Liquid purification or separation – Processes – Separating
Reexamination Certificate
2000-12-27
2002-10-01
Lithgow, Thomas M. (Department: 1724)
Liquid purification or separation
Processes
Separating
C210S785000, C210S388000, C210S489000, C210S492000, C210S499000, C209S401000, C175S066000, C175S072000
Reexamination Certificate
active
06458283
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to methods for separating drilling fluid from a mixture of such fluid and lost circulation materials and to apparatuses useful in such methods.
2. Description of Related Art
Often in drilling a wellbore, the circulation of drilling fluid to and then away from the drill bit ceases due to the porosity of the formation and/or due to fracturing of the formation through which the wellbore is being drilled. Drilling fluid pumps into the fractured formation rather than being returned to the surface. When circulation is lost, it is usually supposed that the lost circulation occurred at some specific depth where the formation is “weak”, and that the fracture extends horizontally away from the borehole. Expressions used to describe rocks that are susceptible to lost returns include terms like vugular limestone, unconsolidated sand, “rotten” shale, and the like. Whether fractures induced by excessive mud pressure are parallel to the axis of the borehole (vertical) or perpendicular to the axis of the borehole (horizontal) is a subject of some controversy.
To fill or seal off a porous formation or to fill or seal off a wellbore fracture so that a proper route for drilling fluid circulation is re-established, a wide variety of “lost circulation materials” have been pumped into wellbores. For purposes of classification, some lost circulation materials can generally be divided into fibers, flakes, granules, and mixtures.
The choice of lost circulation material to use in a given case is influenced to some degree by cost and availability in a given drilling area. Cottonseed hulls, for example, are used widely in areas where cotton is grown and drilling in the same area involves mud loss to permeable and cavernous formations. Sawdust is used in areas where lumber is manufactured. Coarse granular material (¼ inch or ½ inch walnut or pecan shells), coarse fiber (shredded hard wood or cedar), medium fiber (shredded redwood or sugar cane), fine fiber (leather, flax, nylon, asbestos) and coarse flake (1 inch cellophane flake) have also been used.
With the bridging agents available today which can be applied through the mud pumps, mud losses to natural and induced fractures, up to ¼ inch in width, can be plugged. Beer bottles have been successfully applied to a severe loss zone as bridging agents directly down a hole. Rigid hollow objects filled with drilling fluid or a lighter liquid would be strong enough and yet have a density near that of the mud, making it possible for the flow of mud to carry them intact to restrictions in the loss zone. Granular lost circulation material also includes coarse walnut or almond shells—up to ¼ inch to ½ inch in size; coarse-to-medium wood or cane fiber, medium-to-fine fiber, e.g. wood, cane, nylon, leather; and large cellophane flakes.
The evaluation of lost circulation materials is based on performance tests. In these tests, mud containing lost circulation material is applied under pressure to a simulated formation. Observations are made as to the efficiency of the seal formed at or in a fracture at various concentrations of additive, and the volume of mud lost before a seal is effected. Fibers and flakes have been found to be effective for stopping loss in a highly permeable type of formation, and FIBERTEX™, HYSEAL, and JELFLAKE™ materials that are commercially available are in general use for the same type of loss in the field. Other known lost circulation materials include mica, cellophane, perlite, bagasse fiber, nut shells, feathers, textile fiber blend, and granular materials. Granular materials are more effective than fibers or flakes in some instances for sealing fractures at high pressure. Often the width of fracture that can be sealed depends upon concentration, as well as type, of sealing material. Accordingly, WALL-NUT™ material is used routinely for combating loss of weighted mud because weighted mud tends to induce fracturing of a formation. A general purpose lost circulation material may be characterized by the following criteria: it should contain high-strength granules with a definite size distribution; it should form a seal at both high and low differential pressures; and it should be equally effective in sealing unconsolidated formations and fractures or voids in hard formations.
In one prior art material a mixture of fibers, flakes and granules called KWIK-SEAL™ material is used. Often high filtrate squeezes for lost circulation depend upon tightly packed and substantially dehydrated solids to effect a required seal. This approach to combatting lost returns may be used in either high pressured or normally pressured drilling areas. Other prior art lost circulation materials are commercially available FLOSAL™, HY-SEAL, ZEOGEL™ materials (in proper concentration). In some methods lost circulation materials are mostly granular when drilling with heavy mud, and mostly fibers and flakes when drilling with low-density mud.
A great variety of materials, mixtures and formulas that are pumpable at the surface and develop shear-strength when pumped into place downhole have been used for curing lost circulation. Often an amount of such a material pumped into a wellbore is referred to as a “plug”. The plug may develop a “rubbery gel” or a “putty-like consistency,” and breathe” as varied pressures are imposed on natural and induced fractures in the formation. Some as a class have acquired the generic label of “gunk.”
One common plug uses bentonite and diesel oil as a base. With certain lost circulation materials, cement and polymers have been added as refinements for some applications. Various ingredients for such a plug are referred to as: DOB=diesel oil bentonite; M-DOB=mud-diesel oil bentonite; DOBC=diesel oil bentonite cement; and M-DOBC=mud-diesel oil bentonite cement. In certain applications diesel oil is used as spacer between gunk and mud or water. The DOB or DOBC slurry is pumped to the bottom of the drill pipe (which is placed somewhat above the loss zone, or at the bottom of the last casing), rams are closed, and the gunk followed by water is squeezed into the formation, or mud is pumped from the annulus as gunk is pumped from the drill pipe and the mixture is squeezed into the formation. Various polymers have been substituted for part of the bentonite in the gunk formula with the hope that the “rubberiness” of the gel and the “breathability” of the plug will thus be enhanced.
When circulation is lost while drilling with oil mud, the same type of squeeze can be applied using water as the continuous liquid, with GELTONE™ commercially available material instead of bentonite as the critical solid in the slurry. High-shear strength is imparted to the slurry when the GELTONE becomes wetted with oil.
Typical known shale shaker screens or screen assemblies with square mesh openings often are clogged or plugged when attempts are made to separate lost circulation materials from a mixture of them with fluid that has been pumped down a wellbore. Stringy, fibrous, and/or fibril material (“fibrous” material) can wrap around a wire of a screen and/or bridge a mesh opening without passing through the screen. In certain particular circumstances there have been problems with commercially available ULTRA-SEAL lost circulation material that has fibrous material in it. If components of this material which swell are caught between two screen mesh layers, they swell and plug the screen rather than going through the screen with the drilling fluid (while drilled cuttings, debris, etc. move on the top of the sceen assembly and exit the shaker or vibratory separator apparatus). Although the prior art discloses the use of screens with non-square openings for use on shale shakers for treating mixtures of drilling fluid and drilling solids, the present inventors are unaware of the use of prior art screen(s) and/or screen assemblies with non-square mesh openings in methods for separating fluid and fibrous lost circulation materials fr
Adams Thomas C.
Largent David W.
Schulte David L.
Lithgow Thomas M.
McClung Guy
Varco I/P Inc.
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