Restoring Well Integrity in a Critical Gas Pilot Injection Well

Abstract
Appearance of abnormally high pressure in the 9 5/8?? X 13 3/8?? casing annulus of a pilot gas injection well located offshore Abu Dhabi, alerted staff to a potentially serious situation from both safety and economic viewpoints. The subject well is a critical component of a pilot project to test the effectiveness of gas injection towards improving oil recovery. Based on this and other data, decisions on long-term reservoir development involving considerable investment will be made. The well was drilled and completed in 1999 as dual gas injector into two, Lower Cretaceous limestone reservoirs and put under gas injection in 2001. Abnormally high pressure was observed in the 9 5/8?? x 13 3/8?? annulus after three months of injection. A multidisciplinary task force was formed with a mandate from senior management to locate the source of leaking gas, the communication path to surface and recommend remedial actions. A risk assessment ranked the danger to the 80 million U.S. dollar platform and personal as high. As a result, gas injection was halted and visits to the platform minimized. Evidence gathered from the well records and newly acquired diagnostic logs suggested the existence of poor or no cement behind the 7?? liner and 9 5/8?? casing providing a conduit for gas to surface. Moreover, the existence of two abandoned holes penetrating the shallowest reservoir and located at short distance from the currently active bore hole were recognized as potential pathways for gas to surface. Extensive remedial cementing operations were performed including: (1) block squeezes behind one and two sets of production casing above the source reservoir, (2) cement circulation squeeze between production and intermediate casings through mechanically drilled holes. The high annulus pressure was thought eliminated but soon returned after injection commenced. A decision was made to convert the nearby un-perforated Gas Observer to water injection for the shallowest reservoir and continue gas injection into the deepest reservoir of the original well.

Introduction
Located in a giant carbonate oil field offshore Abu Dhabi, ZA41X is a dual gas injector surrounded by four oil producers forming a five spot pattern (Figure 1). The pattern is one of several pilots examining the effectiveness of gas injection in improving oil recovery in the eastern flank of two highly porous but poor permeability limestone reservoirs. Shortly after gas injection commenced, high pressure (550 psi) was observed in the 9 5/8?? X 13 3/8?? casing annulus. Samples taken from the annulus were sea water. This was not deemed a risk until the effluent turned into methane gas, a discovery made during testing and sampling of the annulus six months later. Gas injection was halted and a risk assessment carried out which found the well and associated tower unsafe. Responding to the safety concern and at the direction of mangement, a task force was formed to determine the root cause of the 9 5/8?? X 13 3/8?? sustained annulus pressure in the gas injector and recommend remedial actions and completion scenarios. Reviewing the well history the task force discovered a number of poor practices which were the source of this well's casing integrity problem. This paper presents the data collection, interpretation and multiple attempts to restore the well integrity.