Study On The Pressure Bearing Capacity Of Weak Formations

In a well structure design, the safe drilling and economic benefit should both be considered. The open hole section may contain weak formation, especially in deep and ultra-deep well. In order to reduce the casing program, the pressure-bearing capacity of weak formation should be improved via sealing. Generally, the pressure-bearing capacity of weak formation is evaluated by the lab experiments. However, the lab experiments condition could not fully simulate the geological environment and mechanical state. Thus, it is significant to establish a set of calculation method to quantitative evaluate the pressure-bearing capacity of weak formation for well structure design.Focusing on three questions:the formation pressure-bearing capacity, the fracture pressure-bearing capacity and the sealing zone pressure-bearing capacity after sealing, this paper indicated the pressure-bearing capacity of weak formation was determined by the least of these three pressure-bearing capacity to establish a calculation method to quantitatively evaluate the pressure-bearing capacity of three weak formation types (low tensile strength formation, micro-fracture formation and fracture formation).First, this work researched on the sealing zone pressure-bearing capacity of weak formation. By analyzing the mechanical state and stability of sealing zone according to granular materials mechanics, two sealing failure mechanisms were summed up, namely instability due to sealed layer displacement and structural damage. By analyzing the mechanical model of these two sealing failure mechanisms, the calculation model of sealing zone pressure-bearing capacity was established. And the calculation results show:the hardness, grain size and surface friction coefficient of the sealing materials is the key factor for sealing zone stability.Second, this work researched on the fracture pressure-bearing capacity of weak formation. Based on fracture mechanics, the fracture model is simplified into two-dimensional plane strain model. The mechanism and condition of fracture propagation were analyzed in both longitudinal and altitudinal directions to establish the calculation model of fracture pressure-bearing capacity. Moreover, the key factors affecting the fractures were obtained in both directions after calculating the stress intensity factor on fracture tip after sealing, and therefore revealed a method and construction technology to prevent fracture extension:combine sealing the mouth and certain place of the fracture to reduce the fracture inner pressure to improve the fracture pressure-bearing capacity.Third, this work researched on the formation pressure-bearing capacity of weak formation. The effect on rock stress status surrounding the wellbore were studied and calculated systematically, due to fracture induced stress field. Then the redistribution of hoop stress was calculated. Also the effect of wellbore strengthening and different sealing operation condition on sealing pressure-bearing capacity were looked into. And the calculation model of formation pressure-bearing capacity was summarized.With the aid of Visual C#, a set of software was developed to analyze formation pressure-bearing and sealing pressure-bearing capacity, which can determine fracture pressure profile by calculating formation rock mechanics parameters, in situ stress and other basic data according to routine logging information. Based on these basic geological parameters, the fracture pressure profile can be corrected on the basis of established formation sealing pressure-bearing capacity models.The whole set of calculation method to quantitatively evaluate the sealing pressure-bearing capacity of weak formation could provide theoretical guidance to well structure design. And the complete set of software could provide calculation tools to well structure design. Meanwhile the calculation results could amend the fracture pressure profile after sealing.