Abstract

Cement sealing of the casing-borehole annulus and gas migration control following primary cementing are closely related to real-time downhole fluid loss rates during the cement transition period.

A series of cement fluid loss tests on washed mud filter cake clearly illustrate that downhole fluid loss rates are strongly dependent on fluid loss rates of drilling fluid. Unless altered by reactive washes, the initial fluid loss rate of the cement is equal to the rate established by the drilling fluid filter cake, but after 60 minutes cement filtration time control contributed by cement fluid loss additive is readily apparent.

Using technology similar to hydraulic fracturing fluid loss rate theory, a method of predicting the real time fluid loss rate for cement on drilling fluid filter cake has been developed. This method uses data derived from (1) standard high pressure-high temperature (HP-HT) tests on the cement slurry, (2) standard HP-HT tests on the drilling fluid, (3) laboratory drilling fluid filter cake thickness, and (4) estimated downhole drilling fluid cake thickness.

A method was derived combining static gel strength development rate, downhole fluid loss rate, and well geometry to give a dimensionless evaluation number. This evaluation number, defined as the slurry response number (SRN), is a relative prediction of cement slurry performance for controlling gas migration. Its practical use is illustrated with a case history. A field survey shows a firm correlation between this new evaluation number and successful gas migration control.

The results of HP-HT fluid loss tests on drilling fluid filter cake are given and compared to standard fluid loss tests and to simultaneously measured static gel strength. The influence of pressure on fluid loss rates and static gel strength rates is illustrated. Mechanisms by which fluid loss affects cement bonding and gas migration are discussed.

A method for determining slurry permeability during the transition period is given and results relating fluid loss to slurry permeability and specific static gel strength levels are shown.

Introduction

Annular gas flow and gas migration have been topics of much discussion and a great deal of research in the industry for several years. Many specialized cementing techniques and cement systems have been designed to combat this problem. These systems have all had some degree of success, but none has been completely successful. The theory of how cement loses its hydrostatic pressure is well accepted throughout the industry. Theories on how the cement gels, the influence of permeability, and how gas channels are formed through the cement, however, are still points of contention.

Prerequisite to consistent evaluation of gas migration control methods is agreement on the basic mechanics of gas migration. This paper compares current theories of gas migration, and with mathematical equations, demonstrates that channels in a cement column are the combined result of static gel strength development and downhole volume losses—and not the result of cement slurry permeability.

Static gel strength development of cements has been investigated in detail in past work, and some small amount of realistic downhole fluid loss of cements with drilling fluid filter cakes has been investigated.

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