Two phase rate-transient analysis of a hydraulically fractured coal seam gas well: A case study from the Ordos Basin, China

Abstract

Apart from implications related to coal seam gas (CSG) reservoir engineering such as stress- and desorption- dependent permeability, 2-phase flow of gas and water adds further complications to rate-transient analysis (RTA) techniques in CSG reservoirs. This paper introduces the concept of area of influence (AI) to calculate water saturation in transient formation linear flow for 2-phase RTA in hydraulically fractured CSG wells. The relevant equations are derived and discussed and the application of the AI is demonstrated using reservoir simulation. The area of influence allows a superior estimation of water saturation compared to the dynamic drainage area (DDA) in under-saturated CSG reservoirs and hence improves the estimation of fracture half-length.A practical workflow is also developed and presented to estimate reservoir and hydraulic fracture properties using 2-phase RTA for hydraulically fractured vertical wells. The presented workflow works in a reverse order by first analysing boundary-dominated flow regime and then proceeds to other existing flow regimes developed at earlier production history of the well. Although analysis of each flow regime provides a standalone method to calculate some of reservoir properties, the full development of all flow regimes allows crosschecking and brings more confidence into calculation. In this workflow, the reservoir properties derived from later flow regimes, feeds earlier flow regimes to extract other reservoir variables such as fracture half-length. A hydraulically fractured CSG well from the Ordos Basin, China, is analysed using AI and the proposed workflow to demonstrate the field application of this study.