Multi-fractured horizontal well for improved coalbed methane production in eastern Ordos basin, China: Field observations and numerical simulations

Abstract

Eastern margin of Ordos basin has been an active commercial coalbed methane (CBM) production area in China, where coal seams are typically characterized with thin layers, low permeability and high gas content. Hydraulically fractured vertical wells (HFVWs) are the primary means for recovery of CBM in this area, which however are usually subject to low gas production and unpromising economics. Although the multi-fractured horizontal well (MFHW) has proven as an effective solution for producing hydrocarbon resources from both tight and shale reservoirs, its applicability in efficient development of CBM reservoirs remains yet unclear.This paper presents a pilot field application of MFHW to improve CBM recovery in eastern Ordos basin. A horizontal well stimulated with 6 stages of hydraulic fractures was observed to significantly outperform HFVWs in terms of depressurization efficiency, gas production and ultimate recovery factor. Peak gas rate of the MFHW is more than five times higher than an HFVW. History matches of the MFHW and representative HFVWs were conducted to obtain key reservoir properties that are subsequently used for production predictions. Numerical simulation predicts that cumulative gas production for the MFHW is 3–15 times as that for each of the HFVWs during a production duration of 15 years. A total of 500 stochastic geological scenarios for the study area were simulated to analyze the effect of geological parameters on the superiority of an MFHW over a group of four HFVWs (which are at equivalent well construction outlays) in terms of cumulative gas production. A correlation was then developed using the alternating conditional expectation (ACE) regression method, resulting in a linear relationship between an indicator of the superiority with permeability and desorption pressure in a log-log coordinate system. This correlation can be potentially used as a criterion for fast determination of the optimal completion option in the target CBM seams. The regression results also show that the superiority of an MFHW over HFVWs is especially highlighted in thin coal seams with low permeability and low gas content. Finally, the effect of the size and spacing of stimulated area (SA) on MFHW producibility was analyzed, showing that increasing the spacing between SAs is an economically viable option when the budget for fracturing operations is limited.