Mature vs. Unknown, Where Are We Really? – An Ecuadorian Field Case
Authors: Ziritt J. L. 
Published: December 2007

Shushufindi is the largest field in the Oriente Basin of Ecuador. It was discovered by Texaco in 1969 and the development-drilling program was initiated in February 1972 with a nominal well spacing of 50 acres. Official production started in August 1972. The main targets were the productive reservoirs in the field, the U and T sands of the Napo Formation.

The official OOIP estimated is over 3 billion barrels and the original reserves were estimated in 1589 million barrels of oil. By the end of 2002 the field had produced more than one billion barrels of oil. The remaining reserves as estimated are approximately 550 million barrels of oil. Even do early in the life of the field –December 1984–, peripheral water injection was implemented, it was discontinued in 1999 do to its limited effect.

Initial reservoir pressures were 3867 and 4042 psia for U and T sands respectively. By the end of 2002 pressures have dropped to 1800 psia for U, and 2500 psia for T, but they are still above the saturation pressures of 1065 and 1265 psia for U and T respectively. The reservoirs are considered under saturated and the main driving mechanism is a strong and active water influx coming from an aquifer that has been supporting the historical field performance. The operator-ship of this field was transferred to Petroproducción in 1992 after 20 years of development and production. The daily average production in 2002 was 61,000 BOPD with 55% BSW from 61 wells.

Engineers have advanced many concepts of how to recover the almost 550 million bbl left of proven reserves, to improve as much as possible the 33.8% recovery factor, and/or to even increase the proven reserves. The merits of various approaches have been controversial. Some feel that well density is the most important factor in oil recovery, and they argue that more infill drilling –tighter well spacing– can increase recovery significantly. Others see water injection and EOR techniques tailored to specific areas as the critical need. Still others point to improved completion and stimulation techniques.

Al these factors play important roles in oil recovery but to keep the economic benefits, particularly in mature fields, production programs must carefully optimize infill drilling, secondary and tertiary recovery, and workover operations, to minimize the large investments associated with these applications.

An integrated reservoir study of Shushufindi was engaged to optimize the performance of the field but the reported results are not satisfactory in helping understanding the reservoir behavior and field performance. Even do several factors as the presence of incised valley and channel systems, different OOWC elevations, scattered pressure data, different pressure trends, and the difficulty in matching pressure and water cut insinuate the existence of barriers and/or compartments, they still haven't been identified.

The management of mature reservoirs demands a clear picture of the reserves that have been developed and the potential that is still there. But despite the advance of petroleum technology, only after a thorough study of the primary and secondary performance history can the reservoir characteristics be adequately defined.