| Abstract |
Although the well's prognosticated temperature differentials and chemistry are incorporated in the casing string design, the statistics gathered from the different wells drilled indicate an increase in number of casing failures as compared IO earlier geothermal drilling, i.e. 1976 - 1983. Aside, from the source of the casings and cement, the only differences are: current wells are drilled with higher build-up rates (Bm) and with tighter smaller target radius m) from I.5o/100ft BUR x 300m TR to 2.5o/30m BUR x 2Sm TR; and, larger diameter casings are utilized in most of the wells for production casing, i.e. from 9-5/5" OD to 13-3/5" OD. Based on the study, as the diameter of the casing increases, failure due to mechanical wear and tear and axial stress greatly increases due to increase in drilling side load and casing bending load from high dogleg severity factor, respectively. The cumulative effect of casing wear and tear and the axial stress from bending loads correspondingly reduces the casing's strength to with stand stresses from external pressures, tensile stress and axial thermal stress. From the numerous corrective and preventative measures adopted, the most effective to prevent casing failure as tested in the different wells are: the tie-back system; increase in wall thickness of the rubber casing specially for casings to be set within the casing-to-casing section; and, the proper placement of drill pipe /casing rubber protectors. |