Numerical study on hydraulic fracturing in tight gas formation in consideration of thermal effects and THM coupled processes

• Numerical tools to model the heat transport in a hydraulic fracture.
• Numerical tools to model the heat exchange between fracture and surrounding rocks.
• Modeling temperature warm back after shut-in.
• Modeling the cooling effect of injection fluid on propagation of the fracture.

This paper introduces and explicates a new thermal module that has been developed to study numerically the heat transport in a hydraulic fracture and heat exchange between the fracture and the surrounding reservoir rocks, as well as the THM coupled processes that take place during hydraulic fracturing in a tight gas reservoir. The new module was embedded in the simulator FLAC3Dplus and validated by comparison with several analytical examples. In this paper the new module was subsequently used to model the stimulation job that was executed in a tight gas reservoir (for anonymity reasons only known as X6) in the Northern German Basin. Its basis is the history-matching of the in-situ measured wellhead pressure. The paper presents the exceptional ability of the new thermal module to simulate the entire thermal evolution process during hydraulic fracturing, including a period of temperature recovery towards the original geothermal reservoir temperature (also termed temperature warm back) after shut-in. Further observations by this paper include the significant temperature increase at the fracture front during treatment. This is caused by a delay in the arrival of the fracturing fluid at the fracture tip. In addition, the paper draws a comparison between two simulations: one executed with a thermal module to investigate the THM coupled effects on fracture development and the other without taking the thermal effects into consideration. The basis for the comparison is that the models use (in terms of geometry and stratigraphy) identical mechanical, hydraulic and thermal parameters in both simulations. Results show that the cooling effect of the injection fluid leads to contraction of the rock formations thereby causing them to experience an extra load of tensile stress. In this regard, the hydraulic fracture will easily propagate in the direction of minimum horizontal stress (where rock stress also seems to have decreased remarkably). Finally, the paper observed that simulations carried out using the new thermal module achieved a wider but shorter fracture than that obtained in simulations without consideration of heat effects. Nevertheless, the fracture heights in both cases were almost identical due to the existence of caprocks with similar integrity. Trends indicate that the simulation without thermal module caused a greater leak off volume at the end of the operation, since the fracture surface attained in this case was larger.