Real-Time Subsurface Risk Re-Assessment During Exploratory Drilling: Case of a Deep Play Exploration in Niger Delta

Abstract

Abstract Many wells drilled in Niger Delta up to depths around 12000 ftss are hydrostatic or near hydrostatic, but some wells have encountered very high overpressure. Beyond this depth, formation pressure gradient just below 0.9 psi/ft have been measured, near well Total Depth (TD). Drilling through highly over-pressured sands require robust drilling practices, since taking a kick could present more challenging drilling risks and potential to create HSE incidents. This paper presents a case history of an exploration well with the above-stated drilling situation with unusual pressure trend. It has a near-hydrostatic overburden that terminates at depth in a sharp pore pressure ramp with significant overpressures. Below the ramp, pore pressure sharply decreased back to near-hydrostatic in a thick reservoir interval. Below this reservoir interval, pore pressure ramped back to high overpressure down to well TD. Given this abnormal occurrence in the Niger-Delta, there was the need to evaluate the potential risk of over-pressured gas crossflow from the geo-pressured reservoirs to the normally pressured reservoirs through the Open-Hole (OH) section of the wellbore. This was critical to the drill-ahead decision that had to be made to extend the well into the exploration target sands or call well TD, prematurely. The result of a quick subsurface risks assessment carried out during drilling showed that the shallower sands are competent formations that can serve as natural thief zones for crossflowing over-pressured gas. The risk of reservoir pressure inflation that could lead to formation top seal failure and Internal blow-out in the subsurface was also evaluated. This provided a technical business solution that aided the timely decision to drill-ahead and extend the well into the planned exploration targets. The well discovered significant gas volumes, and was considered, ‘a near-Big Cat volumes discovery’, which has significantly increased the Nation’s gas resources. Landing well TD prematurely, instead of drilling to planned TD, would have cost us the volumes from the deeper exploration target sands. This real-time intervention ensured a quick decision was made and eliminated Non-Productive Time (NPT) that could have resulted from idle time. Finally, the risk re-assessment further demonstrated the value of effective collaboration in decision-based subsurface uncertainty management.