Abstract
It has been a common practice in the industry to obtain core samples to determine gas content for coalbed methane (CBM) projects; however, due to economic issues, permeability testing has usually been avoided. To obtain core samples, the core hole is drilled, samples are collected, and the core hole is plugged per state regulations. Obtaining permeability test results in an openhole core hole on the other hand is time consuming and does not provide consistent results. However, if permeability testing initially could be completed in the core hole, not only could time and money be saved, but permeability data, core data, and logging information could all be tied together from the same wellbore. An informed decision could then be made based on this additional information to help determine whether to move forward with the project or move to another area.
The authors present a process that includes drilling a 3 1/2-in. core hole, collecting a 3-in. core, cementing 2 7/8-in. tubing, and conducting permeability testing using an injection/falloff technique. A unique cement blend is presented that reduces cement loss to the cleat system while providing a lightweight blend with "zero" free water and low friction pressures for placement. A test assembly consisting of slimhole packer, downhole shut-in device, and downhole pressure gauges is run in the hole on 1 1/2-in. tubing to provide isolation, eliminate wellbore storage, and record bottomhole pressure and temperature. A self-contained permeability test trailer capable of pumping and recording rates as low as 0.03 gal/min is essential to this process. The objective of the test is to inject fluid at matrix rates without fracturing the coal seam, then analyze the data using conventional well testing software and a CBM reservoir simulator to arrive at permeability and reservoir pressure. Production forecasting can then be simulated for project review.
