Abstract

Management of equivalent circulating density (ECD) is one of the key issues in designing and drilling extended reach well (ERW). The paper focuses on ECD fluctuation occurred while repeated formation and erosion of cuttings deposit bed in drilling ERW. Numerical studies of hydraulics calculation were carried out in combination with cuttings transport model along the whole trajectory of ERW. While the cause of ECD fluctuation is primarily thought to be the mud pumps being turned on and off, our calculations showed that repeated formation and erosion of cuttings deposit bed, due to the repetition of drilling and drilling interruption for making a connection, causes a significant level of irregular fluctuation of ECD. To avoid severe unexpected ECD fluctuation in drilling ERW, the authors recommend that the schedule of mud circulation operations for hole cleaning and ROP management should be carefully designed by hydraulics calculations in consideration of transient behavior of cuttings transport.

Introduction

ECD Management is one of the key issues in designing and drilling ERW. High ECD experienced while drilling a long highly inclined or horizontal section increases risks of drilling problems such as lost circulation, formation failure and consequently formation damage. In some situations, the reduction in mud flowrate to control ECD and to eliminate circulation losses may cause insufficient hole cleaning and negative effects on torque and drag. ECD is thought to be a more significant issue in Drilling ERW due to the following reasons (Mims et al. 2002):

  • Long measured depth relative to the vertical depth.

  • ERWs are generally shallow so that the formations often have little integrity.

  • ERWs generally use larger diameter drillpipe for hydraulics reason and/or prevention of buckling.

  • More aggressive flowrate is generally required for sufficient hole cleaning.

As well as the statically high ECD, fluctuation of ECD is known to cause borehole instability analogous to the formation fatigue failure. Borehole instability induced by ECD fluctuation could be a serious problem especially in drilling brittle formations and shallow ERWs in which the formations are prone to have little integrity.

It was reported in the field of extended reach drilling (ERD) that the actual ECD measured by pressure while drilling (PWD) was higher than the predicted ECD from modeling and that this high ECD was attributed to poor hole cleaning (Mallary et al. 2002). While the cause of ECD fluctuation is primarily thought to be the mud pumps being turned on and off, hole cleaning should be a vital factor affecting on ECD. From our previous numerical study on cuttings transport (Naganawa and Nomura 2006), the authors come to suppose that repeated formation and erosion of cuttings deposit bed, due to the repetition of drilling and drilling interruption for making a connection, may cause a significant level of irregular fluctuation of ECD. And this kind of ECD fluctuation could not be estimated nor detected from conventional steady-state hydraulics calculations.

This paper presents numerical studies on ECD fluctuation occurred while repeated formation and erosion of cuttings deposit bed in drilling ERW. We carried out a couple of simulation studies of hydraulics calculations in combination with cuttings transport model. From the result of simulation, we demonstrate the importance of thorough planning of ERD in which the schedule of mud circulation operations for hole cleaning and management of rate of penetration (ROP) should be carefully designed by hydraulics calculations in consideration of cuttings transport.

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