The spectroscopy and predissociation of the vibroCicJeve1.s of the 'A; excited stctes , Of NH3 and ND3 have been studied vta A,X dispersed emission spectra, C'-A dispersed emission spectra and C'-A stimulated emission pumping, in each case following two-photon excitation to a selected intermediate level. The predissociation lifetimes span more than two orders of magnitude. The lowest levels predissociate by H( D) atom quantum tunnelling, the higher levels by vibrational rearrangement. A model calcul?tion is presented to illustrate this mechanism for the u1 = 1 levels of the A state, which are very short-lived.
Offshore reservoirs requiring sand control pose a major completion challenge because of extremely high cost and risk involved in remedial treatments, particularly in sub-sea completion and/or deep-water environments. It is therefore of utmost importance to ensure sand control without sacrificing flow conformance, recoverable reserves and well deliverability throughout the expected life of the completion. A major trend in these environments is towards open-hole, horizontal, gravel-packed completions. Although gravel packing stabilizes the wellbore, it can also entrap the filter-cake formed by the reservoir drilling fluid, potentially resulting in high drawdown requirements (flow initiation pressures) and/or low production rates (retained permeabilities). The cleanup procedures in the industry have varied significantly from no cleanup at all to complicated two-stage breaker treatments involving post-completion coiled tubing intervention, with no guidelines existing in the literature. In this paper, we present experimental results and field cases involving filter-cake flow-back through gravel packs with and without cleanup. Effects of various parameters, including gravel size (40/60, 20/40, and 12/20), formation permeability, drill-solids type (clays, quartz) and concentration, and the type of cleanup fluid have been investigated. Flow initiation pressure and retained permeabilities during flow back are reported as a function of these parameters. The experimental results show that the flow initiation pressure is a strong function of gravel size and the type of drill solids. It is concluded that, in clean (low-to-no clay content) formations of large grains and high permeabilities (~ several darcies) requiring large gravel sizes (e.g., 12/20), an enzyme or an oxidizer treatment is sufficient based on laboratory results and productivity predictions. This conclusion is also supported by several field applications as shown. In lower permeability (~ 100–250 md) formations of small sand sizes requiring smaller gravel (e.g., 40/60) elimination of both the fluid loss control agent (starch) and bridging agent (CaCO3) is necessary based on high flow initiation pressures and low retained permeabilities. In intermediate permeability (~ 500–800 md) formations of medium size sand-grains typically requiring 20/40 gravel, the results depend strongly on the type of drill solids: in clean formations (no clays in drilling fluid), an enzyme or an oxidizer treatment is sufficient, while in dirty formations removal of both CaCO3 and starch is necessary. These results are also supported by field case histories presented in the paper. Introduction Gravel packing has been gaining wider popularity in open-hole horizontal completions where sand control is required, particularly in sub-sea completion and/or deep-water environment. The cost of intervention in such cases makes risk mitigation a much more pronounced task. Until recently, a large majority of horizontal sand control completions have utilized standalone screens. However, because a substantial fraction of these wells have failed prematurely (either productivity loss due to screen plugging or loss of sand control due to screen erosion),1 many operators have changed their primary completion technique in these wells from standalone screens to gravel packing. This is particularly true in formations containing a large fraction of non-pay (shale, mudstone/siltstone) and/or have a wide particle size distribution.2
Gravel-packing of open-hole highly-deviated or horizontal wells is increasingly becoming a common practice, especially in deep water and sub-sea completion environments where production rates may reach up to 50,000 BOPD or 250 MMSCFD. In these wells, reliability of the sand face completion, in addition to other factors, is of utmost importance due to the prohibitively high cost of intervention or side-tracking and the very high hydrocarbon recoveries required per well. To date the norm in gravel-packing such wells is water-packing or shunt-packing with water-based fluids. With both techniques, filter-cake removal treatments are conventionally done through coiled tubing after gravel packing, pulling out of the hole with the service tool and running in with the production/injection tubing. Furthermore, because conventional gravel-pack carrier fluids are water-based (brine or viscous fluids), water-based drilling fluids are traditionally used to drill the reservoir section to ensure compatibility and improve wellbore cleanup, even if the upper hole is drilled with a synthetic/oil-based drilling fluid. In this paper, we discuss several novel techniques that can substantially improve return on investment in gravel packing of open-hole horizontal completions, through reduced cost and process time, improved fluid management practices, increased productivity and/or reduced risk of future interventions, so mitigating against the risk of sand face completion failure or under-performance. The proposed techniques include:Simultaneous gravel-packing and filter-cake removal with water-based carrier fluids when the reservoir is drilled with a water-based drilling fluid: laboratory data relevant to gravel-packing are given and field case histories are discussed in detail.Simultaneous gravel-packing and cake cleanup with either water or a synthetic/oil-based carrier fluid when the reservoir is drilled with a synthetic/oil-based drilling fluid: laboratory data on cake removal while gravel packing are presented for both water-based and oil-based carrier fluids along with data on kinetics of cake removal.a new service tool that utilizes wash-pipe as continuous tubing and thus allows spotting of breaker treatments immediately after gravel packing: detailed description of the tool and its operation is given.Gravel-packing of highly-deviated or horizontal wells above fracturing pressure. Benefits offered by each of the proposed techniques are discussed in detail along with their current limitations. Introduction A great majority of the highly-deviated and horizontal wells are being completed as open holes, primarily because of their much higher damage tolerance, higher well productivities at high mobilities (kh/µ) and lower cost compared to cased holes. Although most of these wells in areas requiring sand control have been completed with standalone screens, a rapidly increasing fraction of them are now being gravel packed, particularly in deep water, high production rate and/or sub-sea completion environments (currently ca. 40%, and projected to be ca. 60% by 2003/2004). The major drivers for this current trend are the prohibitively high cost of intervention and much higher reliability associated with gravel packs.1,2
Summary Reservoirs requiring sand control pose a major challenge for selecting a suitable completion method. Horizontal openhole completions have been successfully used in such reservoirs to eliminate sand production while maximizing productivity/injectivity and well deliverability throughout the expected life of the completion and minimizing risk and complexity. Although horizontal, openhole, sand-control completions, ranging from preperforated/slotted liners to gravel packs, have been applied widely in the last decade and many case histories have been discussed in the literature, a systematic methodology for selecting these completion methods remains to be documented. It is the objective of this paper to propose such a design methodology by unifying the broad experience and understanding from a global, technically integrated perspective. The paper first discusses a generalized and unified methodology for determining when to install sand control, what to install for sand control, and how to install it in horizontal openhole completions. Specific factors recognized as affecting "when" are in-situ stresses, pore-pressure decline (sand prediction), expected well life, production rate, hydrocarbon and well type, gross product value, sand tolerance capacity, environmental limitations, and intervention capabilities, while the integration of all these factors has an impact on the overall risk analysis. In addition to many of the previous factors, critical drivers affecting "what" are identified as wellbore architecture, reservoir lithology and petrophysical properties, and sandface equipment reliability. Additional parameters impacting "how" are reservoir drilling fluid, displacement and cleanup methodology, screen type, operational implementation/assurance (risk management, operational timing, and location logistics), torque and drag analysis, and gravel-placement simulations. Secondly, examples of this methodology are presented in detailed case histories pertaining to different types of horizontal, openhole, sandface completions, including slotted liners, stand-alone screens (including expandable), and gravel packs, as well as various integrated cleanup methods, along with a summary of the lessons learned by each company. Introduction Horizontal openhole completions have been widely used in the oil and gas industry for effectively exploiting hydrocarbon reserves in both sandstone and carbonate formations during the last 2 decades. In sandstones, a major issue has been whether sand control is required during the life of a particular well, and if so, what technique to use to minimize overall completion and remediation costs, thus increasing profitability. Most recently, Bennett1 has developed a crossplot (see Fig. 1) that looks at the likelihood of wellbore failure with respect to formation quality and has used this to provide guidelines as to sandface completion methodology based on experiences gained in North Sea and Gulf of Mexico wells. Earlier, others2 developed a questionnaire-based approach for pooling industry experience and providing a database of events from which to learn and share. Unfortunately, because of the anecdotal emphasis this encouraged, little remains or has been developed since, especially in light of the fact that the industry's pace of technological development and understanding has once again accelerated in the last 3 to 4 years. While we see that a tremendous level of expertise and experience has been gained and although a large number of horizontal-well applications in sandstone formations and numerous publications exist, a well-defined set of guidelines for selecting the most suitable sand-control technique in openhole horizontal wells has not been published to date. It is, therefore, the objective of this paper to provide a unified set of guidelines based on five operators' and a completion service provider's experience and engineering expertise. The paper is organized as follows. First, we discuss the sand-prediction methodology that should be used to determine whether sand control is prescribed, and if so, when in the life of the well/reservoir it would be required. Once it is established that sand control is needed, the next step is to decide between gravel packing (GP) and stand-alone screens (SAS), for which we offer criteria based on current field experience, knowledge, and experimental data.3 For GP, the next step is selecting between two methodology techniques used in openhole horizontal wells - water and shunt packing. This is then followed by screen selection for the respective techniques (i.e., water-packing, shunt-packing, and SAS completions). Numerous case histories, both successes and failures, are given to support the selection methodology. Finally, suggestions for future work are made and conclusions are drawn.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMany of the recently discovered reservoirs in deepwater/subsea environments are prime candidates for horizontal open-hole gravel packs. Presence of multiple reactive shale breaks and penetration of different sand bodies along these holes introduce a formidable challenge for selection of proper carrier fluids, considering that most of these wells require oilbased (OB) drilling fluids.Various procedures were practiced for gravel-packing wells drilled with OB fluids, most utilizing water-based (WB) carrier fluids. Primary concern in using WB carrier fluids is the destabilization of the shales.If the displacements to WB fluids are performed prior to running in hole with the sandface completion assembly, inability to run the screen assembly to target zone is the risk. Consequently, operators were forced to use a two-step process, whereby a predrilled liner is run in hole in OB fluid environment, followed by displacements to WB fluids and gravel-packing with WB fluids. This approach introduces additional rig time and increases completion costs.If the displacements to WB fluids are performed after running in hole with completion assembly, primary challenge is the prevention of screen plugging. This necessitates a comparison of the benefits and risks of displacements to solids-free oil-based fluids and conditioning of the OB drilling fluid, considering logistics.An additional consideration in gravel packing with WB fluids in reactive-shale environments is the risk of intermixing of gravel with shales, thus reduced gravel-pack permeability. Various approaches may be taken to minimize this risk. The type of carrier fluid must also be kept in mind from a formation and gravel pack damage standpoints, should losses be experienced during gravel packing.Another approach in reactive shale environments is to use an oil-based carrier fluid and avoid exposure of the open hole to WB fluids both prior to and during gravel packing. This approach, practiced in two applications, also has its limitations.In this paper, a critical review of gravel-packing practices in oil-based drilling environments is provided, along with some of the recent developments and recommendations for future applications based on lessons learned from earlier practices.
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