Keshen is a high-pressure/high temperature (HP/HT) tight-sandstone gas reservoir with reservoir pressure over 110 MPa and temperature over 175°C. The sandstone is very hard, with unconfined compressive strength (UCS) greater than 100 MPa. Given the HP/HT nature and natural fracture system in the reservoir, with aid of stimulation, many wells produced at a very high rate with the mean value exceeding 500,000 m3/d. In the last few years, many production wells in this reservoir experienced severe sanding issues that contradicted the conventional understanding that sanding would not occur in such hard rock. The sanding wells exhibited large fluctuations of production rate and wellhead pressure, erosion of chocks and nozzles, and eventually major or even complete loss of production. A solution to address the sanding issues was urgently needed because the sanding issues had caused a major decline in production and resulted in significant economic loss.

Due to the unconventional nature of the sanding issues, the typical sanding prediction methods based on solely on evaluating rock failure were not adequate to reveal the underlying sanding mechanism and develop a viable operational solution to address the sanding issues. To this end, a new workflow was formulated and applied to this study. The workflow started with detailed data mining on the massive amount of drilling, completion, stimulation, and production data from this reservoir to investigate possible relationships of drilling practices, completion options, and production schedules to the occurrence and severity of sanding issues. The analysis revealed that downhole flow velocity and production drawdown were the two major controlling factors in the occurrence of sand production. Further geomechanics simulation and particle migration simulation with a multiphase dynamic flow simulator confirmed that the production drawdown would cause failure of the rock near the wellbore and the gas flow could transport the sand debris to the wellbore and lift it up to the surface. In addition, the fluctuation of production rate was caused by blockage due to accumulation in the wells and production tubing of sand particles that were flushed out after downhole pressure buildup.

Based on the analysis, the threshold of flow velocity and the threshold of drawdown were identified, and these thresholds can be used in the reservoir management to address the sanding issues.

The experience in Keshen shows that sanding is possible in HP/HT high-productivity sandstone gas reservoirs, even in extremely hard formation, which overturns some prior conceptions on sanding. The information shared from this paper could bring up the attention of those operating similar HP/HT tight sandstone reservoirs around the world.

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