An Overpressure Case History Using a Novel Risk-Analysis Process
Authors: Dodds K.J. , Fletcher A. , Hennig A. , Bekele E. , Johnson M. , Abriel W. , Higgs W. , Strudley A. 
Published: October 2008

The economic consequence of exploitation in areas with an unspecified risk of abnormal pressure profiles range from increased drilling costs to unrealized prospect potential. Optimised planning practices will impact not only the costs of drilling but also the quality of the reservoir evaluation and productivity assessment.

Numerous exploratory wells in the Carnarvon Basin have encountered unprognosed high pore pressures in the past resulting in increased concerns about well control and safety, and as a consequence higher drilling costs. As a result of the diversity of potential overpressure mechanisms, there is a variety of opinion on the risk of encountering abnormal pressure in particular areas, and no standard way to capture and incorporate this information into planning and drilling decisions.

At the end of 1999 a project was defined to evaluate the risk of overpressure of a prospect. This project had the primary focus of evaluating the risk of encountering high pore pressures when drilling an exploratory well. This risk assessment was directed towards a decision point for well design, which had cost implications in anticipation of drilling.

In the process of technical assessment of abnormal pressure in this prospect, it was important to include all of the necessary data and technical practices, which could contribute an understanding of the prediction. The application of a cooperative multi-functional team and a software infrastructure ("Juniper") was instrumental in allowing this process to take place. A visual decision tree with risk assessment allowed for input from all contributors and for the inter-relationship of the inputs. This comprised the methodology for assessment.

The methodology highlighted, explicitly, areas of high uncertainty where evidence neither for nor against overpressure was available. These areas became the focus of technical work as they had potential for significant impact on the risk assessment. A sub-team extensively reviewed the pressure tests, sonic logs, borehole influxes and mud weights of nearby wells. The results of the well study had a bearing on the choice of analogue, the geological model, and pressure prediction. A geological modelling sub-process was carried out to test the significance of compaction disequilibrium, organic maturation and permeability on overpressure generation. A geophysical sub-process was initiated to complement the pressure modelling risk element. Surface seismic data was depth processed to relate velocities to trends obtained from the well study.

The methodology was found to be a highly effective technique for recording the decision processes and as a tool for interdisciplinary communication in a cooperative and nonthreatening environment. The result of the study was to initially identify the critical risk areas and subsequently reduce the risk of overpressure in the prospect as perceived by all parties (geoscientists, drilling engineers, management and joint venture parties). The common view on risk prompted a reassessment of the risk profiles on all the related wells in the program and allowed revision of the drilling programs and significant cost savings relative to the original forecasts for the program.