Temporary Storage of Natural Gas in Coal Wells

Abstract

SPE Members Abstract A research project, nicknamed "ReGas," has been initiated to determine the technical and economic viability of using coal wells to temporarily store surplus methane gas. Coal wells are being tested to determine methane injectivity and flow back characteristics. Based on computer simulated studies, ReGas has several potential applications. ReGas could be used as a "shock absorber" to insulate gas fields from short term fluctuations in downstream operations. Furthermore, preliminary scoping economic analysis suggests that, if almost all of the methane injected into a coal well can be recovered, ReGas offers profit opportunities from predictable changes in gas prices (say from summer to winter months). Furthermore, ReGas could conceivably minimize water influx problems caused by shutting in coal wells for a prolonged period of time. Control over gas deliverability could also provide a gas field operator greater flexibility, which may improve operating efficiency. Results from two short (7 days) ReGas pilot tests are presented, and one field performance is simulated using a computer model. Longer term field tests are needed, and are planned. The impact of prolonged high rate gas injection on wellbore skin is also being studied. Some of the factors that may impact the viability of a coal well to temporarily store natural gas are discussed. Introduction Gas fields are directly coupled to downstream operations. Consequently, even short term perturbations in market conditions directly impact field activities. For example, if gas demand is low for a few days due to pipeline constrains or lack of customer demand, the field operator is forced to shut-in and/or increase back pressure on wells. Furthermore, since storage of natural gas is practically difficult, a gas producer is usually not able to take advantage of price swings, even when gas prices and demand are predictable. Storage of natural gas is not a new concept. Introduced in 1915, pipeline companies have used depleted hydrocarbon fields, aquifers, and salt caverns to store natural gas so that they can ensure adequate supplies for their customers. In this paper, the possibility of using coal wells to temporarily store natural gas is discussed. Originally proposed in 1988 as U.S. Patent 4,756,367 issued to Amoco, injection and subsequent recovery of natural gas from coal wells was nicknamed "ReGas" for convenience. The following brief introduction on the physical and reservoir properties of coal is to facilitate subsequent discussions on ReGas. Coal as a Reservoir Rock An underground coal seam is a dual porosity reservoir rock. While the primary porosity of a coal seam is associated with its micro-porous matrix, the permeability of a coal deposit is entirely the result of a system of natural fractures known as cleats. The porosity of the coal cleat system is very low. For example, it is estimated that the cleat porosity of the sub-bituminous coals in the San Juan basin of Colorado is only of the order of 1%. Therefore, storage of free gas in the coal fracture network is clearly not attractive. What makes coal a unique reservoir rock is its ability to adsorb methane gas. Adsorption is the process wherein gas molecules are physically held on to a solid surface by weak molecular forces at near liquid densities. It is widely recognized that micro-porous coal structure has very high surface area. P. 709^