Keshen Gas Field is part of Kuqa Foreland thrust belt, located in northern margin of the Tarim basin, West China. It has extreme reservoir conditions of ultra-deep(6500-8000m), high temperature(170-190°C), high pressure(110-120MPa). Because of its low porosity(2-7%), low matrix permeability(0.001-0.5md), hydraulic fracturing has become an important way to enhance single well performance. However, sandblasting during sleeve hydraulic fracturing often induces the erosion of casing, which can further result in the decrease of casing callapse strength and deformation of casing. Understanding the effects of erosion on casing strength is of fundamental importance to realize efficient usage of sleeve hydraulic fracturing in the mentioned tight gas reservoir.
In this study, experiments were conducted using self-made jet erosion device to investigate the effects of proppant impact angle, slurry flow velocity and poppant content on the weight loss rate of TP140 steel. The slurry was a mixture of 40/60 mesh high density ceramsite proppant and 0.2wt% guanidine gel fracturing fluid commonly used in Tarim oilfield. Based on the experimental results, an erosion rate calculation model for TP140 casing was established. The erosion rate was a function of slurry flow velocity, proppant impact angle and proppant content.
The erosion rate calculation model was programmed and coupled to the numerical simulation software (FLUENT). The flow and erosion rate distribution around sleeve nozzle and perforation during hydraulic fracturing was discovered using numerical method. Results showed that during the fracturing process, fracturing fluid flowed from sliding sleeve nozzle to casing straightly, resulting in the largest loss of casing wall thickness in the area of sleeve nozzle outlet. Relatively speaking, the erosion of perforation is little during fracturing. And a case study was presented. For well KeS2-2-12, its total fracturing fluid volume was 2500m3 and delivery rate was 6m3/min, the wall thickness loss of TP140 casing would be 1.04~2.50mm when fracturing operation was completed.
The residual collapse strength of casing suffered erosion was calculated. The results showed that for the worst case that the wall thickness loss of casing was 2.5mm, the residual callapse strength of casing was only 64.8% of the original strength before fracturing. Thus, it can be seen that erosion caused by sandblasting during hydraulic fracturing has a great influence on the TP140 casing strength, which brings us the challenge of well integrity. Therefore, hydraulic fracturing parameters should be optimized to reduce the erosion.
Nowadays, hydraulic fracturing is a common and effective technology in tight gas reservoir. This paper discovered the effects of erosion on TP140 casing strength due to sandblasting during sleeve hydraulic fracturing in Keshen tight gas reservoir, providing a good reference for sleeve hydraulic fracturing parameters optimizing.