考虑水泥环完整性的油气井最大允许井口压力计算方法

A method of calculating maximum allowable wellhead operating pressure for oil and gas wells considering cement sheath integrity

在最大允许井口压力计算方法中考虑水泥环的完整性,以厚壁圆筒的弹塑性分析理论为基础,建立了套管-水泥环-地层组合体力学模型,将(内层)水泥环内壁发生破坏作为环空允许带压的限制条件。基于Drucker-Prager与拉伸破坏准则计算了某实例井各环空的最大允许井口压力,并将其结果与采用API RP 90标准推荐做法的计算结果进行了对比。计算结果表明:如果环空带压值增大,对于套管-水泥环-地层组合体而言,垂深最深与最浅处的水泥环内壁将最有可能率先发生破坏;采用本文方法的计算结果有可能会小于依据API RP 90标准的计算结果,依据API RP 90标准计算得到A、B环空的最大允许井口压力分别为17.04MPa和6.39MPa,而采用本文方法计算得到A、B环空的最大允许井口压力分别为15.1MPa和17.5MPa。

A method of calculating maximum allowable wellhead operating pressure(MAWOP)considering cement sheath integrity is proposed in this paper.Based on the elasto-plastic analysis of thick cylinder,mechanical model of combination of casing,cement sheath and formation is established.In this model,MAWOP is the annular casing pressure under which the internal wall of(inner)cement sheath fails.Then MAWOP of each annulus in anoffshore gas well are calculated according to Drucker-Prager and tensile failure criterion.The comparisons between the above results and the results calculated according to API RP 90 are made.A case study shows that,within the combination of casing,cement sheath and formation,the internal wall of cement sheath at the top and the bottom in vertical depth are most likely to be failed at first if annular casing pressure increases.It is also revealed that,MAWOP calculated results by the method provided in this paper is possibly less than those calculated according to API RP 90.Specifically speaking,MAWOP of"A"and"B"annulus is calculated as 17.04 MPa and 6.39 MPa,respectively,according to API RP 90.In addition,MAWOP of"A"and"B"annulus is calculated as 15.1 MPa and 17.5 MPa,respectively,with the method provided in this paper.The study presented in this paper will provide theoretical support for the safety in production of oil and gas wells.