Relief-Well Requirements To Kill a High-Rate Gas Blowout From a Deepwater Reservoir (includes associated paper 19889 )
Authors: Warriner Richard A. , Cassity T.G. 
Published: December 2007

Relief-well requirements were investigated for a dynamic kill ofa high-rate gas blowout from a deepwater reservoir to define any necessary special procedures or equipment. Results of the investigation show that a high injection rate and a special-designlarge-diameter injection riser are required to dynamically kill such ablowout with seawater. The injection riser is necessary to limit surfacepump pressure during the high-rate kill operation. Procedures to completethe killing operation hydrostatically with heavy fluid following thedynamic kill are outlined.

Introduction

Over the past several years, a number of wells have been drilledin deepwater gas reservoirs to provide test data on high flow ratereservoir performance. Casing programs for these wells are designedspecifically to enhance the ability to produce at high rates. Typically, 13 3/8 -in. [34-cm]casing is set just above the reservoir anda 12 1/4 -in. [31 -cm] hole is drilled through the productive interval. The probability of a blowout occurring while drilling andcompleting, one of these wells is small. The difficulties associated withcon-trolling such a blowout, however, can be significant. To assess thesedifficulties. a detailed investigation of relief-well requirements forsuch a blowout was conducted.

For this investigation. a worst-case scenario was defined todetermine maximum requirements to establish control of the blowout. Theprima - assumptions of the scenario are thatthe well islocated in deep water.the reservoir is at a relatively shallowdepth,the permeability of the formation is high,13 3/8/,-in- 134-cmi casing is set near the top of the reservoir,thereservoir contains gas, andthe blowout is unrestricted at theseafloor.

(The drilling rig, riser. and blowout preventer (BOP) stack either are not present or are damaged beyond usefulness and offer noresistance to the flow.]

Fig. 1 illustrates a model well for the worst-case scenario. Ascan be seen, water depth is 1,000 ft [305 m] and the reservoir topis relatively shallow at 4,300 ft [ 1311 m) 13 3/8-in. [34-cm] casingis set at 4,200 ft [ 1280 m]. The fracture pressure at the casing shoeis, 2,850 psi [ 19 650 kpal and the reservoir pressure is 2,250 psi[15 513 kPa]. Reservoir temperature is 136deg.F [58deg.C].

Flow calculations show that an uncontrolled blowout from sucha well could cause a gas flow rate as high as 1,850 MMscf/D152.98 × 106 std M3/d]. The assumptions for the flow-ratecalculations were145 ft [44 ml of reservoir pay,no skin damage,static reservoir pressure of 2,250 psi f 15 5 14 kpal.no wateror condensate production, and2.00-darcy 1.97-um2 averagepermeability.

The gas plume resulting from such a blowout could directlyaffect an area larger than 425 ft [130 m] in diameter al the oceansurface. The central boil of the plume could be more than 200 ft[61 m] in diameter and as high as 11 ft [3 m] above sea level. Gasconcentration at the surface could exceed the lower flammable limit, depending on winds. thus creating hazardous conditions- Suchsurface conditions eliminate the possibility of positioning a rig overthe blowout well and attempting to re-enter or repair the wellheadand BOP'S.

This paper investigates the relief-well requirements and defines the required special procedures and equipment for a directdynamic kill of such a blowout well,

Kill Method Selection

A subsea blowout can be brought under control by vertical re-entry into the blowout well by a rig positioned over the well, reestablishing pressure integrity of the wellhead/BOP by subseameans, drilling a relief well for a reservoir flood kill, or drillinga relief well for a direct dynamic kill.

Vertical re-entry into the blowout wellbore was not consideredbecause of surface problems associated with the gas plume andflammable gas concentrations. Re-establishment of the wellhead/BOPpressure integrity by subsea means was not considered practicalbecause of water depth and high water currents induced near thewellhead by the gas flow.

The reservoir flood kill method requires drilling a relief wellparallel to and near the blowout wellbore. Seawater is injected intothe reservoir to reduce formation productivity and gradually killthe damaged wellbore. The method has been used successfully inthe past and would be considered in the planning of a relief well. Fig. 2 shows that a flood kill relief well would be an S-shapeddirectional well to run parallel to the blowout wellbore through theproduction interval. The relief-well hole angle would be higher thanin the direct kill method in the example. Hole stability problemscould also be a consideration. Reservoir thickness in the blowout well and variable formation permeabilities are potentiallyuncertain parameters that could reduce the chances of an effective floodkill.

A significant uncertainty associated with the flood kill methodis the possibility that the flood kill operation could inadvertentlybecome a direct kill operation. This could occur either byaccidental intersection of the blowout wellbore by the relief well or byfrac-turing into the blowout wellbore during flooding operations. Thepotential for a flood kill becoming a direct kill adds considerablerisk to the operation if the relief-well rigs are not equipped withthe pumping capability for this contingency.

The direct dynamic kill method, while requiring approximatelytwice the pumping capability of the flood kill method, representsthe safest approach for the blowout case discussed here. The directkill method avoids the downhole uncertainties of the flood killmethod and the added surface equipment requirements aremanageable. The relief-well hole angle requirement for the direct kill method is less than that for the flood kill method: this allows greater flexibility in determining surface positions for the relief-well rigs.

The main requirement for the direct kill is to establish directcommunication with the blowout well. Direct intersection can bachievedeither by drilling directly into the blowout wellbore orby establishing a hydraulic conduit between the blowout well andthe relief well (fracture). Direct wellbore intersections can beachieved with existing equipment and procedures. Proper planningof both the potential blowout well and relief wells can improve chances of success should a blowout occur.

JPT

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