In this study, a 1-D transient state mechanistic model of cuttings transport with foam in inclined wells has been developed. The model is solved numerically to predict the optimum foam flow rate (liquid and gas rate) and rheological properties that would maximize cuttings transport efficiency in inclined well. A detailed sensitivity analysis of the effect of gas and liquid flowing rates, drilling rate, foam rheological properties, borehole geometry, wellbore inclination and the rate of gas and liquid influx from the reservoir on the cutting transport efficiency in inclined wells was presented.
The term underbalanced drilling (UBD) refers to a drilling operation in which the pressure of the drilling fluid is designed to be lower than the pressure of the formation1. Underbalanced drilling is designed such that an underbalanced drilling condition is achieved throughout the entire drilling and completion operation2.
Field applications have proven that UBD techniques has many advantages including minimized formation damage, increased drilling rate, improved formation evaluation while drilling, minimized lost circulation, reduced occurrence of differential pipe and logging tools sticking, enhanced detection of all producing zones, and enhanced earlier production from reservoir3–11.
Based on the type of drilling fluids used, underbalanced drilling operation can be classified into four categories12: Air/gas drilling, gasified liquid drilling, foam drilling and the flow drilling, which involves using fluids with density below the formation's hydrostatic pressure gradient.
Foam as a drilling fluid is commonly used for underbalanced drilling because of its low variable density which makes adjustment of foam density possible in order to keep control of the circulating bottomhole pressure and its high effective viscosity which gives a superior cuttings lifting and transport ability. Apart from having a good lifting ability and the ability to maintain an underbalanced drilling condition, foam is also used to remove formation fluids that enter the borehole while drilling and also serve as an insulating medium if loss circulation is a problem. Applications of foam as a drilling fluid in many drilling operations, and the results from various field cases are well documented 13–24.
In this paper, a transient mechanistic model is presented for the prediction of foam drilling performance in inclined wells. The new model considers foam rheological properties, drag coefficient of cuttings in foam, formation fluid influx, drillpipe eccentricity, inclination effect and drilling rate and thereby provides an effective numerical solution method to simulate the hydraulics of foam drilling in inclined wells.
The literature review focuses on mechanism of cuttings transport in wells. It also looks into foam drilling with focus on the foam rheology, foam flow and cutting transport with foam.
Cuttings Transport Problems associated with inefficient cutting transport are; reduction in penetration rate, wear of bit, pipe stuck, high torque and drag and other hole problems25–29. The elimination or minimization of these problems associated with ineffective cuttings transport requires proper understanding of the cuttings transport mechanism.
