Abstract
Coalbed methane (CBM) plays usually consist of multiple zones with geologic variability within the areal extent of the play. One of the key indicators of well productivity potential is permeability, which is highly variable zone by zone and area by area. Permeability is also important for understanding gas in place (GIP) and production variability, designing the completion, optimizing well spacing, and diagnosing production problems. Numerous tests may be required to develop a statistically valid permeability data set. The longer rig time and lower-than-expected success ratio of conventional tests are considered the main obstacles in obtaining such a data set.
The wireline-conveyed formation tester (WFT) used with a dual packer provides measurements of the permeability, skin, pressure, fracture flow regime, reservoir heterogeneity, and state of saturation of the coal seam through drawdown and buildup. Compared with conventional testing, improved operational efficiency is achieved through more efficient wireline conveyance, improved packer isolation, reduced wellbore storage, ability to conduct downhole fluid analysis, precise downhole rate control, and surface readout and real-time data monitoring and interpretation for informed decisions.
Measurement in the openhole conditions allows operational flexibility. The packers are typically set in the bounding shale or sand. To accommodate a multilayer coal seam with varying thickness and typical washout in the bounding shale, several wireline trips may be required to change the tool configuration and distance between the packers at surface. Previously run openhole logs such as dual-axis caliper, acoustic/resistivity borehole imaging, advance dipole sonic, and 3D induction resistivity are used for the coal seam selection and packer positioning.
Conducting a WFT productivity test in a low-permeability coal seam provides atypical challenges from conventional oil and gas applications. The method was tested using field data and pressure transient analysis from six CBM wells in eastern and central Kalimantan of Indonesia.
