(159o) Improved Shale Gas Material Balance Equation Considering Multiple Factors

Shale gas is one of the most wildly developed unconventional resources, which is mainly stored in the shale matrix and microfractures, and exists in the form of free, adsorbed and dissolved states. Reservoir engineers nowadays pay attention to accurately obtain the reserves to formulate reasonable development plans. Material balance method is usually used to calculate the OGIP of a gas well.

The conventional format of material balance is the sample straight line plot of p/Z* vs. cumulative gas production (Gp) proposed by King (1993). And later a simplified method of p/Z** vs Gp is introduced by Moghadam (2011). However, these two methods are derived based on the law of conservation of volume, but the volume of shale gas is not conserved due to the influence of the volume change of the adsorption phase during the development process. Moreover, both of these methods consider the free gas in the matrix and the microfractures as a whole, which optimistically estimates the free gas reserves in the matrix, and at the same time overestimates the adsorbed gas reserves in the matrix. In addition, studies by scholars have shown that solid kerogen contains a large amount of dissolved gas, and multi-component adsorption also has a greater impact on the calculation results of reserves. Unfortunately, these factors have not been taken into account.

Based on the law of conservation of mass, this paper establishes a shale gas reservoir material balance equation considering free gas in the matrix and microfractures, adsorbed gas in the matrix, expansion of rocks and bound water, dissolved gas in solid kerogen. And then the equation is organized as the sample straight line plot of p/Z* vs. cumulative gas production (Gp) to calculate OGIP by slope and intercept.

Through the application of the shale gas reservoir in Fuling, China, the free gas in the fractures occupies about 10% of the total reserves, and the dissolved gas occupies about 20% of the total reserves. Considering the change in the volume of the adsorbed phase, the calculated total reserves increase by 20%. Considering multi-component adsorption, the calculated adsorbed gas reserves increase by 30%.

The results show that in order to obtain more accurate reserves, cracks, changes in adsorption phase volume, multi-component adsorption, and dissolved gas in solid kerogen must be taken into account. Moreover, the material balance method proposed in this paper also provides a theoretical basis for the flow material balance method considering multiple factors, so that the production dynamic data can be used to calculate the produced reserves, solve the problem of difficult to obtain formation pressure, and improve the application value of this method.