bluuebunny/crossref_metadata_2025_split huggingface dataset:
Selective Sweeps in Multilocus Models of Quantitative Traits
Authors: Pavlidis Pavlos , Metzler Dirk , Stephan Wolfgang 
Published: June 2012

Abstract
We study the trajectory of an allele that affects a polygenic trait selected toward a phenotypic optimum. Furthermore, conditioning on this trajectory we analyze the effect of the selected mutation on linked neutral variation. We examine the well-characterized two-locus two-allele model but we also provide results for diallelic models with up to eight loci. First, when the optimum phenotype is that of the double heterozygote in a two-locus model, and there is no dominance or epistasis of effects on the trait, the trajectories of selected mutations rarely reach fixation; instead, a polymorphic equilibrium at both loci is approached. Whether a polymorphic equilibrium is reached (rather than fixation at both loci) depends on the intensity of selection and the relative distances to the optimum of the homozygotes at each locus. Furthermore, if both loci have similar effects on the trait, fixation of an allele at a given locus is less likely when it starts at low frequency and the other locus is polymorphic (with alleles at intermediate frequencies). Weaker selection increases the probability of fixation of the studied allele, as the polymorphic equilibrium is less stable in this case. When we do not require the double heterozygote to be at the optimum we find that the polymorphic equilibrium is more difficult to reach, and fixation becomes more likely. Second, increasing the number of loci decreases the probability of fixation, because adaptation to the optimum is possible by various combinations of alleles. Summaries of the genealogy (height, total length, and imbalance) and of sequence polymorphism (number of polymorphisms, frequency spectrum, and haplotype structure) next to a selected locus depend on the frequency that the selected mutation approaches at equilibrium. We conclude that multilocus response to selection may in some cases prevent selective sweeps from being completed, as described in previous studies, but that conditions causing this to happen strongly depend on the genetic architecture of the trait, and that fixation of selected mutations is likely in many instances.
Gene-Expression Changes Caused by Inbreeding Protect Against Inbreeding Depression in Drosophila
Authors: García Carlos , Ávila Victoria , Quesada Humberto , Caballero Armando 
Published: June 2012

Abstract
We present a transcriptomic analysis aimed at investigating whether the changes in gene expression that occur under inbreeding generally reduce or enhance inbreeding depression. Discerning between these two alternatives can be addressed only when both changes in expression due to inbreeding and to inbreeding depression are estimated simultaneously. We used Affymetrix 2.0 arrays to study the changes in gene expression associated with both inbreeding and inbreeding depression for fitness in four sets of inbred sublines of Drosophila melanogaster. We found that for most genes showing changes in expression associated with inbreeding, the least depressed sublines were those showing the largest departures in expression from that of the outbred control. This suggests a pattern consistent with a protective role of expression changes against inbreeding effects, and would reveal a new dimension of the transcriptomics of inbreeding. The variation in depression observed could then be due not only to the genetic damages primarily originating that depression, but also possibly to differences in the ability to carry out the appropriate adjustments in gene expression to cope with the inbreeding. We also found that these expression changes with a putative protective role against inbreeding effects show a clear specificity on RNA synthesis and splicing and energy derivation functions.
Improving Horizontal Wells Offshore Operations and Production Management Through the Deployment of Inflow Control Devices in Bouri Field, Libya
Authors: Amari Mustafa , Misherghi Nasser Ali , Algdamsi Hossein Ali , Sharaf Hosam Eldin , Abid Mohamed Lakhdar , Porturas Francisco , Zeglam Schlumberger Adel 
Published: June 2012

AbstractWith the advances made in drilling long horizontal wells over the past decades it has become economically attractive to produce oil from thin oil rims. However, the production from these types of reservoirs presents several challenges. Gas coning is one of the most important ones. Horizontal well drilling traditionally helps to improve the oil recovery and avoid problems of premature gas/water breakthrough. In Bouri field, offshore Libya, the main concern of the operator was to establish an advanced method of controlling gas and water encroachment in a fractured carbonate reservoir characterized by high vertical permeability. This paper describes the first Inflow Control Device (ICD) installation for Mellitah Oil & Gas, and the first such application in Libya Offshore field. It was an integral part of a well completion aimed at evenly distributing inflow in a horizontal well, and at limiting the negative effects after occurrence of expected gas breakthrough. Due to small clearances involved, the ICD deployment presented a significant operational challenge. Despite the higher initial completion costs associated with ICDs, they can provide a cost-effective way to reduce long-term operating costs and increase yield. Production targets are achieved with longer, but fewer wells, maintenance and overhead. From a reservoir management point of view, ICDs can improve the productivity index (PI) by maximizing reservoir contact, minimizing gas coning by operating at lower drawdown, and increasing overall efficiency. Swellingpackers were used to compartmentalize the horizontal and build sections, allowing better drawdown control and eliminating crossflow issues. The completion required re-thinking of the established acid-wash treatment procedures, ultimately improving the overall well clean-up. Integrated analysis methods using steady-state wellbore hydraulic and 3D dynamic simulators were performed to generate flow profiles and calculate ICD pressure drop along the horizontal section. The models were updated using results from logging-while-drilling (LWD) and with real-time modifications to the initial design.To verify the inflow profile along the length of the ICD completion, production logging (PLT) was conducted. The inflow profiles compared favorably with those predicted by the models.
Cementing in Front of Soluble Salt Zones
Authors: Simão C. A. , Miranda C. R. , Vargas A. A. , Pereira R. F. , Santos R. L. , Soares M. A. , Conceição A. C. 
Published: June 2012

AbstractCementing in front Salt Zones requires an appropriate cement slurry design to assure minimum dissolution of the formation by the cement slurry and also minimum deleterious impact on the properties of the cement. The influence of halite on the cement slurry properties is already well discussed in the literature, but there’s no much information regarding the influence of other salts like taquidrite, carnalite, which are much more soluble than halite and are found in Brazil Offshore Fields.The first stage of the work consisted in evaluating the influence of different levels and types of salt on the hydration process and its effects on the behavior of the main physical properties. It was produced fourteen cement pastes, five of them containing NaCl at the contents of 5, 10, 15, 20 and 36% by weight of water and eight containing KCl at contents of 1, 3, 5, 7, 10, 15, 20 and 34% by weight of water. It was also prepared a cement slurry with no salt added to be used as reference. The dissolution rate of the different salts core formation in these cement slurries was determined. The influence on the cement slurry properties, such as free fluid, thickening time, compressive strength and rheological parameters due to halite, carnalite and taquidrite incorporation were also determined.This paper presents the studies conducted to design cement slurries for cementing well through salt layers (halite, carnalite and taquihydryte) located in the Aguilhadas Field in Northeast, Brazil. Based on the results, about 500 m of salt were cemented with a semi-saturated cement slurry which provided an excellent quality of the cementing, proved by acoustic logs.Salt cores of halite, carnalite and taquihydryte were used to determine the dissolution rate of salt into the cement slurry. X-Ray tomography was used to visualize and to quantify the salt dissolution in the dynamic tests.The shear bond strength between halite core and hardened cement slurry with 15% NaCl was determined showing good results.
Reaming on Demand - Selective Activation of an Integrated Under Reamer at the Grane Field in the North Sea
Authors: Pragt Jos , Herberg Wolfgang , Meister Matthias , Clemmensen Carl , Grindhaug Gaute , Hanken Knut 
Published: June 2012

Abstract
The Integrated Under-Reamer (IUR) was developed in 2007 in a joint project between an Operator and Service company. The tool was designed to allow for unlimited opening and closing of the tool to provide selective hole opening in unstable formations e.g. unstable shale or swelling salt. Furthermore the tool should be able to close for pull out of hole (POOH) while keeping the circulation continuously on.
To accommodate for this the tool was designed to be fully integrated into the modular bus structure of the downhole BHA. Electronics were developed to activate and deactivate the tool from downlink commands sent downhole and distributed via the communication bus to the IUR. Sensors in the IUR detect the status of the tool: such as position of the cutter blades and the health of the tool. This data is transmitted to surface using the MWD pulser. The data is displayed in real-time for immediate review by the drilling team. Additional diagnostic data are stored in memory for post run analysis.
Advanced autonomous fail safe control was designed to make sure the tool can always be closed either by downlink or other procedures to allow unrestricted pulling out into smaller hole or the casing shoe. In the case of communication problems a procedure is available for back-reaming with automatically closing the blades.
A case history is presented where a water injection well needed increased clearance to run 6 5/8" completion screens. Using the IUR, the well was opened up while drilling from a 8 ½" pilot hole to 9.05". The IUR was de-activated and re-activated successfully several times during drilling the section. The operational procedure is described, and caliper data are presented. The well was completed on plan. At the end an outlook will be given on future reaming on demand applications.
Offshore ESP Selection Criteria: An Industry Study
Authors: Romer Michael C. , Johnson Mark E. , Underwood Pat C. , Albers Amanda L. , Bacon Russ M. 
Published: June 2012

Abstract
Most offshore wells that require artificial lift are gas lifted, as gas typically is readily available and compared to other lift systems, gas lifting is relatively inexpensive and low maintenance. However, electric submersible pumps (ESPs) can efficiently and economically increase oil production and reserves recovery under the appropriate operating conditions. This may translate to a lower abandonment pressure in the long term—possibly reducing the total number of wells required to deplete an asset.
Since few ESPs currently are installed in offshore wells, an ESP screening "Rules of Thumb" was created as a simple guide for prioritizing offshore ESP candidates. The selection criteria focus on feasibility of installation, operability conditions and operating practices to maximize run life, and economic considerations. ExxonMobil† and industry experience from North America, South America, West Africa, Asia, Australia, the Middle East, and the North Sea provided the basis for the study.
Development and Characterization of a Modified Polymeric Material for HP/HT Downhole Sealing Applications
Authors: Ren Jiaxiang (Jason) , Gerrard Dave , Duan Ping , Vu Chau , Leung Hoi Yin 
Published: June 2012

Abstract
Emerging ultra high-pressure/high-temperature applications in downhole service have created the need for the development of new sealing materials. To address this emerging need, a series of modified polymeric materials were developed. The new material displayed strong mechanical properties below glass transition temperature (Tg). Above Tg, the material displayed stable elastomeric properties up to 400° C. The thermal rating of the material is beyond the available commercial thermoplastics and elastomeric materials. Furthermore, the Tg of the material can be adjusted from 97° C to 215° C. The material was aged in a crude oil at high temperature. Dynamic Mechanical Analysis (DMA) measurements were performed on the material before and after aging. The material displayed excellent resistance to the crude oil. In addition, API extrusion-resistance measurements were performed on the new material at various temperatures and various pressures. The extrusion resistance of the new material was superior to the available commercial elastomeric materials, including perfluoroelastomers. Thus, the material has great potential for use in emerging high-temperature sealing applications and other oil and gas applications. This paper describes the work involved in conducting this development work and test results.
The Application of Light and Ultra-Light Weight Proppant in Horizontal Well Sand Control: Unified Model and Case Histories
Authors: Chen Zhongming 
Published: June 2012

AbstractUnconsolidated sand reservoirs, with the characteristics of high permeability, large porosity, and low fracture gradient, are common and present challenge sand control tasks facing completion engineers. Avoiding fracturing such formation and preventing excessive fluid leakoff are the primary concerns to achieve successful gravel placement during job execution. Gravel packing these wells needs innovative technologies and practices. The light and ultra-light weight proppants, with their unique properties and advantages over conventional gravel, are important options to ensure wellbore be successfully packed under low pump rate.Light and ultra-light weight particles transport phenomena and their Alpha and Beta wave packing behaviors were studied. An unified particle transport model is developed to accurately model Alpha and Beta wave behaviors. The paper presents the validation of unified model by wide ranges of field case histories under different pump rates and particle over fluid density ratios. Particle density and geometry, fluid properties, and wellbore configuration are the dominant factors to affect wave building up process. The paper presents the guidelines to design proper pumping schedule and optimal Alpha wave dune height under given well condition.Carrying fluid leakoff and excessive Alpha wave dune height are the causes of many early time screenout and job failures. Underachieved gravel packing will reduce the effectiveness of sand control and result in screen damage during early production. Job histories are presented in the paper to demonstrate the practical considerations of job design and execution, and the use of light and ultra-light weight proppant over conventional gravel to ultimately increase the effectiveness of sand control.
Advantages of Integrating Geomechanics Modeling and Hydraulics Modeling in Well Planning and Execution
Authors: Hargis Virgil , Fowler William 
Published: June 2012

Abstract
Geomechanics and hydraulics modeling are commonly used to optimize drilling performance and to reduce non-productive time (NPT). The geomechanics model indicates safe mud weight parameters, optimum casing points, and wellbore stability analyses. The hydraulics model simulates the wellbore and drilling fluid parameters to optimize the flow rate and rate of penetration (ROP) within equivalent circulating density (ECD) constraints. Real-time drilling data are used to update the models: the pore-pressure/shear-failure gradient and fracture gradient boundaries for geomechanics; and ECD, circulating system pressure drops, and annular cuttings load parameters for hydraulics. Each real-time model strives to ensure that operations are optimal, safe, and cost effective, as well as to provide updated models for future planning purposes.
This paper focuses on collaboration between geomechanics and hydraulics analysts in the planning, drilling, and postdrilling phases of the well. A geomechanics model determines the safe mud weight operating window and casing points, whereas the hydraulics model simulates ECD for defined mud properties, drilling parameters, and wellbore geometry. A synthesis of the two models enables the previously mentioned information to be compared and merged before spudding the well. Real-time collaboration between models enables each model to be updated with pertinent information from the other, ensuring that drilling operations are performed within optimum mud weight and ECD constraints. The geomechanics model can use real-time ECD data from the hydraulics model when a pressure-while-drilling (PWD) tool is not being used. In turn, the hydraulics model requires updating when the safe mud weight limits are being exceeded. The two models can each use information from the other when performing the post-well analysis and planning for the next well. Both models are valuable tools when used individually, but they become powerful, superior companions when used in conjunction with one another.
A Comprehensive Theoretical and Experimental Study on Fluid Displacement for Oilwell Cementing Operations
Authors: Aranha Pedro Esteves , de Miranda Cristiane Richard , Cardoso Walter F. , Campos Gilson , Martins André Leibsohn , Gomes Frederico C. , de Araujo Simone Bochner , Carvalho Marcio S. 
Published: June 2012

Abstract
Displacing fluids in downhole conditions and for long distances is a complex task, affecting several steps on well construction. Cementing gains relevance in the moment that fluid contamination compromises cement sheath integrity and consequently zonal isolation.
Density and rheology design for all the fluids involved are essential to achieve operational success. Properties hierarchy and preferred flow regimes have been empirically defined and tend to provide reasonable generic results. Challenging operations, including ultra deepwaters and their narrow operational windows scenarios, require further knowledge of the physics involved in order to prevent undesirable events
This article presents the in house development of a software for annular two-phase flow, which analyzes fluid displacement in typical vertical and directional offshore wells, for Newtonian and non Newtonian liquids, laminar and turbulent flow regimes. The formulation proposed provides accurate results for a wide range of input parameters, including the cases in which the ratio between the inner and outer radii of the annulus is small. Operational guidelines for fluid and placement schedules are provided.
The computational work is validated by unique results obtained from an experimental test rig where detailed displacement tests were conducted. Contamination degrees were measured after the displacement of a sequence of fluids through 1192 m of vertical well. Effect of fluid density and rheology hierarchy, flow regimes and displacement concepts were investigated. The results provide relevant information for the industry and fundamental understanding on displacement of Newtonian and non Newtonian liquids through annular sections.
Centralizer Selection and Placement Optimization
Authors: Liu Gefei , Weber Lawrence 
Published: June 2012

Abstract
Casing centralization is one of the key elements to ensure the quality of a cementing job by preventing mud channeling and poor zonal isolation. Centralizers can also assist in running of the casing and prevention of differential sticking. The challenge that both operators and service companies face is to place the right amount of centralizers at the optimum positions on the casing. While operators may be conservative, service companies tend to be liberal, in selecting the number of centralizers for a given application.
Casing standoff depends on well path and hole size, casing size, centralizer properties, and many other variables, but most importantly, centralizer placement. Experience plus computer modeling enable both centralizer vendors and operators to conduct centralizer optimization prior to field execution. One approach is to specify spacing using our experience, then let software check if it yields satisfactory standoff profile. Alternatively, we can specify required standoff and let the software tell us how to space centralizers. "Specify spacing" mode gives the simple-to-install centralizer placement because of its constant spacing result. However, this method may compromise the quality of standoff or quantity of centralizers. "Specify standoff" mode ensures the minimum standoff of casing between centralizers, while yielding difficult-to-follow placement program.
To get the best elements from both approaches, we have developed an optimum placement solution, which is specifying standoff with incremental spacing requirement. For high impact operations such as deepwater and use on inline bowspring centralizers, once a casing schematic is available, the centralizer placement is re-evaluated to optimize exact placement of each centralizer.
With the help of computer modeling, centralizer placement can be evaluated and optimized. This practice will result in better cementing job and cost-effective centralizer placement for directional wells. This paper will report examples of how such optimization can be achieved.
Borehole Instability in Tertiary Shales in the Norwegian North Sea
Authors: Nes Olav-Magnar , Bøe Reidar , Sønstebø Eyvind F. , Gran Kjetil , Wold Sturla , Saasen Arild , Fjogstad Arild 
Published: June 2012

AbstractSevere hole stability problems were encountered in a recent exploration well in the Norwegian North Sea. The problems occurred when drilling through Tertiary shale sections interbedded with permeable sand layers. Drilling was initially performed with water based drilling fluid. However, being unable to reach the section target after more than two weeks of operation, the section was plugged back and a sidetrack was drilled using an oil based drilling fluid without encountering major operational problems.On the basis of the post-drill analysis of drilling data, well logs, drill cuttings and borehole cavings sampled from the well, this paper describes how the complex combination of drilling fluid salt concentration and geological constraints may be utilized to ensure successful future drilling operations in this part of the North Sea.Cuttings and preserved cavings collected during the drilling operation were selected from several depth intervals identified as potentially troublesome from drilling experience and log data. Determination of cuttings mineralogy enabled better prediction of how the time dependency of the stable drilling fluid density window is influenced by interaction between the shale and the drilling fluid. Mechanical strength is a key input parameter when predicting borehole stability. Dedicated rock mechanical punch measurements on cavings were used to confirm the prediction of strength from log data alone. Examination of caving surfaces revealed the possible presence of in-situ fractured rock. Such fractures would require special measures while drilling to maintain stability.Subsequently a borehole stability sensitivity analysis was performed focusing on time dependent stability in the shale formations. The analysis used cuttings and cavings properties and logs as input. In particular, the modelling showed how the optimum KCl concentration in the drilling fluid changes with depth. The modelling further identified a relatively large sensitivity towards borehole inclination – even at fairly small inclinations. This paper thus illustrates the significance of properly accounting for rock mechanical aspects when planning new wells.