--> New Method For Disassociate Rate And Permeability Evaluation In Gas Hydrate Reservoir

AAPG Asia Pacific Region Geosciences Technology Workshop:
Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?

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New Method For Disassociate Rate And Permeability Evaluation In Gas Hydrate Reservoir

Abstract

Understanding gas hydrate as an alternative energy is of increasing interest substantially in China, especially with remarkable success of gas hydrate production test in Shenhu region in South China Sea in 2017, much more investment and technical focus has been put in the exploration and production evaluation in the gas hydrate reservoir. To evaluate the productivity of gas hydrate, two key parameters, the dynamic change of permeability and corresponding dissociate rate of the hydrate, need to be understood well. In the literate review, to measure these two parameters, the core lab is the main tool for this evaluation, but the drawback of this method is that the measurement must be done in the artificial manner, because the real gas hydrate formation is difficult to restore in the laboratory. On the other hand, formation evaluation of integrated approach progressed very well on the original state of static reservoir properties in hydrate, but this method can’t capture the dynamic change when the hydrate is under disassociate process. This paper proposed a novel method of permeability and disassociate rate measurement by using Interval Pressure Transient Test (IPTT)*(*Schlumberger). The methodology overcomes the obstacles that core lab and formation evaluation encounter and measure and monitor the dynamic change downhole in the real formation during the mini-production period. There are two methods to characterize gas hydrate disassociate rate, one is called energy balance model, and the other is called kinetic model; the method that IPTT used is kinetic model Kim-Bishnoi. It can measure the different pressure drawdown and corresponding gas and water fraction of total flow during the test, and then the result can feed the parameter into Kim-Bishnoi equation to derive the disassociate rate of hydrate. As for the permeability evaluation, numerical method for pressure transient analysis is proposed in this paper to address two major limitations that the conventional analytical method has: complex geometry and non-linear diffusion problem. In the gas hydrate reservoir, non-Darcy flow, multi-phase flow, and unconsolidated formation can’t be ignored for PTA, integrating the core lab data with numerical solution can better predict average permeability and capture the permeability change during hydrate disassociation. Last but not the least, the paper emphasizes the importance of an integrated approach; i.e., the various petrophysical measurements give a clear picture of the original static properties in the gas hydrate reservoir, and the combination with the dynamic measurement can not only contribute to the productivity evaluation, but also characterize and capture the dynamic change of the reservoir for better understanding of gas hydrate behavior under reservoir condition downhole.