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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents formation treatment technology that helps (1) reduce well-construction costs, (2) improve and maintain wellbore integrity during drilling, and (3) enable economical production from reservoirs with challenging drilling and cementing conditions. The technology presented helps stabilize and improve wellbore conditions by several different downhole mechanisms and types of sealing materials. This paper emphasizes one of the most promising of these mechanisms: increased near-wellbore fracture gradients (NWFG), or wellbore pressure containment (WPC) caused by stress alterations in the rock surrounding cracks induced by sealants forced into the cracks to slightly widen them near the wellbore. Industry studies 1,2 sometimes refer to these stress alterations as "stress cages." The alteration process 3 has been described as a WPC failure mechanism in which tensile force switches to compressive force as the fracture is sealed near the wellbore. Each side of the wellbore connecting the two fracture wings is moved by the compressive force in directions perpendicular to the fracture plane and proportionally to the increase in fracture width. As a result of this WPC improvement, engineers may plan wells with fewer casing/liner strings and with upper hole sections of smaller diameter. Under certain conditions, it is possible to construct the "one-trip" well, wherein only one bit-run is needed to drill to total depth (TD) from the surface-casing shoe. The new technology can also help prevent or mitigate unstable hole conditions that interfere with the drilling process.Wellbore-stabilization technologies presented are designed for wells with narrow mud-weight windows (MWW) that can lead to well-construction incidents such as (1) loss of circulation, (2) failed shoe tests, (3) poor annular zonal isolation, (4) the need for extra strings of casing to enable drilling through depleted formations, and (5) lack of sufficient mud weight to prevent hole collapse and gas kicks. The key process in the system is placement of sealant in wellbore cracks to strengthen the wellbore to enable the use of a greater range of mud weights, i.e., a wider MWW.Results from a numerical simulation analysis model are reported. The model is an aid to well planners in their efforts to design wells with (1) fewer casing strings, and (2) smalldiameter upper-hole casings.Case histories from around the world indicate that this process can result in substantial cost savings and that it represents a real breakthrough in drilling and completion technology. Improved primary cementing and sustained annular zonal isolation during production are also described. The fracture gradient (FG)-increasing treatments were applied during drilling to prevent lost circulation while cement slurry was being placed in setting casing and liner strings. Production cost savings included shorter periods to start of production, improved well integrity, prevention of annular wellhead pressures, no skin damage in reservo...

This paper describes the design, development and application of a new sealant system for multilateral well junctions. The sealant system has been successfully applied in offshore multilateral well installations for North Sea operators. In recent years, the oilfield industry has shown an increasing interest in multilateral wells. Multilateral wells are an economically viable solution for many offshore operators because multilateral wells increase reservoir drainage and extend the wellhead slot coverage on offshore platforms. Multilateral wells can range from simple openhole completions to sophisticated cased-hole completions that have selective access to all laterals. In contrast to the openhole completions and slotted-liner completions, cased-hole completions enable the operator to have more control over production because cased-hole completions allow re-entry for perforating and production logging operations. However, zonal isolation is difficult to achieve at a multilateral junction where the hydraulic seal between the formation and the casing interior is maintained by the sealant only. The sealant system must be resistant to many destructive forces during completion and production. Mathematical models were used to quantify stresses that are exerted on the sealant during completion and operational processes in a multilateral well. A procedure was established to scale the stresses to laboratory conditions. Based on the procedures, it became apparent that the multilateral sealant required high elasticity and high impact resistance, as well as standard oilfield cementing properties. This paper presents how the sealant system was specifically designed and developed to meet the requirements of a cased-hole multilateral completions. Field jobs in which the sealant system was successfully placed are presented. These jobs show that the sealant system withstood stresses at the junction in North Sea multilateral completions. Introduction Multilateral wells have recently been developed to increase production. Multilateral wells can be vertical or deviated (including horizontal) wellbores connected to one or more subordinate laterals. Drilling and completion equipment have been developed that allow multiple laterals to be drilled from a main cased and cemented wellbore. Each of the lateral wellbores can include a cemented liner that is connected to the principal wellbore (Figure 1). Multilateral wells have been successfully drilled and operated; however, one operational problem involves zonal isolation of the multilateral junction. The casing and liners are cemented in the principal and lateral wellbores, respectively, by introducing cement slurries in the annular clearances between the walls of the wellbores and the casing and liners. In the past, conventional well cement slurries were used. Although cement is a strong material, it cannot withstand repetitive impacts and stresses that occur during drilling, milling and other well operations in the laterals. Once the set cement is shattered, it may allow leakage of fluid through some portions of the wellbores. Improved methods of cementing multilateral wells were necessary. A sealant research project was initiated to determine testing methods and fluid compositions. A number of test methods were used including API fluid testing, impact resistance tests and chemical resistance tests. The multilateral sealant required high elasticity and impact resistance. Therefore, a highly elastic sealant and a brittle sealant were chosen as extremes to guide the development of the multilateral sealant. Elastomer-based slurries were chosen for testing because they are known to provide high elasticity to set materials. P. 243^

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