Analytical and Experimental Investigation of the Critical Length in Casing–Liner Overlap

Abstract

Offshore drilling operations exhibit various difficulties attributed to shallow flows worldwide. One of the most common practices for drilling offshore wells is to use liners and liner hangers rather than using full casing strings. This reduces the cost of drilling operation. Liners and liner hangers are required to pass certain standards prior to their deployment in the field. This ensures their ability to withstand harsh downhole conditions and maintain the integrity of the well. A liner hanger contains an integrated seal assembly that acts as a barrier to prevent fluid migration. The cement that is placed within the casing–liner overlap is also considered a barrier, and it is critical that it maintains the integrity of the well by mitigating fluid migration to other formations and to the surface. The failure of this dual barrier (cement and seal assembly) system to seal the annular space can result in serious problems that might jeopardize a well’s integrity. Typically, in field applications, the length of a casing–liner overlap is chosen arbitrarily. In some cases, shorter overlaps (50 to 200 ft) are chosen because of the lower cost and easy identification of leaks during pressure tests. However, some loss of well control incidents (particularly the incident that motivated this study) have been linked to fluid leakages along the casing–liner overlap. This paper investigates the critical length of the casing–liner overlap by modeling gas leakage through the cement placed within the overlap using analytical and experimental approaches. Leakage scenarios were developed to mimic gas migration within the cement in the casing–liner overlap. The results showed that the longer the casing–liner overlap, the higher the leakage time. The results also showed that the current casing pressure test duration of 30 min may not be adequate to verify the integrity of the cement within the overlap. Based on the results and analyses, it is recommended to increase the pressure test duration to 90 min. In addition, the results suggest that the length of the casing–liner overlap should not be less than 300 ft to maintain the integrity of the well in the case of gas influx. Further details are highlighted in the results section. In practice, the current rationale behind the selection of a casing–liner overlap length is not sustainable. Thus, the major advantage of this study is that with field data, it provides both scientific and research-based evidence that can be used to inform the decision behind the selection of the casing–liner overlap length, especially in gas migration-prone zones.

Numerical Modeling of Gas Migration through Cement Sheath and Microannulus

“…The leakage time is defined as "the time it takes the gas to travel f rom the top to bottom of the cemented annulus". 19 5.1.1. Cement Permeability.…”

“…This section includes the parametric study of the cement matrix permeability, cement porosity, microannulus gap size, and casing/liner overlap length with regard to how they impact the leakage time. The leakage time is defined as “ the time it takes the gas to travel from the top to bottom of the cemented annulus ” …”

“…To model the gas flow through cement, the pressure squared method was used to overcome the limitation of the basic form of the diffusivity equation. The finite volume approach was implemented to solve the partial differential 40 studied the effect of confining pressure on microannulus size Opedal et al 41 studied the sealability of neat Class G cement Stormont et al 13 extensively investigated the effect of confining and casing pressures on hydraulic aperture size Ahmed et al 18 investigated the ability of cement to act as a secondary barrier in case of failure of the seal assembly Al Ramadan et al 19 studied the sealability of neat cement and the effect of anti-gas migration additive to prevent gas leakage Corina et al 12 evaluated cement sealability at varying temperatures when using anti-gas migration additives Kwatia et al 3 evaluated the sealability of different cement recipes that can prevent gas migration Corina et al 42 studied the effect of pipe roughness on the sealing ability of cement plugs Kremieniewski et al 43 formulated new cement slurries that can prevent gas migration in gas wells 16 and Tao et al 46 modified the aforementioned model to obtain the effective system permeability Rocha-Valadez et al 15 analytical solution for an SCP numerical model Aas et al 17 analytical leakage model through microannulus Ford et al 47 leakage calculator based on analytical models Stormont et al 13 Fluid flow in porous media is usually described through Darcy's law which is written as…”

“…Limited experimental and modeling studies have been conducted to investigate the cement sealability and to understand the leak through the developed pathways. − Therefore, more experimental and numerical studies are needed to confirm the oil well cement’s ability to seal the annulus. It is supposed to form a barrier that can prevent fluid propagation and movement.…”

Influence of Casing Eccentricity on the Mechanical Integrity of Cement Sheaths in Fractured Wells

“…The results showed that the longer the casing−liner overlap, the higher the leakage time; the results also showed that the current casing pressure test duration of 30 min may not be adequate to verify the integrity of the cement within the overlap. 7 Kiran et al presented a broad review of research and field experiences related to well integrity. 8 Liu et al established the calculation model of a cement sheath stress under casing eccentricity and pointed out that casing eccentricity would significantly increase the stress of a cement sheath, thus leading to yield failure of the cement sheath.…”