水泥浆失重压力评价技术研究与应用

固井后水泥浆“失重”将导致作用于气层的液柱压力下降，可能引发欠平衡，发生早期气窜。进行准确的失重压力评价是制定固井憋压候凝技术、实现压稳防窜的关键。而目前国内外采用的失重压力经验公式和水泥浆失重测量装置并不能很好地反映井下条件水泥浆失重规律。基于此，采用等比例缩小尺寸方法，研发了一套高温高压水泥浆失重压力评价实验装置，开展了直井和斜井中典型水泥浆失重压力实验。结果表明，初凝时刻，稳定性差的水泥浆液柱压力可能低于等高清水柱，而稳定性好的水泥浆液柱压力可能高于等高清水柱。按照经验法设计环空憋压值，可能憋压不足引发气窜，也可能憋压值偏高压漏地层。因此，提出了一种分段计算水泥浆失重压力的方法，在四川盆地磨溪—高石梯区块高压气井得到检验，指导了环空憋压候凝。固井质量合格，固井后未发生环空气窜。

Study and Application of a Technology for Evaluating Pressure Loss of Cement Plug

Pressure loss of cement column after cementing often results in reduction of pressure exerted on the gas zones by the liquid cement slurry column. This pressure loss may cause underbalance of pressure to happen and hence gas channeling at the early stage of cementing. Accurate evaluation of the pressure loss is a key to the formulation of techniques regarding the extent of pressurization and WOC and to the success of well killing and prevention of gas channeling. Empirical formulas for calculating pressure loss of cement slurry and results obtained from apparatus presently in use for measuring pressure loss of cement slurry do not reflect the pattern of pressure loss of cement slurry in downhole conditions. To address these problems, an apparatus for evaluating the pressure loss of cement slurry at HTHP conditions was developed by scaling down the sizes of existed apparatus. With this new apparatus, experiments have been performed on the typical pressure loss behavior of cement slurries in vertical and horizontal wells. It was found that at time of initial gelling, the pressure exerted by a column of cement slurry with poor stability was possibly less than the pressure exerted by a column of water of the same height. When designing the value of pressure exerted at the annulus using empirical formulas, the designed pressure may not be able to balance the pressure inside the annulus, thereby resulting in gas channeling, or the designed pressure may be higher than necessary to fracture the formations. A new method was developed to segmentally calculate the pressure loss of a cement slurry. This new method was used in calculating the pressure loss of a high-pressure gas well in the Block Moxi-Gaoshiti in Sichuan Basin, providing helpful guide to the pressurization in annulus and WOC operations. The job quality of well cementing was certified and no gas channeling happened after well cementing.