Research On Well Deviation Mechanism In Gas Drilling Of Daqing Oilfield

In recent years, gas drilling is used widely due to the advantages of high penetration rate and low leakage. But well deviation control has become a bottleneck restraining the development of gas drilling technology. Field data show that well deviation is more serious in gas drilling than conventional drilling. The circulation medium change which results in the change of force and deformation on bottom-hole assembly (BHA) and the existence of negative pressure differential, the mechanism of well deviation in gas drilling is different from conventional drilling. So, theoretically studies on well deviation mechanism, formulate prevent measures become pretty important.First, the rule of well deviation in gas drilling is summarized according to the statistical results of field data, i.e. well deviation is serious and the growth rate is fast, the control is difficult. And the influential factors including uncontrollable factors and controllable factors are analyzed.Second, the effects of uncontrollable factors including rock-breaking mechanism, bottom temperature field, the anisotropy of stratum, formation dip on well deviation are studied based on the stress condition of bottom hole rock in gas drilling. It is indicated that the stress strain field distribution of hole and bottom rock changes which lead to the stress difference is large, the anisotropy is magnified. This is the main reason of gas drilling tend to deviate and the difficult control. New breaking modes for rock are another cause of serious deviation. The effect of formation dip and properties in gas drilling are more remarkable.Third, the effects of controllable factors including cuttings, well bore enlargement, drilling assembly, WOB, deviation angle and water export on well deviation are discussed. Well bore erosion by cutting particles and bottom stress concentration result in well bore enlargement. The difficulty of well trajectory control is increased, thus causes the well deviation. Cushion is formed by cutting bed after formation water out. And it plays a very important role in well deviation.Finally, well deviation control technologies are researched from the aspects of formation deflecting force, cushion and drilling assembly. Air hammer drilling technology is suggested.