Design of Cement Pulsation Treatment in Gas Wells-Model and Field Validation

Abstract

Abstract Top cement pulsation (TCP) is an auxiliary cementing technology for enhancing zonal isolation by applying low-frequency hydraulic pressure pulses to the top of the well's annulus 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 pressurizing the well's bottom. As a result, the cement slurry prolongs its liquid state, the thickening time is delayed, and transition time is shortened. The combination of these effects improve cement quality and eliminate gas flow after cementing. Using a hydraulic analogy between low-frequency reciprocation of Bingham fluid and the plug flow, a mathematical model describing the TCP treatment has been developed. The model employs a new formula describing plug flow pressure loss for the pulsed cement slurry. Also, equations have been developed to calculate the compressibility of the well annulus filled with cement and drilling mud. The TCP mathematical model calculates downhole transmission of the top displacement amplitude and pressure attenuation. The model gives the basis for TCP treatment design and efficiency prediction for a given well program and slurry properties. Also presented is an experimental verification of the TCP design model using full-scale pulsation of thixotropic slurry at the LSU well facility. Further verification is given by the field data from TCP treatments of cements in "instrumented" wells equipped with pressure and temperature sensors downhole. Introduction Gas migration behind casing is a common problem in petroleum wells. Early gas migration, also known as flow-after-cementing, may induce the hazard of 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, at the end of cement placement in the annulus, the slurry exerts pressure against the formation equal to the hydrostatic head. However, once the slurry becomes motionless, it starts developing gel structure, transforming itself from a liquid state to a solid state. The gelation builds a bond at the annular walls that reduces the downhole transmission of hydrostatic pressure(3–5). Simultaneously, the slurry reduces its volume due to filtration and shrinkage and tends to move downwards(6–8). However, the downward motion is opposed, which leads to the reduction of pressure along the slurry column and at the bottom(9). The effect may also be enhanced by abnormal temperature gradients(10). The reduction in bottomhole pressure may eliminate pressure overbalance, leading to invasion and migration of formation gas through the cement or between the cement and the rock surface. In 1995, Haberman proposed the concept of top cement pulsation-applying low frequency and small-amplitude pressure pulses at the top of the well's annulus using water or air surges(11, 12). The treatment prolongs hydrostatic pressure transmission, keeps cement in liquid state, reduces transition time, maintains hydrostatic pressure overbalance, and as a result, prevents gas invasion and migration. Development and commercialization of the TCP technology required a method for designing the treatment.