Problems In Casing Collapse

The principal cause of casing collapse in the Bolivar Coastal field is compaction of the Miocene producing section and the resultant subsidence of the overlying sediments. The problems discussed concern methods of repairing wells with damaged casing as well as preventive measures being taken or proposed for new wells and workovers. Repairs to existing wells involve chiefly operational problems in placing reliners, milling and sidetracking, and redrilling. Preventive measures are discussed both for wells completed in the compaction zone and those completed in sands below the compaction zone. These measures include the use of slip joints, corrugate casing, plastic pack, and stress relieving. Illustrations of the various methods of combating collapse are presented. Introduction In all wells there are natural forces and, occasionally, induced forces which may' cause collapse of the casing. Consequently, it has long been common practice to design casing strings with sufficient strength to resist these forces. To combat large pressure differentials which may exist, the proper wall thickness and grade of steel are selected. Where corrosion is anticipated, coatings, inhibitors, or non-corrosive metals may be utilized. If primary cementation is poor and isolation squeeze jobs are required, techniques have been developed to reduce collapse possibilities to a minimum. These conditions and problems are common to many areas and have, for the most part, been successfully solved. However, in a very few of the world's oil fields there exist active earth forces which are of such magnitude that ordinary methods of combating casing collapse are futile. These forces are created by actual movement of the rock or sediments penetrated by the well. At least two basic types of earth movements in oil fields have been recognized: first, displacement along active fault planes; and second, subsidence of the complete sedimentary section above a zone of compaction. This latter type of movement is the primary cause of casing collapse in the Bolivar Coastal field, and to date has resulted in the repair or redrilling of over three hundred wells. It is the objective of this paper to discuss the techniques which have been used to repair these wells and the protective measures being taken or proposed for future repairs and new completions.