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 paper, 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. Then, the flow field analysis was conducted to investigate the impact of the perforation phase on erosion and chambering. Finally, pre-fracturing and post-fracturing erosion data were obtained by the downhole video imaging technology, which was employed to validate against simulation results. Based on above technics, the erosional pattern of perforation would be clearly revealed.
The CFD simulation results demonstrated that, perforations were rapidly eroded to droplets (a typical streamline shape) from the initial round shape because of the much higher erosion rate on the upside of the perforation (near toe) than the downside (near heel). The CFD modelling also noted that there was huge difference of erosion rate between perforations at different phases. Perforations at phase 0 degree (high side of the horizontal wellbore) suffered the minimum erosion rate while perforations at phase 180 degree (down side of the horizontal wellbore) got the maximum. As for perforations at phase 90 and 270 degree, their erosion rates were intermediate，and the values were similar. What's more, the above simulation results were verified by the downhole video imaging data. Most post-fracturing perforations look like droplets. The most eroded perforation was near phase 180 degree with an average diameter of 18 mm which was 1.7 times larger than that of perforations near phase 0 degree (11 mm), while post-fracturing perforations at phase 90 and 270 degree were 13.5mm. Therefore, the 180 degree perforation phasing would help to reduce the 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.
