Case Study of a Quad-Lateral Horizontal Well in the Lennox Field: A Triassic Oil-Rim Reservoir
Authors: Yaliz Ayk , Chapman Thomas , Downie Jim 
Published: December 2007

The Lennox Field, located in the East Irish Sea basin UK, originally contained 184 mmstb of oil in-place within a 143 ft thick oil rim overlain by a large gas cap up to 750 ft thick. The GIIP is estimated to be 497 bcf. The oil and gas are trapped in the Triassic Ormskirk Sandstone Formation comprising high permeability aeolian and fluvial sandstone facies. The top reservoir occurs at a shallow depth (c.2500 ft) and contains a light, saturated oil (45° API) with a GOR of 650 scf/stb. The field came on stream in February 1996 and is now being developed with nine horizontal oil producers including four multi-laterals, and two crestal gas injectors. The total oil production to date (January 2002) is 43 mmstb. The current oil rim development phase will be followed by a gas cap blow-down phase anticipated to commence in 2004. Water cut from the field is currently around 2%, and some free gas production has been observed recently in a number of wells.

Horizontal producing wells have been used exclusively, and the potential to improve oil recovery with multi-lateral wells was recognised from an early date. The challenge with this development is to manage voidage replacement whilst minimising early cusping of gas into the producing oil wells. Four wells have been drilled as multi-lateral producers (two being bi-lateral, one tri-lateral and one quad-lateral) at a depth of 3365 ft TVDSS, 35 ft above the oil-water contact. The first two multi-laterals were drilled with a vertical drilling tolerance of +/- 10 ft. This tolerance was reduced to +/- 2 ft in the two latest wells, L10 and L11, in order to minimise the chances of sumps being created along the wellpath. The decision to narrow the vertical corridor was based upon the operator's (BHPBilliton Petroleum) experience in the earlier wells where standing water was known to accumulate in sumps causing a reduction in well productivity.

L10 was drilled as a quad-lateral horizontal infill well by successfully placing 4.5 km of 81/2 inch hole section within a +/- 2 ft corridor. In order to minimize the effect of inherent deviation survey errors the toe end of the horizontal well was placed 10 ft higher than its heel. The development of three open hole horizontal side-tracks using the Geo-Pilot rotary steerable system was a first in the industry. The well was completed in the last lateral leaving the first three laterals open.

The objectives of the well L10 were to provide an additional oil producer to maintain plateau, to reduce the average producing GOR of the field, and to increase oil recovery. All of these objectives were fully met. The total horizontal reservoir section drilled was 14730 ft giving a total well Kh of 11.5million mD-ft over the producing section. The well is currently producing at around 22,000 bbl/d with a drawdown of 25 psi giving a well productivity index (PI) of 880 bbl/d/psi. The successful completion of the well enabled the operator to increase field reserves and to cut down production in other producers so that the overall field GOR has been reduced to manageable levels.