Infinite acting aquifer (IAA) – Implications for coalbed methane production at the Syncline of Umbita, Colombia

Abstract

The Syncline of Umbita (SU) in Boyaca, Eastern Cordillera of Colombia contains reserves of coalbed methane (CBM) estimated in 122 Bcf of gas, the major CBM play in the Cundi-Boyacense plateau. Despite its potential, the role of groundwater on gas desorption is yet to be addressed. The main constraint is water yield, the more sustainable, lower the potential for gas desorption since sustainable yield or additional water influx into reservoir is expected to inhibit the dewatering of the coal seams and thereby limit the coalbed methane recovery. Such flow attributes are known as “infinite acting aquifer” (IAA) type-of-flow, a condition with sustainable groundwater yield without imminent flow boundaries, also known in CBM as “ineffective-type of water”.Presence of IAA at the SU is primarily inferred by sustainable water yield of the coal reservoir interjected from prolific aquifers above and below by means of vertical hydraulic communication or cross-formational flow. In addition, conditions of sustainable water yield are reinforced by converging linear type-of-flow, which develops within the U-Tube-like hydraulic setting with vented ends conformed by the complete, quasi-symetric syncline in the Umbita region.Assessing conditions of IAA is achieved by weighing drawdown vs. dewatering under multiple production scenarios, which is best accomplished with the use of numerical methods. Therefore, a series of flow simulations were undertook forwarded to determining whether IAA would be a limiting factor for commercial development of methane in the SU. The results suggest that “infinite acting aquifer” type-of-flow likely drives groundwater flow in the syncline which may impact gas desorption. Main evidence provided by simulation is the numerical feasibility of having a coal reservoir with conditions of sustainable water yield under extreme dewatering regimes, conditions that are supported by induced recharge from aquifers above and below via cross-formational flow and development of converging linear type-of flow.This work provides a preliminary assessment regarding the hydrodynamics and reservoir depressurization style of what constitutes a unique field analog of a hydraulically-complex CBM reservoir. The main goal is to assist in future hydrogeological investigations and reservoir characterization that set the basis for CBM degassing of the coal reservoir to economically produce methane in the Syncline of Umbita.