Lost circulation has plagued the industry since the beginning of drilling. Historically, severity of losses has been categorized based on the amount of barrels lost to the formation, i.e., seepage, partial, and total. Though helpful, this strategy doesn't help understand the underlying drive mechanism(s) for losses and doesn't provide enough data to propose a solution. The recently adopted category is focused on the lost-circulation mechanism based on the properties of the exposed formation; these classifications are losses to 1) pore throats, 2) induced or natural fractures, and 3) vugs or caverns. This study provides an integrated workflow to predict expected losses for such classification/mechanism of losses. Mud loss through fracturing is categorized based on fracture types, i.e., natural or induced fractures. Different models are used with respect to the fluid-loss mechanism in natural and induced fractures. These models take into account the effect of fracture breathing. In addition, mud loss through the pores on the wellbore and the fracture face is modeled based on formation and mud-cake filtration properties, coupled with the fracture losses. Losses through induced fractures generally occur when ECD exceeds fracture gradient. This happens due to erroneous prediction of the mud weight window, lack of the key information, harsh downhole pressure/temperature and some other operational factors. A field case from deep water Gulf of Mexico is presented in this paper showing how an inaccurate mud window can yield in drastic mud losses. In addition, rock properties and field in-situ stress govern fracture width and fracture propagation. Losses to vugs/caverns are usually total losses due to very large openings in the rock; recommendations are provided on how to control severe losses. Lost circulation not only causes the adverse effect of mud loss itself; it can also lead to several other issues, such as formation damage, stuck pipe, hole collapse, and well-control incidents. The current industry trend is moving towards drilling more low-pressure zones, and lost-circulation planning is becoming a vital part of these projects. Knowledge of the type and the expected amount of mud loss can help engineers select the most appropriate and effective solution and preplan accordingly. This information also provides criteria to evaluate the effectiveness of the applied lostcirculation strategy. In this study we review LCM treatments, wellbore strengthening, MPD, and CwD as some of the most common remedial techniques.

Casing drilling is used as an alternative to conventional drilling with drillpipe in order to reduce non-productive time. The smaller annular space in casing drilling elevates the annular pressure loss considerably at similar flow rates in conventional drilling. Consequently, the Equivalent Circulating Density (ECD) is more affected by annular drilling fluid dynamics in casing drilling than the conventional drilling. The higher ECD experienced in casing drilling brings concerns about exceeding fracture gradient which can lead to induced lost circulation. However, several field observations demonstrate successful application of casing drilling in combating lost circulation and strengthening the wellbore.Smearing effect theory backed by smaller cuttings at the shale shaker, eccentric casing wear, and discrepancy between analytical and field measurements are three main evidences for potential significant eccentricity in casing drilling operations. This paper demonstrates the inherent eccentricity of casing drilling as one of the parameters that controls the annular pressure losses. Eccentricity reduces the velocity in the narrow section of annulus. Similarly, it reduces the annular pressure losses considerably. In addition, controlling the fluid rheological properties as well as the flow rate are recommended to manage the casing drilling hydraulics. This comprehensive study of pressure loss and velocity profile at various annular sizes can help analyzing several field observations and designing the hydraulics of drilling operations.

Drilling depleted reservoirs is often encountered with a host of problems leading to increase in cost and non-productive time. One of these faced by drillers is lost circulation of drilling fluids which can lead to bigger issues such as differential sticking and well control events. Field applications show that wellbore strengthening effectively helps reduce mud loss volume by increasing the safe mud weight window. Wellbore strengthening applications are usually designed based on induced fracture characteristics (i.e., fracture length, fracture width and plug location within fracture). In general, these fracture characteristics depend on several parameters, e.g., in-situ stress magnitude, in-situ stress anisotropy, mechanical properties, rock texture, wellbore geometry, mud weight, wellbore trajectory, pore pressure, natural fractures, formation anisotropy and among others. Analytical models available in the literature oversimplify fracture initiation and propagation process with assumptions such as: isotropic stress field, no near wellbore stress perturbation effects, vertical or horizontal wells only (no deviation/inclination), constant fracture length and constant pressure within the fracture. For more accurate predictions, different numerical methods, i.e., finite element, boundary element, etc., have been utilized to determine fracture width distribution. However these calculations can be computationally costly or hard to implement in near real time. The aim of this study is to provide a fast running, semi-analytical workflow to accurately predict fracture width distribution and fracture re-initiation pressure (FRIP). The algorithm and workflow can account for near wellbore stress perturbations, far field stress anisotropy, and wellbore inclination/deviation. The semi-analytical algorithm is based on singular integral formulation of stress field and solved using Gauss-Chebyshev polynomials. Proposed model is computationally efficient and accurate. The model also provides a comprehensive perspective on the formation strengthening scenarios; a tool for improved LCM design and how they are applicable during drilling operation (in particular through depleted zones). Sensitivity analysis included in this paper quantifies the effect of different rock property, in-situ stress field/anisotropy and wellbore geometry/deviation on the fracture width distribution and FRIP. Additionally, the case study presented in this paper demonstrates the applicability of the proposed workflow in the field.

Casing Drilling is an innovative drilling method wherein the well is drilled and cased simultaneously. The small annulus of Casing Drilling can create a controllable dynamic ECD (Equivalent Circulating Density). Casing Drilling technology permits the same desired ECD as conventional drilling to be achieved using a lower, but optimized, flow rate, rheological properties, and mud weight. In this paper, the frictional pressure loss during Casing Drilling operation is evaluated using Computational Fluid Dynamics. Annular pressure losses have received substantial attention in theoretical analyses, laboratory assessment and actual well measurements. Combinations of casing motion, annular eccentricities, wall roughness and fluid temperatures along the length of the annulus affect fluid flow regimes that control annular pressure losses. Current analytical solutions have limited applicability for complex conditions with pipe rotation and eccentricity. In this study, the pressure losses during Casing Drilling operation are investigated using computational fluid dynamics. The results are compared against the available analytical solution and field data. The effect of pipe rotation and eccentricity on the frictional pressure loss is investigated as well. According to the simulation results, the pipe rotation reduces the frictional pressure loss for Yield-Power-law fluid which would be beneficiary during the casing drilling operation. It is found that the pipe eccentricity has a significant effect on the ECD calculation. The industry is moving towards more challenging jobs in narrow pressure window scenarios such as deep-water and HPHT applications. Drilling with casing/liner is among the primary options to complete these sections due to strengthening effects associated with plastering the wellbore wall and also eliminating conventional drill pipe trip. Having accurate models for ECD including the effects of pipe rotation and eccentricity in the narrow annulus is essential to the success of these challenging jobs.

Casing while Drilling (CwD) is an efficient method by which to increase the fracture gradient in narrow pore-fracture pressure sedimentary basins and deep offshore applications. It offers hydraulic improvements and the ability to plaster cuttings to the wellbore wall, which can enhance the wellbore's hoop stress by wedging the created fractures. Although successful field applications of increasing wellbore integrity have been reported, uncertainties remain regarding the mechanisms and how to operationally capture the maximum attainable wellbore pressure. These uncertainties include the hydraulic complexities of fluids, role of Particle Size Distribution (PSD) and how it relates to the plastering effect, type of drilling fluids, borehole shape, role of lost circulation materials (LCM), and casing eccentricity.This paper presents numerical, analytical and experimental methods to study the contributing factors in CwD applications. Laboratory experiments were conducted to evaluate the Particle Size Distribution (PSD) and filtration rate of the mud mixed with cuttings from a recently drilled well. Several tests were conducted using Permeable Plug Testing (PPT) equipment to evaluate the role of different LCMs, to fill the PSD gap and to capture the strengthening effect.In addition, advance finite-element methods were used to model the near wellbore area and hoop stress changes with consideration of the formation's poro-elastic properties. Furthermore, the frictional pressure lost during the CwD operation was evaluated using Computational Fluid Dynamics. Analytical models were used to investigate different boundary conditions when applying finite-element analysis.The numerical simulations and laboratory experiments in this work were based on a recently drilled well in South Louisiana, where severely depleted sections were drilled successfully. Previous drilling records in this area report multiple problems with lost circulation, tight holes and other wellbore stability issues. Results from the numerical models and laboratory experiments agree well with field observations. The analysis presented in this paper indicates that an optimum PSD can significantly mitigate lost circulation and minimize the need to add LCM.