The Importance of Well Construction and Well Integrity for Reservoir Management— A Mature Field Experience in Sudan

Abstract

This paper describes the importance of well construction & well integrity and its relationship to reservoir management. Productivity enhancement studies in combination with reservoir simulation modeling on the Greater Heglig fields have revealed that well performances and production related problems were largely related to poorly designed wells and poor cementing practices. As a result, water channeling and cross flow across wellbore dominated true well performance characteristics contributing to very high water cuts in the majority of the producers in Greater Heglig fields. Separating the mechanically induced well behaviour from reservoir behaviour helped history matching the wells greatly, findings of which were subsequently validated during the study through running of ultra sonic imaging tool. The ultra sonic logging campaign proved the existence of channels, micro annuli's and cross flow across the wellbore causing a "water channeling phenomena" of up to 90% water cut across majority of the wells. As part of the productivity enhancement program for the Greater Heglig fields, a total of 23 sidetrack candidates have now been identified to capture the remaining developed reserves of ca. 30.0 MMstb, which will otherwise remain unproducible from the existing wellbore's. In addition to this, fit for purpose sidetrack well designs and construction together with good cementing practices will be required to ensure well integrity to improve reservoir management of the Greater Heglig fields. Introduction Greater Heglig is one of the major oil fields of GNPOC, Sudan. It was discovered in 1982 by Chevron and was put on-stream in June 1999. Its area encompasses some 380 sq. km, containing 8 producing oil fields with Heglig as the main field (Fig. 1). It holds around 800 MMSTB oil inplace in 2P category. The field was put on production based on the recommendation of FDP. To-date, 61 wells have been drilled in Greater Heglig field. Field production peaked to 64,000 kbopd in 2001. Plateau rate of around 50 kbopd was maintained for three years. It has produced cumulative oil of 123 MMSTB till April 2006 and is currently producing around 40 kbopd with 85% water cut (Fig. 2). Field is in mature stage of its producing life and is facing "Mid-life Crisis" mainly due to higher water cut. This paper focuses on finding out the reasons for high water cut and other field related problems. It also focuses on identifying bypassed oil and suggesting suitable reservoir management plan to improve field recovery. Geological Setting and Stratigraphy Greater Heglig oil field is located in the sedimentary basin of Muglad in interior Sudan. This area forms part of the Cretaceous-Tertiary Muglad basin in south-central Sudan. The basin was initiated as an extensional graben to the immediate south of the Central African Shear zone. An early phase of extensional tectonics led to rapid subsidence and lacustrine basin fills comprising the rich source rocks of the Lower Cretaceous Barremian-Neocomian Sharaf Formation and Albian-Aptian Abu Gabra Formation. The reservoir section in the Late Albian to Cenomanian Bentiu Formation accumulated as widespread sheet sandstones in response to a cessation of active extension and during a period of regional sag. It marks a fundamental change from an internally draining lake basin to larger scale sediment dispersal patterns transporting sediment out of the basin in the north and south. A sudden change from the sandstone-dominated successions of the Bentiu Formation to the shale-dominated interval of the Aradeiba Formation marks the onset of a second phase of extension and increased subsidence during the early Turonian. The Bentiu Formation comprises the main reservoir interval and is characterized by stacked successions of thick, amalgamated cross-bedded sandstones and intervening laterally extensive, thinner mudrock intervals. Sandstones of the overlying Aradeiba Formation are characteristically isolated within an otherwise mudrock-dominated succession. No significant thickening of stratigraphic units across faults is evident in the Heglig area. It is likely that subtle difference in subsidence due to differential compaction across buried grabens and half grabens will have influence on sediment dispersal patterns during Bentiu and Aradeiba times.