Beyond 8 km Departure Wells: The Necessary Rig & Equipment
Authors: Gammage J.H. , Modi S. , Klop G.W. 
Published: December 2007

Drilling rigs were traditionally designed and built to drill vertical wells. Their horsepower rating and capacity is still quoted in terms of vertical depth. Such rigs and associated tubulars have limitations when drilling ERD wells and require modifications to give them the necessary capacities to drill to 8 km departure and beyond. This paper will initiate the conceptual design of an ERD drilling rig capable of reaching these departures.

Introduction

It is often not possible to build the ideal new rig for a particular drilling programme, we will therefore consider the limitations of existing (conventional) rigs for drilling ERD wells and consider how to extract the maximum potential from such existing equipment and the necessary additions or modifications to give them the required capacities. While drawing heavily on the experience with Deutag's rig T-47 at BP's Wytch Farm oilfield (Fig. 1), where one well in the current ERD programme has already been drilled to an 8 km departure at 1,600 m TVD and one is planned to reach 10 km in 1997 (Fig. 2), information has been drawn from other such projects and also from those that are still as yet in the conceptual stage.

This paper looks primarily at the surface and downhole equipment supplied traditionally by the Drilling Contractor. Due to the ongoing advances in ERD technology any recommendations for rig design or modifications made today are unlikely to be valid in a year's time. From experience at Wytch Farm it would be prudent to build a "redundancy factor" of, say, 25% into all aspects of rig design to allow for future developments. But, wherever possible, extra equipment and associated power requirements should be minimised by looking at ways in which to reduce loads through mud pump pressure and drillstring torque-and-drag reduction.

Conventional hole sizes have been used throughout since there is currently limited small-hole equipment available with which to drill these wells. Smaller hole sizes might be considered for a mature ERD application where the risks are better understood. The attached Figures consider a theoretical well with 33,000 ft (10 km) of 12 1/4" hole and a final measured depth of 49,500 ft (15 km) in 8 1/2" hole.

1. Drilling Mud and the Mud System

Hole cleaning is of critical importance when drilling the high angle tangent sections of ERD wells. To obtain the necessary annular velocities higher flow rates result in a higher demand on the mud pumps. Experience at Wytch Farm shows that above a tangent angle of 80 degrees in 12 1/4" hole flow rates of at least 1,100 gpm are required to clean the hole as it is being drilled. On an existing rig it is therefore often necessary to:increase the number of mud pumps from two to three or moreincrease the power rating of the pumps from 1,600 to 2,000 or 2,200 HPincrease the pressure rating of the pumps and surface system from 5,000 to 6,000 or 7,500 psi.

Apart from the capital cost of option "c" the maintenance costs can be expected to double for the higher pressure equipment. It is unforeseen that the full output from even three 2,200 HP mud pumps would be required simultaneously but at the anticipated higher surface pressures consumable parts replacement occurs more frequently and the extra pump(s) may allow less disruption to the drilling programme from pump repairs. The extra capacity may also allow only one liner or plunger size to be used for the complete well programme.

While there may be constraints on the maximum number of mud pumps installed, the chosen solution will inevitably be a compromise between mud pump capacity and drillpipe size(s).

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