Usari BQI Field Redevelopment
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 101067, "Usari BQI Redevelopment - Paradigm Shift," by F.R. Alege, SPE, W.R. Brock, SPE, J. Linscott, B. Olopade, C. Etta, SPE, and L.K. Chen, SPE, Mobil Producing Nigeria, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September.
Developing low-gravity-oil reservoirs is not common in the Niger Delta. The Usari Base Qua Iboe (BQI) reservoir is an example of a heavier-crude-oil reservoir (18 to 20°API) in a field of approximately 35 more-conventional reservoirs (32 to 44°API). Until recently, this waterdrive reservoir had been developed only partially because of the crude-oil gravity, adverse oil/water mobility, and a strategy of low producing rates to avoid producing at high water cuts in the 70-ft-thick oil column. A study of analog fields in Nigeria and Western Australia led to a redevelopment campaign, which resulted in a nearly 10-fold increase in production and a significant improvement in reserves in less than 12 months.

Introduction
The Usari field, 15.5 miles offshore from the Qua Iboe Terminal in Akwa Ibom State in southeastern Nigeria, was discovered in 1964. In addition to the discovery well, four appraisal and 32 development wells have been drilled. Twelve wells penetrated the BQI reservoir and provided good well control for delineating the reservoir and establishing fluid contacts.
In total, 35 reservoirs have been discovered in the Usari field. These reservoirs have been subdivided into three main zones on the basis of fluid properties, pressure regimes, and geologic setting. These categories are commonly referred to as the shallow (18 reservoirs), intermediate (15 reservoirs), and deep (two reservoirs). The fluid gravities for these categories range from 20°API for the BQI (shallow) to 44°API for the 2-US6 (deep). Fluid viscosity ranges from 5.6 cp in the BQI to approximately 0.3 cp for the intermediate reservoirs and 0.2 cp for the deep reservoir.
The BQI reservoir, with more than 20% of Usari's original oil in place, has a relatively low solution-gas/oil ratio (183 scf/STB). The oil is more viscous (5.6 cp) and dense (20°API) than other area crude oils. The reservoir has a 70-ft-thick oil column with a gas cap approximately 60% of the size of the oil accumulation. The reservoir is mainly upper-to-lower shoreface sands with good lateral continuity and high permeability with a moderate-to-strong waterdrive. The low-API-gravity oil (density close to that of water) and the low gas content make gas lifting a requirement to produce this viscous oil.

History
The first BQI reservoir-development efforts began in 1967 after drilling the Usari-3 appraisal well and installing the first wellhead platform in the field. This vertical well was completed as a producer in the BQI reservoir but died because of low wellhead pressure in less than 1 year with a lack of gas lifting capability. Well Usari-3 subsequently was plugged and abandoned.
Development of the BQI reservoir was resurrected in 1999 with two horizontal wells, Wells 20B and 21B, followed by horizontal Wells 23C and 24C in 2001. These four wells were landed approximately in mid-column and had horizontal completion intervals ranging from 1,500 to 2,000 ft. The wells had 7-in. casing cemented to surface and were completed with 2 7/8-in. tubing equipped with gas lift mandrels.