Abstract

Fluctuations in the borehole pressure has been generally recognized as one of the major problems in aerated mud drilling due to its detrimental impact on borehole stability and formation damage. This paper focuses on analyses of bottomhole pressure fluctuations due to circulation break and backpressure variation. The pressure fluctuations due to circulation break is characterized by the gradient loss defined as ECD less EMW. The pressure fluctuation due to backpressure change is evaluated using the pressure instability factor defined as the ratio of change in the bottomhole pressure to the change in the surface choke pressure.

Sensitivity analyses with an analytical model indicate that the gradient loss decreases with depth, increases with both liquid pumping rate and gas injection rate, and decreases with backpressure. The analyses also show that the pressure instability factor increases with depth, decreases with both liquid pumping rate and gas injection rate, and increases with backpressure. At a given depth and backpressure, it is the combination of liquid and gas flow rate, rather than the injection GLR, that determines the pressure instability factor. Following a change in the backpressure, the transition time for borehole pressure to stabilize at a new level depends on several factors including well depth, well geometry, and liquid pumping rate.

This paper presents an easy-to-use method for predicting pressure fluctuations and identifies key factors affecting the pressure stability in aerated mud drilling. The results can be applied to designing of hydraulics for Underbalanced drilling (UBD) and managed-pressure drilling (MPD) wells.

Introduction

Aerated muds are mixtures of liquid (normally water or oil), gas (normally air or nitrogen), and solids (drill cuttings). The IADC Fluids Selection Guidelines list aerated muds as liquid-gas mixture with a density ranging from 4.0 to 6.9 ppg. This constitutes a liquid pumping rate between 100 gpm and 300 gpm and a gas injection rate ranging from 250 scfm to 1250 scfm in a typical hole size such as a 6" holes drilled with 3.5" drill pipes.

Aerated muds are currently used for Underbalanced drilling (UBD) and managed pressure drilling (MPD) operations for reducing formation damage and lost circulation.1,2 Fluctuation in the borehole pressure has been recognized as one of the major problems in aerated mud drilling due to its detrimental impact on borehole stability and formation damage.3,4 Severe wellbore damages and failures can result from the pressure stability problem. However, a systematic analysis of the subject problem has not been found from literature. This paper fills the gap.

A careful analysis of the pressure stability problem requires an accurate hydraulics model for multiphase flow. Although both steady state flow and transient flow simulators are available in drilling industry for aerated mud drilling hydraulics calculations,4–9 the results from these simulators are frequently conflicting10 due to different assumptions that were made in mathematical formulations. In addition, tedious procedure has to be followed to analyze the behavior of aerated mud with transient flow simulator. Guo et al.11 presented a closed form hydraulics equation and demonstrated excellent accuracy of the equation when applied to two field cases covering a deep horizontal well and a shallow horizontal well drilled with aerated muds. This hydraulics equation provided a basis for the analyzing pressure stability in this study.

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