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--- wellsite_geostop [20/03/2023 15:59] – created mike_gss
+++ wellsite_geostop [20/03/2023 16:14] (current) – mike_gss
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 The Gorm Field in the Danish sector of the North Sea demonstrates such a scenario. The reservoir is a chalk limestone overlain by a seal of claystones with a thin (c.2m thick) “transition zone” of marl in between. The reservoir in underpressured with respect to the claystones which means if penetrated by the drill, the heavy mud necessary to control the claystones is too dense to avoid entering the reservoir (by overcoming the lower internal pore pressure of the chalks) and damaging it. In addition – mechanically – the situation normally results in the drill becoming “stuck” in the formation. This usually results in an expensive and time-consuming operation to rectify the situation, often requiring the redrilling of the original well and the abandonment of much expensive drilling hardware.
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+//Penetrating the reservoir (limestones) without casing off the overlying borehole results in formation damage, lost equipment and lost time. Ideally drilling into the marly transition zone and then stopping to set casing before drilling ahead is the solution but this interval is only c.2 metres thick and its precise depth below the surface cannot be accurately calculated.//
 The essence of the problem is that the drill needs to stop as close to the reservoir as possible, but not to actually penetrate it. Ideally, the drilling would cease within the 2m window of the “transition zone” marls and the casing set within that unit. The exact depth to be drilled to the top of the “transitional zone” or the reservoir is unknown to the accuracy required and cannot be determined by seismic interpretation. Further exploitation of the reservoir was becoming uneconomic. Each development well was costing in the region of $2 million extra to cover the loss of drilling mud and drilling equipment and the operator was questioning the field’s future.
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 A solution was found by examining closely-spaced samples from nearby wells and producing a high-resolution biozonation scheme which utilised a pragmatic set of fossil events, specific to the Gorm Field and the surrounding areas, and to use a wellsite biostratigrapher to monitor closely-spaced samples taken on the approach to the reservoir section. It can be seen (see below) that these fossil events accurately define the target horizon which is the “transitional marls” immediately above the reservoir. More importantly, it also shows a series of closely-spaced subzones defined by separate fossil events which can be used to “flag” the approach of the target horizon. These subzones – because they are an expression of the local geology – may even be used to estimate rock thickness and provide estimates in actual distances to be drilled between marker events.
-The technique was used on the subsequent 12 production wells drilled with no “lost holes” thereby estimating a saving of $24 million by not losing any redrilling time or drilling equipment.
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+//Chart showing closely-spaced biostratigraphic events to warn of the impending arrival of the drill string in the target zone (green unit) just above the underpressured reservoir (blue unit). When the markers for higher zones like TM7 to TM4 were observed in the samples, the ROP could be controlled to allow closely-spaced samples to be taken to look for markers TM3 d, c, b, a and ax. When markers for TM2 were seen drilling could be stopped quickly without risk of penetrating the reservoir (TM1).//
+The technique was used on the subsequent 12 production wells drilled with no “lost holes” thereby estimating a saving of $24 million by not losing any redrilling time or drilling equipment. The estimated life span of the field was also increased because of the increased confidence in future safe drilling.