Study On CBM Vertical Well Drainage Controlling In Different Reservoir Types

Coalbed Methane Vertical Well is the most common type of CBM development well type for the moment. A reasonable drainage working system can reduce coal reservoir damage in the drainage process, which is an important safeguard of CBM Vertical Wells to obtain high and stable gas production. The diversity of coal reservoir petrophysical and structures, the heterogeneity of reservoir properties, the differences of storing environment and the variability of the main control factors of the reservoir petrophysical parameters in the drainage process, result in the producing demand of CBM Wells in the light of these differences to make different drainage working system.Making drainage working system of different stage according to the change of storage environment of different types of reservoirs is the key to improve the CBM Wells gas production.This paper based on the predecessors’ study and the production characteristics of CBM Vertical Wells to redivision of the drainage stages. Considering the difference of geological boundary in drainage process, coal reservoirs were divided into layered and block the reservoir types. According to the CBM development geology, rock mechanics theory, the horizontal and vertical fracture morphology were analysised for different structure coal in hydraulic fracturing process. According to permeability, the difference of reservoir types and principle of seepage flow, it has got the pressure propagation regularity of CBM Wells in the horizontal and vertical of different types reservoirs,namely: pressure propagation regularity was different in drainage process of CBM Wells different types of reservoirs. When the coal reservoir has good permeability,pressure spread easily, and the lateral pressure transmission can be thought of concentric circles centered on wellbore, longitudinally the water pressure and air pressure can be thought of a logarithmic distribution; With poor permeability, the spread of lateral pressure goes through concentric circles first, then turn to the ellipse, the longitudinal pressure propagation is relatively complex, which has phenomenon of pressure differentiation. According to the positive and negative effect of permeability influence factors in the process of CBM well drainage process, The mathematical model of CBM Vertical Wells is established in the balance-water production and control of bottom holeflowing pressure and not deflated stage, on the basis of that obtaining the permeability change regulations of the differences of drainage reservoir, namely: the balance-water production stage permeability of a certain distance away from the wellbore some point with a logarithmic function to reduce in the drainage time, the permeability increases a little in the gas/water phase; According to the principle of seepage theory and the theory of gas recovery, respectively setting up the CBM Wells mathematical models of reasonable pressure drop rate in the balance-water production stage, the reasonable deflated casing pressure in the control of bottom hole flowing pressure control and not deflated stage and the reasonable pressure drop rate in the stable increasing production stage, and expand the "five periods, three pressure types and four points" of drainage control theory. On the basis of which, obtaining the different types of reservoir characteristics on the dynamic change of drainage process, put forward to reasonable drainage working system under different drainage reservoir types.Comparing the theory calculation of different drainage phases with the actual drainage results under different drainage reservoir types validate the precision of the established mathematical model.The study of reasonable drainage working system under different drainage reservoir types, provides a reference and lesson for vertical wells drainage control.