A Geosteering Success Story From the North Sea
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 159132, ’Geosteering the Impossible Well: A Success Story From the North Sea,’ by Soazig Leveque and Christophe Dupuis, Schlumberger; Thomas Stærmose, Estela Vazquez Esmerode, and Peter Sommer Linnet, DONG E&P; and Graham Raeper, Schlumberger, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8-10 October. The paper has not been peer reviewed.

The Nini East field represent a significant drilling challenge. The target is a post-depositionally remobilized sand reservoir with a thickness of 2‒15 m. None of the standard well-placement methods or current mapping tools was a suitable solution for the well objectives. A field test of the next-generation bed-boundary mapping tool was launched to geosteer two upcoming production wells. The two producers achieved an outstanding 0.99 and 0.96 net pay ratio with no sidetracks or delays and were completed within budget.

Introduction

The Nini East field is located 3 km south of the border between the Danish and Norwegian sectors of the North Sea (Fig. 1) and was discovered in 2001, when oil was found in the Kolga-member sandstones of the Upper Paleocene/Lower Eocene Sele formation.

The challenges in drilling horizontal wells in this type of reservoir include a complex geological setting—leading, in some wells, to a low net-pay ratio—combined with the unstable reservoir- capping Sele shale, which increases the risk of having to drill sidetracks.

The Kolga-member reservoir of the Nini East field consists of a glauconite-rich sandstone with unique characteristics. Although initially deposited as traditional gravity-flow sands, the Kolga sands have been remobilized under burial, leading to sand fluidization and injection along weakness zones. These traits create difficulty in defining the exact stratigraphic location of the sand.

With these challenges in mind, the Dansk Olie og Naturgas company (DONG E&P) launched the development of the Nini East field in 2009 with a drilling campaign including two horizontal producers and one injector in the Kolga sand. The two main goals were to increase the percentage of horizontal section in pay while avoiding the Sele shale and maintaining the oil/water-contact (OWC) standoff.

Methodology

The operator decided that much-needed uncertainty reduction could only come from significant investment in seismic-data processing and inversion, combined with the new- generation deep-directional-electromagnetic (DDE) service-placement workflow. The seismic data were reprocessed and inverted to improve the imaging of the injectite sands. The seismic resolution was improved from approximately 10 m on the conventional seismic data to approximately 5 m on the inverted cube (Fig. 2).

The primary method of trajectory adjustment involves correlating the logging-while-drilling (LWD) log response acquired on a drilling well with offset data acquired along previously drilled wells in the area. This implicitly relies on the assumption that the formation is continuous between the offset wells and the current drilling location—an assumption that may not hold in faulted reservoirs, pinchouts, unconformities, or highly remobilized reservoirs such as those found in the Nini East field.