The Erha-7 well is a deepwater exploration well that was ultimately drilled with a dynamically-positioned rig in 1,074 m water depth within Nigeria's Offshore Mining License (OML) 133 (formerly OPL-209). During the early well planning process, the site investigation team identified numerous, extensive shallow hazards stacked in the area surrounding the Erha-7 geologic targets. These hazards were evaluated by the drill team, site investigation team, and the business unit to optimize the well location and minimize the risk of encountering shallow gas-charged sands. The final well location allowed vertical drilling of the riserless conductor-hole interval and required directional drilling below the conductor to intersect vertically stacked geologic targets. Because of the close proximity to numerous shallow hazards and the limited seismic resolution, the final well location was still deemed to possess a moderate risk of encountering gas-charged shallow sands. This paper discusses the shallow hazards planning involved with the Erha-7 deepwater well. It summarizes the limited industry experience regarding deepwater shallow gas flows and the associated safety considerations. The paper presents the modeling and evaluation of shallow gas flows and dynamic kills used to quantify the potential benefits of drilling a pilot hole, and discusses the sensitivities associated with performing an effective dynamic kill. Finally, a discussion of the dynamic kill plans developed to prevent and effectively mitigate a shallow gas flow. Introduction The Erha field was discovered in 1999 in approximately 1,074 m of water in Nigeria's Offshore Mining License (OML) 133 (formerly OPL-209). The Erha-7 well was a near-field wildcat designed to test a prospective accumulation of hydrocarbons to the north of the main field. Fig. 1 shows the location of OML-133, the Erha field, and the location of the Erha-7 well. Fig. 2 is a bathymetry map of the Erha-7 area. Fig. 3 is a seafloor rendering of the area illustrating the seafloor channel complex. The drilling rig contracted for the Erha-7 well was the fifth generation dynamically positioned drillship Deepwater Discovery. Site investigation revealed numerous, high-amplitude seismic anomalies associated with three different stratigraphic intervals and depths below the mudline (BML). The anomalies are typical of those associated with confined and unconfined deepwater channel complexes. The Erha-7 well was positioned on the proximal margin of a large, sediment filled, confined-channel complex. The channel complex consisted of multiple stacked channels that had migrated laterally through time, making it difficult to find a location within reach of the Erha-7 reservoirs that would not penetrate potentially gas-charged shallow sands. The attenuation of seismic data below high amplitudes in the confined-channel complex and limited lateral resolution further complicated the task. Fig. 4 illustrates the potential shallow hazards surrounding the Erha-7 location, and Fig. 5 shows the summary of shallow hazards prepared by the site investigation team.

Since 1996, ExxonMobil has conducted an extensive deepwater exploration / appraisal program in West Africa. In an effort to achieve improvement in production testing performance, ExxonMobil's West Africa Deepwater Drill Team pursued key initiatives to deliver quality well test operational results. These initiatives included performing effective long-term and near-term planning, highgrading equipment quality control and inspection practices, making appropriate use of selective new technology, enhancing standardized procedures by capturing lessons learned and best practices, performing rig-site engineering supervision of production testing operations, and communicating captured follow-up recommendations to service contractors. To execute these continuous improvement initiatives, a single dedicated test engineer was assigned to plan and oversee all West Africa well testing operations. As a result of this increased focus, the average non-productive time (NPT) per test has been reduced from approximately 30% to 6% and the overall test time has been reduced by about 40%. The methods and resources used to achieve these improvements are discussed in detail in this paper. Overview of Well Test Performance and Strategy ExxonMobil is a leading holder of deepwater acreage in West Africa, with interests in 17 blocks totaling nearly 16 million acres in the countries of Angola, Nigeria, Equatorial Guinea, and Republic of Congo (Figure 1). Since 1996, ExxonMobil as operator has drilled 37 exploration / appraisal deepwater wells resulting in the discovery of significant hydrocarbon reserves. These wells were drilled with both moored and dynamically positioned drillships and semisubmersible rigs in water depths ranging from 2000–6000 ft (600–1800 m). By mid-2002, this has resulted in ExxonMobil conducting more than 20 deepwater production tests offshore West Africa. Deepwater tests in West Africa require significant planning due to the region's remoteness and limited contractor and equipment availability. The use of specialized deepwater test equipment results in daily spread-rates during testing operations that can exceed $450K per day. Additionally, because of the exploratory nature of the drilling activity in this region, production test timing and well characteristics are unpredictable. As such, one of the most challenging aspects of testing West Africa deepwater wells has been to maintain testing operations flexible enough to accommodate a wide range of production conditions on very short notice. Early Test Performance Prior to the year 2000 and before the assignment of a dedicated test engineer, ExxonMobil well testing performance for offshore West Africa was on par with industry. The typical completion and test phase required around 13 days with NPT averaging approximately 30% (excluding flow periods) (Figure 2). Goals and Initiatives However, the numerous well tests conducted by ExxonMobil offered an opportunity to evaluate and strive to improve testing performance. As a result, in late 1999 / early 2000, West Africa Exploration Deepwater Drill Team Management established two key goals:Reduce NPT associated with completion and test phase operations from approximately 30% to around 5%.Improve testing operational efficiency through the implementation of best practices and lessons learned. To achieve these goals, the Drill Team assigned a drilling engineer from within the team to focus specifically on well test operations. The test engineer's responsibilities were to include oversight of all West African testing operations and implement specific initiatives aimed at achieving the two key goals. Post-Initiative Results As a result of the methods described in this paper, the typical completion and test phase was reduced to around 8 days with NPT averaging approximately 6% (excluding flow periods) (Figure 3). This equated to nearly $20M of cost savings in well tests performed from 2000 through mid-2002 and validated the benefit of assigning a dedicated test engineer to testing operations.

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