Exploration of upside in a stranded discovery: Lochranza, a Donan Field satellite

Abstract

The Lochranza Field was developed using seismic amplitude analysis, evolving conceptual geological models and the implementation of horizontal well technology, built on the knowledge gained from the adjacent Donan Field redevelopment. Subtle depositional and structural complexities were, however, encountered in the Lochranza development wells. These had the potential to impact on the successful targeting of reservoir sands.Thinning sands and erratic lateral sand pinch-outs at the margins of the deep-water Balmoral Fan complex, small-scale sand injection and subtle structural complexity across the Lochranza Field were identified in the first phase of development. These introduced greater interpretation uncertainty and made further development challenging. This highlighted the importance of considering alternative geological scenarios, whilst these insights aided the identification of infill well opportunities.These uncertainties were partially mitigated by the planned development well trajectory, the data acquisition programme and the ability to geosteer based upon the geology encountered. It proved important to be mindful of different geological scenarios whilst geosteering, guided by the real-time dataset, keeping the 3D geological model peripheral to decision-making to limit the impact of anchoring bias.Identification of infill targets used a pragmatic approach based upon empirical data that showed that well recovery efficiency could be characterized by net pay length, stand-off from the oil–water contact (OWC) and connected hydrocarbon volume. Infill opportunities were defined probabilistically and subsequently supported by 3D reservoir simulation. This assessment was helped significantly by additional appraisal being undertaken as part of development well drilling.

Tertiary deep-marine reservoirs of the North Sea region: an introduction

“…† The productivity and connectivity of low NTG interchannel areas can be good when specifically targeted with bespoke production wells. In many instances, the vertical connectivity of the sheet sandbodies can be enhanced by sand-injection modification (Ings et al 2015).…”

“…However, despite this apparent complexity, careful well placement of long horizontal wells cross-cutting stratigraphy has ensured that the connected volumes are optimally accessed. Ings et al (2015) document the progress in geological understanding of the Lochranza Field following various stages of data acquisition. An initial model, based on nearby 'analogue' fields, was of sheet-like proximal lobe deposits, possibly modified by underlying bathymetric relief.…”

“…In particular, progressively better seismic definition clarified an irregular roof structure resulting from sediment remobilization and offset channel architecture, allowing horizontal wells requiring a large aquifer stand-off to be more accurately placed, thereby maximizing reservoir exposure. The internal connectivity of the reservoir can also be modified by sand injection, as suggested for the Lochranza Field by Ings et al (2015), and may also provide a leak point and access to higher reservoir intervals (Murphy & Wood 2011). …”

“…In addition, lobe reservoirs can show higher connectivity that a simple layer-cake or offset and shale-draped sand-body model might suggest (e.g. Charles & Ryzhikov 2015;Ings et al 2015). In both of these cases, the higher connectivity was unanticipated and subsequently ascribed to an unknown contribution by small-scale injection features, lobe amalgamation on axial scours or small-scale fault juxtaposition.…”

“…We have now come to expect that directional drillers never deviate by more than a handful of metres from the planned well path. Excellent examples of drilling programmes that exploited these technologies are provided by Moore (2014) and Ings et al (2015). Even the less demanding wells described by Rose & Pyle (2014) would have been much harder to achieve with 1990s technology.…”