High-efficiency casing perforation oil and gas wells

The energetic capabilities of a high-speed jet of an aqueous solution of polyethylene oxide (PEO) with varying concentration and different outflow pressures from a jetforming nozzle were investigated using the length of the forming channel in the model of the casing of an oil and gas well, cement sheath and rock layer, as well as impact of the jet force on a metal plate fixed on a physical pendulum. The experimental data made it possible to obtain a calculated dependence in a dimensionless form to determine the quality (initial sections) of jets of aqueous solutions with different concentrations and molecular weights of PEO, considering the real parameters of the jet-forming nozzles of the hydroperforator. A comprehensive study of the perforation process made it possible to substantiate the mechanism of the high destructive capacity of a high-speed jet of polymer solution. It has been established that the mechanism of the high destructive capacity of the polymer water jet is not due to the Toms effect, but caused by the destructive action of the dynamic pressure of the polymer water jet «reinforced» by strongly unfolded macromolecular chains under the action of a tensile flow in the inlet area of the jet forming nozzle of the hydroperforator. Keywords: perforator; jet nozzle; jet quality; casing; cement sheath; rock; Toms effect.

Case Studies on Multistring Isolation Evaluation in P&A Operations

Cement sheath quality assessment is a critical initial step in plug and abandonment (P&A) operations during oil and gas well decommissioning. However, the technologies commonly used require unimpeded access to the casing annuli, thus enforcing the need for production tubing pulling or inner casing milling. Cement integrity or isolation evaluation through multilayered well casing strings will provide the opportunity to significantly reduce operational time and costs and to greatly simplify the traditional P&A process. As desired by the industry for years, recent advancements in isolation evaluation have proven the feasibility to assess cement sheath quality without the removal of production tubing or inner casing. The new development, consisting of a sophisticated logging apparatus with a novel processing methodology, led to a groundbreaking technology evaluating zonal isolation through multiple casing strings in wells. The logging tool is deployed in the borehole using E-line, slickline, or coiled tubing. Then, the acoustic energy that is emitted and received by the tool travels through the tubing and surrounding annulus to reach the isolation barrier behind the casing. A proprietary frequency-domain processing algorithm successfully identifies the desired signal by discriminating it from overwhelming undesired signals such as tubing arrivals. The latest development stage further enables the segmentation of the measurements, providing an improved sensitivity to detect the azimuthal variations in the cement sheath quality. Case histories of applying omnidirectional and segmented multistring isolation evaluation technology in field trials in the North Sea are presented in the paper. The measurement accuracy has been verified through side-by-side comparisons with industry-standard cement bond log (CBL) and ultrasonic logs recorded after the tubing was removed. Additionally, the technology has been proven applicable to various casing or tubing weight and size combinations with tubing eccentric inside the casing. Thus, it is practicable in actual well configurations and suitable for the deviated well sections as well. In conclusion, this innovative technology that exhibits quantitative assessments of bonding or isolation conditions of wells in multistring configurations provides a cost-effective solution during P&A and further demonstrates a great potential to accelerate along the path to a rigless P&A operation.

The Influence of Formation Creeping on Wellbore Integrity

Creep, the time-dependent deformation of rock, will increase the pressure applied on the interface between the cement and formation. The objective of this paper is to study the influence of the formation creeping effect on the cement sheath integrity and zonal isolation. It focuses on the failure behavior of the cement sheath in the long period after drilling. The paper also investigates the changing of mechanical properties of cement to avoid loss of zonal isolation. The interface pressure between the cement and formation cannot be measured directly in the field, so it will be valuable to predict this pressure through alternative methods. A Casing-Cement -Formation System (CCFS) analytical model based on linear-elasticity and Cam-Clay plasticity model was built. The CCFS model includes four layers, casing layer-cement layer- plastic creeping layer and the formation layer. This plastic- transition layer is formed because of formation creeping. The axial stress and tangential stress distribution of the cement sheath were calculated by the CCFS model. The contact pressure between the cement sheath and formation was calculated. Mohr-Coulomb yielding criterion was applied to estimate failure behavior for the cement sheath. Two case studies were performed with the new CCFS model and previous CCFS model that do not consider the formation creeping effect. The comparison between two models showed that without considering the formation creeping effect, we might underestimate failure of the cement sheath. The simulation result by our CCFS analytical model indicated that the creeping effect would make the interface between the casing and cement vulnerable to shear failure. We changed the Young's modulus and Poisson's ratio for the failed case to investigate the influence of mechanical properties of the cement material. The result showed that a lower Young's modulus and higher Poisson's ratio were preferred for improving zonal isolation. Instead of pursuing how creeping happens, this paper accepts formation creeping as a fact in the whole life of the well. The geomechanical impacts of the plastic-creeping formation, although undetectable from the surface observations, may cause detrimental consequences to cement integrity.

Investigation of Stress State During Cement Hardening and its Effect on Failure of Cement Sheath in Shale Gas Wells

Consideration of initial stress state after cement hardening provides a vital basis for the prediction of cement failure, which has been overlooked in previously published methodologies partly due to the difficulties in examining this problem rationally. In the present study, the hoop stress at casing-cement interface during cement hardening is investigated experimentally based on the full-scale casing-cement sheath-formation system (CCFS) facility, which is equipped with the real-time stress-strain measurement capability. The hoop stress at casing-cement interface during cement hardening drops sharply, rather than equating with the initial annulus pressure of cement slurry. It presents a higher drawdown under higher annulus pressure and thinner casing, and a lower drawdown under elastic cement slurry and thicker cement sheath. Furthermore, an analytical model taking the effect of cement hardening into account is developed to predict the integrity of cement sheath. Reliability of the model is verified by comparison with field observations. Excellent agreements are observed. The results illustrate that the tensile cracks are likely to occur at the inner cement (inner surface of cement sheath) by the effect of cement hardening, since the hoop stress at inner cement during cement hardening drops greatly and even becomes tensile. A detailed sensitivity analysis illustrates that an elastic cement slurry with a lower elastic modulus works more effectively, which can resolve the SCP problem in shale gas wells.

High Density Foamed Cement Application for Channeling and Behind-the-Casing Flows Minimization in the Production Zone

This article describes the application of relatively high-density foamed cement for cementing wells in the Volga and Urals region. Good cementing practices with high density or conventional density cement slurry is required to ensure mud displacement in fluid saturated intervals of reservoir formations (Benge et al; 1982). With this requirement met, the cement column should circumferentially cover the annulus at this very interval which is exposed to the highest loads. However, due to limited physical and mechanical properties of conventional cement slurries in both liquid and solid state, in certain cases conventional slurries do not solve the problems encountered by the Customer, namely elimination of annular flow between the casing and cement sheath. High-density foamed cement is considered as an improved alternative to conventional cement slurries, and results in a high quality and durable sealing of gas and oil saturated production zones for the life of the well. Proprietary software and process equipment are used for the mixing of the foamed cement slurry with a variety of foaming properties. This process enables the use of a base cement slurry with higher density (up to 2.1 g/cm3) for delivering foamed cement slurries in a wide range of densities. To avoid possible cross flows behind the casing, pilot tests were conducted, where a conventional cement slurry (1.80–1.90 g/cm3) was replaced with a high-density foamed cement slurry with equivalent density with a foam quality of approx. 10% making the cement sheath elastic with improved adhesion to both the casing string and the formation (Spaulding et al; 2018). Pilot tests, incorporating the cementing of several production casings, were conducted where only foamed cement slurries with various foam quality were used in the entire cementing interval. No conventional (non-foamed) cement systems were used in these cases.

Mud-Removal Efficiency Boost by Engineered Scrubbing Spacer in Cementing Operations at Caspian Sea

Long-term well integrity and zonal isolation are the ultimate objectives for cementing in the well construction process. Effective mud removal plays an essential role in obtaining competent zonal isolation and hence should not be overlooked and underestimated. The negative consequences of poor mud removal can lead to microannulus, channeling, or gas migration, which might require costly time-consuming remediation. The conventional approach of optimizing spacers based on chemical interactions with the mud layer does not always yield desired results and, thus, demanding further improvement. In this paper we discuss the approach taken to boost the mud removal efficiency by implementing an innovative engineered scrubbing spacer containing fibers in a challenging environment, resulting in notable improvement in long-term cement sheath integrity. The engineered scrubbing fibers were thoroughly tested in the laboratory to ensure spacer stability and efficiency. The new spacer with an additional scrubbing capability was introduced to one of the major operators on the Caspian shelf and after successful implementation, it has now been used on more than 20 cementing operations. Scrubbing fibers concentration was optimized through thorough laboratory testing covering flowability, dispersibility, and mud removal efficiency; later, it was applied on most of the cement operations, including 4½-in. liners characterized by a very narrow annular gap across the hanger sections. Cement evaluation log results from those cementing operations demonstrated an improvement in mud removal efficiency, suggesting no issues associated with microannulus, channeling, or gas migration, thus confirming the effectiveness of the newly implemented engineered scrubbing spacer. The typical challenges associated with meeting the zonal isolation requirement on one of the offshore fields of the Caspian shelf, and the success of the approach taken to overcome those challenges by implementing the new engineered scrubbing spacer are discussed. The comparison of cement bond evaluation log results of the jobs where conventional spacer systems were used vs. those where the spacer with scrubbing capability was used are also presented, demonstrating the clear difference and improvement.