Abstract
Casing deformation is a key issue that restricts the efficient development of shale gas in the Sichuan Basin, southwest China. In this study, we take the Ning209 block as an example to analyze the characteristics of casing deformation distribution and the mitigation effect of using reduced treating parameters on casing deformation. The geological structure and in-situ stress characteristics of this block indicate that the high horizontal stress difference and high pore pressure may be the main cause of casing deformation. Hence, based on the fault likelihood and 3D in-situ stress model, a geomechanical probability model of fault slip is established to identify the areas with high risk of faults to explain the distribution of casing deformation and investigate the impact of reducing pumping rate on fault slip. The results show that the faults in the upper region of the block have a higher slip probability than the faults in the lower region, consistent with field casing deformation observation. The high stress difference and high pore pressure are the main factors causing a high slip probability of faults. After reducing pumping rate, slip probability is found to change from medium risk to low risk for faults in the lower region, which led to a significant reduction of casing deformation in the lower region. Reducing pumping rate can play a significant role in mitigating casing deformation caused by medium-risk faults, which has been proved by the field practices. This paper proposes a comprehensive method for preventing casing deformation by combining the fault slip risk assessment with the treating parameters optimization, which bridges the gap between prediction and control for mitigating casing deformation in the field.
Highlights
The Changning-Weiyuan national shale gas demonstration area is located in the Sichuan Basin, southwest China
It can be seen that under high stress difference and high pore pressure conditions, the slip risks of faults in the upper region are clearly higher than that in the lower region, which is the reason that casing deformation locations are mainly concentrated in the upper region
(1) The treating parameter optimization conducted in the Ning209 block shows that reducing the pumping rate has played a significant role in mitigating casing deformation
Summary
The Changning-Weiyuan national shale gas demonstration area is located in the Sichuan Basin, southwest China. Since 2009, horizontal drilling and hydraulic fracturing have been used to develop shale gas in this block. Serious casing deformation problems occurred during hydraulic fracturing operations. As of December 2018, among 113 wells that have completed fracturing in the Changning block, casing deformation occurred in 39 wells, accounting for 34%. As of October 2018, among 74 wells that have completed fracturing in the Weiyuan block, casing deformation occurred in 36 wells, accounting for 48%. As a result, casing deformation has become a key engineering problem that needs to be resolved urgently for shale gas development in the Changning-Weiyuan block. Does casing deformation decrease the number of viable fracturing stages and reduces the overall life cycle of the well, which in turn seriously restricts the efficient development of shale gas resources
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