Fluidized Sandpacking

Abstract

A method for obtaining an improved pack emplacement when packing a well with sand or gravel. A slurry of sand or gravel is pumped through the end of a tubing string that is extended to near the bottom of the interval to be packed; as the slurry is pumped, the tubing string is raised at a rate that is correlated with the pumping rate in order to maintain a fluidized bed of sand around the end of the tubing string as it moves through the interval to be packed.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of completing wells. More particularly, the invention concerns the placement of a pack of granular material in a well which penetrates an unconsolidated earth formation.

2. Description of the Prior Art

In producing petroleum from subterranean reservoirs, many wells are drilled into or through loosely cemented or unconsolidated formations. When such a well is placed on production, sand is often carried from the formation by the fluid and deposited in the well bore. Some sand is usually entrained in the produced fluid, thus causing severe erosion and damage to equipment which is employed in the production of the fluid. The production of the sand interfers with the normal production operations and gives rise to numerous operating problems. It can be generally said that in the production of a fluid from a loosely consolidated formation, workovers are frequently necessary to remove sand from the bottom of the borehole. If the erosion of the sand around the casing in the bottom of a borehole is sufficiently great, it will leave the casing inadequately supported and the casing may eventually buckle or collapse. In some instances, the sand problem is so severe that the well has to be abandoned.

Various methods have been attempted or suggested to date for solving the problem of sand production in wells that penetrate unconsolidated formations. One of the most widely practiced methods is to place packs of granular material such as sand or gravel behind a slotted liner or a perforated casing in such wells to prevent an inflow of material from the surrounding incompetent earth formation.

Such packs are installed by pumping a slurry of granular material into the well so that the grain-suspending liquid is forced into the surrounding earth formation while the suspended grains are screened out on the face of the earth formations. In a borehole that contains a perforated casing, the slurry must flow through the perforations that extend laterally through the casing and through a surrounding sheath of cement.

In accordance with conventional practice, the slurry of granular packing is pumped through a stationary tubing string positioned so that the lower end of the string is located at or above the top of the interval to be packed. This placement of the tubing string is believed to be desirable in order to avoid the pack's being formed around a significant length of the tubing string thus binding the tubing string within the well.

The use of a tubing string that ends above the interval to be packed causes the slurry to flow downward past the perforations in the casing or liner. The jetting effect of fluid emerging from the tubing string causes a well mixed turbulent zone of slurry within a distance of a few casing diameters from the end of the tubing string. However, below this jet mixed zone, slurry moves down the casing in a relatively slow flow and forms a deposit within the well bore as lower perforations cease taking slurry. This well bore deposit can build up to the lower end of the turbulent slurry zone in the region of the tubing string. Thus, in the conventional practice it is inevitable that a relatively large proportion of the sand remains within the well rather than being displaced into the formation.

SUMMARY OF THE INVENTION

The present invention provides an improved method for packing a well with a granular packing material. According to the invention a string of tubing is extended into the wellbore to near the bottom of the interval to be packed and a slurry of packing material is then pumped through the tubing string while the string is being raised at a rate that is correlated with the pumping rate to maintain a fluidized bed of packing material around the end of the tubing string as it moves through the interval to be packed. In a preferred embodiment of the present invention, when packing a well having a cemented and perforated casing the slurry is pumped at a pressure great enough to create a cylindrical fracture between the casing and/or cement and the walls of the formation penetrated by the well.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 we see an unconsolidated earth formation 10 such as an unconsolidated sand formation penetrated by a well 9 having a well bore 11 which has been cased with a tubular casing 12 that is fixed in place with a cement 13. The well bore 11 has been opened into fluid communication with the formation 10 through a number of perforations 14-18 which penetrate the casing 12 and the surrounding cement 13. There are a number of cavities or zones of high porosity 19-23 in the formation 10 adjacent to the perforations 14-18 which zones are of a type that may be caused by sloughing or realignment of sand particles from the unconsolidated earth formation 10 as fluid flows into the well bore 11 through the perforations 14-18.

The well 9 is provided with equipment suitable for the practice of this invention. A string of tubing 24 extends from the surface down the well bore 11 to near the lowermost perforation to be packed 14. At the surface the tubing 24 passes through a wellhead closure assembly 25 that is operatively connected to the casing 12 and which is adapted to carry a wellhead seal-off means 36 of a type which may provide a fluid-tight seal-off of the annulus between the casing 12 and the tubing 24 while allowing the tubing 24 to be raised and lowered through the wellhead assembly 25. The top of the tubing 24 is connected in fluid communication with a flexible conduit means such as a hose 26 adapted to maintain fluid communication between the tubing 24 and a conduit means such as a flow pipe 27 as the tubing 24 is raised and lowered. The flow pipe 27 is in fluid communication with a slurry pumping apparatus of a conventional type (not shown).

The tubing 24 is operatively connected to a raising and lowering means of a conventional type which may be a workover rig comprising a draw works 28 powered by a motor means 29 and operatively connected to a wire line 30 that is strung through a crown block 31 mounted near the top of a mast or derrick means 32 and then through a traveling block 33 that is connected by suitable means to a hook 34 which may be coupled to a tubing elevator 35 operatively connected to the tubing string 24.

To pack zones 19-23 behind the perforated casing 12 with a granular packing material 37 according to the process of this invention, the tubing 24 is lowered into the well bore 11 until it extends to a point adjacent the lowermost perforation 14 in the interval to be packed. Preferably, the lower end of the tubing 24 is spotted about 12 to 24 inches above the lowermost perforation 14. The annulus between tubing 24 and casing 12 is preferably closed to fluid flow above the interval to be packed as by wellhead seal-off means 36 or any other suitable means. A slurry of granular packing material 37 is then pumped through the tubing 24 into the well bore 11 at a rate sufficient to create a zone of turbulent flow extending below the lowest perforation 14. Injection rates in the range of about 1 barrel per minute to about 3 barrels per minute are normally suitable for this purpose; however, the injection rate used in the practice of this invention may range from the minimum rate at which a slurry of a given packing material may be pumped through the tubing 24 without plugging to the maximum rate which can be achieved without fracturing the formation 10. As the slurry is pumped through the tubing 24, the tubing 24 is slowly raised through the interval to be packed at a substantially constant rate correlated with the slurry pumping rate so that when the lower end of the tubing 24 reaches top of the interval to be packed all of the granular packing material intended to be used in packing the interval has been used. This raising of the tubing 24 may be done either incrementally or substantially continuously.

Referring to FIG. 2, we see an illustration of the flow of slurry in the well bore 11 near the end of the tubing 24. Since the annulus between the casing 12 and the tubing 24 is closed to fluid flow at some point above the interval to be packed, as by the seal-off means 36 in the wellhead closure assembly 25 of FIG. 1, the fluid pumped down the tubing 24 into the well bore 11 can flow out the well bore only through openings such as perforations 14-18 in the walls of the casing 15. As the slurry of packing material 37 flows through the perforations 14-18 into the cavities 19-23 behind the casing 15, the grain suspending liquid component of the slurry is forced into the surrounding earth formation 10 while the packing material 37 is screened out on the face of the formation 10.

As the slurry is pumped out of the tubing 24, a zone 38 of turbulent flow extends across the diameter of the well bore 11 and into any adjacent perforations, such as perforations 15 and 16 for a distance of about 1 to 2 feet below the end of the tubing 24. Within the turbulent zone 38 packing material 37 is churned into a fluidized bed in which the grain density (i.e., the number of grains per unit volume) is substantially uniform. Each perforation that is adjacent to the turbulent zone 38 tends to be penetrated by a substantially equal amount of packing material 37 as the fluidized packing material 37 flows into the perforation. Above the turbulent zone 38, there is a zone 39 in the well bore 11 in which the direction of flow is mainly upward. In this zone 39, the velocity of fluid flow decreases at each perforation, such as perforation 17, in proportion to the amount of slurry entering that perforation and becomes substantially zero at about the depth of the uppermost perforation 18. Because of the decrease in slurry flow velocity in the zone 39 the flowing fluid in this zone 39 soon becomes free of suspended packing material 37. Thus, only a small proportion of the packing material 37 is carried above the top of the turbulent zone 38. Below the turbulent zone 38, the well bore 11 becomes filled with packing material 37 that has settled out of the slurry.

As the tubing string is moved upward through the interval to be packed, the zone 38 of turbulent flow is advanced up through the perforated interval of the casing 15 assuring that each of the cavities 14-18 behind the perforated casing 15 is filled with a substantially equal number of grains of the packing material 37 per unit of volume if the character of the formation is uniform. The upward movement of the tubing 24 and the fluidized bed of sand in the turbulent zone 38 avoids the sticking of the tubing 24 by keeping it above any sand that is not part of a fluidized bed. The process also prevents a premature filling of the well bore 11 with packing material 37. Since the packing material 37 is introduced at the bottom, no static deposit is allowed to form at the depth of any of the perforations 14-18 until after the perforation has been filled with the fluidized bed of packing material.

In an embodiment of this invention, when packing an interval in a cemented and perforated casing 12, the grain suspending liquid component of the slurry of packing material 37 is forced into the surrounding earth formation 10 at an injection pressure less than fracturing pressure of the formation 10 but great enough to compress the formation 10, moving the face thereof radially outward relative to the casing 12 and cement 13 and creating a narrow void space or "cylindrical fracture" substantially surrounding the cement 13. The pressure at the face of the formation 10 required to create a cylindrical fracture 40 may be determined by injecting a quantity of the grain suspending liquid into the formation prior to the injection of the slurry of packing material 37 at a relatively low injection pressure and then increasing the bottom hole injection pressure (without exceeding the pressure at which fractures will be created which extend laterally into the formation 10 which lateral fracturing pressure may be known from other wells in the area or may be determined by methods well known in the art) while determining the injectivity of the well 9 from data obtained by measuring the injection pressure and the injection rate. Tests show that a marked increase in injectivity can be expected to occur at that pressure at which a cylindrical fracture is formed.

In operating the present process, the slurry of grains to be used in forming a sandpack or gravel pack can comprise a slurry of grains of particulate matter which are coated with a solution of epoxy resin components in a resin component solvent and are suspended in a liquid hydrocarbon that has a limited solubility with the resin component (as disclosed in a copending patent application of E. H. Bruist et al, Ser. No. 867,631, filed Oct. 20, 1969.)

While the process of this invention has been described primarily with respect to packing an interval in a well wherein a casing has been cemented and then perforated, it should be understood that the method is equally applicable to the placement of granular packing material in wells completed in other ways, for example, as with an uncemented slotted liner or other perforated pipe member.

In summary, the present invention provides an improved method for packing a well bore with a granular material in a well that is at least partially cased with a tubular casing. The method comprises the steps of extending a string of tubing into the well bore to a point adjacent the bottom of an interval of the wellbore to be packed, closing the annular space between the string of tubing and the casing above the interval to be packed, pumping a slurry of a granular packing material through the string of tubing at a pumping rate sufficient to create a zone of turbulent flow around the end of the string of tubing which extends across the diameter of the wellbore and in which turbulent zone the granular packing material is churned into a fluidized bed in which the grain density is substantially uniform, and simultaneously raising the string of tubing, preferably at a rate that is correlated with the pumping rate to maintain a fluidized bed of packing material around the end of the string of tubing as it moves through the interval to be packed. In a wellbore cased with a tubular string of casing that is cemented in place and perforated through an interval to be packed, an embodiment of the invention may comprise the additional steps of determining a cylindrical fracture forming pressure less than the lateral fracturing pressure of said formation which is sufficient to cause the surrounding earth formation to move outwardly relative to the cemented string of casing leaving a narrow space between the cemented string of casing and the earth formation, and adjusting the slurry pumping rate while pumping the slurry to maintain a pressure on the earth formation substantially equal to the cylindrical fracture forming pressure.

Claims

We claim as our invention:

1. In a wellbore cased with tubular string of casing that is cemented in place and perforated through an interval to be packed to provide fluid communication with a surrounding earth formation, a method for packing the wellbore with a granular material comprising the steps of:

extending a string of tubing into the wellbore to a point adjacent to the bottom of a selected interval of the wellbore to be packed, an annular space being defined between the string of tubing and the tubular casing;

closing the annular space between the string of tubing and the casing above the interval to be packed;

determining a cylindrical fracture forming pressure less than the lateral fracturing pressure of said formation which is sufficient to cause the surrounding earth formation to move outwardly relative to the cemented string of casing leaving a narrow space between the cement and the earth formation;

pumping a slurry of a granular packing material through the string of tubing at a slurry pumping rate sufficient to create a zone of turbulent flow around the end of the string of tubing which extends across the diameter of the wellbore and in which turbulent zone the granular packing material is churned into a fluidized bed in which the grain density is substantially uniform;

adjusting said slurry pumping rate while pumping said slurry to maintain a pressure on said earth formation substantially equal to said cylindrical fracture forming pressure; and

simultaneously raising the string of tubing through the interval to be packed.

2. The method of claim 1 wherein the slurry of granular packing material comprises a slurry of grains of particulate matter which are coated with a solution of epoxy resin components and which are suspended in a liquid hydrocarbon.

3. In a wellbore penetrating a fluid containing subsurface earth formation which is cased opposite the fluid containing formation with a perforate pipe member having a plurality of perforations vertically spaced along the length thereof, there being at least one cavity external of the perforate pipe member adjacent one or more of the plurality of perforations, a method for packing the interior of the perforate pipe member, the perforations in the perforate pipe member and the cavity external of the perforate pipe member with a granular material comprising the steps of:

extending a string of tubing into the well bore to a point within the perforate pipe member adjacent the lowermost of the vertically spaced perforations;

pumping a slurry of a granular packing material through the string of tubing at a slurry pumping rate sufficient to create a zone of turbulent flow within the perforate pipe member around the end of the string of tubing in which zone of turbulent flow granular packing material is churned into a fluidized bed in which the grain density is substantially uniform, said zone extending across the diameter of the perforate pipe member adjacent the lowermost perforation, through the lowermost perforation and into the cavity external of the perforate pipe member adjacent the lowermost perforation; and, thereafter,

raising the string of tubing through the perforate pipe member while continuing to pump the slurry at the slurry pumping rate whereby the zone of turbulent flow is sequentially moved past each of the vertically spaced perforations above the lowermost perforation, each perforation and the interior of the perforate pipe member as well as the cavity external of the perforate pipe member adjacent that perforation being filled with granular packing material as the zone of turbulent flow is moved past the perforation.

4. The method of claim 3 including the step of closing the well bore external of the string of tubing to fluid flow at a point above the perforate pipe member before pumping said slurry.

5. The method of claim 2 wherein said slurry pumping rate is a rate within the range of 1 barrel per minute to 3 barrels per minute.

6. The method of claim 3 wherein the step of extending a string of tubing in the well bore to a point within the perforate pipe member adjacent the lowermost of the vertically spaced perforations comprises lowering a string of tubing into the well bore to a point at which the lowermost end of the string of tubing is less than about 2 feet above the lowermost perforation.

7. The method of claim 3 wherein the step of raising the string of tubing comprises raising the string of tubing through the perforate pipe member at a substantially constant rate.

8. The method of claim 7 including the steps of:

determining a selected amount of granular material to be used in packing the well bore adjacent the fluid-containing formation, and

correlating the substantially constant rate at which the tubing is raised with the slurry pumping rate so that when the lower end of the tubing reaches the top of the well bore portion to be packed an amount of granular material equal to the selected amount of granular material has been pumped into the well bore.