There are many factors that impact on hole cleaning while drilling directional wells for hydrocarbon recovery. Those that are most prevalent include:
annular drilling fluid velocity
hole inclination angle
rate of penetration
drilling fluid properties
characteristics of drilled cuttings
In this paper the authors discuss the above factors and their field limitations. In addition, the key issues that need to be addressed for the development of a universal and reliable model in hole cleaning are identified.
Drilling directional oil/gas wells, including horizontal, has become a routine process in all geographic areas of the world. While the overall directional drilling technology has advanced significantly over the past quarter century, hole cleaning problems are still a deterrent to extended reach drilling (ERD).
Inadequate hole cleaning can contribute to several major drilling problems which include: increase in torque and drag that can limit the reach to target, mechanical pipe sticking and difficulties in casing/cementing and logging operations that can increase well cost significantly. Although, there have been many experimental studies and few attempts on mechanistic modeling in cuttings transport, field reports still indicate that hole cleaning is a frequently occurring problem depending on annular drilling fluid velocity, drillstring dynamics, inclination angle of well, drilling mud properties, drilled cuttings size, eccentricity of the annulus and drilling rate. These factors and their limitations in actual field practices are the main points of discussion of this paper.
Annular Fluid Velocity. Regardless of all other factors that impact on hole cleaning, drilling fluid flow rate dominates the cuttings transport process. It is expected that an increase in flow rate will always cause more efficient removal of drilled cuttings out of the annular space. However, an upper limit of the flow rate is dictated by:
rig hydraulic power availability
permissible equivalent circulating density (ECD)
susceptibility of the open hole section to hydraulic erosion
Drillstring Rotation. It has been demonstrated in lab studies and reported in field cases, that drillstring rotation with the induced modes of vibrations (torsional, longitudinal, and lateral) has moderate to significant effects on hole cleaning in directional wells. The level of enhancement in drilled cuttings removal due to drillstring rotation is a function of the combination of mud rheology, cuttings size, flow rate, and the dynamic behavior of the string. It is believed that the whirling motion of the string as it rotates is the major contributor to the cleaning process. Mechanical agitation of the cuttings bed and its exposure to higher fluid velocities are the beneficiaries of this motion.
Although there is a definite gain in hole cleaning due to drillpipe rotation, it must be recognized also that there are limitations that may have to be imposed as well. For example, pipe rotation cannot be activated during the sliding mode while building hole angle. Also, pipe rotation induces cyclic stresses that can accelerate pipe failure due to fatigue, causes excessive casing wear and in some cases mechanical destruction of the walls of open hole sections.