Wells in the Gullfaks field, situated in the Norwegian sector of the North Sea, are characterized by long, highly deviated producing intervals. Cased hole completions with 177.8 mm production liners are dominant, but since 1994 several wells have been completed as open hole in one or two intervals. The Gullfaks field is producing from a reservoir that consists of a multitude of high permeability loosely consolidated sandstone formations. Formation permeability varies from 50 md to as much as 10 darcies, while reservoir pressure ranges from 220 bar to 320 bar. Sand production from the loosely consolidated formations is prevented with traditional gravel-packs. Internal gravel-packs in the Gullfaks field are performed with under-balanced snubbing, thereby eliminating the need to kill the well at any stage of the operation. Leakoff in the different formation sands is well defined by log permeability and reservoir pressure; high choke pressure during the under-balanced operation aids in controlling the pre-determined return rate throughout the job. This paper describes an approach where gravel-pack treatments pumped in the Gullfaks field have been designed and evaluated using a pseudo three-dimensional gravel placement simulator. With reservoir pressure given as a range rather than a specific value, a sensitivity study yielded the best estimate for reservoir pressure. Following verification of reservoir parameters and simulator capabilities, new jobs were optimally designed using the simulator so that potential pitfalls in traditional designs could be avoided. Designs were modified to achieve a successful annular pack, also permitting a priori knowledge of safe operational limits. This further allows better control over fluid selection, rate determination, tool position and other parameters that are critical to achieving a successful outcome of the treatment. Post-job evaluation of gravel-pack treatments using this approach confirms the validity of the designs. Three wells in the Gullfaks field operated by Statoil, serve as the basis for the case studies presented in this paper. Recommendations are made on how to extend the technique to similar fields. P. 149
Production decline in several wells in the Gullfaks field in the Norwegian sector of the North Sea has been attributed to various formation damage mechanisms. These include fines migration, scale formation, relative permeability changes after floodwater breakthrough, possible emulsion formation and byproducts of bacteria. To address this decline, many types of treatment operations are employed. This paper describes how a software program for design of matrix acidizing treatments has been validated for the Gullfaks field. Emphasis has been given to sensitivity analysis of this particular software as a function of variations in critical design parameters. The paper also describes how this program, StimCADE, has been used as an integrated part of a chain of analytical engineering applications, introduces a complete design methodology for possible identification of the damage mechanisms. The method revolves around analyzing each well's historical production data, immediately before and after a given treatment operation, for changes in reservoir skin using nodal analysis. The actual treatment parameters of the chosen job are input to the StimCADE simulator. The pre-treatment actual and predicted skin values are calibrated to one another by adjusting various input reservoir parameters in either one or both of the nodal analysis and StimCADE programs. Differences between the post-treatment actual and predicted skin values are interpreted to determine the formation damage mechanism. This improved damage determination is used to design more effective future treatments. The subsequent skin reduction prediction from the StimCADE program is input into nodal analysis to assess the production and thus economic impact of the new design. Four wells in the Gullfaks field in the Norwegian Sea, operated by Statoil, serve as the basis for a case study. Recommendations are made on how to extend the technique to other similar fields. P. 413
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractGravel-packs can be damaged by a variety of mechanisms which include scale formation, fines migration, paraffin and asphaltene deposition resulting in a damage skin of 10-300. Frequently damage removal treatments do not remove this damage completely from all the producing zones, resulting in a short treatment life and/or inadequate production. This can be attributed to poor diversion and/or improper damage characterization. This paper focuses on several aspects of improved damage removal in gravel-packed wells. A methodology to assist in the proper identification of the damage mechanism and the design process for more effective treatments is presented. An integrated approach using an overall systems analysis coupled with log, water analysis and numerical simulations of the gravel-pack/matrix damage removal is recommended.A numerical model to simulate acid treatments of gravelpacked well is presented and used to evaluate several treatments performed on wells from the Gullfaks field in the North Sea. Results indicated greater than 75% of the damage is in the gravel pack. The model incorporates a detailed wellbore model integrated with a reservoir mode for simulation of acidizing treatments in horizontal and vertical gravel-packed wells. An algorithm for quantifying the damage in the gravelpack is discussed in detail. Various diversion techniques and fluid formulations are evaluated using the simulator and compared to field results with emphasis on pre and posttreatment production logs.The results of this study helped us establish guidelines for identifying the damage mechanism and in designing optimal treatments for matrix stimulation of gravel-packed wells.
This paper was prepared for presentation at the 1999 SPE European Formation Damage Conference held in The Hague, The Netherlands, 31 May–1 June 1999.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractExcessive water production is an increasing environmental problem in mature oil fields in the Norwegian sector of the North Sea. Throughout the past 10 years, several mechanical and chemical water-shutoff solutions have been developed and implemented with variable success. For chemical water shutoff, the two major challenges have been placement optimization and choice of a technically feasible system. A cement squeeze is traditionally the most usual means for permanent isolation of a naturally perforated section. Cement is often the preferred solution because of its well-established properties and proven performance. An increasing problem has been water production in gravel-packed wells. Chemical shutoff in a proppant pack cannot easily be achieved with standard cement slurries or specialized squeeze slurries because of the risk of bridging and incomplete placement. A polymeric system can be easily placed in a proppant pack, but there is a concern regarding the systems ability to withstand high differential pressure following onset of production.A water-shutoff treatment utilizing deep penetrating squeeze cement slurry is presented for the isolation of a highly permeable gravel pack. The objective of the treatment was to eliminate the negative impact on the environment caused by production from a formation with 100% water cut and to initiate new production from an oil-saturated layer immediately above the producing formation. To access the desired interval, one would have to perforate through a section of blank pipe in the existing gravel pack. The perforation could be easily achieved; the challenge was to prevent continued production influx from the gravel pack producing with 100% water cut prior to the water-shutoff treatment.The evaluation of the treatment has provided valuable information and qualified a new technique for water shutoff in gravel-packed completions. Benefits from this treatment are reduced negative impact on the environment through reduction in the volume of produced water and increased oil recovery.
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