Evaluation of a method of placing cross-sectional barriers during permanent plugging of wells

Abstract

When oil and gas wells reach the end of their production life, they need to be permanently plugged and abandoned. The requirements for a permanent barrier state that it must cover the entire cross-section of the wellbore, including all annuli. This thesis evaluates a new method of establishing a cross-sectional barrier in areas with poor, non-sealing annular cement. The traditional method is to mill away the section with poor cement and set an open hole cement plug, but due to the ECD effect of milling fluids, this is not always desirable. In some formations on the Gullfaks field, the operational pressure window is too small for section milling. An alternative solution was therefore tried out on a well that needed plugging. This method, referred to as punch and squeeze, consisted of perforating the section of poorly cemented casing and squeezing cement into the annulus. In this thesis, the equipment and techniques used are presented and evaluated, along with general theory relevant to plugging and abandonment. On Gullfaks, two punch and squeeze techniques were used. In the first, cement was pumped through a packer plug and squeezed into the perforations. The second technique involved setting a balanced cement plug over the perforations, and squeezing this plug into the perforations. The main conclusions are that the technique using a packer plug is safer with regards to well control, involves less waiting on cement and gives a better annular seal than the balanced plug alternative. Also, a cement evaluation log should be run before the squeeze jobs are performed, and the log results should be used when determining where to perforate. All things considered, it was found that the punch and squeeze method can succeed in creating a length of cross-sectional cement, but still involves some uncertainty and the technique can be further optimized.