井眼冲蚀模型的建立与影响因素分析

ESTABLISHMENT OF HOLE WASHOUT MODEL AND ITS INFLUENCE FACTOR ANALYSIS

随着钻井技术的不断进步,井眼尺寸越来越小,井深不断增加,井径扩大的预测显得尤为重要。针对以往预测模型的不足,根据对钻井液引起井径扩大的原因的讨论,认为钻井液自身流体切应力引起井眼冲蚀的可能性并不大,而引起井径扩大的真正原因是钻井液中的固相颗粒对井壁的碰撞,因此提出了固相冲蚀井径扩大理论。文章建立了固相颗粒冲击井壁的物理数学模型,对模型进行求解,分析了影响冲击效果的因素。研究表明,钻井液中固相颗粒对井壁岩石的冲蚀作用具有选择性,固相颗粒对强度较大和颗粒较粗的井壁岩石冲击作用很弱,但对颗粒较小的砂岩和泥岩具有较强的冲蚀能力,并且冲击作用对颗粒大小非常敏感,随着井壁岩石颗粒直径的减小,冲击作用力将呈指数增加。这些结论对指导现场施工中的固控、泵排量标准等具有实际指导意义。

Along with the continuous development in drilling technology, the hole size is getting smaller and smaller and drilling depth increase uninteruptedly, so that the prediction of borehole diameter enlargement is even more important. In light of the shortcoming of the prediction model set up in the past and the discussion on the reason of borehole diameter enlargement caused by drilling fluid, it was considered that the probability of arousing borehole diameter enlargement by the fluid shearing stress of drilling fluid itself was small, and its genuine reason was the impact of solid particle's in drilling fluid against the borehole wall, thus putting forward a borehole diameter enlargement theory of solid particle washout. A physical-mathematical model of impacting against the borehole wall by solid particles was set up and the factors affecting washout result were analyzed by solving the model. Through investigation, it was shown that the washing action of the solid particles in drilling fluid against the borehole wall is of selectivity, i. e. it is very weak to the wall rocks with relatively great strength and grains but strong to those with relatively small grains as sandstones and mudstones; and the washing action is highly sensitive to grain size and the washout acting force will be exponentially increased along with the decrease in grain diameter of wall rocks. All the conclusions are of great importance to guiding the solid control and pumping rate standards in the operation on the spot.