Abstract

A typical well scheme for a deep well usually has five casing sections that can include a liner string. However the number of casing strings is given by the number of intermediate sections required to isolate unstable formations prior to reach the pay zone.

The industry has defined the casing sizes following a standard well design of five casing string (Table 2) that can include liners depending on the well requirements. The clearance between casing string is based on a standard drift which is related to unobstructed tolerance through pipes that is associate to the casing geometry such us ovality and casing straightness, this design criteria does not take in account other phenomenons that can compromise the effectiveness of a well design such us the effect of surges forces while running casing strings.

In prescence of competent formations and shallow wells, the design citeria explained above does not represent risk in term of loss circulation during running casing or cementing operations. However in the Mezosoic fields at Souht of Mexico (Figure 1), the geological structures are domain by unstables clay domes and eventually salt domes (Figure 2) that are located between 3,200 m - 3,700m. Also the drilling environment is characterized by narrow mud windown. As a consequence the wells are completed with six casing string.

The wells drilled in this fields are classified as high risk wells due to the potential hole problems related that include massive lost circulation. The geometry of the wells is also an issue since the use of non conventional casing sizes to isolate unstable formations left low clearance between casing strings that induce lost circulation when running casing.

This paper describes the actions taken to reduce massive loss circulation and define an “Anti surge methodology to mitigate severe loss circulation” when running non-conventional casing sizes with reduced clearance between casing strings in some fields located at South of Mexico.

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