Restoration and Remediation of Abandoned Petroleum Drill Sites - A Nigerian Case Study

Abstract

In mid 2002, the Federal Government of Nigeria through the Special Adviser on Petroleum Matters gave a directive that all Oil and Gas Operators should embark on abandonment and remediation/restoration programs to address all abandoned drill locations cutting across land, swamp and offshore environments. Before this directive, our company operations had commenced a comprehensive effort in 1999 to address these sites. Across the Nigerian Niger Delta, abandoned drill locations (including well stubs) number well over several tens of thousands. The Niger Delta area of the country lies in the southern part of Nigeria and oil and gas operations have been carried out in this region over the past four decades. Community unrests have arisen from lack of commensurate development in this region. In addition, new environmental regulations with very restrictive requirements have been enacted. This case study consists of two phases. The first phase is to address the land sites which involve demolition of existing camp/concrete structures, removal of existing well heads, backfilling of existing pits (including large borrow pits), leveling of entire drill site to match natural contour level, spreading of native humus soil and revegetation with native plant species. The second phase will address the abandoned swamp and offshore wells by ensuring that they are completely plugged and abandoned using a more cost-effective approach such as the use of highly compressed sodium bentonite. Thereafter, the wells will be cut below mudline and removed. This paper discusses the issues involved in abandoning and restoring the land sites and the well stubs in swamp and offshore environments. Experience obtained in carrying along all stakeholders particularly the host communities in the Niger Delta and the regulatory agencies is presented. Other alternatives to pit closure will be mentioned and the environmental and economic benefits of using highly compressed sodium bentonite (Zonite) for plugging well stubs will be discussed. Introduction The Niger Delta is situated on the Gulf of Guinea on the west coast of central Africa (Figure 1). During the Tertiary it was built out into the Atlantic Ocean at the mouth of the Niger-Benue river system, an area of catchment that encompasses more than a million square kilometers of predominantly savannah-covered lowlands. The delta is one of the world's largest, with the subaerial portion covering about 75,000 km2 and extending more than 300 km from apex to mouth (see Figure 1 also). The regressive wedge of clastic sediments which it comprises is thought to reach a maximum thickness of about 12 km. The Niger Delta forms one of the world's major hydrocarbon provinces, with proven ultimate recoverable reserves of approximately 35 billion barrels of oil and an underevaluated, but vast gas resource base. The oil and gas resources are closely associated and are present throughout the delta (see Figure 2). The oils are of the light waxy type, typical of deltas (see Figure 3). Geochemically, they appear to belong to one family, although heavier bacterially degraded oils are found at shallow depths, where the formation temperature is below 80° C. The Need for Closure Before the regulatory directive, the company, based on their ‘Operational Excellence’ philosophy, had pro-actively commenced remediation and restoration of some of these abandoned land sites. By doing so, the company developed improved relationship with major stakeholders. In addition, this effort has gone a long way to solidify their leadership role in the industry from a safety and environmental performance standpoint. History A large number of exploratory wells were drilled on land, swamp and offshore locations. The practice then was the use of Water Based Mud (WBM) and bigger holes. At the land sites, earth materials were borrowed from offsite locations to the drill sites for civil work construction. Large borrow pits were created as a result.

An overview of the coal seam gas developments in Queensland

“…The predicted pressure was reported to be consistent with the results of their laboratory experiments on plug heights of 3-10 feet although their data exhibited a lot of scatter. Moreover, the reported parabolic relationship is in contrast to the work of Chevron researchers, who assumed that the relationship between the height of bentonite and the pressure it can seal is linear [75][76][77] . Recent analysis by Hywel-Evans and Towler [78] suggests that while the frictional strength is a parabolic function of plug height, while shear strength is linear with height, lending validity to Chevron's assumption for long plugs.…”

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“…The heat source comprises a coaxial borehole heat exchanger (BHE) exploiting the thermal contents of an abandoned oil well in form of geothermal energy. The numerical analysis presented in [28] was adopted in this study using the geometrical parameters of a typical abandoned oil well in the Niger Delta region of Nigeria [29]. The main features of the abandoned well and the BHE are highlighted in Table 1.…”