A New Approach to Evaluate Stimulation Effectiveness in Unconventional Reservoirs
Authors: Shi Juntai , Li Shuang , Sun Fei , Li Jing , Li Xiangfang , Zang Jiali , Li Yanzun , Zhang Fuxu 
Published: March 2015

Abstract
Unconventional resources such as shale gas and tight oil are becoming a major contribution to the world energy. The development of unconventional resources has been boosted by the advancement in horizontal multi-stage fracturing technology. Field results show that about 40% of the perforation clusters were not producing in shale reservoirs. However, it is very challenging to identify the successful stage and its corresponding production contribution to overall recovery along horizontal well. Hence, a detailed study for characterization of the effective stimulation stages is significantly necessary.
This paper presents a new approach that can be used to evaluate stimulation effectiveness of horizontal wells. The transient pressure model for multi-fractured horizontal well (MFHW) in shale gas reservoir is derived by utilizing the point source function, Newman Green's function and Laplace transformation, where the difference of fracture flow rate between fractures is considered. Well bottom hole pressure is calculated based on the numerical inversion algorithm and a template curve is made. Effects of effective stimulation properties on well bottom hole pressure are considered. Results demonstrate that, in the log-log plot of dimensionless pseudo pressure derivative versus dimensionless time, the less the fracture interval is, the less value of the first horizontal line before achieving the second horizontal line with y-axis value 0.5, the more obvious the bilinear flow is, and the longer the bilinear flow lasts for; the closer the ineffective fracture locates to the pressure gauge position, the more sensitively the type curve in the linear flow regime changes from the type curve for MFHW with the same number of evenly spaced fractures; and the further the most effective fracture locates from the pressure gauge position and the lower the fracture flow rate at the pressure gauge position is, the earlier the bilinear flow appears, the more obvious the bilinear flow is, and the longer the bilinear flow lasts for. We compare bottom hole flowing pressure of multi-fractured horizontal well from a numerical model with the template curve to verify the reliability of the proposed model.
This work can provide a quantitative assessment of the stimulation effectiveness in shale gas and tight oil reservoirs. Also, it can provide guidance on optimization of hydraulic fracturing treatment design in new well development.

1 Introduction
The technology of multi-fractured horizontal well (MFHW), which was multistage fractured along the well, has become the mainstream of the unconventional resources. Meanwhile, the complicated mechanism of fluid flow in multistage fracture and the interference between each other make the fractured horizontal well testing analysis become more complex. Also, field results show that about 40% of the perforation clusters were not producing in shale reservoirs. So understanding pressure transient behavior is of importance to provide an insight into a long-term production performance, which could be approach to identify the successful stage and its corresponding production contribution to overall recovery along horizontal well.