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GENERAL GUIDELINES FOR EFFECTIVE MILLING
Good Cutting Return
The following are guidelines for the optimum use of drilling mud to circulate steel cuttings out of the wellbore.
! A minimum annular velocity of 120 feet per minute should be maintained.
! Oil-base mud should be avoided whenever possible.
! Ordinarily, no difficulty is encountered in raising drilling cuttings or the usual cavings (if any) using normal drilling practices. Most difficulties are encountered when a light ring with a small mud pump is used in remedial work. A common remedy in such cases is to add bentonite to the mud so that the effective viscosity becomes sufficient to raise the cuttings or cavings.
! If "bird nesting" occurs around the drill string, pull up the kelly and circulate down until proper cutting return is achieved. During long milling jobs this procedure should be repeated frequently to maintain rate of penetration.
! Always inspect I.D. of subs or auxiliary tools to make sure they are full-bore and have no restrictions. Changes in cross sections can reduce fluid volume and result in poor cutting return.
! Reverse circulation is another way to remove cutting accumulation in the fish. In some cases, junk baskets, bailer, or snatchers are used to remove or catch cuttings.
How to Read Cuttings?
The ideal cuttings is usually 3 /32" to 1 /4" thick and 2" to 4" long. If cuttings are thin or "hair-like" and penetration rates are low, weight on the tool should be increased.
If fish-scale type cuttings are being returned when pilot milling, the penetration rates should improve by decreasing weight and increasing rotary table speed. This is more common when milling H-40, J-55 and K55 pipe. When milling up N-80 or P-110, finer cutting return can be expected.
Recommendations on Weights and Speeds
Generally the most efficient rates are obtained by running the rotary at 80 to 100 RPM. Milling with washover shoes is an exception; they are usually more efficient when run at 60 to 80 RPM. (As with all milling tools, speed and weight will be dictated by actual conditions.)
Always start rotating about one foot above the fish. Lower onto the fish and vary the weight to improve the penetration. Whenever possible, maintain a constant milling weight. Feed the drum slowly, allowing the drawworks to "creep"; do not drill off.
The wear pattern on the mill is a great indicator of its performance downhole. If the dressed blades show a hook wear pattern, the mill is working efficiently. If a tapered pattern exits, ease off the weight.
Some Factors That Affect Milling Rates
The type and stability of the fish (cemented or not), the weight on the mill, the speed at which it is run, proper carbide dressing of the mill, as well as the weight, stiffness and vibration of the drill collars, are all factors which will affect milling rates. The hardness of the fish or cement will also affect a mill's performance.
When milling cemented casing, penetration rates can be increased by using higher weights and speeds. Unlimited casing should be milled at lower speeds with less weight. When severely corroded casing is encountered, high-speed, light run will prevent tearing or splintering the fish.
Encountering Rubber in the Hole
Rubber always presents problems during milling. When milling rates decrease, cut back or shut off pump pressure. Pull kelly up and spud the mill to help get a bit on the rubber. When necessary, pull the kelly, remove the mill, and clean the fish by running a drill bit in the string.