Research Of Wear Casing Burst Strength After Cement Defects

Casing and cement play a strong role in separating the different fluid pressure system formation, isolating the leakage layer and collapsed layer, maintaining the circular flow of drilling mud in order to facilitate the conduct of the drilling process and the normal operation of the whole oil wells after completion. However, casing wear and cementing defect is the inevitable phenomenon during drilling process. At present, drilling practice of deep wells, ultra deep wells, directional wells and horizontal well are becoming more and more, working condition of down-hole casing is extremely harsh, the problem of casing wear is especially prominent. Because the worn-casing is difficult to be replaced, it still need to be used continually underground, once the casing happens serious eccentric wear, the capacity of resisting internal pressure and external pressure decreased significantly, when drilling in the high-pressure oil and gas reservoir, the worn-casing tends to rupture under the high-pressure of the mineshaft, well control measures could not be carried out securely, it may endanger the personnel safety, causing environmental damage and significant economic loss. Therefore, studying the influence of casing wear and cementing defects on the casing strength of resisting internal pressure, assessing the safety risk of high-pressure mineshaft, provide a scientific proof for reasonable casing maintenance and development of well control measures, have become an urgent task that oil and gas field drilling engineering faces in the current and future.Firstly, we consulted the related literature at home and abroad about the wear casing, and investigated the casing’s use and cementing quality defects in the major oil field to make sure the scope of the worn and the shape of the main worn zone. Then, for9-5/8" casing, select the six thicknesses, including7.92mm,10.03mm,11.99mm,15.11mm,17.07mm and20.24mm, we established casing-cement-formation system, wear casing-cement-formation system, casing-cement partial deletion-formation system, wear casing-cement partial deletion-formation system. For the casing-cement-formation system, firstly, built theoretical model, the corresponding program and deduced corresponding stress formula; then compared Ansys analysis results and theoretical calculation results; at the last, analyzed and predicted the initial burst strength of the casing. For the wear casing-cement-formation system, analyzed and forecasted the initial and limit burst strength of the casing For the casing-cement partial deletion-formation system, analyzed and forecasted the initial burst strength of the casing. For the wear casing-cement partial deletion-formation system, analyzed the initial burst strength of the casing and summarized corresponding law.The main results and conclusions are as follows:Ⅰ、casing-cement-formation system:(l)Radial stress is continuous between the casing and the cement, the cement and the formation. There are distinct boundaries within the tangential stress and von Mises stress between the casing and the cement, and the cement and the formation.(2)For a given casing thickness, initial burst strength of the casing increases linearly with the increase of confining pressure.(3)For a given confining pressure, initial burst strength of the casing increases linearly with the increase of casing thickness.(4)Establish prediction formula of the initial burst strength of the casing.Ⅱ、wear casing-cement-formation system:(1)For a given confining pressure, initial burst strength of the casing decreases nonlinear with the increase of casing wear amount. When the wear loss is less than20%, the descent gradient of the initial burst strength of the casing is great. When the wear loss is greater than20%, the descent gradient of the initial burst strength of the casing is small.(2)Initial burst strength of the casing increases linearly with the increase of confining pressure.(3)For a given confining pressure, limit burst strength of the casing decreases nonlinear with the increase of casing wear amount. When the wear loss is less than20%, the descent gradient of the limit burst strength of the casing is great. When the wear loss is greater than20%, the descent gradient of the limit burst strength of the casing is small.(4)Limit burst strength of the casing increases linearly with the increase of confining pressure.(5)Establish prediction formula of the initial burst strength of the casing.(6)Establish prediction formula of the limit burst strength of the casing.Ⅲ、 casing-cement partial deletion-formation system:(1)When the cement is intact, the burst strength of the casing is maximum, that indicate the integrity cement has a protective effect on the casing.(2)When there is trace amounts of missing on the cement, the initial burst strength of the casing will decrease sharply.(3) For a given confining pressure, with the increase of cement missing amount, the initial burst strength of the casing will firstly decrease and then increase, and finally decrease leveled.(4)When the cement is missing a certain amount, it will undermine the initial burst strength of the casing, than indicate the cement is not provide necessary protection.(5)When the cement is missing more than a certain amount, the cement has no protective effect.(6)When the cement missing in danger angle, the more the confining pressure, the greater the casing damage.(7)Establish prediction formula of the initial burst strength of the casing.Ⅳ、wear casing-cement partial deletion-formation system:(1)With the increase of cement missing amount, the initial burst strength of the casing will firstly decrease and then increase, and finally decrease leveled.(2)The greater the confining pressure, the greater fluctuation the initial burst strength of the casing.(3)When the cement missing amount is small, the initial burst strength of the casing has a small extent.(4)For a given confining pressure, when the cement missing amount is greater than20%, the trends of the initial burst strength of different casing almost the same.