Drilling a Horizontal Well Using Air/Foam Techniques
Authors: Shale Les , Teleco Eastman , Curry David 
Published: October 2008

ABSTRACT
To provide a dedicated horizontal well for testing and training, a well was drilled in Aberdeen, Scotland, using airlfoam drilling techniques and employing a significant amount of new air drilling technology. Well design was determined by consultation within the industry to ensure optimum utility of the well. The well design and drilling techniques are chronicled. A subsea wellhead was set on 30" conductor pipe. A vertical 12-114" top hole section was drilled to 979 ft by a combination of air hammer and rotary techniques. 9-518" casing was set prior to drilling the 8-112" build and horizontal sections using steerable air motors. Both surface and downhole equipment used to drill the well are described in some detail. The actual drilling is then reviewed, concentrating on the performance of the air motors and the survey systems. The overall performance of the drilling techniques adopted allowed the well to be completed as designed and within budget. As evidenced by this success, air drilling now offers a viable means for drilling horizontal wells.

INTRODUCTION
The International Drilling and Downhole Technology Centre (IDDTC) provides an open-access facility for full scale testing of oilfield equipment and training of personnel. Horizontal wells are playing a rapidly increasing role in the production of hydrocarbons around the world.1,2 However, if the full potential of horizontal wells is to be achieved, much remains to be done to develop alternative drilling techniques, completion and production equipment. It was always envisaged that a horizontal well would form part of IDDTC's technical specification. During 1990, two wells were drilled at IDDTC's site, one to 300 ft and one to 4900 ft. Both were drilled using foam as the drilling fluid. Drilling the first near-vertical, high pressure well revealed unexpected geological difficulties. Electric logs showed several massive washouts associated with zones of naturally shattered rock. The formations were hard and abrasive, being principally metamorphic with some igneous intrusions (schists and gneiss). Garnet concentrations ranging from 15 % to 25% were encountered over the first 2000 ft. The formation dipped 10 to 20 degrees from SSW to NNE. Abrasive wear of downhole equipment, bits and stabilizers, was high, typically losing 114" on gauge per 300 ft drilled. Both wells had a tendency to deviate towards the SSW, building angle at around 1°/100 ft. In thedeeper 4900 ft well, water influx of around 35 bbllhr was experienced from 400 ft to TD, and any attempt below 400 ft to circulate liquid resulted in total losses.2 These drilling problems prompted a review of the initial design for a horizontal test well. A market study was performed in 1991 that indicated major shifts in the design requirement for a horizontal test well, reflecting rapid changes in horizontal well technology. A new well design, which differed considerably from the initial, was produced: a shallow, medium-radius well with an 8-112" open hole completion.

WELL DESIGN and OBJECTIVES
Optimum design for the test well was determined by a market-based appraisal. Opinions were sought on a total of 10 different aspects of the well's technical specifications: wellbore diameter, radius of curvature, completion method (cased or open hole), casing shoe inclination, wellbore direction, fluid injection capability, depth, length of horizontal section, pressure capacity, and the need for a tangent section.