Overview Solids Transport Study and Application in Oil-Gas Industry-Theoretical Work
Authors: Li Jeff , Luft Bernard 
Published: March 2015

Abstract
The transport of solids is often encountered in well drilling, completion, well intervention and production processes. The process is affected by many variables and the complexity of the phenomena presents challenges to the field engineer who is trying to determine how these parameters affect solids-transport even if only one of these variables is changed during an operation. In the early stage, solids-transport studies mainly focused on finding the minimum critical velocity to prevent the formation of a stationary debris bed in the wellbore annulus for conventional rotary drilling with mud fluids. Over the past 10 years, the research and development focus has been on obtaining information related to the prediction of the equilibrium solids concentration profile along the whole well path during tripping in, on the wiper trip speed during tripping out, and on the prediction of the hole-cleaning time. Various correlations and models have been developed that would guide the field engineer to design and optimize the hole cleaning process. However, due to the complexity of the process, a comprehensive and proven model still does not exist at present.
In this paper, based on 20 years of research experience, the authors present a critical review of the state of the art of previous approaches to studying the solids transport during both drilling and well interventions. It mainly focuses on theoretical studies. An overview of the experimental study of solids transport is given in a companion paper (Li et al. 2014). The methodology used to develop a model to simulate the hole cleaning process is also discussed. To date, completely satisfactory solids transport software, is still not available for the oil industry. Hopefully, this paper will provide a useful and helpful general guide for future research related to this subject and will aid in a better understanding of the lack of previous studies and what issues should be focused on in future research.

Introduction
The transport of solids is encountered in various industries which include the mining (Wilson et al. 2006), sedimentation (García, 2008), oil transport, well drilling, well cementing, well completion, and well intervention and production applications. Coal water slurry, iron ore, mineral concentrate, sand removal, limestone or cement kiln feeding in the cement industry, dredging and tailings disposal and heavy bitumen transport are examples of typical slurry hydraulic transport in a pipeline. Figure 1 shows the proppant image which was taken from a downhole camera after a fracturing job in a horizontal wellbore. The sand bed is in the form of dunes that has to be cleaned out the wellbore before the well can start to produce. Otherwise the sands could block the perforations and reduce the production rate. In some cases if the sands are not cleaned out before the production, a high production rate could clean the well. However, once the sand enters the surface lines, it could cause erosion problems for the surface pipeline and other equipment.
The cuttings transport in a conventional drilling application involves a drilling period and a hole-cleaning period. In the drilling period, the bit cuts the rock formation and generates the cuttings. As the drilling progresses, the cuttings are transported toward to the surface. Depending on the flow rate and the well deviation angle, cuttings usually form a cuttings bed in the highly deviated section of a wellbore (between 60o and 90o) and form sand dunes between 40o and 60o of the build section. In the vertical wellbore (between 0o and 40o), the particles are usually fully fluidized. Once the drilling is completed, all cuttings should be cleaned out of the hole. The cuttings could be removed out of the hole when the bit remains at the last position (called the stationary circulation hole cleaning mode) or pull the drilling string out of the hole, at the same time the fluid continues to be circulated (called the wiper trip hole cleaning mode). For conventional drilling, the stationary circulation mode is usually applied during the hole-cleaning period since the pump rate is relatively high (annular fluid velocity &gt;1 m/s) and the drill pipe can also be rotated to aid the solids transport.