Utilizing Innovative Flexible Sealant Technology in Rigless Plug and Abandonment

Abstract

Abstract This paper presents Petroleum Development of Oman's (PDO's) rigless plug and abandonment (P&A) technologies and practices resulting from a corporate goal to find a cost-effective means to abandon its oil wells. PDO is abandoning aging wells and must balance environmental and financial objectives. Technically sound abandonment practices are essential for long-term environmental protection. Many operators are revising procedures to ensure that depleted reservoirs remain permanently sealed. During a 60-well pilot project in Oman, PDO optimized its practices by employing new cementing and plugging technologies to minimize the cost of abandonment while ensuring long-term environmental protection from once-prolific oil wells. PDO used a new expanding, flexible sealant system on the P&A project, conveying the flexible sealant with a coiled tubing (CT) unit. The new sealant has improved elastic and expansion properties to meet PDO's requirement for a long-term mechanical durability and zonal isolation. The presence of expanding material leads to an optimum configuration to prevent loss of zonal isolation resulting from changes in downhole conditions over time. The presence of flexible and expanding materials increases the elasticity and decreases compressibility and permeability of the set cement while retaining good compressive strength. Use of a CT unit eliminated the need for a rig, which reduced the time and cost of operations. The novel flexible sealant system provided the long-term isolation necessary to meet environmental standards. This paper presents case histories on the use of this new flexible, expanding sealant and its influence on the P&A project. Introduction The use of cement for P&A of an oil well has been known to the oil industry for many years. The main function of the cement is to provide support and to prevent any fluid communication between drilled formations to provide long-term zonal isolation. Zonal isolation1 must be maintained during the life of the well and after its abandonment. Mechanical failure of the cement itself, debonding of the casing from the cement, or debonding of the cement from the formation can cause the loss of zonal isolation in absence of chemical attack. Mechanical failure can lead to the formation of cracks while debonding leads to the formation of a microannulus.2 Both mechanisms create a high-conductivity path for any fluid or gas. Mathematical modeling is the traditional method of quantifying the deformation mechanism and the amount of damage generated downhole. This modeling determines the properties the cement must have to prevent loss of integrity. Variation in downhole conditions can induce sufficient stresses to affect the integrity of the cement sheath, which will result in loss of zonal isolation. A weak formation will not mechanically support the cement deformation. In a recent study, Bosma3 also recommended that the selection of the well sealant should be engineered with respect to all the mechanical properties of the sealant, rather than on compressive strength alone. Mechanical damage can also follow excessive shrinkage of the cement sheath, as demonstrated by Thiercelin,4 and nonshrinking sealant material is recommended. Abandonment Requirements The life of a well has numerous stages: discovery of a new accumulation of oil or gas, drilling and achieving first production, successful enhanced recovery operations, and finally abandonment of wells and production facilities. Yet the effects of abandonment practices are felt for many more years than the relatively brief producing life of the average well. Well abandonments are becoming increasingly frequent as aging fields reach their productive and economic limits.5 From 2000 to 2010, offshore abandonments will encompass 27,000 wells on 4900 platforms worldwide. Onshore, tens of thousands of wells will have to be abandoned in the future.