Liquid purification or separation – Filter – Movable medium
Reexamination Certificate
2001-10-03
2002-10-01
Lithgow, Thomas M. (Department: 1724)
Liquid purification or separation
Filter
Movable medium
C210S780000, C210S785000, C210S489000, C210S499000, C209S401000, C209S403000, C175S066000, C175S072000
Reexamination Certificate
active
06457588
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.
In certain prior art screens, square mesh wire cloths have been used with relatively fine wire diameters in multiple layers because of their resistance to blinding, e.g. with one or more support layers and two screening layers. The ratio between these meshes has generally been between about 1.1 and 1.7. Certain of these screen combinations have difficulty in handling LCM material and the material often blinds these screens. Oblong opening meshes with length to width ratios between 1.55 and 2.0 have also been used in the past for their resistance to blinding. As the ratio increases between the meshes, blinding decreases but strength also decreases. As the wire diameter increases, the resistance to blinding decreases, but the strength increases. However, when multiple layers of oblong cloths have been used in certain prior art screens, the ratio of the length of the openings at adjacent layers has been 2 or greater and of the width of the openings of adjacent layers has been 1.6 or greater. Attempts have been made to use oblong meshes with increasingly large ratios.
Typical known shale shaker screens or screen assemblies with square mesh openings often are clogged 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 screen 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 from drilled cuttings and believe it is not obvious to use screens with non-square openings with relatively more opening area to successfully treat such fluids.
FIGS. 1A and 1B
show a prior art screen
22
as disclosed in U.S. Pat. No. 2,723,032 with a coarse mesh wire screen, or cloth
23
that provides a backing screen or cloth of the unit. A fine mesh wire screen
24
is superimposed or mounted upon the backing screen
23
. The screen unit
22
has its coarse backing wire mesh or cloth coated or covered preferably with rubber or some suitable rubber or synthetic rubber composition. The strands are indicated at
25
and the covering or coating at
26
. Since all of the strands
23
are coated or covered, there is, of course, rubber-to-rubber contact between these strands of the coarser mesh screen
23
. The backing screen of cloth
23
is of the roller flat-top type and of any coarse size such, for example, as three or four mesh. The mesh of the finer mesh wire screen
24
varies, in accordance with the separating job to be done. For example, the mesh of the fine wire screen or cloth
24
may vary from the order of minus 20 (−20) to the order of minus 325 (−325).
FIGS. 2A and 2B
disclose a screen
30
as disclosed in U.S. Pat. No. 4,696,751 with a first mesh screen with rectangular dimensions of width and length. A second screen
38
is held in superimposed abutting relationship to the first screen
32
. The second
38
has width and length dimensions. The length dimensions of the first screen is larger than length dimension of the second screen, and the width dimension of the first screen is smaller than the width dimension of the second screen.
FIGS. 3A and 3B
disclose screens
50
and
53
shown in U.S. Pat. No. 5,626,234 which has an upper cloth
51
and lower cloth
52
. The upper cloth
51
is formed from woven stainless steel wire in the range 0.19 mm to 0.036 mm diameter and 60-325 mesh, (i.e. number of strands per inch) while the lower cloth
52
is formed from woven phosphor bronze wire in the range 0.45 mm to 0.19 mm diameter and 20-40 mesh. A screen
53
in
FIG. 3B
has an upper cloth
54
like the upper cloth
51
(
FIG. 3A
) and a lower cloth
55
woven from stainless steel wire having a nominal diameter in the range 0.20 to 0.45 mm diameter and typical 30 mesh, and is coated with an epoxy based material, or Molybdenum Disulphide, or Teflon (Registered Trade Mark), to a thickness in the range 5 to 50 microns typically 20 to 40 microns. Multiple passes of the wire through a coating process or through a succession of such processes may be necessary to achieve the desired coating thickness. The wires
57
,
58
,
59
are shown in cross-section to show the outer material coatings
67
,
68
,
69
(not to scale). The wire
64
is shown with the coating scraped from one end.
There has long been a need for a method for efficiently and effectively separating fluid and fibrous lost circulation materials from a mixture of them with drilled cuttings, debris, etc. There has long been a need, recognized by the present inventors, for such a method that does not result in clogged or plugged screen assemblies used for such separating.
SUMMARY OF THE PRESENT INVENTION
The present invention, in certain aspects, discloses methods for separating fluid and fibrous lost circulation material from a mixture of fluid, drilled cuttings and fibrous lost circulation material, the methods including: introducing the mixture onto a screen assembly that is vibrating, e.g., vibrated in a shale shaker, for passage of the fluid and fibrous lost circulation material through the screen assembly and for separation from the mixture of drilled cuttings, the screen assembly having at least two layers of screening material one on top of the other and including at least a first layer over a second layer, each layer having screen mesh with rectangular openings as viewed from above, the first layer having first rectangular openings, and the second layer having second rectangular openings, each of the first rectangular openings having a first width and a first length, and each of the second rectangular openings having a second width and a second length, the ratio of the first length to the first width ranging between 1.55 and 2.00, the ratio of the se
Adams Thomas C.
Largent David W.
Schulte David L.
Lithgow Thomas M.
McClung Guy
Varco I/P Inc.
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