Study on Perforation Erosion in Hydraulically Fractured Shale Gas Wells Through Downhole Video Imaging and Computational Fluid Dynamic Modelling

Abstract The limited-entry technology was a key technology in the multistage fracturing of shale oil and gas horizontal wells to help distribute fracture fluid among multiple perforation clusters evenly. However, perforations could be eroded and enlarged because of high-speed and continuous abrasion by the proppants, resulting in unequal inflow and uneven cracks propagation between perforation clusters, which caused low efficiency of the fracture treatment. Considering that the perforation erosion has not been fully understood yet, special attention should be paid to this aspect of research. In this work, post-fracturing perforations erosion images were collected by the downhole video imaging technology, which was employed to qualitatively analyze the geometrical morphology. Then, the erosion data (perforation diameter & perforation area) at different phases were utilized to quantitatively summarize the perforation erosion & chambering laws. Finally, perforation erosion models were built using computational fluid dynamics (CFD) to simulate the distribution of the flow field when high-speed fracturing fluid with proppants injected perforations, which would help interpret the analysis of the results. Based on above technics, the erosional pattern of perforation would be clearly revealed. One thousand and two perforation erosion images from medium to deep shale gas wells in Sichuan and Chongqing regions demonstrated that post-fracturing perforations appeared in four different geometrical morphology, including round (under-erosion, 10%), droplets (normal-erosion, 52%), near-droplets (over-erosion, 25%) and irregularity (abnormal-erosion,13%). Due to the influence of the proppant concentration profile, perforations located near the downside of the horizontal wellbore suffered more severe erosion, for example, the most eroded perforation was near phase 180 degree with an average diameter of 18.4 mm which was 1.8 times larger than that of perforations near phase 0 degree (10.4 mm). What's more, the CFD simulation results were in perfect agreement with the downhole video imaging data. The simulated flow field distribution map showed that there were huge difference in erosion rates between perforations at different phases. Perforations at phase 0 degree (high side of the horizontal wellbore) suffered the minimum erosion rates while perforations at phase 180 degree (down side of the horizontal wellbore) got the maximum. Therefore, the 180 degree perforation phasing would help reduce uneven erosion and to avoid oversized perforations. It was a novel approach to integrate the CFD modelling and downhole video imaging technology to quantitatively characterize the perforation erosion pattern, which presented a better dynamic understanding of perforation erosion rather than the previous static analysis by field observations or simulations. This approach would help the optimization of perforation designs and the improvement of field fracturing effect.