固井早期气窜预测新方法及其应用

水泥浆静胶凝强度过渡时间是固井早期气窜的主要时期,但若初凝前发生较显著的塑性体积收缩,造成孔隙压力大幅度下降,井内压力出现较大程度欠平衡,引发气窜的可能性仍然很高。为此,基于初凝前套管、地层在井下的实际受力工况,建立了水泥浆凝固过程中的"套管—水泥浆—地层"物理模型,并求解得到了初凝前水泥浆塑性体积收缩与孔隙压力下降之间的解析关系。立足于该关系,在综合考虑水泥浆性能、井底温度、压力及套管力学性能等特定工况的前提下,建立了针对初凝前整个塑性态的固井早期气窜问题的气窜预测新方法,即把固井环空水泥浆初凝前的"有效液柱压力与启动压力之和大于地层孔隙压力"作为判别条件。最后,利用X区块2口井的资料进行了气窜风险预测分析,分析结果与施工结果一致。该方法可有效预测固井早期环空气窜风险,为针对性地调整水泥浆体系和措施,避免固井后井口环空带压,保障油气井固井质量和安全提供了分析依据。

A new method of predicting gas channeling at the early stage of cementing

The static gel strength transition time of cement slurry is the main period of gas channeling at the early stage of cementing. If significant plastic state volume shrinkage happens to the slurry prior to the initial setting, pore pressure will drop dramatically,which triggers severe under-balanced pressure in a well, as a result, gas channeling is still more likely to occur. Therefore, based on the actual stress conditions of casing and formations before the initial setting, a physical model of "casing-slurry-formation" was established for cement slurry solidification and the analytical relationship was figured out between plastic state shrinkage and pore pressure drop. On this basis, considering the specific conditions of gas wells such as slurry performance, downhole temperature, downhole pressure, and casing mechanical properties, a new method was developed to predict the early-stage gas channeling in the whole plastic state cementing by a criterion that "effective hydrostatic pressure plus starting pressure is greater than pore pressure". Finally, the risk of gas channeling was analyzed and predicted by use of the data of two wells in block X. The result was consistent with that in actual operation. In conclusion, this study provides good references for engineers to adjust the cement slurry system, avoid wellhead annular pressure, and ensure the cementing quality and safety.