Soft Chalk Research Articles

The mosaic tesserae in the industrial Byzantine wine press, Yavne, Israel: A natural unusually hard chalk or a chemically transformed chalk?

Many of the surfaces of a large industrial scale wine press from the Byzantine period in Yavne, Israel, are covered in mosaic tesserae. These surfaces are part of the grape presses, storage pools, fermentation pools and also the walkways between these structures. The calcitic tesserae resemble limestone in color, texture and fracture properties. We were therefore surprised to discover that they are more disordered at the atomic level than consolidated limestone and in fact they are as disordered as porous and friable chalk. In addition SEM examination of fracture surfaces reveals abundant nannofossils, mostly coccoliths and foraminifera; properties that are also reminiscent of chalk. The nanofossil identifications shows that the tesserae are from the Middle Eocene, and this includes the chalky Maresha Formation that outcrops about 15kms from the site. SEM examination of polished surfaces shows that the larger pores of tesserae are lined with euhedral calcite crystals. Vickers hardness measurements show that the tesserae are much harder than chalk, and are comparable to limestone. There are two scenarios for explaining these seemingly contradictory observations. One scenario is that a chalky limestone geological formation with all these properties exists somewhere in the Mediterranean region and is the source of the raw material for the tesserae. The second scenario is that the Byzantine constructors of the wine press transformed a chalk into a harder and more consolidated material. Chalk transformation processes akin to plaster formation can be excluded based on the analyses of the stable carbon isotopic compositions of the tesserae, as well as their 14C concentrations. A chemical transformation using a weak acid that would only affect the calcite in the vicinity of the accessible pores, and leave the bulk of the chalk and its microfossils in a good state of preservation, is conceivable. In fact a by-product of producing wine is vinegar and the acetic acid of the vinegar could conceivably have been used for this purpose. If indeed a chemical transformation of soft chalk into a hard durable material was used, this would represent a “lost” technology that might even be useful in present times.

Enhancing Fracture Conductivity in Soft Chalk Formations With Diammonium Phosphate Treatment: A Study at High Temperature, Pressure, and Stresses

Summary This study aims to address the problem of fracture hydraulic conductivity decline in soft formations using a diammonium hydrogen phosphate (DAP) solution. A naturally weak carbonate, Austin chalk was chosen as an ideal specimen. Flat chalk samples with reduced elastic modulus and roughness were evaluated before and after aging with 1 M DAP for 72 hours at 75°C and 1,000 psi. The fracture gas conductivity of DAP-aged and untreated samples was measured at various flow rates and stresses while recording sample compaction using linear variable differential transformers (LVDTs). The study found that DAP aging increased the reduced elastic modulus of chalk specimens up to 330% of the original value, improving their resistance to deformation and failure under stress by 200 psi. The hydraulic conductivity of DAP-aged samples was at least twice that of untreated samples, with an extended hydraulic fracture conductivity seven times higher than that of the untreated ones. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis revealed that DAP reacted with the chalk to form hydroxyapatite (HAP), which binds the calcite grains, yielding a stiffer, more deformation-resisting rock surface. Overall, the study demonstrates the potential of chemically enhancing and extending the fracture hydraulic conductivity of weak carbonates using DAP.

“They Shall Come into the Hollows of the Earth” (Isa 2:19): Bar Kokhba-Period Hiding Complexes at Biblical Tels—Tel Lavnin as a Case Study

Hiding complexes in Judea have been objects of considerable scholarly interest since the 1970s. By now, we are well acquainted with their main features and spatial distribution. Most hiding complexes in the Judean foothills were cut beneath the houses in Jewish villages. They were entered via shafts carved out of the nari rock, leading to underground passages quarried in the soft chalk beneath. Following recent intensive looting at Tel Lavnin, a site located in ‘Adullam Park, south of the Ela Valley, inspectors of the Antiquities Theft Prevention Unit of the Israel Antiquities Authority documented three hiding complexes. In this paper, we present these hiding complexes and the objects discovered in them. We discuss these complexes’ special architectural features and ponder why particular architectural methods were chosen. We then compare the complexes of Tel Lavnin to complexes documented elsewhere in Judea. We propose that they constitute an architectural subtype of hiding complexes from the Bar Kokhba Revolt and predict that others like them will be discovered in the future.

Applications of well logging techniques to evaluate the groundwater aquifers in the area between southwest Bani Sweif and west Asyoute governorate, upper Egypt.

The relevance of detecting aquifer characterization and aquifer potential has risen with the application of well logging techniques as the demand for water has increased. Apart from pumping data, 16 geophysical well logs (resistivity, gamma ray, self-potential, and nuclear logs) are utilized to achieve the main goal of estimating petrophysical parameters (porosity (∅), effective porosity (∅𝑒𝑓𝑓), hydraulic conductivity (k), permeability (K) and shale volume (𝑉𝑠ℎ)) of the Apollonia (Eocene fractured limestone aquifer), Upper and Lower Bahariya (Nubian sandstone aquifer) formations. The findings show that the Eocene fractured limestone aquifer has a carbonate in soft chalk with high K and ∅ as well as high k of 4974.39 mD at well ST-188 (a highly productive aquifer), whereas the Lower Bahariya formation is mostly fine to medium-grained sandstone and clay, suggesting that it formed in shallow marine environments with low K. On the other hand, Upper Bahariya was noted that having poor sand succession. As results shown, K of the most examined wells ranges from 2609.69 to 1486.812 mD at ST-258 and ST-38 wells respectively. The study appears that 𝑉𝑠ℎ, along with ∅ and K features, is regarded as an important attribute in aquifer evaluation where, estimated 𝑉𝑠ℎ over the studied area is ranges between 5.96% and 31.7% at ST-48 and WBS1 wells respectively. Overall, this study provides an important insight into the importance of evaluating groundwater aquifers over time to help in making decisions to save and protect groundwater aquifers in the future.

Cellular Materials on Carbonate Rocks

The experimental studies’results of the structure and properties of cellular concrete with one-stage preparation of foam concrete mixtures are presented. The effect of the filler on the foam concrete properties has been evaluated. The possibility of producing high-quality chalk foam concrete from soft crushed chalk with a fraction of 2.5 mm has been proven. The influence of the slip viscosity, the type and amount of the foaming agent, the type of cement on the properties of small foam concrete has been established.

Water weakening of soft and stiff outcrop chalk induced by electrical double layer disjoining pressure

In this work we assessed the strength and stiffness of high and low porosity outcrop chalk plugs saturated with three different fluids. Plugs were triaxially tested in a Hoek cell at varying confining stress to probe different parts of the failure envelope as described in the p-q’ space (brittle, transition and yield failure modes). Stiffness of the plugs was derived from bulk density and P and S-wave ultrasonic velocities measured throughout the mechanical test. Selection of the saturating fluids was aimed at testing the hypothesis that water weakening of chalk arises due to a physico-chemical process acting on the vicinity of the chalk grain contact, the electrical double layer (EDL) disjoining pressure. This pressure depends both on the salinity and ionic species dissolved in the saturating water. Fluids used were designed to exert varying levels of disjoining pressure between the chalk grains: calcium chloride brine (highest), calcite equilibrated water (moderate) and as reference Isopar-L or Ethylene glycol (no induced disjoining pressure). We found that both strength and stiffness parameters are affected by the level of the EDL disjoining pressure: The higher the pressure between grains induced by the saturating fluid, the weaker and softer were the core plugs. Water related reduction in strength and stiffness were more prevalent in the soft chalk samples and for the samples mechanically tested at low confining stress. We attribute this selectivity to the smaller cementation area between grains (assessed by Biot's coefficient) for the softer chalk.

Experimental Investigation of Drilling Lateral Boreholes in Chalk Rocks with High-Pressure Jets

Increasing reservoir connectivity to the wellbore and bypassing the damaged area is crucial in improving the productivity of the wells and enhancing the swept area. This has become feasible by a new technology called radial jet drilling (RJD), in which relatively long, small-diameter laterals can be drilled radially from the main wellbore. In this study, the authors attempt to gain a better understanding of the efficiency of a high-velocity jet drilling on chalk destruction, and also identify parameters controlling the jet drilling. For this purpose, two distinct outcrop chalks from Austin, Texas (US) and Northern Province, Welton (UK) are used in this study, which are analogs to the reservoir chalk in the North Sea. In conjunction with the jet drilling experiments, basic rock mechanics testing is carried out in order to correlate the rock strength and stiffness properties to the jet drilling performance. Jet drilling of boreholes is evaluated not only by varying the fluid and nozzle type and the fluid pressure at the nozzle, but also varying the jet drilling setup under unconfined and also confined stress fields resembling reservoir condition. Results of our study show a clear correlation of the rock strength (and stiffness) on the threshold pressure and specific energy required to break the rock. Tight chalk requires more than 30% higher pump pressure than used in soft chalk for breaking the chalk, having more than twice the strength properties. Soft chalk presents larger borehole size and better rate of penetration, both with water and acid-aided fluid, owing to its higher matrix permeability value, as well as lower mechanical properties that favor diffusion of the jet drilling fluid into the rock and faster erosion/breakage compared with tight chalk. Static nozzles create a larger surface area compared with rotating nozzles. The penetration rate of the nozzle is improved significantly under stress confinement. In addition, jet drilling in the direction of minimum principal stress (σ3) appears to be faster owing to localization of shear failure around the drilled hole induced by the differential stresses compared with the jet drilling in the direction of maximum principal stress (σ1) under isotropic stress or ambient conditions.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ТЕПЛОВОГО И ВЕЩЕСТВЕННОГО ПОЛЕЙ НА ИЗМЕНЕНИЕ ФИЗИКО-МЕХАНИЧЕСКИХ ХАРАКТЕРИСТИК МЕЛА ДЛЯ ОЦЕНКИ ЕГО РАЗРАБАТЫВАЕМОСТИ

Experimental studies were performed and the dependences of the influence of thermal and material fields on the change in the following physical and mechanical characteristics of the chalk were established: density, the ultimate compressive strength, ultimate tensile strength, ultimate shear strength, elastic modulus, Poisson's ratio, and stickiness. The geomechanical and energy criteria characterizing the chalk workability are calculated: the coefficient of hardness, the indicator of the difficulty of rock destruction, the indicator of the difficulty of rock excavation, the energy intensity of digging and rock destruction. A classification of chalk by workability was developed, in which 5 categories of rocks are distinguished: I -soft chalk, of very low strength; II - dense semi-solid chalk, low strength; III - solid dense chalk of increased strength; IV - solid strong chalk; V - rocky chalk

Adaptive brachiopod morphologies in four key environments of the Late Cretaceous–Danian Chalk Sea of northern Europe: A comparative study

Rich brachiopod faunas are known from the four main, but highly different, palaeoenvironments in the Late Cretaceous–Danian Chalk Sea of northern Europe: Maastrichtian soft-bottom chalk (Møns Klint, Stevns Klint), lower Danian bryozoan mounds (Stevns Klint), middle Danian coral mounds (Faxe), all in the Danish Basin, and a late Campanian rocky shore (Ivö Klack) in the Kristianstad Basin, southern Sweden. There is almost no genus or species overlap between the rocky shore fauna and the three other faunas; the former is interpreted as endemic, having evolved in the small basin which is situated adjacent to the eastern end of the Danish Basin. The Maastrichtian and Danian faunas differ significantly at the species level, reflecting the mass extinction at the Cretaceous–Paleogene (K/Pg) boundary. In spite of having inhabited very different environments, the brachiopod faunas are remarkably similar in terms of modes of life and population structure, and by a marked predominance of micromorphic species. The main difference is the total absence in the Danian faunas of highly specialised, secondarily free-living hemispherical species, which are a characteristic element of the Maastrichtian chalk fauna. This adaptation was developed in several unrelated taxonomic groups during the c. 40-Myr-long existence of the Late Cretaceous interval of the Chalk Sea. No species of this group survived the K/Pg mass extinction. Identification of brachiopod morphogroups reported as characterising different environments is thus successful for the rocky shore environment, but tends to break down between the soft chalk sea floor and the bryozoan or coral mounds. This is interpreted to reflect the availability of a multitude of small, hard substrates, notably slender fragments of bryozoans, in all the three of the last-named environments and by the fact that the survivors of the mass extinction were represented mainly by micromorphic species in addition to a few generalists from which the new, Danian fauna rapidly evolved. Three new major environments, each characterised by distinct brachiopod morphotypes, are recognised in addition to the seven described previously in the literature, namely the sublittoral chalk sea floor, the bryozoan mounds, and the cool and deep-water coral mounds. The rocky shore may be considered as representing a fourth new distinct environment.

Function‐First Exposure to the Wrist and Forearm Musculature

The objective of the present study was to assess whether a more functional presentation of the initial exposure of the anatomy of the forearm leads to greater student satisfaction and improved assessment performance. Traditionally, medical curricula and medical texts take a structural approach to teaching anatomy; this is particularly true for the study of the muscles. These approaches generally present the muscles in terms of layers and compartments. This method, despite its long‐standing reputation as effective, may not be the best approach for all students. Students may gain a deeper understanding of muscle structure and function after being taught from a more functional approach during their initial exposure, i.e. when it is linked more strongly to clinically‐relevant function, not just proximal and distal attachments, innervation, layers or compartment in which it resides. This study attempts to elucidate the differences on the understanding of human forearm anatomy, if any, between a traditional layer‐based approach and a function‐based approach. First‐ and second‐year medical students at the Oakland University William Beaumont School of Medicine (OUWB) were invited to participate in the evaluation of a control and experimental forearm anatomy learning module. After random assignment to one of the Soft Chalk online modules, participants completed a pre‐test, interacted with the module, participated in a post‐test and completed a usability study. The control group was exposed to typical layer‐based anatomy of the forearm, while the functional group was presented with a function‐based approach to the same anatomy. There were a total of 19 volunteer participants, seven in the functional group and twelve in the traditional group. Analysis using a 2‐Sample T‐Test and within groups using a Paired T‐Test indicates an insignificant difference in outcomes on the post‐test between the groups (p=0.765). Usability study results indicate that compared to the traditional group, the functional group found the module easier to navigate. Participants were also pleased with the clarity provided by the manner in which the wrist and associated musculature was presented relative to the more traditional, layer‐based presentation. Although the number of participants was not enough to produce significant results, preliminary trends would seem to support the notion that novel, ‘function‐first’ approaches to anatomy presentation may benefit students and enhance retention of material. Ideally, such results will encourage medical educators to explore alternative teaching methods to best serve their students.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Late Saalian and Weichselian glaciations in the German Baltic Sea documented by Pleistocene successions at the southeastern margin of the Arkona Basin

Seismic profiles and numerous sediment cores of a planned offshore wind farm area northeast of Rügen Island provide a unique insight into the distribution and thickness of glacial deposits in the southern Baltic Sea. Fine gravel analysis enabled recognition of five tills that correspond to different ice advances. On the basis of a 3D structural model, the main Pleistocene events could be reconstructed and correlated with stratigraphical records in the circum‐Baltic area comprising Marine Isotope Stages (MIS) 6 to 2. The oldest till was deposited during the Late Saalian Warthe advance (qs2; c. 150–130 ka). It is only preserved in ENE–WSW trending subglacial channels incised into Upper Cretaceous soft chalk. Isolated rafts and remnants of two Middle Weichselian tills occur locally, whereas Eemian interglacial sediments are missing. The older till (qw*) has an unusual fine gravel composition and might have formed during ice advances known only from northern Denmark and northern Poland (c. 65–60 ka). The younger till represents the Warnow advance (qw0) in NE Germany that can be correlated with the Danish Ristinge advance (c. 55–50 ka). Furthermore, two tills deposited during the Late Weichselian Last Glacial Maximum (c. 24–16 ka) occur. One can be correlated with the Pomeranian advance (qw2), while the other has characteristics typical for the Mecklenburg advance (qw3). These advances eroded older Pleistocene deposits, thus probably also removing the till of the Brandenburg/Frankfurt advance (qw1) – in contrast to the onshore areas. The Pomeranian till is limited to the southeast, close to the shallow Adlergrund, but the Mecklenburg till covers the whole study area. This first detailed lithostratigraphical classification of till units from the Arkona Basin allows for the identification and correlation of Pleistocene successions in other parts of the southern Baltic Sea.

Hydrogeophysical characterization of the porous and fractured media (chalk aquifer in the Beauvais, France)

This study is aimed to investigate the principal aquifer in the northern part of Paris (Beauvais region) which is materialized by the Senonian chalk deposits. This aquifer supplies water to the principal region in the north and in particular the Picardie and Artois region. In order to characterize the chalk aquifer of this area, a wide pluridisciplinary research program based on the well logging and near surface geophysics surveys (seismic refraction and electrical resistivity tomography) is in progress. These surveys were carried out within the experimental site of the LaSalle Beauvais Polytechnic Institute and its hydrogeological boreholes. The heterogeneous distribution of the seismic velocities in the complex aquifer and of the physico-chemical properties (conductivity, temperature and gamma-ray) of the chalk aquifer were revealed by the analysis and the interpretation of the seismic lines and data logging, which are linked to changes in lithology and water properties at the place of the hydrogeological wells. The fracturation investigation identified during this work makes it possible to consider the relationship between the degree of the fracturation and the variation of the hydrogeological parameters (hydraulic conductivity, porosity) and physico-chemical parameters (electrical conductivity and temperature). The interface between vadose and saturated zones was assessed by the measurement of the water conductivity, the variation of the average temperature and by the cuttings analysis in particular at the level of the capillary fringe of the chalk aquifer. The presence of the saturated depth is confirmed by the piezometric investigation. The interpretation of the well logging by using gamma-ray is consistent with the seismic and electrical analysis: the three principal layers are composed of silt and clay with cherts, soft chalk, as well as hard chalk.

156 Use of Soft Chalk technology in equine science classes to provide an interactive study tool

156 Use of Soft Chalk technology in equine science classes to provide an interactive study tool

Royston Cave: an holistic approach to conservation

Cut into the soft chalk beneath the market town of Royston in Hertfordshire, Royston Cave is decorated with religious and mystical low relief carvings, probably dating to the late fourteenth century. Discovered in the eighteenth century, the carvings have been deteriorating for most of their recorded recent history and key areas are at risk of being lost. A conservation programme has been undertaken to identify and control the underlying causes of deterioration and to develop conservation measures. Work has included examination of the cave structure and materials, a study of the below ground water sources and drainage repairs, investigations into the microclimate and measures to stabilise conditions, assessment of micro and macro biological growth with interventions to control damage, as well as conservation treatment to stabilise carvings and prevent imminent loss. This case study presents a summary of how conservation projects can require new and different project coordination skills to bring together professionals from different fields for a successful outcome.

Clay mineral-grain size-calcite cement relationships in the Upper Cretaceous Chalk, UK: a preliminary investigation

AbstractThe idea is tested that the evolution of the Chalk’s clay mineral assemblage during diagenesis can be deduced by examining the relationships between its clay mineralogy, particle size distribution pattern, and the timing and trace element chemistry of the calcite cement. The preliminary results from five different examples of cementation developed at different stages of diagenesis in chalks with smectite-dominated clay assemblages suggest that this is a promising line of investigation. Soft chalks with minor amount of anoxic series calcite cement poor in Mg, Fe and Mn are associated with neoformed trioctahedral smectite and/or dioctahedral nontronite and talc. Hard ground chalk with extensive anoxic series calcite cement enriched in Mg and relatively high Fe, Mn and Sr are associated with neoformed glauconitesensu lato, berthierine and dioctahedral smectite, possibly enriched in Fe. The chalk associated with large ammonites shows extensive suboxic series calcite cement enriched in Mg, Mn and Fe that show no obvious correlation with its clay mineralogy. Nodular chalks with patchy suboxic series calcite cement enriched in Fe are associated with neoformed dioctahedral smectite, possibly enriched in Al, and berthierine. Regionally hardened chalk with extensive suboxic calcite cement and relatively high trace element contents contain pressure dissolution seams enriched in kaolin and berthierine. Laser-based particle-size distribution patterns suggest that each type of lithification has a typical complex clay mineral population, indicating that subtleties in mineralogy are not being identified and that there could be some control on the size and shape of the clay crystals by the different types of cementation.

Cliff collapse at Stevns Klint, south-east Denmark

The scenic coastal cliff of Stevns Klint is a classical study locality that stretches for 15 km along the east coast of Sjælland and it holds arguably the best exposed Cretaceous−Tertiary boundary in the world (Fig. 1; Damholt & Surlyk 2012). The famous boundary separates the soft Cretaceous chalk from the harder overlying Tertiary bryozoan limestone (Fig. 2), and the difference between the two rock types controls the character of the frequent cliff falls. The relatively soft chalk at the base is eroded by storm waves and is subject to general debris shedding. The overlying bryozoan limestone, with its hardgrounds and flint layers, is more resistant to erosion and is strong enough to form overhanging projections of the coastal cliff that result in large and small recurring collapses.

Chondrichthyans from the Menuha Formation (Late Cretaceous: Santonian–Early Campanian) of the Makhtesh Ramon region, southern Israel

Exposures of the Menuha Formation (Santonian–Early Campanian, Mount Scopus Group) in the Makhtesh Ramon region of the southern Negev have produced numerous chondrichthyan teeth. The isolated teeth represent at least ten different species: Cretalamna appendiculata, Cretoxyrhina mantelli, Squalicorax falcatus?, S. kaupi, Scapanorhynchus rapax, S. raphiodon?, Carcharias samhammeri, Carcharias cf. C. holmdelensis?, and two other fish (Hadrodus priscus and a pycnodont). This assemblage has important implications for Late Cretaceous chondrichthyan palaeobiogeography. The majority of teeth were contained within a glauconite-rich, yellow-brown, soft chalk that included oysters (Pycnodonte vesicularis?), trace fossils (Planolites, Thalassinoides, and Chondrites), phosphatic peloids, and foraminiferans (globigerinids). The teeth were collected mainly through surface-sampling and sieving. The Menuha Formation probably represents a temperate to subtropical, shallow, open-shelf environment deposited during the formation of the Ramon anticline. Reworked conglomeratic chalks in the western section represent marginal facies derived from this structural uplift. With little to no published material describing the chondrichthyan fauna of the Menuha Formation, these data improve interpretations of its palaeoenvironment. Interpretation of the palaeoenvironment of the formation is important for understanding the larger stratigraphic/tectonic framework of the Ramon monocline region of southern Israel.

Zones of required investigation for earthquake-related hazards in Jerusalem

Jerusalem was hit by earthquakes several times in its history, in the course of which none of the holy sites of the three main faiths of the western world escaped damage. Intensities of the last ML 6.2, July 11, 1927 Dead Sea earthquake, reached MSK VIII in the Old City of Jerusalem and the surrounding villages. As future strong earthquakes are inevitable, the need for the evaluation of earthquake-related hazards is obvious. Only general geotechnical properties of the section exposed in the mountainous area of Jerusalem are available; therefore, the hazard assessment was conducted from a geological perspective. The hazards identified in this study are: (1) amplification of seismic acceleration due to soft rock and soil conditions; (2) amplification due to mountainous topography; (3) dynamic instability of natural slopes; and (4) potential failure of slopes that have undergone engineering development and were weakened due to damaging, steepening, overloading, and wetting beyond their natural state. We formulated relative grades of vulnerability for each of the hazards and delineated the zones that require further specific investigation. For practical use we constructed a summary map that combines the different hazard categories. Looking at the summary map, the ground at the central N–S axis zone across Jerusalem is the least vulnerable. The bedrock there is mostly hard carbonate, the topography is mild, and thus only the alluvial cover, if thicker than 3 m, should be considered sensitive. Yet although the natural hazard in this area is limited, the risk should not be underrated. Much of the city lies there, including buildings constructed before antiseismic codes were regulated, and traditional engineering practice should not be taken for granted as antiseismic proof either. Eastwards, the shear wave velocity (Vs) contrast between the hard and soft rocks as well as the notable topography in places, impose the potential for amplification. Slopes, either naturally or artificially cutting into the soft chalk, may expose the area to dynamic instability; thus, the ongoing extensive development of the city in this direction should certainly take into account all of this. West of the central axis, the potential of failure of both steep natural and urbanized slopes appears. Being a plausible direction for future urban expansion, these areas specifically call for careful environmental and engineering planning. For engineering purposes, however, a specific site investigation is still necessary. Nevertheless, the summary map established in this study sets up for Jerusalem, for the first time, a practical tool for environmental and municipal planning, emergency response planning, and civil protection.

Horizontal-Well Completion and Stimulation Techniques - Review Emphasizing Low-Permeability Carbonates

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 108075, "Horizontal- Well Completion and Stimulation Techniques - A Review With Emphasis on Low-Permeability Carbonates," by Valdo Ferreira Rodrigues, SPE, and Luis Fernando Neumann, SPE, Petroleo Brasileiro S.A.; Daniel Torres, SPE, Halliburton Energy Services; Cesar Guimaraes, SPE, Schlumberger; and Ricardo Sadovski Torres, BJ Services, prepared for the 2007 SPE Latin American and Caribbean Petroleum Engineering Conference, Buenos Aires, 15–18 April. A brief review is presented of available techniques to complete and stimulate horizontal wells, emphasizing low-permeability carbonate reservoirs. These techniques can be applied in unconventional reservoirs, particularly in tight formations. Considerations are presented to select completion and stimulation techniques. Cased and cemented horizontal wells, in addition to wells completed as open hole and with perforated/slotted liners, are addressed. Introduction This study focused on fracturing stimulations. It assumed that a horizontal well already is justified and that its completion and stimulation are being discussed. The macroaspects of field/area management are stressed as the completion and stimulation drivers. Key parameters for designing, evaluating, and implementing horizontal-well completion and stimulation are presented, emphasizing common failures and controversial aspects. North Sea Carbonate Reservoirs The North Sea low-permeability chalk reservoirs are used as a reference. The Dan, Halfdan, South Arne, Valhall, and Eldfisk fields are the focus. The main characteristics of these fields include shallow water (43 to 69 m), dry completions, high volumes of original oil in place (1.6 to 2.9 billion bbl), low-permeability carbonates (0.2 to 10 md) with microfractures in the central areas (10 to 120 md), high porosities (up to 48%), soft to very soft chalks, small to medium net-pay thickness (15 to 65 m), high oil saturation (up to 97%), and light oils (approximately 36°API). These fields are distinguished from most by overpressured soft chalks, which are subjected to a high degree of compaction under pore-pressure depletion, resulting in loss of drilling fluids, rapid production decline, well failures, and seafloor subsidence. However, the positive effects of rock compaction as reservoir-drive energy outweigh the negative effects, with the primary recovery factor as high as 30%. In general, North Sea chalks experienced an evolution from vertical and directional wells stimulated with acid treatments to multiple-fractured horizontal wells. Campos Basin Carbonate Reservoirs The Campos basin, offshore Brazil, was discovered in 1974, and first production was by an early-production system in August 1977. The history of low-permeability carbonate-reservoir completion and stimulation in this basin is similar to that of North Sea counterparts. In the 1980s, the completions were vertical or directional wells stimulated with massive acid fracturing or matrix acidizing. Rapid production decline was the main problem in this period. In 1989, a very large hydraulic-fracturing treatment was performed in a subsea directional well on the basis of a comprehensive study. Unfortunately, a previously unidentified fault very close to the well took most of the pumped proppant. From 1990 to 1995, stimulations were characterized by low-volume matrix treatments (up to 75 gal/ft of 15% HCl) with similar results. Beginning in 1995, with the drilling and completion (with preperforated liner) of the first horizontal well in the Quissamã formation, standard stimulation treatments comprised alternating stages of nonviscous acid, viscous acid (up to 30 gal/ft of 15% HCl), and acid with a diversion feature pumped below fracture rates, which were spotted by use of a dedicated stimulating string or coiled tubing inside the horizontal liner.

Productivity Preservation Through Hydraulic Propped Fractures in the Eldfisk North Sea Chalk Field

Summary The Eldfisk oil field is a high-porosity, low-permeability reservoir of soft chalk in the Norwegian North Sea. Commercial oil production from the Eldfisk field is dependent on well stimulations. To date, pseudolimited-entry acid stimulation (PLEAS) has been the standard completion technique. This technique initiates relatively high (5,000 BOPD) individual-well oil-production rates. Over time, however, many of the Eldfisk wells have experienced rapidly declining oil production characterized by wellbore skin increase and fracture closure. Solids production and casing collapse have also been problematic. In late 1997, an alternative completion technique involving four hydraulic propped-fracture stages was successfully executed in an Eldfisk horizontal well. This paper presents a simple methodology to evaluate sustained productivity enhancement of propped fractures vs. acid stimulations in soft chalk formations based on the use of analytical modeling in combination with empirical production-data analysis.