Spatial Variation Of San Juan Basin Fruitland Coalbed Methane Pressure Dependent Permeability: Magnitude And Functional Form
Authors: Gierhart Roger R. , Clarkson Christopher R. , Seidle John P. 
Published: February 2016

The San Juan basin Fruitland coalbed methane (CBM) resource is the most significant CBM play discovered in the world to date and comprises areas within both Colorado and New Mexico of the United States. It contains three distinctly different performance areas within the 6,800 mile2 (17,600 km2) enclosed by the Fruitland outcrop. Two of these areas are considered in this topic: "Fairway" and "Colorado Type II". Explaining well performance in these areas has required the examination of a mechanism whereby coal permeability increases over time. Field data and pressure transient analysis (PTA) for Fairway wells have revealed that coal permeability does increase over time and is an exponential function of reservoir pressure depletion.

While evidence for pressure dependent permeability in CBM reservoirs has been presented in the literature before, this work seeks to compare the magnitude and functional form in two different reservoir units.

In the high productivity Fairway, well data monitored and gathered by a Supervisory Control & Data Acquisition (SCADA) system, including data from two non-producing pressure observation wells, reveal a representative area of the Fairway to exhibit an exponential increase in permeability with respect to reservoir pressure depletion. Time-lapse PTA confirm this phenomenon exists in the lower productivity Type II area as well. Significantly, the Type II area is one of continued, active development via infill drilling. A regulatory requirement of this infill program is that operators measure reservoir pressure in the infill wells drilled after July, 2000 at a prescribed frequency. While the SCADA system also monitors wells in the Type II area, there are no suitable pressure observation wells there. Further, the permeability in the Type II region tends to be significantly lower than in the Fairway. As a consequence, an offset pressure monitor well would tend not to be representative of average reservoir pressure in the drainage area of a Type II producing well. This precludes the Fairway approach and requires a time-lapse PTA approach.

While the rate of change of permeability with respect to reservoir pressure appears to be of exponential functional form in the Fairway, until recently it could be demonstrated only that the permeability increased with respect to reservoir pressure in the Type II area of the field. Insufficient testing precluded any claims as to the functional form. In the Type II area, second quarter 2007 pressure transient testing on each of ten wells drilled circa 2000–02 have provided a third data point of permeability vs. reservoir pressure. For these ten wells, one can begin to claim the rate of increase is exponential - as in the high capacity Fairway region of the basin. Over time, many other Type II infill wells will be tested to confirm more conclusively the functional form of permeability with respect to reservoir pressure. This will be of interest with respect to greater data density as well as with respect to obtaining data at ever lower reservoir pressures.

Introduction

Coalbed methane reservoirs are among the most complex reservoirs the industry manages. An important tool for understanding and predicting the performance of CBM reservoirs is numerical reservoir simulation. This requires the quantification of the change in permeability during the lives of the CBM wells. Other investigators1–9 (select references given only) have studied this phenomenon, often from a rock mechanics viewpoint. The time-lapse PTA presented in this discussion should serve as a check on these investigators' models and aid in their fine tuning. The data presented here show that permeability may increase by more than an order of magnitude over the lives of the wells which can impact dramatically their predicted performance. This work is limited to the coals of the San Juan basin.