ABSTRACT

The Andrew field is a small oil and gas field with a 58-m oil column, a 66-m gas cap, and a simple dome structure, producing entirely from horizontal wells. It has been a successful development for BP and the Andrew field partners, with plateau oil production extending 18 months beyond the predicted onset of field decline. Development success has been helped substantially by focusing presanction activity on key reservoir uncertainties and business decisions. The decisions that resulted were to drill all horizontal producers to optimize low gas-oil-ratio (GOR) oil recovery, to closely manage the reservoir under production, to delay gas coning and water breakthrough, and to collect sufficient surveillance data to allow regular updating of the reservoir management plan. The objective of the Andrew development is to maximize oil recovery before going to gas-cap blowdown. The challenge is to manage the GOR throughout the life of the field. Central to this are well design, location, numbers, and the drawdown strategy. The horizontal wells produce at higher rates (average 10 MBOPD) and at relatively lower drawdown pressures (100 psi). They recover increased reserves per well (13 MMBO per well), compared with a conventional well. Project economics were improved as well numbers were reduced from 24 in a conventional well case to 10 horizontal producers. Low GOR oil production has been maximized by well positioning relative to the gas-oil contact (GOC) and oil-water contact (OWC); by drilling long wells that enter the reservoir on the crest and exit through the flank of the field; and by completion design, perforation strategy, careful well management, and drilling two additional infill wells. As a result, the recovery factor has risen from 45% at sanction in July 1996 to 49% by the end of 2002. The final field recovery factor is expected