SS - Sakhalin-1: Chayvo Field Subsurface Development and Learnings
Authors: Venner Bridget , Coyne Patrick L. 
Published: July 2009

The Exxon Neftegas Limited (ENL), an ExxonMobil affiliate, operatedSakhalin-1 Chayvo field is located off the northeast coast of Sakhalin Island, Russia in water depths of 15-25 meters. Drilling began in 2003 and productioncommenced in 2005. As of January 2009, thirty-nine extended reach wells havebeen drilled and the field has been on production for over three years. Mostinitial reservoir assumptions have been confirmed and some have been modified. This paper describes the evolution of the reservoir description and the highlysuccessful subsurface development plan for the Chayvo Field.

The major oil reservoir in the Chayvo Field is Zone XVII/XVIII. Thissandstone reservoir was deposited in a shallow near shore marine environment. The reservoir is characterized by stacked prograding highstand deltaclinoforms. Production behaviour indicates a high degree of lateralconnectivity, something that was an uncertainty prior to development.

Zone XVII/XVIII has a ~100 meter thick oil rim overlain by a large gas capand underlain by an extensive aquifer. The initial development plan called forthe reservoir to be developed with extended reach horizontal drill wells andsupported by high angle gas injection wells. The completion design commingledall the net pay in zones XVII/XVIII. Typical zone XVII/XVIII horizontalcompletion intervals are 2000 - 3000 meters in length. Inflow profiles werecontrolled by selective perforations in cased and cemented completions.

The initial completion design encountered problems while cementing theproduction liners. Consequently, the completion design was changed to open holepre-perforated liners and sand screens with the production profile controlledby inflow control devices (ICD's) and external swell packers.

Oil production was generally consistent with pre-development forecasts. Because of the large overlying gas cap, gas-oil ratios (GOR) were expected toincrease early during field life. However, the GOR in the field increasedfaster than forecast. A multi-functional team addressed this issue tounderstand the reasons for this increase and develop mitigating actions. Basedon the available data, the team was able to develop most likely models toexplain the GOR behaviour. As a result, completion interval optimization, arigorous reservoir management program of choking and cycling wells, anddrilling some additional production wells has mitigated the GORperformance.

The improved understanding of Zone XVII/XVIII was also possible because of athorough data acquisition program that included MWD logs, permanent downholepressure and temperature gauges, obtaining " pressures while drilling" in selected wells, and a key well production logging program.

Introduction

Early production from the Chavyo Phase-1 oil development began in 2005 froman interim processing facility (IPF) at rates of up to 50,000 barrels per day. Full production was initiated in December 2006 ramping up to full field plateauby March 2007 of 250,000 barrels per day. As of year end 2008, over 181 millionbarrels of crude and condensate have been produced. Production rates, asexpected, have declined somewhat from that plateau, but have been maintained atabout the 200,000 barrel per day range due to a combination of selectivedrilling of additional wells and careful reservoir management.