In this review paper, a variety of reservoir applications are indicatedwhere horizontal wells (HWs) can have advantages over the use of conventionalvertical wells. Analytical methods for calculating critical coning rates inhomogeneous reservoirs are reviewed, and shown to give a very large range ofresults for HWs. Some aspects of assigning effective parameters to flexiblegridding schemes for HWs are presented. The potential significance of two-phasepressure drop within the wellbore on GOR performance is discussed, and a rangeof uncertainty by a factor of six is indicated between the use of variouscorrelations for calculating the well pressure drop. In the final sectionstudies are summarized for a gas coning application using Eclipse, a commercialsimulator. Conditions under which the wellbore pressure drop becomes importantare demonstrated.


Horizontal wells (HWs) have provided a major breakthrough in new techniquesfor developing oil fields; the drilling technology was first pioneered by Elfaquitaine and the Institut Francais du Petrole (IFP) with application in theoffshore Raspo Mare field in the Adriatic (circa 1977). Since then thetechnology has expanded enormously with one assessment indicating over 800horizontal wells being drilled in 1991. Some 2,500 HWs are known to have beendrilled to the end of 1992 in the USA. Drilling costs have decreased to be onlyabout 1.3 times those for vertical wells, even with horizontal sections of2,000 feet or more. Although the drilling techniques in association with Measurement While Drilling (MWD) have progressed to remarkable accuracy, and inmore recent years clearer insights have been established on completionpractices, the reliable prediction of the reservoir engineering and performanceattributes of horizontal wells remains a severe challenge. For example, in the Prudhoe Bay field where reduction of gas coning under an expanding gas capprovides an incentive, Sherrard et al., the use of horizontalwells has sometimes not been as beneficial as first anticipated. Part of thishas been attributed to a rapid build-up in gas/oil ratio after gas breakthroughinto the well. In vertical wells, however, techniques for an optimalrecompletion of the well to offset gas coning behavior can be used withreasonable confidence, thus the economic benefits for horizontal wells are notalways clearly assured. This raises the important question of optimalrecompletion strategies for long HWs.

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