Weak Sand Research Articles

Experimental studies on interference of two angular footings resting on surface of two-layer cohesionless soil deposit

The paper presents an experimental study pertaining to the interference of two closely spaced small scaled model square and rectangular footings, each of two different sizes, and resting on the surface of a two-layered sand deposit. The effect of the parameters such as the depth of the upper weak sand layer D and the clear spacing between the footings S on the bearing capacity and settlement behavior of the interacting footings were studied. The study indicates that the bearing capacity and settlement values of the closely spaced footings on the layered deposit decrease with increase in D/B ratio, where B is the width of footing. The results are presented in terms of efficiency and settlement factors, ξγ and ξδ, and their variations with S/B ratio are presented. The present experimental observations are generally found to be in good agreement with similar studies available in the literature.

ダイレイタンシーを考慮したCIP法による砂層における断層関連撓曲の発生シミュレーション

An active fault that develops in a plain is often covered with a weak sand layer. The large-scale fault slip in a bedrock may spread to the surface and will frequently generate a fault rupture. Depending on the case, the fault slip occasionally appears on the surface as a fault flexure. This study identifies the reason for the formation of a fault flexure on the sand layer using numerical simulation. The simulation technique employs the constrained interpolation profile (CIP) method a new technique that overcomes the flaws such as numerical diffusion in the finite difference method. Furthermore, we study the influence of fault parameters on the configuration of the fault-related flexure. The experimental results indicate that the fault-related flexure is generated when a shear-zone migration occurs in the sand. We ascertain that the velocity of the shear-zone migration is proportional to the velocity of the fault slip. As a result, we conclude that this migration is probably one of causes of fault-related flexure generation at faulting. Therefore, we conclude that the velocity and acceleration of the fault slip affect the configuration of the fault-related flexure.

A preliminary experimental study on sand production under true triaxial stress conditions

Sand production prediction is becoming a regular study in reservoirs with weak or unconsolidated sands. Three main approaches for sanding prediction are analytical, numerical and experimental methods. Laboratory experiments have proven to provide more realistic results, with these being used to understand sanding mechanisms and validate analytical and numerical methods. A large number of experimental studies have been carried out by researchers worldwide—most of which have been performed on cylindrical-shape samples under uniaxial (i.e. σ1 ≠ 0, σ2 = σ3 = 0) or triaxial (i.e. σ1 ≠ 0, σ2 = σ3 ≠ 0) stress conditions. In general, a sanding experiment under true-triaxial stresses (i.e. σ1 ≠ σ2 ≠ σ3 ≠ 0) is more realistic in simulating downhole conditions. This stress condition can be simulated in the laboratory on a cubic sample. The first part of this paper provides a comprehensive but brief literature review on past sanding laboratory experiments. This will be followed by the introducition of a unique true-triaxial stress cell (TTSC) which was modified and used for sanding simulations in the laboratory. The applied modifications will be illustrated and the test procedure described. The sample preparation for testing synthetic samples will be explained and some preliminary results obtained will be presented.

Rock engineering systems adopted for sanding prediction in perforation tunnels

Sand production is an important issue in reservoirs with weak or unconsolidated sand formations. Production of sand not only causes several problems in maintaining wellbore integrity but also is a problem during production where damages through the tubing and surface facilities are likely to occur due to the sand grains being transported along this path. The rock engineering systems (RES), initially introduced in mining and civil related geomechanics problems, is one approach to analysing the interrelationship between different parameters involved in a rock engineering project. This is the approach that was adopted in this work to study and predict the sanding potential in perforation tunnels. Sanding mechanism in perforation tunnels during production was reviewed and all effective parameters were identified. An interaction matrix was introduced to study the sanding mechanism through the interrelation between pairs of parameters. The interaction matrix was coded using a semi-quantitative rating approach to determine the interaction between each pair of parameters. The interaction intensity and dominance of each parameter in the system were studied through the cause-effect diagram to classify the parameters. This will assist in finding a better engineering action to mitigate or eliminate instabilities. A sensitivity analysis was conducted on a data set, and major parameters playing in sand production in a perforation tunnel were identified using analytical formulae. The results of sensitivity analysis were compared with the cause-effect diagram derived from the interaction matrix. A good agreement between the two methods was observed. This shows the usefulness of RES for identifying potential sanding solutions through the interaction matrix analysis.

Through-Tubing Completions Maximize Production

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 92392, "Unique ‘Through Tubing’ Completions Maximize Production and Flexibility," by M.O. Johnson, SPE, and P.G. Hyatt, SPE, BP plc; T.O. Stagg, SPE, Orbis Engineering; and L.L. Gantt, SPE, ConocoPhillips, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23–25 February. The challenge with through-tubing side-track completions has been dealing with small clearances and the lack of ready-made equipment. Over the last 10 years and after more than 450 coiled-tubing (CT) -drilling sidetracks on the North Slope of Alaska, a number of unique completion designs have been developed to maximize production and achieve vertical/zonal-isolation. Introduction Low-cost reservoir access is a key component to sustaining production from maturing fields. CT drilling (CTD) and through-tubing rotary drilling (TTRD) can achieve significant cost savings by sidetracking through existing production tubing. However, the critical completion phase of these sidetracks is challenged by small clearances and custom equipment. During the course of completing more than 450 CTD sidetracks through 4 1/2-in. and 3 1/2-in. production tubing in Alaska, a number of innovative completion designs have been developed to maximize production, achieve zonal isolation, allow for selective multilateral production, and preserve the parent wellbore for additional sidetrack opportunities. Continuous innovation and close collaboration with the service industry has yielded successful solutions for challenging through-tubing completions. These proven techniques have made CTD the preferred method for re-entry sidetracks on the North Slope. The completion options discussed in the full-length paper may make low-cost through-tubing sidetracks more feasible for other mature fields. CTD Applications Reservoirs on the North Slope range in depth from 8,600 to 9,200 ft true vertical depth (TVD) and have a combination of gas-cap and waterflood/aquifer drive. Most sidetrack completions are required to provide vertical and zonal isolation with a cemented liner. Exceptions occur in predominantly waterflood areas or in shallower viscous-oil sands (4,000 to 7,000 ft TVD). In these areas, a simple slotted liner to hold open weak sands and shales within the reservoir often is appropriate. CTD has been successful in both types of reservoirs by sidetracking away from depleted bottomhole locations and targeting areas of undrained oil to increase oil rate and reserves recovery. North Slope CTD typically drills a sidetrack 2,500 ft measured depth (MD) in length that includes a short high-dogleg-severity build section followed by a precisely placed horizontal lateral. A new world-record sidetrack depth of 17,515 ft MD was reached with CTD in March of 2004. By drilling through tubing to save decompletion/recompletion cost and with high-build-rate capability to stay within the reservoir, CTD is able to achieve a 30% cost savings compared to an equivalent rotary-rig sidetrack. The continuous pipe also is well suited for drilling in severe-lost-circulation environments and in underbalanced mode.

DEM analysis of crushing around driven piles in granular materials

The capacity of driven piles in breakable weak sands is less than that of similar piles in strong sands. Numerical simulations using the PFC2D program, based on discrete element method (DEM), were used to investigate the penetration resistance of driven piles. The penetration resistance for an unbreakable granular material and a crushable granular material was examined. A soil composed of weak granular particles had a lower penetration resistance than a similar soil composed of uncrushab]e particles. Particle crushing caused the production of particles of different sizes. Breakage and particle rearrangement induced stress relaxation and caused the broken granular material to develop a lower resistance to pile penetration. Including a simplified tensile failure criterion in the DEM allowed crushing around piles to be analysed and visualised.

Prediction and Evaluation of Sanding and Casing Deformation in a GOM-Shelf Well

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Sand production is an important well-completion issue affecting well economics and, possibly, casing stability. This study compares predictions of sanding and casing deformation for a well in the U.S. Gulf of Mexico (GOM) shelf prepared before completion of the well with the results of a post-sanding event for this well. The sanding event was predicted adequately considering the uncertainty of the data entered into the model. The casing deformation monitored by the post-sanding survey also was consistent with the prediction of helical buckling caused by depletion of fluids only when cavities of vertical extent greater than 7 ft exist (e.g., because of sand production) around the wellbore. This study supports the use of these predictive methods in well-completion design. Introduction The driver for this study is a concern that sand production from GOM wells in weak sands may lead to casing failure. In Ekofisk, for example, casing failures caused by compaction have been studied, but these failures were not caused by solids production. There have been surprisingly few papers written about casing failures correlated with sand production. One is the Canadian heavy-oil-sands work, although it addresses massive amounts of sand produced (thousands of barrels). For this paper, we found an opportunity to assemble different aspects that had been studied separately into a more complete technical analysis. The subject has gained importance recently because of a renewed interest in managing sand production from wells rather than installing costly gravel packs. Oil and/or gas production in weakly consolidated sandstone formations can lead to sand production when the reservoir pressure is depleted to the point at which the effective stress on the near-wellbore region exceeds the formation disaggregation strength. Sand production can lead to costly wellbore-cleanup operations, erosion damage to tubing and surface equipment, and significant sand-handling expenses. Thus, the prediction of sanding is an important design consideration when deciding whether a well should be completed with sand-control measures. This Technology Today Series paper is based on paper 78236 prepared for the 2002 SPE/ISRM Rock Mechanics Conference.

Application of Threadlike Micellar Solution for Civil Engineering Usages

Utilizing the thickening ability of threadlike micellar solution, which exhibits Maxwell-type viscoelastic behavior, we have developed a new thickening agent VISCOTOP for civil engineering usages. In contrast to usual thickening agent, VISCOTOP is able to increase viscosity of fresh concrete mixture without hardening retardation. In addition, we have found that VISCOTOP has various potential abilities in civil engineering usages because of its gel-like properties produced in the water. The cement slurry protected with VISCOTOP has unique properties; it can be poured into water without diffusion; it does not permeate into porous ground. The elastic property of VISCOTOP solution protects falling down of weak sand wall.

Experimental investigation and numerical analysis of compaction behaviour of moulding sand

The compaction behaviour of green sand was studied to assess the applicability of the Cooper–Eaton equation under conditions close to those of real foundry operations. The results indicate that the Cooper–Eaton equation describes the behaviour reasonably well, allowing prediction of the pressure required to achieve sand moulds of reasonable density. A corresponding distinct element method numerical simulation was also attempted. The study contributes useful information to the understanding of green sand compaction. It is clear that spherical sand is preferable to improve filling behaviour and that the gap between mould wall and pattern must be sufficiently large to avoid prevent weak sand compaction.

Zum verbleib des düngemittelstickstoffs im verlaufe der winterroggen-vegetation: For the comparison of nitrogen fertilizer in the course of winter rye vegetation

In the Static Nutrient Deficiency Trial Thyrow (weak silty sand) 3 trial blocks were selected: without fertilizer, NPK + lime and NPK + lime + manure. They differ in nutrient content and graduate themselves considerably in the content of organic soil substance. In the field plots there are micro plots with the nitrogen stages 60, 120 and 160 kg ha− 1, as the 15N marked ammoniumsulfat becomes incorporated. The nitrogen fertilizer of the first input only migrated 0 – 10 cm deep into the layer of the ground. Next to the plant N-intake it is defeated considerably by an apparent nitrogen immobilisation in the soil. The second nitrogen input is directly received by the plant and is affected by a nitrogen establishment; as it is a mineral nitrogen, also in the upper crumb lay, hard to prove. Against that the soil-born nitrogen is distributed above the depth, here until 60 cm, and becomes increasingly assimilated in the second vegetation half by rye. The yield increase during the vegetation, corn and straw yield for the harvest time follows the soil quality (trial blocks) as well as the actual fertilizer use. The nitrogen from the fertilizer pool corresponds a polynom 2, degrees, while the nitrogen in the soil is subjected in the nitrogen intake of a linear function. For the harvest the nitrogen uptake (grain and straw) form both, soil and fertilizer-N, in the variant NPK + lime + manure is the biggest.

Description of fluid flow around a wellbore with stress-dependent porosity and permeability

Stress-dependent porosity and permeability effects have been widely studied at the laboratory scale, as they can significantly affect reserve estimates, well production rate and profitability. Based on current experimental data and theories, a general analytical method of calculating stress-dependent porosity and permeability is developed and applied to a wellbore producing oil from unconsolidated or weakly consolidated sand, with the aid of a coupled geomechanical model by which stress distributions around the wellbore can be specified. For clean weak sand, nonlinear elastic theory is appropriate for calculations of stress-dependent rock properties such as compressibility, porosity and permeability. When evaluated in terms of pore pressure variations, the stress-dependent aspect of porosity and permeability may be negligible as far as stress analysis concerned. With input of different stress-compressibility relationships, the model can be used to help screen those reservoirs for which the effect of stress on permeability should be considered during geomechanical analysis (sand production prediction, reservoir stress arching and shear, plasticity onset, etc.). Also, it can be used for analyzing formation compaction that results from the decrease of stress-dependent porosity. The model limitations have been discussed and it is believed that a microscopic approach based on particulate mechanics may be valuable for future research. Different boundary conditions commonly used in current geomechanics models have been compared and discussed in the development of the poro-inelastic geomechanics model, and boundary restraint is demonstrated to be a critical factor to stress solutions.

Anatomy of a ‘fossil’ landslide from the Pleistocene of SE Spain

Pleistocene landslide failures are rarely recorded in the literature due to difficulties in recognition in the field. An adaptation of standard landslide classification based on activity is used to identify and examine the significance of a ‘fossil’ Pleistocene landslide from the Mocatán catchment of the Sorbas Basin, SE Spain. Recognition of the landslide is based on identifying key, but subtle anomalies in the geology and drainage. The fossil landslide at Mocatán developed principally in response to a rapid increase in valley incision rates associated with capture-induced base-level changes in the master drainage. This was facilitated by the weak sand and silt lithology of the study area. The landslide was a significant morphological modifier of the environment. Holocene drainage follows the original landslide morphology, whilst more recent gullying and piping exploits planes of weakness developed within the internal body of the landslide feature. In addition, the landslide removed much of the Pleistocene divide, facilitating future river capture.

Perforating Unconsolidated Sands: An Experimental and Theoretical Investigation

Summary This paper describes the experiments and modeling undertaken to develop a perforating strategy for unconsolidated sand formations. The experimental program consists of single-shot perforations in cylindrical samples of three sizes (4-, 7-, and 10.5-in. diameters) of unconsolidated sand saturated with kerosene at residual brine saturation and placed under a high effective stress in a pressure vessel. The amount of sand produced during perforating, as well as that produced during post-shot flow, is measured, and a video probe is used to observe post-shot sand production. Computerized tomography (CT) scan equipment is used to map the geometry of the cavity. The sand features behind the casing are examined and interpreted, providing essential insights into the interaction of the charge with the sand sample. A theoretical model of the failure mode of unconsolidated sands has been developed that supports the observations. The main results so far from this study are as follows. Perforating does not generate a tunnel in unconsolidated sands. The intense pressures set up by shaped-charge penetration in the weak sand sample fluidize it and cause the initial tunnel to collapse. Production flow creates a dilated zone around the tunnel entrance. At a critical flow rate, a large volume of sand is produced and the sample collapses. This occurs when flowing out of a hole in an unperforated sample as well. The implications of these results for future work and for field applications are also addressed here.

Bearing capacity of foundations on a weak sand layer overlying a strong deposit. Technical note : Hanna, A M Can Geotech J, V19, N3, Aug 1982, P392–396

Bearing capacity of foundations on a weak sand layer overlying a strong deposit. Technical note : Hanna, A M Can Geotech J, V19, N3, Aug 1982, P392–396

Method for identifying weak sands

A method and apparatus for obtaining improved measurements of shear waves in low-velocity lossy formations surrounding a borehole. The method comprises forming an interface between the borehole wall and a solid homogeneous material having a lower velocity than the formation. The geometry and velocity of the homogeneous material provide enhancement of the shear waves at the interface and thereby minimize the effect of interference from undesired compressional waves. Further improvement is obtained by using shear waves whose particles move perpendicular to the travel of the wave, parallel to the plane of the waves and perpendicular to the surface of the material. These are commonly referred to as SV shear waves.

Bearing capacity of foundations on a weak sand layer overlying a strong deposit

The ultimate bearing capacity of footings resting on subsoils consisting of a weak sand layer overlying a strong deposit has been investigated. Based on model tests of strip and circular footings in a loose or compact sand layer overlying a dense sand deposit, the classical equation of bearing capacity of footings on homogeneous sand was extended to cover cases of these footings in layered sands where the upper layer is the weaker. The theory compared well with the available model test results. Design charts are presented.

Foundations on strong sand overlying weak sand : Hanna, A M J Geotech Engng Div ASCE, V107, NGT7, July 1981, P915–927

Foundations on strong sand overlying weak sand : Hanna, A M J Geotech Engng Div ASCE, V107, NGT7, July 1981, P915–927

Foundations on Strong Sand Overlying Weak Sand

The ultimate bearing capacity of footings on subsoil consisting of a strong sand layer overlying a weak sand deposit was investigated. A theory was developed and then results predicted. These predictions compared well with test data on model strip and circular footings resting on dense sand overlying loose, as well as, compact sand. Design charts are presented for use in practical application. Based on these charts, a procedure to determine the ultimate bearing capacity of a strip and circular footings on a strong sand layer overlying a weak sand deposit is described.

Tunnels In Oil Sand

Abstract Deep oil sand deposits may require shafts and tunnels either as access for in-situ oil methods or for direct mining, Only one tunnel, Saline Creek tunnel, has been constructed in oil sand to date (1978). Recent research has made significant contributions to the understanding of oil sand behaviour, but empirical design methods may still have to be used until more experience is gained. Mathematical relationships show some promise in providing a framework for assessing stand-up time and the effects that in-situ recovery processes, such as steam drive, may have. Introduction There are several projects which are being considered for the feasibility or mining oil sand where the depth of overburden exceeds economical mining by open-pit methods (EMR 1977). In these methods shafts, tunnels and underground mining are proposed either to provide access for in-situ processes or for direct mining of oil sand as an ore, However, to date only one Jarge diameter tunnel has been constructed in the oil sands of Alberta and hence the underground behaviour of oil sand poses some uncertainties. The purpose of this paper is to assess tunnel construction in oil sand based on the experience gained from the Saline Creek tunnel (Smith et al. 1978), from observations of the behaviour of oil sand both in natural exposures and man-made excavations and from a review of information available in the literature. The characteristics of oil sand arc discussed, first within the framework of its geological selling and second as a material with unique engineering properties. Consideration is given to the effects of weak clay layers, water-bearing sands and other geological features such as cemented stringers, gas pockets and sink holes as they might affect tunnel construction. Background The construction of shafts and tunnels in Alberta's Athabasca and Cold Lake oil sand deposits has recently assumed practical significance because they may be necessary for access to oil sand under overburden depths too great for economical open-pit mining. A recent study (EMR, 1977) indicates that mining of oil sand as an ore may not be practical until world oil prices are higher than they are at present, but it seems likely that in-situ mining, using shafts and tunnels for access only is feasible. At-least one scheme the Rabbitt Kinetic In-Situ Method (Oilweek, 1978), requires shafts through the oil sand to the underlying Devonian limestone. Conventional design and the construction of shafts and tunnels in soil and in weak or strong rock is a skill largely based on practical experience obtained from previous work. Although oil sand is an oil-impregnated sandstone, observations from present open-pit mine operations have shown that the oil sand does not behave like most sandstones. The oil sand swells and experiences a significant loss of strength as it is exposed by excavation. Recent research at the University of Alberta has provided an understanding of the physical behaviour of oil sand under stress change, and has provided a framework for predicting behaviour.

Estimation of Formation Strength From the Mechanical-Properties Log(incudes associated paper 6400 )

The mechanical-properties log provides a quantitative means for identifyingsands that are strong enough to produce oil and gas without any form ofsand control. The method is based on a correlation of in-situ strengthwith the dynamic elastic moduli computed from sonic and density logs. Introduction To meet current demands for more oil and gas, companies would like to increase production. However, increases in well-production rates are frequentlyaccompanied by sanding problems. For this reason, there is often reluctance to go to higher producingrates.Yet many sands are, in fact, strong enough topermit greater production. It would be highly beneficialto be able to recognize these. It is also important toknow beforehand which sands are too weak to beproduced at the higher rates because once the wellhas made sand, consolidation is difficult and oftenineffective.Many factors must be considered to understandthe sanding problem. The pressure gradient near theperforation, the flow rate per foot, and the scrubbingaction of the fluid being produced all interact toimpose destructive forces on the sand. The ability ofthe sand to withstand these destructive forces isdetermined by two main factors: the intrinsic strengthof the formation, and the capability of the sand toform stable arches around the perforations.The intrinsic formation strength is governed bythe state of the confining stress (as determined bythe difference between the overburden stress and thepore pressure), the grain shape and sorting, and thecementation between the grains. Shaliness maycontribute to the cementation.There is a good correlation between theformations intrinsic strength and the ability of theformation to produce at a high flow rate. Moreover, the consistently lower permeabilities of the strong sandslimit the magnitudes of the flow rates to which theyare exposed. Conversely, weak sands have relativelyhigh permeabilities and are capable of producing athigh flow rates, even with small drawdowns. Forthese, sanding is a potential danger.Although we believe that a criterion based on theintrinsic strength is basic and fundamental, there areother factors that cannot be ignored. For instance, the type of fluid being produced is important. Ourexperience to date has been predominantly with gasand oil production, but it is believed that productionwith a high water cut may require higher intrinsicstrength.Resistance to sanding in a weakly cemented sandcan result from the formation of a stable, load-carryingsand arch spanning the producing cavity. In this respect, published work on laboratory experimentsindicates that the presence of two different fluids(e.g., oil and water) in the formation, with the wetting-phase saturation near irreducible, maycontribute cohesive forces between the sand grains.These forces help sustain a stable arch after some sanding has occurred. The forces result from theinterfacial tension between the two fluids where theirsurface contacts the sand grains, and the effect is topull the sand grains together. JPT P. 283^