Sand Influx Research Articles

Mid- to late Holocene aeolian activity recorded in a coastal dunefield and lacustrine sediments on Andøya, northern Norway

In an effort to reconstruct past aeolian activity, a foredune stratigraphy and a continuous lake sediment record from the largest dunefield on Andøya, northern Norway, have been investigated. The dunefield extends landwards in a north-eastward direction and consists of several parabolic dunes, foredunes and blowouts. The sediment record (169 cm) from the nearby lake Latjønna and the foredune stratigraphy (10 m) covers the last 6200 and 3700 cal. yr BP, respectively. Both sites possess sediments deposited after the Tapes transgression maximum (~6800 cal. yr BP), which reached a level of ~7–8 m a.s.l. at the study site. The lake sediment record consists of several units dominated by sand grains interspersed by more organic-rich beds. The core has been examined by x-ray fluorescence (XRF), magnetic susceptibility (MS) and loss-on-ignition (LOI). Mineral grains were detected by wet sieving of the ignition residue (IR), and the influx of sand grains to Latjønna was calculated based on the weight of sand grains >250 µm/cm divided by the accumulation rate determined from a radiocarbon (14C)-based age–depth model. Phases with high influx of sand to Latjønna are recorded around 4800, 4250, 3000–2000, 1850–1750, 1600–600, 450, 300 and 150 cal. yr BP, which coincides with periods of increased storminess recorded in other studies around the North-Eastern Atlantic region. The two study sites show, however, quite contrasting results; high sedimentation rates in the lake record associated with greater aeolian influx correspond to stability in the foredune stratigraphy reflected by the presence of several palaeosols. Because of this out-of-phase behaviour, it is suggested that the foredune is mainly influenced by summer climate and relative sea level (RSL) change, whereas the lake record is more influenced by niveo-aeolian processes transporting sand grains farther inland during winter.

Holocene aeolian sedimentation and episodic mass-wasting events recorded in lacustrine sediments on Langøya in Vesterålen, northern Norway

In this study, the frequency of mass-wasting events and past storminess has been reconstructed throughout the Holocene (11,500 cal yr BP to present) from lacustrine sediments in lake Trehynnvatnet (33 m a.s.l.), which is located in a glacially carved valley at Nykvåg on the outmost coast of western Langøya, Vesterålen, northern Norway. Sediment cores (∼2–5 m long) have been examined by use of high-resolution magnetic susceptibility and XRF-scanning as well as grain size and loss-on-ignition analysis. In total 35 episodic event layers have been identified throughout the Holocene. The majority of these events are characterized as discrete coarse-grained sediment layers followed by normal grading, and are related to past mass-wasting activity within the catchment. Periods with high mass-wasting activity are dated to 11,000–10,500, 5500–4500, 4000–3500, 3000–2500, 2000–1000 and 500–0 cal yr BP. The continuous input of sand grains (>250 μm) has been systematically investigated throughout the sediment cores. The sand grains are related to catchment samples from the sandy beach deposits in Sandvikbukta c. 750 m away in SW direction, and are suggested to indicate (niveo-) aeolian influx to the lake. The content of sand grains varies greatly throughout the record, although there is a clear increase in influx of sand during the last 2800 years. Periods with high aeolian influx are proposed to indicate increased storminess, which occurred between 1600 and 1550 (350–400 CE), 1400–1300 (450–550 CE), 750–550 (1200–1400 CE) and 250–20 cal yr BP (1700–1930 CE), which to some degree coincides with periods of increased storminess and winter precipitation recorded in other studies around the North Eastern Atlantic region.

Sand production prediction using ratio of shear modulus to bulk compressibility (case study)

Sand production is a serious problem widely existing in oil/gas production. The problems resulting from sand influx include abrasion of downhole tubular/casing, subsurface safety valve and surface equipment; casing/tubing buckling, failure of casing or liners from removal of surrounding formation, compaction and erosion; and loss of production caused by sand bridging in tubing and/or flow lines. There are several methods for predicting sand production. The methods include use of production data, well logs, laboratory testing, acoustic, intrusive sand monitoring devices, and analogy. The methodologies are reviewed and the data needed for predicting sand production are enumerated. The technique used in this paper involves the calculation of shear modulus, bulk compressibility, and the ratio of shear modulus to bulk compressibility. The shear modulus to bulk compressibility ratio has been related empirically to sand influx. This Mechanical Properties Log method works 81% of the time. This technique is supported with examples and case studies from regions around the world known for sand production. The authors collected the information of the “Kaki and Bushgan Oilfield in Iran”, set a sand production prediction to predict sand production potential. The technique has been successfully applied in reservoirs and results have been compared with testing data.

Rapid Adjustment of Submarine Channel Architecture To Changes In Sediment Supply

Abstract Changes in sediment supply and caliber during the last ∼ 130 ka have resulted in a complex architectural evolution of the Y channel system on the western Niger Delta slope. This evolution consists of four phases, each with documented or inferred changes in sediment supply. Phase 1 flows created wide (1,000 m), low-sinuosity (1.1) channel forms with lateral migration and little to no aggradation. During Phase 2, the Y channel system began to aggrade, creating more narrow (300 m) and sinuous (1.4) channel forms with many meander cutoffs. This system was abandoned at ∼ 130 ka, perhaps related to rapid relative sea-level rise during Marine Isotope Stage (MIS) 5. Phase 3 flows were mud-rich and deposited sediment on the outer bends of the channel form, resulting in the narrowing (to 250 m), straightening (to a sinuosity of 1.22), and aggradation of the Y channel system. Renewed influx of sand into the Y channel system occurred with Phase 4 at ∼ 50 ka, during MIS 3 sea-level fall. The onset of Phase 4 is marked by the initiation of the Y′ tributary channel, which re-established sand deposition in the Y channel system. Flows entering the Y channel from the Y′ channel were underfit, resulting in inner levee deposition that is most prevalent on outer banks, acting to further straighten (1.21) and narrow (to 200 m wide) the Y channel. The inner levees accumulated quickly as the flows sought equilibrium, with deposition rates > 200 cm/ky. Marked by the presence of the last sand bed, abandonment occurred at ∼ 19 ka in the Y channel and ∼ 15 ka in the Y′ channel and is likely related to progressive abandonment due to shelf-edge delta avulsion and/or progressive sea level rise associated with Melt Water Pulse 1-A. The muddy, 5-meter-thick Holocene layer has thickness variations that mimic those seen in the sandy part of Phase 4, suggesting that dilute, muddy flows continue to affect the modern Y channel system. This unique dataset allows us to unequivocally link changes in submarine channel architecture to variations in sediment supply and caliber. Changes in the updip sediment routing system (i.e., the channel “plumbing”) are shown to have profound implications for submarine channel architecture and reservoir connectivity.

A shift towards wetter and windier conditions in southern Sweden around the prominent solar minimum 2750 cal a BP

A variety of palaeoclimatic records show a shift towards cooler, wetter and windier conditions in Europe around 2800 cal a BP. The shift broadly coincides with an increase of the atmospheric 14C concentration, suggesting a connection between solar activity variations and climate change. Here we investigate a peat record from Undarsmosse in southern Sweden. In a previous study, based on a low-resolution chronology, this record showed increased aeolian sand influx and Sphagnum spore content around 2800 cal a BP, indicating high storm activity and wetter conditions. We applied the 14C wiggle-match dating technique on the same record to construct a robust chronology to evaluate the temporal relationship to solar forcing. In addition, we performed plant macrofossil analysis to determine local vegetation changes. Based on the new chronology, a shift to a Sphagnum-dominated bog, representing wetter conditions, and the onset of a period with increased storminess occurred around 2700 cal a BP. These changes are, within age model uncertainties, synchronous with climatic changes inferred from other sites in Europe, suggesting a shift in the larger scale atmospheric circulation, possibly triggered by decreased solar activity.

SCALES & SIGNATURES OF EPISODIC SAND BYPASSING AT A TIDE-DOMINATED INLET — FRIPP INLET, SOUTH CAROLINA

This paper provides a case example of meso-scale sand circulation and volumes in a tide-dominated inlet (Fripp Inlet, South Carolina, USA) and the adjacent barrier island. Episodic bypassing is the dominant process controlling shoreline change in this area (Gaudiano & Kana 2001). The scale of bypassing in this case has proven to be more than adequate to restore a highly eroding armored beach along Fripp Island. Upward of 2.7 million cubic meters (m³) shifted from the ebb-tidal delta of Fripp Inlet to a central oceanfront area of Fripp Island between 1989 and 2012. This volume, representing roughly 10 percent of the ebb-tidal delta volume, provided natural nourishment equivalent to nearly 600 cubic meters per meter of shoreline (m³/m) along the nearly 5-kilometer (km) long barrier island. The volume density varied greatly with some beach segments widening by more than 500 m. Prior to the bypassing events, there was no beach at high tide along most of the island. The paper discusses the possible trigger for recent bypassing events and documents changes in Fripp Inlet. Channel rotation into a seawall, southerly deflection of the ebb-delta centroid, and large inputs of sand from the adjacent barrier island appear to have produced conditions promoting large releases of sand to Fripp Island. While the influx of sand has restored much of the oceanfront, full restoration of a sandy beach along the inlet margin will require a channel avulsion or realignment such that the principal erosion-causing process—ebb jet in the main channel—no longer encroaches on the seawall. A channel realignment plan developed by the authors will require upward of 1.2 million m³ of dredging under present conditions but possibly less volume in the near future because of the formation of an incipient new channel.

Enrichment, distribution and sources of heavy metals in the sediments of Deception Bay, Queensland, Australia

Sediment samples from 13 sampling sites in Deception Bay, Australia were analysed for the presence of heavy metals. Enrichment factors, modified contamination indices and Nemerow pollution indices were calculated for each sampling site to determine sediment quality. The results indicate significant pollution of most sites by lead (average enrichment factor (EF) of 13), but there is also enrichment of arsenic (average EF 2.3), zinc (average EF 2.7) and other heavy metals. The modified degree of contamination indices (average 1.0) suggests that there is little contamination. By contrast, the Nemerow pollution index (average 5.8) suggests that Deception Bay is heavily contaminated. Cluster analysis was undertaken to identify groups of elements. Strong correlation between some elements and two distinct clusters of sampling sites based on sediment type was evident. These results have implications for pollution in complex marine environments where there is significant influx of sand and sediment into an estuarine environment.

Available Methodologies Concerning the Treatment and Removal of Sand From Pipelines and Associated Case Studies

Summary Traditional sand-management programs usually consist of several measures employed to mitigate the production of sand from wells, a process commonly referred to as sanding. Such systems include production prediction and monitoring, topside management, sand control, and "intelligent" systems (Gherryo et al. 2009). Sand control can be considered in terms of short- and long-term solutions; examples of long-term solutions (preventative measures) would be the application of gravel packs, screens, inflow control devices, and chemical stabilization methods (Abney et al. 2007). In instances where a sand-management program has not been employed or has been ineffectively delivered, sand can and is deposited in pipelines/riser systems. This can be a gradual process of continual minor deposition over time, or systems can be overwhelmed with a sudden influx of sand as a consequence of formation or completion failure. The result leads to increasing pipeline resistance, reduced flow rates, rising pressure drop in the pipeline system, and erosion, which, in turn, can lead to degradation of the pipeline/riser and topside systems (Abney et al. 2007). The main focus then concerns the short-term sand-control solution of remediating the pipeline system with the goal of removing the deposits. An operator might wish to pursue a sand-removal/cleaning program with the specific goal of performing an in-line inspection as part of an integrity-management scheme. This paper discusses the available methodologies for remediation of a pipeline affected by sand and presents examples where the discussed methodologies have been applied.

Holocene variability in hydrology, vegetation, fire, and eolian activity in the Nebraska Sand Hills, USA

This study combined multiple aquatic and terrestrial proxies, including diatoms, pollen, grain size, and bulk-sediment chemistry to reconstruct the history of three lake sites located in the central Sand Hills of Nebraska, USA. Long-term changes in effective moisture are evident at all sites, with significant changes occurring at ~6000, ~4000, and ~ 2000 cal. yr BP. Both aquatic and terrestrial indicators suggest that effective moisture was low between 10,000 and ~6000 cal. yr BP, and that this time interval was the driest period of the Holocene. The dominance of benthic and tychoplanktic diatom taxa indicates relatively shallow lake-level, high sand influx indicates moderately high eolian activity, and the pollen assemblage suggests xeric grasslands with abundant mud flats. About 6000 cal. yr BP, all three sites experienced an increase in effective moisture. Lake-level rise is indicated by increases in planktic and tychoplanktic diatoms relative to benthic taxa, while greater abundance of grass pollen and charcoal, and decreased eolian flux indicate stabilized dunes with dense vegetation sufficient to fuel local fires. A significant hydrologic shift recorded at all sites occurred at ~4000 cal. yr BP. This event was characterized by substantial lake-level rise, yet decreased grass cover and fire frequency, and increased eolian activity. Water-table rise may have been caused by a combination of factors including: (1) formation of dune-dams that blocked old drainage channels, (2) reduced grass cover and hence reduced evapotranspiration, and (3) changes in the frequency and duration of drought. The most likely cause(s) of the differential response of the terrestrial and aquatic systems at this time is not clear, none-the-less the late Holocene was not nearly as dry as the interval prior to 6000 cal. yr BP. The last ~2000 yr were characterized by several short-term fluctuations in lake level, including an interval of drought between 950 and 750 cal. yr BP, coincident with increased eolian activity during the latter part of the Medieval Climatic Anomaly.

A 7600-year sedimentary record of climatic instability in Dunnet Bay, North Scotland

We present a partial reconstruction of NE Atlantic climatic variability, the first based on stratigraphic evidence obtained from a vegetated dune system of the northern Scottish coast. The dune system at Dunnet Bay is fringed by prominent submerged peat deposits, exposed only at low tide, which record stable climatic periods when lower-energy wind regimes prevailed, allowing the formation of organic soils. By contrast, periods of sand influx, are associated with high-energy conditions. The majority of sand material tested had a narrow range of particle sizes and major element composition, supporting the interpretation of a common Aeolian origin. Sand layers were dated with optically stimulated luminescence (OSL) and peat layers were 14C-dated in four cores (C1–C4) and seven sample sections (S1–S7). The results from the two master sections S1 and S2 (1 km apart) indicate synchronous episodes of climatic stability (7650–6310, 4340–4000, 2690–2290, 100–0 cal a BP) and instability (6310–4340, 4000–2690, 2290–2000, 170–100 cal a BP) in spite of very different mean accumulation rates at the two sites (1 mm a− 1 at S1 and 0.2 mm a− 1 at S2). Phases of sand deposition (i.e. Aeolian activity) at Dunnet tend to be associated with high average levels of chloride in the Greenland Ice Sheet Project 2 ice core (GISP2). Phases of peat development show a fairly good correspondence with low ice-rafting activity in the NE Atlantic and with indicators of wetness measured from a peat bog in SW Sweden. By contrast, comparison of the Dunnet depositional timeline with other regional studies (e.g. Hebrides and Orkney) shows little synchronicity. This suggests that (a) local scale variables have a strong effect on regional patterns of dune instability, and (b) the geological and geographic settings of Dunnet Bay mean that only large climatic regime shifts in the NE Atlantic are capable of triggering instability and being preserved in the depositional record.

Hybrid System Brings Flexibility of CT to Long, Extended-Reach Wells

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 142769, ’New Hybrid System Brings the Flexibility of Coiled Tubing to Long, Extended-Reach Wells,’ by F. McNeil, SPE, S.D. Lindsay, SPE, R. Lyons, and R.P. Gracey, Halliburton, prepared for the 2011 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, 5-6 April. The paper has not been peer reviewed. The number of long extended-reach wells being drilled in the oil and gas industry continues to increase. These wells present complex challenges in completion and intervention procedures because operators demand the same level of performance achieved on shallower wells while providing a cost-effective and safe solution. To reach deep target depths with coiled tubing (CT), smaller coil must be used because of reel capacity, which limits pumping rates because of pressure and velocity limitations. As an alternative, jointed pipe can be used; however, using jointed pipe reduces the overall efficiency of the process because continuous pumping cannot be achieved. A new hybrid system uses both CT and jointed pipe in a single work string. The system incorporates a unique flapper safety valve that enables seamless functioning of the string in a live well. The hybrid system enables larger CT and jointed pipe to be deployed, which results in higher pumping rates and greater depths. Large CT Reel Capacity. A major hurdle when considering large lengths of CT is reel capacity. For example, the maximum length of coil that can be stored on a standard reel [assuming 144-in. reel outer diameter (OD)] is as follows: 1¾-in.-OD CT: 19,670 ft 2-in.-OD CT: 14,795 ft 2⅜-in.-OD CT: 10,151 ft Lockup. Another important consideration in the design of any CT job in a highly deviated deep-reach well is lockup. Lockup occurs when CT forms a helix inside the casing and high wall-contact forces prevent any additional forward movement. While this effect will occur in all CT, smaller CT is more susceptible to complete lockup at shallower depths than larger CT. Therefore, larger CT is more likely to reach target depths. Wellbore Cleanouts. Accumulation of sand and solids in wellbores significantly impairs oil and gas production; an influx of sand can impair or stop the flow of oil from a reservoir. This is commonly dealt with by CT intervention; in fact, nearly half of all CT operations involve well cleanouts. Conventional CT units are limited to small-OD coil when performing remedial operations on deep wells because of limitations in reel size, as well as local laws regarding width, height, length, and weight restrictions.

Multi-proxy analyses of a peat bog on Isla de los Estados, easternmost Tierra del Fuego: a unique record of the variable Southern Hemisphere Westerlies since the last deglaciation

We have analyzed an almost 14,000 year old peat sequence on the island of Isla de los Estados (55° S, 64° W), east of Tierra del Fuego, in the core of the Southern Hemisphere Westerlies. A multitude of methods have been used: high resolution 14C dating; detailed lithologic descriptions including humification degree; loss on ignition; magnetic susceptibility; bulk density; pollen and spore analysis and determination of Aeolian sand influx. By combining proxies for wind and precipitation we have been able to reconstruct how the westerlies have varied over time in the Atlantic sector of the Southern Ocean. It shows that this westerly wind belt was most intense at the onset of the record, 13,600–13,200 cal BP, coinciding with the mid to late part of the Antarctic Cold Reversal, followed by a gradual decline. At 12,200 cal BP the westerlies seem to have shifted to a position south of Tierra del Fuego and this phase, the calmest and driest period on the island throughout the sequence, ended at 10,000 cal BP when the westerlies moved equatorward again. Since then the westerlies have been present but with a variable impact on the 55° S latitude of the Atlantic. Mostly conditions have been fairly similar to today, but occasionally with a wider or narrower and/or weaker or stronger wind belt. At 7200 cal BP wind intensity began to increase and between 4500 and 3500 cal BP these southern latitudes experienced a distinct wind and precipitation maximum, both in terms of perseverance and intensity. Our results show a both wide and strong wind belt, with possible niveo-aeolian activity in Tierra del Fuego in winter, and possibly creating milder summers around the Antarctic Peninsula. In the later part of the Holocene, expansion–contraction phases of the wind belt, especially in winter, seem to have been a common phenomenon.

Model for the genesis of coastal dune fields with vegetation

Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the present work, we use dune modeling in order to investigate the genesis and early development stages of coastal dune fields in the presence of vegetation. The model, which consists of a set of coupled equations for the turbulent wind field over the landscape, the saltation flux and the growth of vegetation cover on the surface, is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx. Different dune morphologies are obtained, depending on the characteristic rate of vegetation growth relative to wind strength: barchans, transverse dunes with trailing ridges, parabolic dunes and vegetated, alongshore sand barriers or foredunes. The existence of a vegetation-free backshore is found to be important for the nucleation timescale of coastal dune generations. The role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed.

Late Holocene morphodynamics in the littoral zone of the Iwik Peninsula area (Banc d'Arguin — Mauritania)

In the littoral zone of the Banc d'Arguin both littoral sedimentary units and man-made deposits (shell-middens) abound. They were formed during the Late Holocene after the final onset of the post-glacial transgression. Here, a geo-archaeological approach is used to study them. The geomorphological and sedimentological characteristics of the coastal features define two distinct coastal sedimentary units in the Iwik–Aouatil zone between an ancient shoreline which formed on the Tafaritian substratum ca 6.7–5.7 cal. ka BP and the present one: (i) very extensive sand flats and (ii) linear relief features, both covered by a thin veneer of anthropogenic Anadara senilis shell-middens. They developed under a stable sea-level regime. The sand flats expanded at variable rates throughout the period. The linear relief features represent beach barriers and coastal dunes which can be dated archaeologically thanks to the superimposed shell-middens, supplemented by 14C dating. Two series of linear sedimentary units are identified. First, a complex north–south system of dunes and hook-ended sand barriers developed from 4.8 to 4.1 cal. ka BP and, secondly, two regular and linear beach barriers developed between 3.7 and 3.3 cal. ka BP. Their morphological characteristics suggest that they were generated through littoral drift processes. Directional changes observed between the first and the second groups demonstrate that the wave refraction pattern was dramatically modified after the closure of the strait between the former Iwik Island and the land. A comparison with the nearby Jerf el Oustani coastal area in the Late Holocene shows that both sites shared similarities in depositional patterns but also differences in the chronology of the sedimentary units. Processes involved in the development of the linear units are indicative of the enhancement of both sand influx and longshore drift due to climatic change.

Jurassic sedimentation in the Cleveland Basin: a review

Jurassic sedimentation in the Cleveland Basin: a review

Palaeoenvironmental implications of the ichnology and geochemistry of the Westbury Formation (Rhaetian), Westbury-on-Severn, south-west England

Abstract: The Westbury Formation (Rhaetian) beds of Westbury Garden Cliff, Westbury-on-Severn, west of Gloucester, Britain, show an unusual combination of features. Both deep water and emergent characteristics are present within the sediments and the trace fossils. The ichnoassemblage consists of abundant Selenichnites, Planolites beverlyensis and Lockeia with rarer Oniscoidichnus, Chondrites, Rhizocorallium irregulare, Taenidium serpentium, an unusual form of Walcottia and Merostomichnites-like traces. These trace fossils display an interesting relationship with the sediments: low-energy Cruziana ichnofacies is found within high-energy sandstones. The sandstones are interbedded with laminated mudstones, apparently deposited in deep water, but some aspects of the ichnoassemblage, preservation and sedimentation indicate shallower water. One new trace fossil, Radichnus allingtona igen. et isp. nov., closely resembles the traces of modern fiddler crabs and imply emergence, by analogy. This ichnofauna is similar to early stage disaster colonisation in recent experiments in Long Island Sound (south of Connecticut, USA) and with storm-influenced deposits within the Cardium Formation (Seebe, Alberta, Canada). This indicates a lagoonal environment with influxes of sand and oxygen. Total organic carbon levels were found to fluctuate greatly between stratigraphic layers but remained relatively high. This implies low oxygen conditions. The abundance of sulphur (in pyrite) also supports an interpretation of anoxic conditions, and low sedimentation rates within the shale layers. A restricted shallow basin or lagoonal environment is proposed for the palaeoenvironment, with fluctuating oxygen influencing diversity.

Morphosedimentological record and human settlements as indicators of West-African Late Holocene climate variations in the littoral zone of the Iwik peninsula (Banc d’Arguin – Mauritania)

Abstract The geomorphological, sedimentological and archaeological studies of coastal features in the Iwik-Aouatil area show two categories of coastal sedimentary units: very extensive sand flats and linear relief features covered by a thin veneer of anthropogenic Anadara senilis shell-middens. These units provide an opportunity to discuss the return to arid conditions in western Sahara during the Late Holocene, when climate changes induced the alternating occurrence of more or less dry episodes, from around 5 ka until today. These constructions reflect the global climate change within the general framework known for the region. Two different processes occurred: (1) constant deposition of wide sand-flats and (2) construction of sedimentary ridges at specific moments. These processes demonstrate the existence of (1) constant sand discharge during the whole period as sand availability is not a limiting factor and (2) a substantially increased sand influx during the 4.2-3.2 ka episode. Because of sea-level stability in the Banc d’Arguin area during the Late Holocene, the variability of morphogenetic agents is considered to be the likely cause of fluctuations, influencing sand input rate and implying that either sand was delivered in greater abundance (aridity crisis and stronger winds) or littoral drift was more powerful (stronger waves and more frequent storms in the northern part of the Atlantic ocean), or both simultaneously. The action of these two driving forces in the depositional processes observed in the Iwik-Aouatil coastal plain is evaluated with respect to the occurrence of episodes of rapid climate changes (RCCs) and North Atlantic Oscillation (NAO) changes. A climate pattern combining episodes of RCC and NAO positive (NAO+) index during the 4.5-3.2 ka period is convenient for both sand supply, which accelerates the southwestward migration of barchan’s lines, and sand transportation, which improves the efficiency of the littoral drift.

Holocene vegetation history of the Sahel: pollen, sedimentological and geochemical data from Jikariya Lake, north‐eastern Nigeria

AbstractAim This study aims to separate regional and local controls on Holocene vegetation development and examine how well pollen records reflect climate change in a semi‐arid region. The relative importance of climate and human activity as agents of vegetation change in the Sahel during the late Holocene is also considered.Location Jikariya Lake, an inter‐dune depression in the Manga Grasslands of north‐eastern Nigeria.Methods Pollen and charcoal were used to provide a record of Holocene vegetation history. Palaeoclimate and hydrological changes were reconstructed from sedimentary and geochemical data. Regional and local influences were separated by comparing the evidence obtained from Jikariya Lake with previously published data from the Manga Grasslands.Results The Manga Grasslands experienced a prolonged wet period during the early and mid‐Holocene, during which swamp forest vegetation with Guinean affinities (Alchornea, Syzygium, Uapaca) occupied the inter‐dune depressions. However, variation in the pollen records between sites suggests that their establishment was dependent on conditions being locally favourable, rather than being directly coupled to regional climate. The pollen records from the Manga Grasslands are more consistent in suggesting the colonization of the dunefields by trees associated with Sudanian savanna (Combretaceae, Detarium) c. 8700 cal. yr bp. The Jikariya Lake pollen data are in accordance with the sedimentological and geochemical data from the region in indicating that the onset of arid conditions occurred progressively during the late Holocene (from c. 4700 cal. yr bp). Abrupt changes in pollen stratigraphy, recorded at other Manga Grasslands sites 3500 cal. yr bp, appear to be the product of the local passing of ecological thresholds. The dunefield vegetation (Sahelian savanna) appears to have been resilient to (or at least palynologically silent regarding) to the climatic variability of the late Holocene.Main conclusions While climate appears to have been the primary control on vegetation development in the Manga Grasslands during the Holocene, local conditions (particularly depression size and sand influx) had a strong influence on the timing of pollen stratigraphic changes. Anthropogenic influences are difficult to detect, even during the late Holocene.

How Much Oil You Can Get From CHOPS

Abstract Cold Heavy Oil Production with Sand (CHOPS) has been applied with very good success to enhance heavy oil production in Canada, China, Venezuela and Kazakhstan. Based on existing laboratory and field information and case histories, the most important physical processes enhancing cold production have been reviewed, summarized and quantified. Several sanding models are developed, including a new porosity cap model for failure propagation, as well as a semi-analytical elastoplastic stress model coupled with an unsteady pressure model for foamy oils. The mechanisms for oil rate enhancement by CHOPS, such as porosity and permeability enhancement that arise from sand removal, propagation of the elastoplastic (remolded) zone, increase of oil velocity relative to the matrix and the effects of foamy oil behaviour, are quantitatively described and compared. The proposed model can be applied to predict how much additional oil one might expect for a given amount of produced sand. It might also serve as a tool for optimizing cold heavy oil production while nevertheless keeping the sand flux at a low level, which could reduce operating expenses such as limiting sand disposal costs. Introduction There may be more than 0.95 × 1012 m3 of heavy oil in the World(1), compared to 0.28 – 0.37 × 1012 m3 of conventional oil, of which over 40% has already been produced(2). Because of high viscosity, primary recovery factors for heavy oils are generally low; if the viscosity is higher than 10,000 – 20,000 cP in situ and the permeability is less than 5 Darcy, it appears that commercial recovery using any conventional non-thermal method is not possible. With careful design and implementation, various thermal recovery schemes can be effective, but high operational costs restrict their applicability. Though it has been long recognized that "...the maximum recovery of oil from an unconsolidated sand is directly dependent upon the maximum recovery of the sand itself...(3), " CHOPS was not widely implemented with commercial success until advanced pumping systems (such as the progressive cavity pumps) were perfected in the late 1980's for slurries containing sand. Since then, because of reasonable recovery factors (~15 – 20%), production rates (3.2 – 47.7 m3/day), effective sand handling and disposal and no heat costs, CHOPS has grown to provide more than 20% of Canada's oil. In 2002, Canada's oil production from all sources was approximately 460,000 m3/d, of which more than 95,400 m3/d was CHOPS production. Heavy oil reservoirs suitable for CHOPS are located in unconsolidated or weakly consolidated sands where sand mobilization can be easily triggered and sand influx sustained for the productive life of the well. Because of several unique characteristics of unconsolidated heavy oil reservoirs, well productivity may be 10 – 20 times higher in CHOPS wells than predicted by conventional Darcy's law flow equations(4). The mechanisms responsible for the enhanced production rate in CHOPS are(5):Porosity and permeability are enhanced as sand is removed from the formation, along with any mechanical skin that may have developed;The oil flow velocity relative to fixed coordinates is increased if the matrix is partially mobilized. Therefore, production rate increases, as predicted from Darcy's law;

Storminess variation during the last 6500 years as reconstructed from an ombrotrophic peat bog in Halland, southwest Sweden

AbstractCores taken from an ombrotrophic peat bog in the coastal zone of Halland, southwest Sweden, were examined for wind transported mineral grains, pollen and humidity indicators. The core covers the period from 6500 cal. yr BP to present. Ombrotrophic conditions existed from ca. 4200 cal. yr BP onwards. Bog surface wetness fluctuated strongly until ca. 3700 cal. yr BP, with an apparent dominance of dry summer conditions from 4800–4500 cal. yr BP. Local wet shifts occurred around 4300, 2800, 2400 and 1500 cal. yr BP, whereas the most recent 600 years of the record show increasingly dry conditions. Mineral grain content, interpreted as aeolian sand influx (ASI), was used as a proxy for (winter) storm frequency and intensity until ca. 1500 cal. yr BP, after which increasing human impact, as reconstructed by pollen analysis, became a second important potential cause for increased sand drift. Strongly increased storminess occurred at 4800, 4200, 2800–2200, 1500, 1100 and 400–50 cal. yr BP, indicating a dominance of cold and stormy winters during these periods. Many of these storm periods apparently coincide with storm events in other sites in southwestern Scandinavia, suggesting that our ASI record reflects a regional scale climatic signal. Furthermore these stormy periods correlate to well‐known cold phases in the North Atlantic region, suggesting a link to large‐scale fluctuations in atmospheric circulation patterns. Copyright © 2006 John Wiley & Sons, Ltd.