Control and Prevention of Inter-Zonal Flow

Abstract

An investigation of the factors affecting the inter-zonal flow of fluids in the casing-wellbore annulus of an oil or gas well is presented. Laboratory tests revealed that failure of the casing-cement bond allows communication of fluid at pressure differentials near 1,000 psi. Perforating with a hollow carrier gun does not shatter or crack the cement sufficiently to allow communication through these cracks and does not seriously affect the casing-cement bond failure pressure. The pressure to cause communication at the casing-cement bond can be increased to 3,000 psi by limiting pipe deformation during cementing and subsequent operations. Communication can be prevented, however, up to pressures sufficient to cause the cement to fail or the casing to collapse, by attaching to the casing a seal ring of deformable rubber molded between two steel flanges. When installed, one flange is attached rigidly to the casing to serve as back-up for the gasket while the other flange is movable. To date, these gasket seal rings have been field-tested in 27 wells. Although communication has been noted in two wells, inter-zonal flow has been prevented in other wells in the same area by installing more seal rings between the communicating zones.LABORATORY INVESTIGATIONSThe laboratory tests performed in this study were divided into three phases. The first phase consisted of tests to determine if perforating would shatter and crack the cement sheath so severely that communication would occur through the cement itself. The second phase consisted of tests to determine more precisely the cause for the casing-cement bond failure. The third phase consisted of tests to evaluate the effectiveness of various deformable seals attached to the casing in preventing communication.EFFECT OF PERFORATINGThe effect of perforating was investigated in tests performed with apparatus shown on Fig. 1. Cement was poured into the annular space between 5 1/2-in. casing and 8 5/8-in. casing and allowed to cure for 72 hours. The apparatus was then perforated with a hollow carrier gun containing eight duPont 26-A shaped charges. Bull plugs were placed over the perforations in the 8 5/8-in. casing and hydraulic pressure was applied inside the 5 1/2-in. casing. The pressure to cause fluid communication past the cement sheath was observed and termed the "bond failure pressure".Several different cement compositions were included in these tests, the results of which are shown in Table 1.JPTP. 579ˆ