The Damage Of Wellbore Pressure And Pressure Fluctuations To Coal Formation During Drilling Process

The Coalbed Methane is a new type of clean energy, which is a typical unconventional natural gas with both source and reservoir. Facing the increasingly serious shortage of petrochemical resource, to speed up the exploration and development of CBM will be one of the most promising and the most realistic way to replace traditional energy in the21st century. There are rich CBM resources in China, which is just less than United States and Russia, so the development and utilization of CBM resources play a crucial role in our country’s energy reserves.As reservoir of coalbed methane, the physical and chemical properties of coal seam have a large difference with that of conventional sandstone reservoir. Coal seam has the properties of brittle, fragile, low mechanical strength, mature cleat, vulnerable to compression, low young’s modulus and high poisson’s ratio, complicated structure, and poor homogeneity, low porosity and permeability, low reservoir pressure, etc. These characteristics determine that the coal seam is easier to influenced by drilling than conventional reservoirs, ie the damage to coal seam is much larger than conventional reservoirs in the process of drilling. The damage to coal seam will directly affecting the desorption, diffusion, migration and post drainage of the coalbed methane.Due to the strong stress sensitivity of coal, small pressure changes will result in great change of coal permeability, therefore, in the well drilling process, drilling fluid and drilling pressure can easily damage coal formation. When the pressure of drilling fluid and drilling strings changes, or wellbore pressure fluctuats in the process of tripping operation, can both cause deformation of coal seam near the wellbore, so that the fracture deformation occurs, resulting in lower reservoir permeability. At the same time, an increase of pressure in the reservoir fracture will lead to crack width become larger, thus exacerbate invasion of drilling fluid to coal seam. After the drilling fluid invade into coal seam, it will adsorption in the coal seam tunnel, which will cause harm to coal and affect the production of coalbed methane, therefore, it is necessary to study the damage caused by pressure change during the process of drilling, and discover the injury reasons from the source, thus to provide supporting evidence for coal reservoir protection measures.This paper studies the mechanical properties and reservoir characteristics of coal samples from Zhengzhuang block of Qinshui Basin, Shanxi Province. It finds that the elastic modulus of those samples are low, poisson’s ratio is high, compressive strength are low, and those coal samples are fragile, breakable, and vulnerable to compression. The permeability and porosity of coal seam are both very low, and has a strong aeolotropism. These characteristics determine that the coal seam in the study area is easy to be damaged in the process of drilling.In order to further make clear damage to coal seams caused by drilling pressure and wellbore pressure fluctuations in the process of drilling, this paper take coal samples from Qinshui Basin, Shanxi Province as research object, using the method of combination of theoretical analysis and experimental tests to research from three aspect. First, in general, the coal-rock has strong stress sensitivity. Through using the evaluation of stress sensitivity in the oil industrial standard for reference, this paper designs experiment to research the stress sensitivity of coal samples from Fenghuangshan and Sihekuang Mines in Qinshui Basin, Shanxi Province. The experimental results show that the stress sensitivity of that two mines are both extremely strong, and stress change makes the coal permeability suffer permanent damage. In the process of drilling, drilling pressure change and wellbore pressure fluctuations both influence the fracture and permeability of coal. Through laboratory experiments, this paper tests the changes of crack width and coal permeability when the pressure changes. And simulate the wellbore pressure fluctuation by changing the pressure suddenly in the experiment, record changes of crack width and coal permeability in the process of pressure fluctuation. The experimental results show that when the pressure rises, the coal permeability is increasing; when the pressure declines, the coal permeability is reducing, and can not return to its initial value. The higher the drilling pressure rangeability, the severer the damage to the coal permeability. Pressure fluctuation make deformation of coal fracture, coal permeability increases rapidly at the moment of pressure increasing, the greater the pressure fluctuations, the severer the damage to coal permeability. The drilling fluids invade into the coal seam will damage the coal seam. This paper analysis the damage mechanism of drilling fluid to fractured coal seam in theory, and measures the damage degree caused by four different drilling fluids in the experiment. The experimental results show that, for the same drilling fluid, the smaller the fracture width, the greater damage to the coal permeability. The damage to coal caused by water drilling fluid is the most small, but the loss amount is the largest. Therefore, from hole stability and sealing capacity of drilling fluid, it is suggested that foam drilling fluid is a good choice. In order to reduce the damage of coal reservoir during drilling should choose suitable drilling technology to drill, prevent too large drilling pressure, and reduce the number of tripping to avoid drilling pressure fluctuations.