|Publisher||Society of Petroleum Engineers [successor to Petroleum Society of Canada]||Language||English
|Content Type||Conference Paper|
|Title||Numerical Modelling of Borehole Ballooning/Breathing-Effect of Fracture Roughness|
|Authors||M. Ozdemirtas, T. Babadagli, E. Kuru, University of Alberta|
Canadian International Petroleum Conference, Jun 12 - 14, 2007 2007, Calgary, Alberta
|Copyright||2007. Petroleum Society of Canada|
Borehole ballooning or breathing is commonly observed during drilling through fractured zones. It refers to small, partial and continuous mud losses and significant rapid mud gains due to annular pressure fluctuations resulting from mud circulation and non-circulation. Better understanding of the factors controlling borehole ballooning and/or breathing is needed for correct interpretation of the symptoms observed while drilling and to avoid mixing this phenomenon with lost circulation and well kicks.
This paper introduces a two dimensional transient model of borehole ballooning and/or breathing. The model considers the effects of Newtonian fluid rheology, and fracture roughness on the fracture volume change as a function of transient wellbore pressure fluctuations inherent in typical drilling operations. Different types of fracture surface roughness that are commonly observed in sedimentary rocks and the degree of roughness identified by a wide variety of fractal dimensions were considered.
The model was solved numerically to investigate the effects of fractures' natural geologic properties (fracture roughness, fracture dimensions, fracture surface deformation law) on the fluid loss and gain rate between the borehole and the fractured formation. Analyses on the importance of fracture roughness and non-linear deformation approximations were provided and situations where the degree of roughness becomes critical were identified.
Mud losses/mud gains have been a major problem in the drilling industry and the identification and treatment of this problem is still a crucial issue due to the high cost of the drilling operations. Several practical solutions have been recommended to avoid drilling fluid losses and gains. However, regardless of the type of treatment, significant rig time can be lost and these solutions can make the control of other drilling parameters required for a precise well design even more complicated.
Borehole ballooning or breathing is a recognized combined mud loss and mud gain phenomenon referring to the small, partial and continuous mud losses and significant rapid mud gains due to annular pressure fluctuations resulting from mud circulation and non-circulation. If the bottomhole pressure or Equivalent Circulating Density (ECD) exceeds fracture initiation pressure during circulation, drilling mud starts to escape into the fractured formation and more mud is required to maintain the hydrostatic head. As soon as the dynamic wellbore conditions disappear and the ECD falls below the Fracture Initiation Pressure (FIP) during a pump-off period because of a connection or flow check operation, sizeable amount of mud is gained back into the wellbore. A large amount of mud gain from formation when pumps are turned off can be diagnosed as a well kick. This misjudgment and its likely treatments can lead to unnecessary costly well control procedures.
Limited number of studies have been published about the mechanisms behind this phenomenon. According to Gill (1989), elastic deformation of the borehole wall due to the bottomhole pressure can explain this incident. Ram Babu (1998) proposed that expansion and contraction of the drilling fluid due to the temperature variations in the wellbore can be diagnosed as borehole ballooning.
|File Size||1,092 KB||13|