimentation and debris. We could also see that the inner 7 in tubing was touching, or nearly touching, the outer 9⅝ in casing over the entire interval. For the shallower interval, below a certain depth, we saw that the 7 in tubing appeared to be touching the 9⅝ in casing approximately every 7–8 m. From measurements inside the 7 in tubing, we estimated the deformation of the outer 9⅝ in casing to be up to a maximum of nearly 5 cm, meaning that the minimum inner diameter of the outer pipe was close to the outer diameter of the inner pipe. Whereas some features revealed by the analyses were “good-to-know” — such as minor pipe deficiencies and deformations — other features might be critical for planned overhaul, or for operations related to abandonment, e.g., touching points were potential sticking points. Knowing where they were may be critical in determining the best depth to cut a pipe before pulling.

Decomposing full-waveform borehole acoustic data with application to data from a North Sea well

The objective of this study is to establish whether sonic data acquired in a cased hole can be used to estimate the material behind a second casing when the annulus between the two sets of casings is fluid filled. We have analyzed full-waveform data acquired using the Schlumberger tool Sonic Scanner for a double casing with a fluid between them, and where the outer annulus, outside the outer casing, might be cement filled or fluid filled. The sonic tool uses a cylindrical array of 104 omnidirectional receivers. The cylindrical array — approximately 4 in. in diameter and 1.8 m (6 ft) long — allows a formal decomposition of the acquired data into quasi-plane waves. Analyzing these plane waves, we have identified subtle but distinct changes in the waveforms. These changes appear to be dependent on the material filling the outer annulus, allowing for the determination of the fill material. The most significant changes relate to the propagating Stoneley waves. The identifications made are confirmed by cement bond log (CBL) analysis done on the exposed outer pipe after pulling the inner pipe. In one instance, for single-casing logging and a clean top-of-cement, like with conventional CBL analysis, we were able to confidently and accurately identify the cement/fluid boundary. In another instance, we were able to identify it as a nondistinct/smooth transition zone. For double-casing logging, we could confidently and accurately identify the cement/fluid boundary behind the second casing. For one case, we have substantiated that this zone has a longer interval of smooth transition from cement to consolidated/unconsolidated barite to a fluid-filled annulus.

North Sea’s Problems Driving Push for Change

North Sea’s Problems Driving Push for Change

Long-term acoustic monitoring at North Sea well site 22/4b

Marine seeps produce underwater sounds as a result of bubble formation and fragmentation upon emission from the seabed. The frequency content and sound levels of these emissions are related to bubble size distribution and emission flux, providing important information on methane release from the seafloor. Long-term passive acoustic monitoring was used to continuously record seep sounds over a 7-month period within the blowout crater at the abandoned well site, 22/4b, in the central North Sea. Also recorded were water column fluid velocities and near-seafloor water conductivity, temperature, and pressure. Acoustic signatures were primarily from ∼1 to 10 kHz. Key features were relatively broad spectral peaks at about 1.0, 1.5, 2.2, 3.1, 3.6 and 5.1 kHz. Temporal variations in spectral levels were apparently associated with tides.The recordings also documented a series of major episodic events including a large and persistent increase (∼10 dB) in overall sound levels and spectral broadening on 8 December 2011. The acoustic temporal pattern of this event was consistent with other recorded large transient events in the literature, and the major event was correlated with dramatic changes in other measurements, including increased water column fluid velocities, increased pressure and decreased salinity, indicating real changes in emission flux. Observed seabed morphology changes reported elsewhere in this special issue, also likely were related to this event. These data demonstrate the dynamic nature of marine seepage systems, show the value of monitoring systems, and provide direct supporting evidence for a violent formation mechanism of many widespread seep-associated seabed features like pockmarks.

Case Study—Equivalent-Circulating-Density Management Strategy Solves Lost-Circulation Issues on Complex Salt Diapirs/Paleocene Reservoir

Summary Drilling horizontal infill wells in the Pierce field in the UK Central North Sea (UKCNS) is challenging because of the narrow drilling window caused by depletion in a highly fractured reservoir, which is complicated further by water injection and the proximity of other production wells. Wellbore strengthening was attempted in the reservoir section of Pierce B5 (B5), although, when a pre-existing fracture weakened further by depletion was encountered, losses occurred. During the next 10 days more than 5,000 bbl of nonaqueous fluid (NAF) was lost downhole. During the recovery from the losses event, 15 individual treatments with lost-circulation materials (LCMs) were made. More than 50 t of LCM was used without success; however, the well was stabilized enough to secure the drilled footage and to complete the well. Two additional horizontal wells in the same field were successfully drilled with a low-equivalent-circulating-density (ECD) management strategy combined with the development of a low-ECD drilling-fluid system and the use of a high-fluid-loss, high-strength (HFHS) pill. The dual problems of barite sag and poor hole cleaning, which can be problematic when reducing ECD through a reduction in fluid rheology, were successfully managed. This low-ECD technique can be applied in other subsea depleted wells in which a narrow drilling window exists. This paper describes how a detailed analysis of the losses event on Pierce B5 provided an improved understanding of the loss mechanism that resulted in a revised ECD-management strategy. An engineered low-ECD NAF with an HFHS pill, combined with learnings on ECD awareness in the planning and execution phases, mitigated the drilling risks on subsequent wells. This has implications when encountering very large fractures that are difficult to overcome, not only on future Central North Sea wells, but also globally for wells drilled in similar geological settings.

Formation of flint horizons in North Sea chalk through marine sedimentation of nano-quartz

AbstractIn the Upper Cretaceous-Danian North Sea chalk, silica composed of nano-size quartz spheres is dispersed in the chalk matrix, and quartz is present in bands and nodules of flint. In the present investigation of the North Sea Danian chalk the nano-quartz in the chalk matrix is compared with the silica in the flint. Samples of chalk and flint layers from four North Sea wells have been investigated. Atomic Force Microscopy (AFM) has been applied to image the quartz in the chalk and in the flint. X-ray diffraction (XRD), including analysis of the positions and profiles of hkl reflections in powder diffraction patterns, has been applied to characterize the lattice of the quartz in both the chalk matrix and in the flint. The quartz in the chalk matrix and in the flint is composed of nano-quartz spheres having identical cell parameters. Based on the results we propose a new model for formation of flint in North Sea chalk: (1) The nano-quartz in the flint, like the nano-quartz in the chalk matrix, has crystallized in the marine Chalk Sea environment. The colloidal quartz particles flocculated and were deposited on the sea floor mixed with calcitic bioclastic material. (2) Regional variations in the concentration of nano-quartz particles in the sediment reflect different degrees of acidification of the Chalk Sea. (3) This resulted in areas where practically all the calcite bioclasts were dissolved leaving a high concentration of nano-quartz particles to form flint layers; where there was less dissolution, indurated chalk with abundant nano-quartz particles is now preserved. (4) The acidification could have been caused by the effects of enhanced atmospheric CO2 linked to massive short-lived volcanic eruptions in the British Tertiary Igneous Province.

Automation of Drawworks and Topdrive Management To Minimize Swab/Surge and Poor-Downhole-Condition Effects

Summary Careless axial and rotational movement of the drillstring can cause formation fracturing or fluid influx, resulting in costly remedial actions. With increasingly complex wellbore geometries and narrow geopressure windows, it is not always obvious for the driller how to estimate accurately the real maneuvering limits of the drawworks and the topdrive, especially under poor downhole conditions. The solution presented in this paper uses continuously updated safeguards applied to the drilling-control system to maintain a downhole pressure within the acceptable limits of the openhole formations. It automatically stops the movement of the drillstring in the case of abnormal hookloads or surface torques. Because automatic actions can be triggered in the case of an unexpected situation, some standard procedures have been fully automated, including friction tests and back reaming. Numerical models are used to constantly calculate the maximum accelerations and velocities, which can be applied to the drillstring in the current drilling conditions. The resulting envelope of protection is dependent on many factors. Therefore, a proper evaluation of the downhole conditions is of paramount importance for the quality of the calculated safeguards. An automatic calibration of the physical models, on the basis of surface measurements, is at the heart of the system. The calibrated mechanical models are used to determine the limits for abnormal surface torques or hookloads. It is, therefore, possible to take actions automatically in the case of overpull, set-down weight, or high torque. In 2008, a preliminary version of the system was tested during the drilling of a well in the North Sea. Even though the control algorithms did work well at that time, it was noticed that, in some circumstances, the drilling-control system would not be ready in time for fast-changing drilling conditions. An improved version of the drawworks and topdrive automation system has been tested during the drilling of three North Sea wells in the spring of 2009. In this last version, the response time of the system has been optimal at all times. The drillers involved in the testing of the system have found the system useful and user friendly.

Probabilistic Well-Time Estimation Revisited

Summary Probabilistic estimation of well duration has been common practice for more than a decade; many papers have been written on the subject, and commercial software is available for the purpose. Is the subject, therefore, mature? The authors suggest that this is not the case and show that several essential aspects of both data characterization and probabilistic analysis have been overlooked in previous contributions. In 2007, Nexen began a study with the objective of improving our process for well-time estimation. Its workscope was to Assemble a statistically significant historical well database.Develop a consistent definition of nonproductive time (NPT) as it relates to the original approved-for-expenditure (AFE) timings.Reanalyze the historical well database for NPT from the original daily drilling reports (DDRs), on the basis of the definition above.Decide how NPT is best characterized (that is, determine the correct choice of input variables).Determine the associated occurrence frequencies and probability-density-function (PDF) parameters from the historical database.Validate the probabilistic model, by comparison of program predictions against the historical data set.Work with a software provider to implement any necessary changes in a commercially available product. A database of 118 central North Sea wells was reanalyzed independently for NPT from the original DDRs. These considerably underestimated the true NPT, by 19.7% on average. Train wrecks (mechanical NPT events of more than 2.5 days) were only 4% by number but contributed 50% of NPT by duration. It was found that "ordinary" mechanical NPT, train wrecks, waiting on weather (WOW) in open water, and WOW with riser connected are all statistically distinct, with very different occurrence frequencies and PDFs. Earlier workers did not observe this distinction or properly validate their models against the well database. Therefore, it is impossible to obtain historically accurate probabilistic well-time predictions (i.e., that are consistent with the historical database) using the previous state of the art. Moreover, such predictions will generally be underestimates. This paper describes an accurate method that overcomes previous limitations. While central North Sea data are used, the analysis techniques are not area specific, and the method may be applied easily to other areas in the oil field.