Cuttings transport in highly deviated wellbores is more challenging and critical than in vertical wells. By optimizing drilling practices and sweep programs, on bottom drilling time can be increased and the need for wiper trips minimized. Data from two geological side tracks drilled from the same vertical wellbore are used to demonstrate the improved efficiencies possible when using weighted sweeps rather than high viscosity sweeps for hole cleaning in deviated wellbores. In the examples discussed in this paper, over $500,000 in savings were realized by the operator through using optimized drilling practices and engineered sweeps. Optimizing the use of rotation and circulation versus continuous sliding also can contribute to improved drilling efficiency. In addition, computer simulation data are presented to support the field observations. The use of sweep reports is an integral component of tracking sweep performance.
Successful drilling of all wells requires efficient hole cleaning. When drilling deviated wells the challenge of maintaining efficient hole cleaning is typically greater than when drilling vertical wells. Some of the primary factors contributing to these greater challenges include drill pipe eccentric in the hole, the need for slide drilling (to build angle and maintain direction), and the resultant skewed flow distribution in the annulus1,2.
The rheological characteristics of drilling fluids lead to a skewed flow distribution in the annulus for drill pipe that is eccentric in the hole. When drilling deviated and horizontal wells the drill pipe is usually located on the low side of the well bore, forcing the majority of the fluid to flow above the drill pipe. Annular velocity modeling of Herschel-Bulkley fluids clearly demonstrates this point. The shear thinning behavior of the fluid, combined with the yield stress characteristics of the fluid do not favor flow in the restriction below the drill pipe. Optimizing the rheology of the fluid to minimize this effect is an important part of well planning and design. It must be acknowledged that rheology modification alone cannot overcome the problem.
A compounding factor associated with drill pipe being eccentric is the fact that drilled cuttings will settle towards the low side of the annulus due to gravity. Regardless of the drilling fluid rheology, it is almost impossible to clean a high angle wellbore without the action of drill pipe rotation3,4. Drill pipe rotation acts to agitate any settled solids back into the flow stream. A solids bed will develop over time in the absence of drill pipe rotation, which can impact on the annular equivalent circulating density (ECD), cause pack-offs and result in stuck pipe. Monitoring annular pressure just above the bit, together with accurate modeling and tracking of pick-up weight, slack-off weight, torque and drag can provide valuable insights into the development of a cuttings bed. This information should be utilized efficiently to detect the onset of solids build-up down hole before the situation becomes critical5.