Research And Application On Calculation Method Of Gas Content In Shale Gas Reservoirs

With the successful development of shale gas in North America,the third energy revolution has arisen.As a large country of shale gas reserves,China has paid great attention to shale gas exploration and development,and has achieved a series of achievements.There are three kinds of gas in the shale reservoir,free form,adsorbed state and dissolved state.The dissolved natural gas can be transformed into adsorbed state under certain conditions.Its content is relatively low,so shale gas mainly consists of two occurrences: free gas and adsorbed gas.In the exploration and development process,the content of shale gas has been the key point to evaluate reserves.The evaluation of free gas content of mainly refers to the existing evaluation methods of sandstone;The adsorption gas mainly uses the adsorption model to evaluate the gas content of the adsorbed gas.For example,Langmuir monolayer adsorption model,BET multi layer adsorption model,pore filling model,DA-Langmuir isothermal adsorption model and improved model based on Langmuir monolayer adsorption model,etc.In this paper,the basic characteristics of shale gas reservoir and the characteristics of shale reservoir pore space are introduced briefly.The complex and relatively special reservoir space of shale is expounded.Then,through series of petrophysics experiments,the influence of this kind of special reservoir space on physical properties is investigated and analyzed.The microscopic characterization method of shale pores based on isothermal adsorption curve is briefly described.The advantages and disadvantages of various methods are compared.It is considered that H-K,DA and QSDFT methods are more suitable for shale pore characterization.The electrical and nuclear magnetic resonance T2 spectra of shale in 5 plug cores are measured.It is found that the T2 spectrum has two or three peaks,and the front peak is obviously higher than the back peak.The main peaks of T2 are below 1ms,which fully reflects the special micro pore space of shale.The latter two peaks aregenerally around 15 ms and 130 ms,which reflect the medium-sized pores and clay layers of the shale respectively.Based on the T2 spectra of nuclear magnetic resonance provides a simplified model of the shale pore,the pore space is divided into two parts,one part is the diameter is below 6nm,the micro pore has strong adsorption capacity,a part of the pore is larger than 6nm and the natural gas mainly as free gas.The calculation accuracy of the free gas content mainly depends on the calculation of water saturation.In this paper,the Archie formula is used to evaluate the water saturation,and its applicability is discussed and verified.With analysis of the data,It is found that the n value is positively related to the sample specific surface and pore volume,and negatively related to the pore size;the m value is positively related to the sample specific surface and pore volume,and negatively related to the pore size.Besides,the stratification direction affects the saturation exponent n,and the n value of the sample of the vertical bedding is smaller,while for the horizontal bedding sample,the n value is larger and more reliable.This paper describes the basic characteristics of adsorption gas and the existing adsorption models briefly.The nuclear magnetic resonance T2 spectra are compared with the pore size distribution,and the functional relationship between the transverse relaxation time and pore size distribution is obtained.The existing three phase isothermal adsorption model of solid,liquid,gas is improved,and the concept of excess adsorption quantity is introduced to make it more in line with the actual formation.The method of calculating the parameters of the model is explored to determine the model parameters.Finally,the above method is applied to the actual well and compared with the core gas content data,and satisfactory results have been achieved.