﻿ 延缓大庆油田标准层套损的套管优选

Casing Optimization for Delaying Casing Damage in the Datum Bed of the Daqing Oilfield
Ai Chi, Hu Chaoyang, Cui Yueming
College of Petroleum Engineering, Northeast Petroleum University, Daqing, Heilongjiang, 163318, China
Abstract: Casing shear damage appeared frequently in the datum bed in the Daqing Oilfield, and an uncementing technology has been adopted to delay casing damage. However, the critical value of marker casing shear damage is related to wall thickness and steel grade of the casing. An analysis was conducted on the elastoplastic mechanical characteristics of casings and formations, and a three-dimensional finite element calculation method was proposed, based on the engineering conditions of uncementing markers, for integral formation movement and shearing casing by using the Lagrange finite element equation, which takes account of the stress equilibrium state after large deformation. Critical values of the casing failure with different wall thickness and steel grade were calculated and compared at two failure conditions, i.e. the casing deformation and casing fracturing. It is shown that the critical slippage of casing with formation shear when the casing was fracturing is much smaller than that of casing deformation. The critical slippage of casing that have the same size and steel grade decreases linearly with the increase of wall thickness, but increases linearly with the increase of elongation rate. To sum up, the casing with small wall thickness and low steel grade should be adopted in datum bed of the Daqing Oilfield to delay casing damage on the condition that strength checks are satisfied.
Key words: datum bed    casing damage    finite element method    geometric model    Daqing Oilfield

1 有限元模型的建立 1.1 几何模型的建立

 图 1 地层和套管的几何形态 Fig.1 Geometry of formation and casing

1.2 有限元计算模型

1.3 材料的力学参数

 钢级 弹性模量/GPa 泊松比 屈服应力/MPa 抗拉强度/MPa 强化阶段模量/MPa 延伸率，% J55 206 0.29 379 517 579.4 24 N80 206 0.29 552 689 731.2 19 P110 206 0.29 759 862 710.6 15

2 套损临界条件

3 延缓剪切套损套管优选 3.1 套管壁厚的优选

 图 2 不同壁厚套管通径与地层剪切套管滑移量的关系 Fig.2 The relationship between formation shear casing slippage and bore size of casing with different wall thickness

 图 3 不同壁厚套管最大Mises应力与地层剪切套管滑移量的关系 Fig.3 The relationship between formation shear casing slippage and maximum Mises stress on casing with different wall thickness

3.2 套管钢级优选

 图 4 139.7 mm壁厚6.20 mm不同钢级套管的最大Mises应力 Fig.4 Maximum Mises stress of 139.7 mm casing with a wall thickness 6.20 mm at different steel grades

3.3 延缓标准层套损的新型套管展望

Φ139.7 mm壁厚6.20 mm的3种钢级套管在地层剪切过程中最大等效应变曲线几乎重合，3种钢级套管在不同滑移量的最大等效应变差值不超过2.4%，但不同钢级的延伸率具有明显差别(见表1)，这表示套管断裂的临界等效应变具有明显差异。分析认为，套管强度等因素对套管的最大等效应变曲线影响较小，标准层套管断裂条件主要受套管延伸率控制。因此，断裂前允许变形程度越大的套管，越有利于延缓标准层剪切套损。

 图 5 Φ139.7 mm壁厚6.20 mm套管延伸率与套损临界滑移量关系 Fig.5 Relationship between the critical casing damage slippage and a rate of elongation of Φ139.7 mm casing with a wall thickness 6.20 mm

4 结论及建议

1) 随着地层套管滑移量增大，不同壁厚套管的通径近似呈线性减小,但套管壁厚对套管通径变化速率影响较小。

2) 套管最大Mises应力在地层剪切初期迅速增大，当套管发生塑性变形时，套管最大Mises增大速度变缓。不同壁厚套管最大Mises应力差别相对较小，壁厚越大的套管所承受的最大Mises应力越大。

3) 大庆油田标准层剪切套损与以往套损情况不同，套管管壁不能有效抵抗地层滑移，只能被动变形，套管延伸率成为影响标准层套损的主要参数。

4) 在标准层不封固条件下，套损临界滑移量随套管壁厚增大和套管钢级升高缓慢减小，虽然临界滑移量差别不大，但薄壁、低钢级套管成本低，在满足强度校核前提下，在标准层使用低钢级薄壁套管能延缓标准层套损。

5) 在满足强度校核的前提下，标准层若能采用高延伸率新型材料套管，可使套损临界滑移量大幅增加，有效延缓标准层剪切套损。

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#### 文章信息

Ai Chi, Hu Chaoyang, Cui Yueming

Casing Optimization for Delaying Casing Damage in the Datum Bed of the Daqing Oilfield

Petroleum Drilling Techniques, 2015, 43(06): 7-12.
http://dx.doi.org/10.11911/syztjs.201506002