The Impact on Drilling Programme Design of Shallow Gas Detection Using Passive Ultrasound Integrity Survey

Abstract

This paper is a case study of field data, assessing the impact of a well integrity survey on an on-going development drilling programme, enabling critical management of an unexpected shallow gas risk. The results and conclusions presented are applicable to the diagnosis and risk management of shallow gas zones and their impact on well construction, design and integrity. The well Operator experienced a severe loss of well control during drilling operations in the Western Block of Talang Jimar field in Sumatra. The field is mature and the well was considered routine in a series of more than 200 drilled over the past 50 years. A kick occurring at a relatively shallow depth around 300m was completely unanticipated. It was judged critical to further drilling to investigate the location and extent of the charged interval. This would identify HS&E and operational risks to be mitigated by modification of both the on-going drilling plan and the well design. The clearest and most precise information was required. When active surface gas venting was discovered at an adjacent well, an ultrasonic annular integrity survey was commissioned to determine the subsurface source of the gas. This ultrasonic survey deploys a number of sensors on wireline in the well, the most important being a passive ultrasound detector. The paper describes the survey technique in brief, the results of the successful survey and the modifications made to the well and mud-weight design to manage the shallow gas risk and the benefits for the drilling programme. The new results presented reveal annular flow outside surface casing with clarity, and the paper explores the underlying reasons for this clarity and its potential benefits for shallow-gas applications. Field data is presented to support the conclusions reached. The methods and technology described in the paper will be of interest, as they should be widely applicable to shallow gas integrity diagnosis, whether a surface gas vent or sub-surface cross flow, in both onshore and offshore applications. Such diagnosis may enhance drilling and production HS&E performance, reducing the risk of well-control incidents.