Debris Management in High-Angle, Extended-Reach, Large-Diameter, Cased Holes During Completions Operations
Authors: Ghalambor A. , Haeri F. , Norman W. D. 
Published: June 2014

Frac-packing has gradually become the standard completion technique for sand control in cased holes. As offshore development moves into more challenging deepwater environments, frac-packing technology continues to evolve and therefore brings new issues with the need for a better understanding to resolve them. One of the challenges is solids removal during the reverse out process in high-angle extended-reach large-diameter cased-hole wells.

In high-angle extended reach deepwater drilling, relatively large diameter connections are utilized to handle the tension load and torque. This geometrical environment, when using Bingham plastic or Power-law fluids and sufficient pump rate allows the flow to carry the solids and avoids formation of debris dunes in the areas adjacent to the tool joints. However, during the reverse circulation process, flow phenomena, such as fingering and dilution of the excess slurry with even small volumes of Newtonian brines coupled with low pump rates can create significant difficulties in the removal of solids from the inside of the workstring.

This problem manifests itself when some of sands from these dunes in the workstring are left in the wellbore after the frac-packing process is completed. During tripping of the pipe to extract the service tool, the solids in the workstring fall out of the tube as it goes from high angle sections to vertical. These solids remain in the wellbore and when the dune is of sufficient size or if pushed downhole, during the installation of the production tubulars, this debris can cause issues such as problems with the seal units testing, tagging sufficient debris to not allow seal latch-in, etc. Many times this situation is unavoidable and a special trip should be included to clean the wellbore.

The objective of this work is to study the effectiveness of circulating viscous sweeps to pick up the debris and clean the hole. This can be done by taking multiple aspects into consideration, including the annular and average fluid velocities at different regions of the eccentric annulus, rheological properties of the circulation fluid, density of the fluid and solids, size of the solids, size of the annulus, position of the drillpipe in the annulus or eccentricity, and wellbore angle. The mathematical approach for this research is based on previous works on drilling cuttings removal techniques. A model has then been developed to fit the described completion environment. The model is to estimate fluid velocity in eccentric annulus, based on fundamental concepts of transport phenomena, in combination with particle velocity in order to get an optimum range for the flow rate using the characteristics of Reynolds number and to find a balance between laminar and turbulent flow regimes to pick up the dunes. An experimental apparatus was also designed to simulate the solids removal process after the frac packing completion. Cuttings, suspended in different type of fluids, were circulated in an inclined annulus. The experimental data showed the mobility of solids all around the eccentric annulus and were used to fine-tune the mathematical model.

Utilizing the model and applying the fluid and solids characteristics for a particular completion, one can adjust and control the circulating pump rate in an optimal range. The model allows for the use of best and worst-case scenarios, to eliminate the dunes and also to save time by cleaning the wellbore with appropriate viscous sweeps during the completion and avoiding debris causing completion problems.