In completion of oil and gas wells, cementing operations are employed to provide zonal isolation, a means to prevent wellbore fluids from contaminating sensitive zones such as freshwater aquifers. Perhaps the most important factor engineers and operators should consider for successful cementing is adequate drilling-fluid removal, or "mud displacement." To help optimize mud removal, the primary technique used is to pump a spacer fluid with modified rheology that creates a favorable fluid-fluid interface to enhance mud displacement. In many instances, it is highly desirable to monitor how this interface evolves over time. Fluid intermingling may inhibit the ability of a fluid to perform its intended purpose, for example, intermixing of spacer fluid with cement slurry may lead to contamination of the cement. This contamination may cause an undesirable failure of the setting of the cement and, consequently, a significant increase in cost because of increased wait time or remedial repair. Therefore, a three-dimensional (3-D) simulator modeling the intermixing of wellbore fluids in a highly eccentric annulus with casing reciprocation and rotation has been developed. The computational system is formulated on a general curvilinear coordinate system whose boundaries can conform to irregular boreholes such as those with washouts. Unlike existing models limited to weakly eccentric annuli without casing movement, the present simulator handles multiple real-world effects and efficiently performs trade-off studies that can enable more economical and effective cementing jobs. The finite difference model provides visual output useful in prejob design and post-job analysis. Among these outputs are 3-D color plots illustrating axial velocity, concentration, viscosity, and density evolution. Introduction Efficient mud displacement is perhaps the most important factor in providing a successful cement job. The primary technique used today is to pump a spacer fluid ahead of the cement slurry. Several other factors that directly impact mud displacement are also known, including wellbore geometry, mud conditioning, casing movement via reciprocation and rotation, casing centralization, and optimizing the pump rate.1,2 However, it is often unknown the extent to which these variables affect mud displacement, especially when applied in combination with one another. Even a relatively straightforward cementing operation can quickly become a challenging scenario with multiple variables. The industry has conducted numerous large-scale physical studies3–8 over the last half-century to empirically evaluate the importance of these factors on displacement efficiency. More recently, however, a number of studies have taken advantage of computational numerical methods to describe the different aspects of the mud displacement process in annular geometries. Tehrani et al.9 discuss combined theoretical and experimental studies of laminar displacement in inclined eccentric annuli. The authors appropriately couple the momentum equation with the concentration equation suggested earlier by Landau and Lifshitz.10 Cui and Liu11 address helical flow in eccentric annuli based on the bipolar coordinate system. Pelipenko and Frigaard12 examine fluidfluid displacement in a two-dimensional (2-D) "narrow annuli" without casing reciprocation or rotation. The well known model discussed by Escudier et al.13,14 considers non- Newtonian viscous helical flow in eccentric annuli for a single fluid.

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In completion of oil and gas wells, zonal isolation requires proper cement placement with adequate bonding to the casing and formation. To achieve a successful cementing operation, the cement slurry should be properly designed to enable effective displacement of the drilling fluid from the annulus between the casing and wellbore. This is a complex process, involving displacement of viscoplastic fluids in eccentric annuli. The rheology, flow rate, and interfacial mixing of these fluids have direct impact on the displacement efficiency. Reliable computational modeling of the dynamics of the displacement process is beneflcial to properly perform pre-job design and post-job analysis of the cementing operation. Furthermore, experimental data are also used to validate numerical predictions. This paper presents a flow visualization study using a helical flow device with adjustable annular eccentricity and rotation of the inner cylinder. Displacement experiments were conducted with a variety of non-Newtonian fluids to simulate the cement slurry -drilling mud displacement process.

The main objectives in well construction are to maximize reservoir deliverability, reduce remedial jobs, and minimize nonproductive time (NPT) during drilling and cementing. One of the key factors to help maximize reservoir deliverability is the ability to reach the target reservoir depth with a minimum number of casing points and maximum production tubing size for the completion. Losses during drilling and cementing should be controlled to reduce unplanned casing points and minimize the NPT. These challenges are severe in depleted and low-fracture-gradient formations. Engineered solutions to control loss circulation and to place cement slurry in these environments are presented and discussed. Controlling loss of circulation during well construction is more than just selecting the proper type of lost circulation material (LCM). The engineered solution discussed in this paper correlates the formation and LCM properties for effective control of losses. In situations where LCM alone may not reduce the losses, the use of chemical sealants such as polymers and special cement systems are discussed. The special cement systems can be designed to meet the specific needs such as acid solubility for easy removal, thixotropy, and filtrate loss. Cement slurry for primary cementing across low-fracture-gradient and depleted formations should be designed to meet the density requirement so that it can be placed in the annulus and losses can be minimized. Ultra-low-density cement slurries, as low as 5.4 lbm/gal, are presented and discussed. Case histories are also presented to illustrate field implementation procedures, and the optimization of cement slurry designs to meet well requirements is discussed as well. The results presented in this paper can be applied in well construction to control loss circulation and cementing across depleted and low-fracture-gradient formations, which ultimately should help reduce NPT and maximize well production. Introduction Controlling loss of circulation during well construction involves more than just selecting the proper type of lost circulation material. A fully engineered approach is recommended. During the planning phase, this approach incorporates borehole stability analysis, equivalent circulating density (ECD) modeling, leakoff flow path geometry modeling, plus drilling fluid and LCM material selection to help minimize effects on ECD. During the execution phase, real-time hydraulics modeling, pressure-while-drilling (PWD) data, connection flow monitoring techniques, and timely application of LCM and chemical treatments are proving to minimize and in some cases eliminate losses in high-risk areas. After successfully drilling a wellbore, the hole should be circulated and cleaned of the gelled drilling fluid and drill cuttings, and then cement slurry should be pumped to cover the entire annulus. The cementing job should be engineered so that the ECD during cement slurry placement does not exceed the fracture gradient. In addition, LCM should be incorporated into the cement slurry to help control losses. The cement slurry density should be low so that the ECD (hydrostatic pressure + friction pressure) is less than the fracture gradient. While placing the cement slurry, the sum of friction pressure and hydrostatic pressure should be lower than the fracture gradient of the formation. Planning Prevention of drilling NPT begins with selecting the optimum drilling fluid, i.e. one that exhibits low or fragile, nonprogressive gel strengths. A common characteristic of these fluids is a minimized requirement for commercial colloidal materials and prevention of colloidal-sized drill solids buildup. Both high-performance water-based and invert emulsion fluids that are low-colloid, polymer-based systems are available.1 The use of geomechanical modeling in well planning can provide the safe mud weight window within which the ECD should be constrained. Static mud weight predictions (to mechanically stabilize the wellbore) are influenced by parameters such as in-situ stress and pore pressure gradients, wellbore orientation, and formation material and strength parameters. However, exposure to the drilling fluid could alter the near-wellbore pore pressure and rock strength and can cause progressive wellbore instability. Obtaining an accurate picture of potential issues can require sophisticated wellbore stability simulators that evaluate time-dependent instability developments and account for fully-coupled mechanical, thermal, and chemical effects.2

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractThe main objectives in well construction are to maximize reservoir deliverability, reduce remedial jobs, and minimize nonproductive time (NPT) during drilling and cementing. One of the key factors to help maximize reservoir deliverability is the ability to reach the target reservoir depth with a minimum number of casing points and maximum production tubing size for the completion. Losses during drilling and cementing should be controlled to reduce unplanned casing points and minimize the NPT.These challenges are severe in depleted and low-fracturegradient formations. Engineered solutions to control loss circulation and to place cement slurry in these environments are presented and discussed. Controlling loss of circulation during well construction is more than just selecting the proper type of lost circulation material (LCM). The engineered solution discussed in this paper correlates the formation and LCM properties for effective control of losses. In situations where LCM alone may not reduce the losses, the use of chemical sealants such as polymers and special cement systems are discussed. The special cement systems can be designed to meet the specific needs such as acid solubility for easy removal, thixotropy, and filtrate loss.Cement slurry for primary cementing across low-fracturegradient and depleted formations should be designed to meet the density requirement so that it can be placed in the annulus and losses can be minimized. Ultra-low-density cement slurries, as low as 5.4 lbm/gal, are presented and discussed. Case histories are also presented to illustrate field implementation procedures, and the optimization of cement slurry designs to meet well requirements is discussed as well.The results presented in this paper can be applied in well construction to control loss circulation and cementing across depleted and low-fracture-gradient formations, which ultimately should help reduce NPT and maximize well production.