Theories and Techniques of Permeability Prediction of Multiphase Medium Coalbed - Methane Reservoirs

SUMMARY
Some innovative achievements were acquired in the physical properties of coal reservoirs, based on coal the section observation under 28 mines, the micro-petrological and pore identification from 57 coal samples, the multiphase medium sorption experiments from 35 samples, the multiphase medium mechanical, adsorption-expanding and permeability experiment of the single-phase and gas-water double-phase medium of 6 set samples, and the analyses of 214 bore logs.
1. The new concepts, i.e., the three-phase medium and ternary structure system of coal reservoirs, were put forward and demonstrated utilizing phylogenetic idea.
Coal reservoir is a three-dimension geological system consisted of three-phase matters, i.e., gas, liquid and coal matrix. Under the stratum in situ, the relationships among the coal matrix, adsorbed methane, free methane and water are relatively steady with dynamic equilibrium state, and total methane contents accord with the thermodynamical conditions or the gas-state equation. When the effects of the fluid pressure or other outside forces are changed, the corresponding changes would occur in the three-phase medium system, namely, the multiphase medium coal rock can be look upon a separate and self-adjustable system.
Coal reservoir is a ternary structure system including the macrofratures, microfractures and pores. The pores are the primary stored room of coalbed methane (CBM), the macrofratures provide the channels for the CBM flow, and the microfractures make up of the bridge connecting the former to the latter.
The macrofratures, microfractures and pores in coal reservoirs were systematically researched in virtue of the fractal theory and method for the first time, and the fractal classification and natural ones of the coal pores and fracture structures adapting to CBM-sorption, diffusion and seepage flow were further suggested.
2. The characteristics of the absorption, diffusion and seepage flow in the multiphase medium coal rocks were re-examined by means of the modern testing methods and theories.
It was found that the evolution of the methane-adsorbing capacities of coals with coal ranks takes a reverse about at the maximum vitrinite reflectance 4.5%R璷,max under the equilibrium moisture. The Langmuir volume of coals increases with increasing coal rank when R璷,max is less than 4.5%, and it is inverse when R璷,max is more than 4.5%. But as for the low-rank coals, the influence of the coal rank on the Langmuir volume is relatively weak, and that of the coal macerals is rather strong.
The permeability experiments showed that the absolute helium permeability of coals is more than the Klinkenberg methane one, whereas the latter is again more than the water permeability. The water-bounded saturation of coal reservoirs in China is higher, and the seepage flow range of the double-phase gas-water medium is very narrow. The relative permeability at the equilibrium spot and the gas permeability under water-bounded are lower.
The key factors affecting the absorptive capacities of the coals to methane were found out. The capacities strengthen with raising the gas pressure (the partial pressure of multiple components), pore volume, special surface area and adsorbing heat, and weaken with increasing the temperature, mineral matters, water and oil. Thereby, the reservoir pressure drop, temperature elevation and gas replacement were suggested as the primary methods for developing coalbed methane.
3. Coalbed gas content and permeability of coal reservoir were predicted based on the characteristics of geophys of coal reservoir
The regress formulae between log responses and coalbed gas content were established utilizing progression regression analysis method, and F-checking and precision-checking shows that digital log is good simulative log.
With cluster analysis method, coalbody structure was classified into primary constructional coal-ruptured coal, mortar coal and mylonitic coal utilizing log curve, and combining the observation under mine and description of coal core.
The mathematical models for predicting coalbed gas content and permeability of coal reservoir were established, and they make good the shortage of the testing of coalbed gas content and the imperfection of breakingup of coal core in the course of coalfield exploration.
4. The mechanical characteristics of multiphase medium coal rocks were discussed based on the modern theory and methods of the rock mechanics.
Mechanical parallel experiments of multiphase medium coal rocks showed that elasticity modulus and compression strength of coal samples with saturated water and gas are less than that of the ones with saturated water, and the latter are as well as less than that of the ones under air dry, nevertheless the poissons ratio is just opposite. Effect stress coefficient (Boit coefficient) reflecting the distortion effect of pore fluid to solid can not explain completely physical chemistry effect of gas and water medium on coal matrix, whereas there existing "corrosion-damage" of intensity and energy among them. On the contrary, the intension of coal reservoirs may strengthen, poissons ratio may decrease, plane stress ponderance of gravity will fall and permeability will improve with the discharge of water and the release of methane in the process of releasing coalbed methane
Parallel permeability experiments of methane and helium showed that permeability increment induced by coal matrix shrinkage effect increases on logarithm form with the reducing fluid pressure. Expand experiments of coals adsorbing CO2 showed that simulative relationship between coal volume strains and fluid pressure accords with the form of Langmuir equation, and the coal matrix shrinkage strains reduce with the increasing coal rank.
Coal reservoir permeability lies on the development of nature fractures, whereas modern crutial stress controls opening and closing of them. The modern ground stress in middle to south of Qinshui basin in Shanxian Province were simulated utilizing FLAC-3D software, integrating stress-permeability experiments and the data of testing permeability of CBM testing wells in the researched region, a coupling mathematical model of permeability to three dimension stress was erected, and original permeability were completely predicted in the said region at the light of the mode.
It is pointed out that the main permeability-controlled factors were the development fractures and pores, effect stress and the capacities of coal matrix shrinkage. Negative effect of effective stress on permeability is more than position one of coal matrix shrinkage on permeability in high-rank coals and theirs permeability reduce with the fall of reservoir pressure, whereas middle-rank coals are opposite.
The development of fractures and the capacity of coal matrix shrinkage in high-rank coals are less than middle-rank coals, nevertheless confining water saturation of the former is bigger and its gas-permeability only accounts for 15.6% of the Klinkenberg one under confining water. As a whole, they are difficult to be improved in comparison with middle-rank ones in the process of releasing coalbed methane.