Continuous Circulation Drilling

Abstract

Abstract In conventional rotary drilling, the circulation of drilling fluid to the well must be stopped each time a new joint or stand of drill pipe is added to the drill string. With a newly developed ‘Coupler’, continuous circulation to the well can now be maintained, while drill pipe is added to (or removed from) the drill string. Development of the ‘Coupler’ became the subject of an ITF (Industry Technology Facilitator) Joint Industry Project in October 2000, supported by five major oil companies plus Coupler, Maris and Varco. Two more oil companies have since joined this JIP. A 'Continuous Circulation System or ‘CCS’, incorporating the ‘Coupler’ will be undergoing trials in May/June, prior to commercial availability in the fourth quarter of 2002. This paper explains this new technology and estimates the economic benefits, which appear to average some $1.0 million per well drilled. The facilitation of other new technologies and improvements in drilling safety are also addressed. The ‘Coupler’ can be retro-fitted with relative ease to over a thousand existing rigs that use a Top Drive (Fig. 1). Up to a thousand more may justify upgrading to a Top Drive in order to use a ‘Continuous Circulation System’. The conversion of the Drilling Industry to Continuous Circulation Drilling may be relatively fast, once the many perceived benefits are proven in practice. Introduction Ever since rotary drilling was introduced some 100 years ago, the circulation of drilling fluid to the well has been interrupted every time a new joint or stand of drill pipe has been added to the drill string. The following are just some of the undesirable effects that are, or can be, caused by this interruption of circulation while breaking out or making up tool joints.The dynamic pressure drop in the annulus, the ECD (Equivalent Circulating Density), is lost;The bottom hole pressure drops; and often ‘undershoots’ the static pressure by hundreds of psi, sometimes inducing a kick and often damaging the exposed formation;When circulation is re-started, the down hole pressure surges and ‘overshoots’ the flowing pressure, causing supercharging of sensitive formations and possible lost circulationIn high angle sections of ERD (Extended Reach Drilling) and Horizontal wells, cuttings fall to the lower side of the hole, reducing the effective diameter and increasing drillstring torque and drag.The mud temperature increases, altering the mud properties, particularly in HPHT (High Pressure High Temperature) wells.;In UBD (Under Balance Drilling) wells, gas flow continues creating an unstable annulus together with cuttings and fluid dropout;The annulus contains columns of cuttings of varying concentration and mud of different temperatures.When re-starting drilling, the bit has to drill through settled cuttings and debris before reaching un-drilled formation again. These, and many more effects, often cause, or lead to, such drilling problems as: bore hole instability and wall collapse, stuck bit and/or BHA (Bottom Hole Assembly), differential sticking, tool failure, drillstring twist-off, formation fracture and lost circulation.