Investigation of Fautls Geomechanical Activity and Its Application to Development Program Optimization in Kelasu Gas Field in Tarim Basin
Authors: Jiang Tongwen , Zhang Hui , Wang Haiying , Yin Guoqing , Yuan Fang , Wang Zhimin 
Published: October 2017

The Kelasu gas field located in northern Tarim Basin had experienced four tectonic evolutions, with the most intense deformation between northern margin of the basin and southern Tianshan Mountains. A series of sandstone faulted anticline gas reservoirs were produced after the Himalayan movement. Faults were the main channel to transport natural gas from Jurassic coal-bearing formation to sandstone reservoir in Cretaceous. Simultaneously, the faults play a key role for fluid flow during the development of the gas field, but it is a huge challenge to evaluate the influence of faults on fluid flow quantitatively with depletion. To solve this problem, an integrated research combined geology, geomechanics and gas reservoir engineering was conducted. Firstly, 6 geological factors associated with connectivity and sealing properties of faults was analyzed to determine the critical factors among them. Secondly, based on 4D geomechanical modeling and 3D stress analysis of faults' plane, a calculation model of faults geomechanical activity index (FGAI) was built. Finally, the relationships between faults geomechanical activity and performance of gas field development were investigated to understand the influence of faults' mechanical behavior on production and water invasion during development in Kelasu gas field.

It is shown that faults geomechanical activity has profound influence on the performance of Kelasu gas field. ➀The faults geomechanical activity is one of key factors to control permeability, which can indicate the difference of permeability around faults and permeability variation during depletion. ➁With the depletion during exploitation the in-situ stress regime in Kelasu gas field changed from strike slip to normal faulting, and the heterogeneity was also gradually increasing which two resulted in the variety and complicate of faults' geomechanical activity. ➂It is found that there is a good correlation between the faults geomechanical activity and water invasion. The water breakthrough was early and gas-water interface rose fast near the faults with higher geomechanical activity index during depletion. ➃The complex relationship between stress field and faults system resulted in a great difference of faults geomechanical activity index in different location of reservoir. FGAI (Faults geomechanical activity index) is the highest in western reservoir, followed in turn by the eastern, northern, southern, so there is the most rapid uplift of gas-water interface in the western, followed in turn by other parts. Based on evaluation of faults geomechanical activity in this area, this reservoir could be divided into three blocks by different water invasion risk. Areas and gas wells with high risk water invasion were warned in advance. ➅For optimization of well placement, we found that FGAI is relatively low in northwestern reservoir, the fault sealing ability is high, the research provided one of basis for the placement of a new gas well.

A fault geomechanical activity index (FGAI) model for the gas reservoir with complex structure and high pore pressure and high in-situ stress was established. And its validity and effectiveness toward development of gas field was proved by production data and information. Based on the quantitative classification and description of faults geomechanical activity to investigate the influence of faults on water invasion, the mechanism of heterogeneous water production was determined in Kelasu gas field. The research provided the sealing evaluation of faults for new wells placement and risk prediction of water breakthrough for gas wells during depletion.