Role of Cement Fluid Loss in Wellbore Completion

Abstract

ABSTRACT The fluid-loss behavior of cement slurries against permeable formations and its relationship to interzonal fluid migration have been investigated. Static fluid-loss testing, designed to simulate cement placed in a wellbore, has shown that the cement fluid-loss rate through a deposited mud filter cake decreases with time. This decrease in fluid-loss rate is dependent upon the designed fluid-loss properties of the cement slurry, and significantly influences the loss of hydrostatic pressure required to control formation fluids. Once the pressure starts to decay, formation fluids can potentially enter the wellbore causing ineffective zonal isolation and, in some cases, gas migration to the surface. The mechanism described indicates that hydrostatic pressure is initially transmitted after cement placement. During this period, fluid loss to the surrounding formation is compensated for by an equal decrease in slurry volume. The resulting changes in the solid-to-water ratio and gelation properties coupled with the growth of the cement filter cake cause the pressure against the formation to eventually decrease. The rate of pressure decrease is dependent upon the fluid-loss rate and the slurry's compressibility. Once the pressure approaches the formation pressure, interzonal fluid flow through the cement slurry occurs with its rate initially governed by the cement fluid-loss rate. Several correlations have been made relating fluid loss to the pressure profile after wellbore completion. The results suggest that an API fluid-loss requirement of 100 mL/30 min, which may be sufficient during cement placement, is actually too high when consideration is given to the fluid loss that occurs after cement placement. Sufficient delays in the loss of pressure are obtained for an API fluid-loss requirement in the range of 10 to 30 mL/30 min with the actual value dependent upon wellbore geometry, overbalance pressure, mud fluid-loss properties and cement slurry design.