(a) Time Temperature Index (TTI) method
The basic idea of this method was first put forward by Lopatin( 197 1), and then it was improved by Waples( 1980) according to the measured data of 402 samples in 3 1 watershed, and its numerical simulation method system was established, so it was also called Lopatin-Waples method. TTI method follows the basic law of chemical reaction kinetics to measure the contribution of temperature and time to coalification, that is, the rate of coalification increases with the increase of heating temperature, and the rate of coalification doubles with the increase of 10℃.
Therefore, the mathematical model of TTI can be expressed as a continuous function:
Coal Gasification in South Shanxi and Its Paleogeothermal System —— Also on the Geological Mechanism of Gas Control of Coal Gasification
T=Ts+G D
Where: t-paleogeothermal temperature of coal seam heated,℃;
Ts—— paleosurface temperature,℃;
G—— paleogeothermal gradient,℃/m;
D- buried depth of coal seam, m.
The above-mentioned continuous function mode is not convenient for routine operation, so in practical work, piecewise integration is usually adopted and calculated according to the following formula:
TTI=∑γn △t
Among them: γ-temperature effect coefficient, taking into account the relationship that the coal rank value doubles every time the temperature rises 10℃, take r = 2；;
N- temperature index, whose size depends on temperature (Table 5-1);
△t—— heating time of coal seam at a certain temperature, Ma.
Table 5-5- 1 TTI Corresponding Relationship between Temperature Index (N) and Coal Seam Heating Temperature Section in Mathematical Model
Note: Every time the temperature increases (decreases) 10℃, the temperature index increases (decreases) 1.
The TTI value itself can be used as the maturity index of coal rank or organic matter, and Waples( 1980) also provides the relationship between TTI value and the maximum oil-immersed reflectance of vitrinite (Table 5-2). In addition, Wood( 1988) expressed the relationship between them by regression fitting as follows:
LG % Ro =—0.006(LG tti)3+0.042(LG tti)2+0. 162(LG tti)—0.397
Table 5-2 Correlation between Time Temperature Index (TTI) and vitrinite reflectance (Ro)
It should be pointed out that although TTI method has been widely used in fossil energy geology for decades, there are still some obvious shortcomings to be improved, and scholars at home and abroad have made some efforts to this end. For example, the assumption that the coalification rate doubles with the increase of 10℃ has its limitations, and it can only be in the range of activation energy 10 ~ 25 kcal/mol (equivalent to the temperature range of 20 ~ 160℃,1kcal = 4186. For another example, the reaction activation energy of thermal degradation of sedimentary organic matter is a function of heating temperature, which gradually increases with the increase of maturity, and the activation energy required for different types of organic matter to reach the same maturity is different, while TTI model takes the reaction activation energy in the whole coalification process as a constant, which is obviously different from the actual situation.
Organic maturity method
This method was established by Hood( 1975) and improved by Bostick et al. (1979) to become a widely used numerical simulation method at home and abroad, so it is also called Hood-Bostick method.
Hood et al. considered the change trend of total reaction activation energy with the increase of heating temperature (18 ~ 33 kcal/Mo 1), and calibrated the organic matter maturity level (LOM) with vitrinite oil immersion reflectivity (Figure 5- 1). The concept of effective heating time is adopted in the model, that is, the heating time when the temperature is not lower than the highest heating temperature 15℃, and the relationship among organic matter maturity, temperature and effective heating time is established (Figure 5-2).
Fig. 5 Relationship between LOM of1and other indicators of organic matter maturity (cited from Zhou Zhongyi, 1990).
I-lignite; Ⅱ1-sub-bituminous coal; Ⅱ 2-high volatile bituminous coal; Ⅱ 3-medium volatile bituminous coal; Ⅱ 4-low volatile bituminous coal; Ⅲ1-semi-anthracite; Ⅲ 2-anthracite
1Btu= 1055.06J
In practical work, LOM method uses graphical method to calculate the maturity of organic matter or vitrinite reflectance (Figure 5-2). The application results at home and abroad show that the paleogeothermal calculated by this method is reliable, which is compared with the results obtained from comprehensive research and the actual situation of the basin (VOTE,1981; Zhou Zhongyi et al., 1983, 1984, 1985).