Natural Productivity Analysis and Well Stimulation Strategy Optimization for the Naturally Fractured Keshen Reservoir

Abstract

The Keshen reservoir is a naturally fractured, deep, tight gas sandstone reservoir under high-tectonic stress. Stimulation is critical to the successful development of the Keshen gas field. However, due to the heterogeneity of the natural fracture system in the reservoir, there is large variation in natural productivity from production wells, which mandates different types of stimulation operations from simple acidizing to massive hydraulic fracturing. Hence, to improve the efficiency of reservoir stimulation and overall performance of the reservoir development, reliably evaluating the natural productivity from production wells and optimizing well stimulation strategy (by selecting appropriate stimulation operations) are required. To this end, single well production history matches were conducted to calculate the natural fracture (NF) permeabilities for 12 acidizing wells with known matrix petrophysical properties. It was found out that the NF permeability is clearly correlated to the intersection angle ((, angle between maximum horizontal stress and NF strike) and Young's modulus (E), based on detailed natural fracture interpretation and geomechanics analysis. By using a correlation with the intersection angle and Young's modulus, the NF permeabilities for seven hydraulic fractured wells with large size treatment were obtained and the natural productivity for each well was predicted. By comparing predicted natural productivity with actual post-stimulation productivity, the productivity enhancement by hydraulic fracturing treatment was determined. The comparison showed that the productivity improvements by hydraulic fracturing treatment were very limited and unnecessary for the wells with high natural productivity, while improvements were substantial for others. Subsequently, the well stimulation selection criterion was formulated, which classified wells into three categories based on the natural productivity, and specified stimulation method was recommended for each well category, respectively. This paper shows an example: the natural productivity of a newly drilled well was predicted, and massive hydraulic fracturing was selected based on the stimulation treatment selected criterion. The well was successfully stimulated with absolute open flow (AOF) of 3.6 MMm3/d, which doubled the natural productivity. The workflow used to estimate the natural productivity of the tectonically active Keshen reservoir and to establish the well stimulation selection criterion, and their application and validation, which are rarely published previously, are presented in this paper.