Top cement pulsation (TCP) is an auxiliary cementing technology for enhancing zonal isolation by applying low-frequency hydraulic pressure pulses to the wellhead immediately after placing cement in the annulus. A properly designed TCP keeps the well overbalanced by delaying the process of cement slurry thickening in the well annulus without pressure pulsation impacting the well's bottom. In the result, the slurry remains longer in liquid state, hydrostatic pressure is sustained, thickening time is delayed, and transition time is shortened. All these effects combined would improve cement quality and eliminate gas flow after cementing.
The paper shows how to design TCP for a given well program and slurry properties. Using hydraulic analogy between low-frequency reciprocation of Bingham fluid and the plug flow, a mathematical model has been derived to describe the top pressure pulse attenuation with depth. The model also includes well compressibility formulas to calculate downhole transmission of the top displacement amplitude. The mathematical mode constitutes a basis for TCP treatment design and prediction.
Presented in the paper is experimental verification of the design model using full-scale pulsation of a tixotropic slurry at the LSU Well facility and field data from TCP treatments in conventional and "instrumented" test wells (equipped with downhole pressure gauges). For the conventional well, the design model would calculate depth of the TCP treatment resulting in a good match between calculated and recorded values of displacement amplitude at the cement top. For the instrumented well a direct match was obtained between the measured and computed pressures at depth.
Gas migration behind casing is a common problem in the petroleum wells. Early gas migration also known as flow-after-cementing may induce hazard of the surface or underground blowouts. Late gas migration resulting from leaking cement would result in the problem of sustained casing pressure.1,2
In the cementing operation, after cement placement in the annulus, the slurry exerts pressure against the formation equal to the hydrostatic head. However, once the slurry becomes motionless, it develops gel structure transforming itself from liquid state to solid state. The gelation builds a bond at the annular walls that partially reduces the downhole transmission of hydrostatic pressure.3,4,5 The transmission is needed since the slurry losses volume due filtration and shrinkage.6,7,8 The effect may be also enhanced by abnormal temperature gradients.9 In the result, the slurry column resists the downward motion leading to reduction of pressure exerted by the column at the bottom. 10 The reduction may cause loss of pressure overbalance followed with invasion and migration of formation gas through the cement or between the cement and the rock surface.
In 1995, Haberman proposed a concept of Top Cement Pulsation.11,12 In this method, a low frequency and smallamplitude pressure pulsation is applied at the top of the cement by cyclic pumping of water or air to the wellhead. The treatment continues for sufficiently long time to keep cement in liquid state, reduce transition time, and maintain hydrostatic pressure overbalance thus preventing gas invasion and migration.