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Effectiveness of Horizontal Wells in Coalbed Methane Plays

Palmer, Ian
Higgs-Palmer Technologies, Albuquerque, NM

     This topic addresses how wellbore damage (skin) and horizontal well geometry can affect production from horizontal CBM wells. Based on field performance from several basins, horizontal wells often give more than 4 times greater gas production than vertical wells. This result depends strongly on near-wellbore damage (i.e., positive skin factor), but the effective skin in a horizontal well is moderated by the well geometry. Sources of positive skin are reviewed.
     As well as skin factor, horizontal/vertical gas production depends on diameter, length, permeability, and seam thicknesses accessed by both horizontal and vertical wells. We will report on a study of these factors, and which have the most influence, using a pseudo-steady-state model.
     The impact of wellbore flow effects, including friction pressure, hydrostatic pressure, and multiphase flow, when coupled with wellbore geometry was also examined. This model predicts that small wellbore diameters and small build radii give better production at lower reservoir pressures, and gives quantitative differences in gas rates. Also we show that undulations and down dip wells can reduce production substantially. This means longer horizontal wells can lose their production advantage at low reservoir pressure.
     Finally, smaller wellbore diameters are stronger, and therefore more stable during drilling and production, and we demonstrate how this translates to safe operating conditions (i.e., without wellbore collapse and fines production). The use of liners is suggested whenever possible, to avoid loss of effective length in the event of well collapse.


AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah