Managing Marine Geohazard Risks Throughout the Business Cycle
Today, the industry is faced with entry into frontier areas with little prior published understanding and potentially complex slope and deepwater settings. In such settings, early effort in the exploration-and-production cycle is required to allow appropriate data to be gathered and assessed. In order to address these issues, BP has adopted a methodology to manage geohazard risks over the life of the license.
In 1964, the rig C.P. Baker was lost in the Gulf of Mexico in a shallow-gas blowout with the loss of 22 lives. That accident, and similar events in the industry around the same time, triggered the development of geophysical site investigation or geohazard methodologies to support safety in tophole drilling and field development through detailed assessment of seabed and near-surface geology. To this end, the Hazards Survey in North America and the Site Survey in Europe became the staple means for evaluating predrill or predevelopment conditions over the following 30 years.
The technologies used in these surveys have continued to be developed. These approaches have generally served the industry well for 50 years. However, as the industry has progressed from operations generally on the continental shelf out onto the continental slope and into ultradeep water, the geohazard issues that need to be addressed by the industry have grown in variety and complexity.
While the scope of possible sources of geohazards has expanded, so has the potential size of license areas to be studied.
If conditions across such blocks on the continental shelf or in ultradeep water were homogeneous, it may be acceptable to continue with the traditional approach of the site survey. However, the conditions in many large blocks are far from homogeneous, and, therefore, a site survey would deliver little understanding of the variability in geohazard conditions and processes that may have implications for the immediate safety of drilling.
The longevity of production operations now faced in a license or field has also been gradually extended through the implementation of improved-recovery techniques. BP’s Magnus field was discovered in the far north of the UK continental shelf in 1974. At the time of first oil in 1983, the projected field life was seen as being out to the mid-1990s. However, another phase of production drilling will be starting from the platform in 2015, and current projected field life is now seen out to the 2020s. However, the last high-resolution seismic data to have been acquired below the platform were acquired in 1984. Before restarting drilling, a prudent operator would ask the question, “What is the possibility that geohazard conditions may have changed over the last 30 years?” The prudent operator, therefore, needs to revisit geohazard risks and the validity of site-investigation data across the full life of the license, from entry to field abandonment, and to update geohazard understanding consistently across the whole time period.
This paper, therefore, sets out an integrated approach to address management of geohazard risks across the life of a license (Fig. 1), an approach that seeks to consistently update understanding of what geohazards might be present, and, thus, where possible, seeks to avoid them directly or mitigate their presence.
Upon entry to a new license area, existing seismic or published geoscience information upon which to build understanding of geohazard complexity may be sparse.
A consistent approach for the rapid evaluation of the potential degree of geohazards complexity before, or upon, entry to a new license area uses an evaluation of four fundamental geoscience attributes: evidence for presence of shallow hydrocarbons, recent-deposition rate (over the last 1 million years), structural complexity, and underlying seismicity. A final attribute is the quality of the database available to review the area: The sparser or poorer the data available, the greater the interpretive uncertainty. Each of these five factors is scored by use of a consistent scoring mechanism, and they can be plotted on a pentagon where the greater the area finally shaded, the greater the fundamental level of underlying geohazard risk.
Geohazard Baseline Review
After initial fundamental evaluation of risk before or upon entry, it is normal to expect that licensewide exploration 3D data acquisition will be a first step to support the exploration effort—if this is not already in place.
Delivery of a geohazards or short-offset volume at this stage is a simple and effective byproduct. Indeed, in the case of wide-azimuth data acquisition, delivery of such a product may be a key intermediate quality-control output to delivery of the final product and may be of significantly greater value to the geohazards interpreter than the final volume used by the explorer.
Once processed, 3D data are available to produce a complete geohazards baseline review (GBR) of the delivered volume. Such baseline reviews need to be performed and communicated efficiently to the exploration team in a way that supports eventual prospect ranking and delivered early enough in the exploration cycle to affect choice of drilling location.
Production of a GBR provides the underlying framework for all later geohazard studies to be built and data requirements to be defined. The GBR, therefore, should be revisited and updated regularly.
Geohazard-Risk-Source Spreadsheet (GRSS)
A GRSS captures individual sources of geohazards, the threat that each may pose to operations, and their effect on those operations. These then form a threefold semiquantitative evaluation of the interpretive confidence that a hazard is present, the likelihood of that geohazard event occurring, and the effect of that event to establish an initial definition of operational risk from the individual source of the hazard.
On the basis that a prospect is identified within the licence that is considered of sufficient value to commit to exploratory drilling, a location will need to be assessed for its safety for drilling.
Local regulatory requirements may establish specific constraints. Otherwise, the level of visible overburden complication may suggest, even in deep water, that site-specific high-resolution 3D-data acquisition is required to support either selection of a location clear of geohazards or accurate definition of the geohazards present to allow their mitigation in well design.
The key is that, outside of regulatory requirements, the operator, rather than applying a rote process to evaluation of a drilling location, should be designing a site-investigation program that specifically addresses the potential hazards faced at that location.
Appraisal: Toward Field Development
At this stage of the life cycle, direct operational experience of initial drilling activities should have been gathered and can be fed back directly into improving predictions of tophole appraisal drilling. Beyond this, however, the addition of potential location-specific site-investigation-survey data, combined with direct operational experiences from initial drilling, will allow a full revision of the GRSS contents. This review should focus on whether the GRSS contents either were too conservative or overlooked possible hazards sources.
At the onset of a field-development project, it is expected that all site-investigation-data needs have been met and plans have been put in place for data acquisition or that the data are already in hand. Ultimately, the different study strands defined in the project GRSS should be brought together into an integrated geological model.
Outputs from a completed integrated study allow proper risk avoidance in concept screening through choice of development layout, for example, or risk mitigation by engineering design.
Development-Project Execution Into Early Production
As a development project moves into the execute phase and the instigation of production drilling or facility installation, the refinement of geohazard understanding needs to continue.
Drilling requires the same screening as used for the exploratory-drilling phase. Experiences from drilling of the first wells from a location need to be captured either directly by presence of tophole witnesses on-site or indirectly by use of remote monitoring facilities. These experiences should be fed back into updated predictions of drilling conditions for ensuing project or production wells to allow appropriate and safe adjustment of drilling practices in accordance with actual conditions encountered. This process needs to be carried through the production phase after the initial development is complete. Variances should always be investigated and reconciled against pre-existing knowledge.
Drilling Renewal and Field Redevelopment
Before the restart of drilling or redevelopment operations, an operator should pause to capture previous operational lessons learned. Reviews of the ongoing integrity of the overburden should be held regularly throughout the life of the field, especially ahead of any engineering operations, and, as a result, the validity of overburden imagery should be considered regularly and carefully for renewal.
Ahead of the instigation of abandonment operations, a review of the potential for change in overburden, or geohazard, conditions should be undertaken. For a single suspended or partially abandoned subsea well, the period since the well was last worked over may have been considerable. The prudent operator will undertake a review of the original operation to understand the condition of the well. It is also prudent to undertake a simple survey of the seabed around the well to look for anomalies that may suggest a change in the integrity of conditions since temporary abandonment.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 173139, “Managing Marine Geohazard Risks Over the Full Business Cycle,” by Andrew W. Hill and Gareth A. Wood, BP America, prepared for the 2015 SPE/IADC Drilling Conference and Exhibition, London, 17–19 March. The paper has not been peer reviewed.