ABSTRACT

Environmental protection is a major consideration in developing today's offshore resources. Protecting the natural habitat from pollutants is a significant factor in offshore drilling and production. It is also important that shallow, potable water supplies and other important minerals be protected from contamination beneath the surface. Today, wireline technique scan effectively monitor tubular goods and annular cement to determine their capacity to confine potentially harmful materials. The location of corrosion, pits, splits, and other corrosive effects on the inside or outside of tubular goods can be pinpointed, and the magnitude quantitatively evaluated with acceptable accuracy. A system of sectorial evaluation permits the entire tubular periphery to be accurately evaluated for cement shear and hydraulic bond, and to determine if effective zone isolation is established. Data from several test wells and practical field examples support the effectiveness of the sectored, pad-mounted bond log instrument. Recognition of zone isolation and/or hydraulic cement integrity is virtually 100% accurate with the proper measurements, and pinpointing problem areas in a casing string permits effective remedial action to be taken. When conditions are irreparable, the well can be properly plugged and abandoned.

CASING OVERVIEW

Steel casing is manufactured by means of two successive processes that convert an iron oxide to an iron and steel alloy. A blast furnace initially transforms the iron ore to molten pig iron, a solution of carbon, iron, silicon, phosphorous and sulfur. The pig iron is then refined by a steelmaking process to reduce the high concentrations of unwanted elements to acceptable levels, steel being the end product. Steel is formed into casing and other tubular goods at a pipe mill. The heat energy used to convert iron ore to steel is ironically the cause of corrosion of steel casing in a wellbore (Fig. l).l

CEMENT OVERVIEW

During well cementing operations, a cement slurry is pumped downhole through the casing and back up the annular space between pipe and borehole wall. The cement is then allowed to "set" or "cure" and, as a result, develop its compressive strength.2 Cement serves two principal function:

  1. shear bond, which mechanically supports the casing, And

  2. hydraulic bond, which restricts fluid movement longitudinally along the casing and between adjacent geological horizons

A fundamental problem in cementing is fully displacing the fluid in the casing annulus. Hydraulic bond at the cement- to-formation interface is largely influenced by the presence or absence of filter cake and by rock type.

Permeable formations usually experience better hydraulic bond because cement slurry loses water to the formation and develops a higher compressive strength.2 High efficiency is often difficult to achieve as a result of the ease with which cement flows through the larger annular regions and its ability to flow through narrow annular regions. Several remedial measures alleviate the problem, Also, well cement.

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