Wellbore Instability in Forties: Diagnosis of Root Causes for Improved Drilling Performance

Abstract

Success in developing and maintaining wells in mature fields is heavily dictated by cost reduction to maintain profitability and maximize production life of the filed. Achieving this relies on better well planning and optimized drilling performances with reduced drilling non productive time. Pertinent to this is the ability to understand mechanisms of wellbore failures, predict and manage operational risks such as wellbore instability, sand production, or hydraulic fracturing. This paper describes how analysis of the historic drilling events have been incorporated to refine mechanical earth models and provides key inputs needed to optimise well plans and adapt drilling practices to changing field conditions. Capturing and classifying historic drilling events is a fundamental key process in understanding the mechanism and causes of well bore failures. A workflow has been engineered to perform such task systematically and efficiently in a field scale multi-well environment. The users are provided with a collaborative common environment to visualize and understand relationships between drilling events in an earth model context. Driven by this new-found understanding, a more accurate prediction of where, when, and how specific problems are likely to manifest themselves can be made. Ultimately it enables closer collaboration between domains and the knowledge gained can be rapidly incorporated into future well planning and drilling operations, while mitigating risks and therefore reducing costs. Introduction Extending the life of mature fields is dependant, not only on carefully selecting locations for optimized production, but also on reducing the cost of operations. As the giant Forties field in the UK sector matures, drilling is becoming increasingly difficult and complex. Increased likelihood of losses as reservoir pressures decline calls for drilling with low mud weight. However the mud weight needs to be high enough to prevent the overburden shales from collapsing as targets are pushed further away from the platform. To compound this, the anisotropic behaviour of shales in the Forties area has had an adverse impact on the wellbore stability in the overburden. With an aggressive drilling campaign for 2009, Apache faces a tough task to improve drilling efficiency and practices to succeed in this challenging drilling environment. This has placed a prime importance on well planning and drilling procedures that translates to better execution in the drilling phase and hence reduction in operational costs. This has been achieved through better understanding of geomechanics behaviour and risk analysis, not on an individual well by well basis but in the context of the drilling environment of the entire field. For the case of the Forties field, this in part involved a comprehensive review of the historic drilling events and performance of over 100 wellbores drilled since the acquisition of the Forties field by Apache in 2003. Clearly a systematic and methodical approach is needed for accurate evaluation of wellbore failures not only because of the number of wellbores involved, but also because wellbore failures can be attributed by a whole host of factors or combination of them. These factors can be broadly defined as field geological factors or drilling operation factors. Without proper diagnosis it would be difficult to confidently predict and mitigate these risks in the well planning phase and ultimately will have an adverse impact on drilling operations.