Developing Small Tight Gas Reservoirs Through Horizontal Drilling
Authors: Paterson R. , Moss J. , Williamson B. 
Published: December 2007

Mobil North Sea Limited has in the last 4 years developed a total of eight tight gas reservoirs in the UK Southern Gas Basin. The economic success of these developments is largely due to the use of an original combination of standard horizontal drilling techniques. The economic viability of a number of these fields has only been possible by optimising the number of wells in each development and minimising drilling costs. Currently in excess of 50,000 ft of horizontal reservoir section is being produced from 18 wellbores. The fields have been developed through a range of production options including subsea monopod and conventional four leg platforms. This paper will document the progression in drilling technology from relatively simple horizontal wellbores to extended reach wells and multi-laterals.

Drilling techniques and equipment including the use of water based drilling fluids, friction reducing lubricants, bottom hole assemblies, drill string design, well planning and well control strategies in the Zechstein carbonates, will be highlighted.

A summary of drilling performance and cost per foot analysis will also be presented.

Introduction

Mobil North Sea Limited (MNSL), operator in the UK sector of the North Sea, has in the last 4 years brought to development a total of eight fields in the UK's Southern Gas Basin (SGB). Early appraisal drilling showed small reservoirs (20 to 300 BCF) with medium to tight Rotliegendes Leman sandstone reservoirs and poor well test results.

The introduction of horizontal drilling technology and the gradual progression to extended reach and multi-lateral drilling has led to successful economic development of each of these fields.

A total of 18 horizontal wells are currently on production from the Rotliegendes Leman sandstone at reservoir depths ranging from 6000 – 8500 ft tvdss.

The economic viability of all of these fields has been considerably enhanced by the introduction of cost reducing methods (minimising number and cost of both wells and their infrastructure) and maximising individual well productivity through careful targeting of probable high deliverability intervals.

Geology Overview

The majority of MNSL's horizontal development wells have been drilled within the Lancelot area complex, which comprises a number of Rotliegendes (Leman sandstone) gas accumulations. The Lancelot complex is located on the southwestern margin of the Southern North Sea Basin mainly in Blocks 48/17a and 48/18a. Figure 1 shows the location of the Lancelot complex within Quad 48 of the SGB.

The Lancelot area lies on the south-west flank of the Sole Pit Trough and straddles the Dowsing Fault zone which runs north-west to south-east through Blocks 48/17 and 48/18. The facies present in the area include; aeolian dune and interdune, fluvial sheet flood, minor playa lake sediments and marine reworked sands. The local stratigraphy consists of a Cretaceous chalk overlying the Jurassic Lias shale and a Triassic Bunter sand/shale sequence. The Zechstein group which comprises four cycles, Z1 to Z4, forms the vertical and lateral seals for the gas in the Lancelot complex. The Zechstein sequence incorporates both the Platten and Haupt dolomites which are commonly overpressured and their presence has the greatest influence on well design and drilling costs. Figure 2 shows a generalised stratigraphy of the Lancelot area.

The Rotliegendes Leman sandstone within the area varies in thickness from 80 - 300 ft with a gas water contact generally present within the sandstone. P. 435