Diffusivity and Covariance Analytical Approaches to Estimate Inflow Performance Relationship for Coalbed Methane Reservoir

Alhamzany, Isna Kamalia; Hutami, Dinar Citra Indar; Permana, I Gde; Wicaksono, Sri Konsep Harum; Alam, Hudan Wira
Petroleum Engineering Master Program, Institute Technology of Bandung, Bandung, Indonesia

A relationship between bottomhole pressure and its production rate is known as Inflow Performance Relationship (IPR). IPR can be used to asses well performance in both oil or gas reservoir, then its result is used in determining the economics of producing a well. This IPR curve and its validation for CBM well had not been developed well. In Coalbed Methane (CBM) production, unlike the conventional gas reservoir, the critical desorption pressure needs to be created by declining bottomhole pressure so that gas can be desorbed from inner matrix then flow into the wellbore.

Calculation for determining Inflow Performance Relation on Coal Bed Methane can be subjected into two methods. Firstly, reservoir on coalbed methane can be modeled same as conventional gas reservoir. The flow is contributed only by porosity and permeability of rocks. In coal seam reservoir, porosity and permeability is represented by cleat (natural fracture). By ignoring the diffusivity, gas flow can be modelled using Vogel's equation. Secondly, diffusivity is accounted into calculation. Diffusion of methane exist on saturated condition. On saturated condition, critical desorption pressure is reached so that methane (CH4) will be released into cleat from micropore. On the otherhand, on undersaturated condition the existing of diffusion is very small and can be neglected.

The objective of this paper is to show the effect of diffusivity while accounted into flow rate calculation. While many equation that used in Coal Bed Methane is more emphasized on Vogel's equation without diffusivity assumption. By comparing covariance of two models, the best result can be obtained. The result shows that the diffusivity covariance difference of Vogel and diffusivity are similar. It can be concluded that the diffusion does not have significant effect to estimate Inflow Performance Relationship.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012