Evaluation of Coal Seam Permeability in Marginal Reservoirs
Recent increases in gas sales prices have made marginal coal gas reservoirs commercially viable. As a result, operators have been completing coal seams that in the past would have been considered to have natural fracture permeability too low to be exploited. These marginal reservoirs are typically stimulated by hydraulically fracturing to increase gas productivity. Due to the high cost of stimulation and the need to minimize costs to exploit marginal reservoirs, it is important to avoid stimulation of coal seams that will not yield commercial gas production rates. Pre-stimulation water injection-falloff testing of multiple seams considered for stimulation in a single well has proven to reduce overall costs. Avoidance of stimulation of one seam generally results in completion cost savings that are greater than the cost of the testing program.
We improved methods analyzing water injection-falloff tests performed in lower permeability coal seams to increase reservoir pressure and natural fracture permeability estimate accuracy. In many cases, water injection-falloff tests, which are the most common permeability evaluation method, increase the permeability several times above the permeability that will control production in marginal seams. It is easy to misinterpret these data to expect substantially greater post-stimulation productivity than possible. Our analysis method is based upon history matching both the injection and falloff data with mathematical models that account for the permeability changes. The analysis also results in in-situ natural fracture porosity estimates. Porosity estimates can be used to predict the volume of water that will be produced.