Horizontal Well Liner

Abstract

A liner for a horizontal well, the liner comprising a substantially hollow elongated body having a plurality of orifices in the liner body that each allow limited ingress of fluid into the liner body. Each orifice has a predetermined shape, size and spacing in the liner body. The shape, size and spacing of the orifices provide that pressure of fluid in an annular space located between the liner body and the horizontal well is substantially uniform along the horizontal well.

Claims

1. A liner for a horizontal well, the liner comprising a substantially hollow elongated body having a plurality of orifices in the liner body, wherein: the orifices allow limited ingress of fluid into the liner body; each orifice has a predetermined shape, size and spacing in the liner body; and the shape, size and spacing of the orifices provide that pressure of fluid in an annular space located between the liner body and the horizontal well is substantially uniform along the horizontal well.

2. A liner for a horizontal well according to claim 1, wherein the shape, size and spacing of the orifices collectively offset pressure losses occurring in the well due to accumulated hydraulic friction and velocity head acting on fluid in the liner.

3. A liner for a horizontal well according to claim 1, wherein the shape, size and spacing of the orifices take into account adopted initial entry loss at an upstream end of the well.

4. A liner for a horizontal well according to claim 1, wherein the shape, size and spacing of the orifices accommodate variations in hydraulic conductivity in geological strata adjacent to the well.

5. A liner for a horizontal well according to claim 1, wherein the shape, size and spacing of the orifices accommodate variations in water table level adjacent to the well.

6. A liner for a horizontal well according to claim 1, wherein the shape, size and spacing of the orifices provide a uniform rate of fluid inflow along the liner body.

7. A liner for a horizontal well according to claim 1, wherein the annular space has a cross sectional area that is between 30% and 50% of a cross sectional area of the liner body.

8. A liner for a horizontal well according to claim 1, wherein the shape, size and spacing of the orifices are determined using a plurality of input parameters, the input parameters including accumulated hydraulic friction acting on fluid flowing along the well.

9. A liner for a horizontal well according to claim 8, wherein the accumulated hydraulic friction is calculated using measurements of flow rate, diameter, length and/or a roughness coefficient of the liner body section.

10. A liner for a horizontal well according to claim 8, wherein the input parameters include variable velocity head along the liner body.

11. A liner for a horizontal well according to claim 10, wherein the variable velocity head is calculated as V.sub.h=V.sup.2/.sub.2g, where V is liner flow velocity and g is acceleration due to gravity.

12. A liner for a horizontal well according to claim 8, wherein approximate cumulative total head loses are calculated for the liner using the formula 0.333Lh.sub.f where L is a total length of the liner and h.sub.f is head loss in meters per unit length of the liner.

13. A liner for a horizontal well according to claim 8, wherein the input parameters include adopted initial entry loss at an upstream end of the well.

14. A liner for a horizontal well according to claim 8, wherein the input parameters include one or more error margin corrections for hydraulic friction.

15. A liner for a horizontal injection well, the liner comprising a substantially hollow elongated body having a plurality of orifices in the liner body, wherein: the orifices permit egress of fluid out from the liner body; each orifice has a shape, size and spacing in the liner body; and the shape, size and spacing of the orifices provide that pressure of fluid in an annular space between the liner body and the well is substantially uniform along the well.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0047] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0048] FIG. 1 is a cross sectional view of a horizontal well having installed therein a well liner according to a preferred embodiment of the present invention;

[0049] FIG. 2 is a side view of the well and liner of FIG. 1 in a horizontal well installation;

[0050] FIG. 3 is a schematic side view of a horizontal well installation constructed in a coastal superficial aquifer; and

[0051] FIG. 4 shows a graph illustrating the relationship between cumulative pressure loss and distance along the length of a horizontal well at design flow rate.

DETAILED DESCRIPTION OF THE DRAWINGS

[0052] Referring to FIG. 1, there is shown in cross section a horizontal well, referred to generally by reference numeral 10. The well 10 has a conventional well screen or slotted casing 12 comprising an elongated tubular body 14 inserted into the well 10. The well screen or casing 12 has a spaced wire wrap or series of slots along its longitudinal length adapted to permit water, or other fluids, to pass through the well screen 12.

[0053] The well 10 additionally has installed therein a well liner 16 according to a preferred embodiment of the present invention. The liner 16 comprises a substantially hollow elongated body 18 that extends, at least in part, along the longitudinal length of the well 10. Preferably, the body 18 extends along the entire longitudinal length of the well 10.

[0054] The liner body 18 has a cross sectional shape that is substantially circular and is made of a material having sufficient rigidity, durability, water-resilience and anti-corrosive properties such as, for example, polyethylene, polyvinyl chloride, fibre re-enforced plastic or a metal-based material such as stainless steel.

[0055] A plurality of orifices 20 are formed in the liner body 18 that are each adapted to permit the inflow of ground water, or other fluid, into a centre 22 of the liner body 18. Whilst four orifices (20.1 to 20.4) are shown in the liner body 18 in cross section in FIG. 1, it will be appreciated that, generally, the liner body 18 will comprise numerous orifices 20 and may, therefore, have more or less than four in cross section.

[0056] The liner 16 is adapted to twist or rotate when inserted into the course of the well 10. The elongated length of the liner 16 may, therefore, not necessarily be straight or concentric once fully installed into the well 10.

[0057] A void 24 is located between an exterior surface 26 of the liner body 18 and an interior surface 29 of the well 10 bore. In the exemplary arrangement shown in FIG. 1, the well screen or slotted casing 12 occupies the void 24, in part. It will be appreciated, however, that for horizontal wells not having a well screen or slotted casing inserted therein, the void 24 will be substantially empty.

[0058] The void 24 has a cross-sectional shape that is, preferably, substantially annular but not necessarily concentric. The total cross sectional area that is occupied by the annular void 24 is variable but is, preferably, between 30% and 50% of the total internal cross sectional area of the liner body 18.

[0059] Referring to FIG. 2, there is shown a horizontal well installation, being indicated generally by reference numeral 30, comprising the well 10, well screen 12 and well liner 16 of FIG. 1.

[0060] The well 10 has been installed below the ground surface 32 and follows a long course along a distance indicated by D. The installed well screen or slotted casing 12 and liner 16 pass a substantial distance through an aquifer 34 below an initial water table 36 and serve as the collection conduit for the well installation 30.

[0061] The well installation 30 also includes a collection sump 38 and pump, or an alternative pump configuration. As shown in FIG. 2, the discharge end 40 of the liner 16 is in flowable communication with the collection sump 38 and/or pump.

[0062] In the case of a sump, an annular seal 44 is installed around the circumferential surface of the liner body 18 which will ensure a water-tight seal between the liner body 18 and well screen body 14 at its discharge end 40. A further seal 46 may also be in the base end 42 of the collection sump 38.

[0063] In use, ground water from the aquifer 34 flows through the porous well screen or slotted casing 12 and through the orifices 20 in the liner 16. The ground water then flows along the course of the liner 16 in the direction indicated generally by reference numeral 48. The ground water collects at a collection location 38 and is extracted using a pump 50, as indicated by reference numeral 52.

[0064] The extraction of ground water causes a degree of formation loss (commonly referred to as drawdown) to occur in the adjacent aquifer 34, as indicated by reference numeral 54. The drawdown causes the zone of saturation adjacent to the well to be reduced to an operating water table level 56.

[0065] Flow in the liner 16 results in friction and velocity head losses that cause accumulated pressure losses within the liner 16. In addition, the orifices 20 at the upstream end 62 of the liner 16 may be designed so that there is an adopted initial inflow loss (or, alternatively, an adopted outflow loss) equivalent to 5 to 10% of the cumulative total friction, velocity head and safety factor loses. The adopted initial entry loss, into the liner 16, is indicated in FIG. 2 by reference numeral 61. The entry loss thru the well screen 12 is indicated as by reference numeral 52.

[0066] The hydraulic grade line 58 indicates the predetermined hydraulic grade within the liner 16 under design flow and adjusted water table conditions.

[0067] Without the presence of the well liner 16, the flow of ground water through the well screen 12 into the collection sump 38 would cause hydraulic friction and velocity head losses to occur along the length D of the well screen or slotted casing 12 and the accumulated pressure losses would be transferred to the surrounding aquifer 34 through the well screen or slotted casing 12. These pressure losses would occur, in particular, towards the discharge end 40 which, in turn, would cause high inflow rates and transfer of the low pressures to the adjacent aquifer 34. The reduction in aquifer pressure would cause a corresponding drawdown to occur adjacent to the well.

[0068] The pressure losses and water table 56 reduction lead to unwanted upwelling to occur in respect to any layer of saline ground water that is present below the freshwater aquifer 34. Upwelling may lead to unwanted saline intrusion and resultant contamination of the extracted ground water.

[0069] The liner 16 that is installed inside the well 10 is intended to offset these pressure losses and related issues. Specifically, the shape, size and spacing of the orifices 20 in the liner body 18 are predetermined so as to restrict inflow and ensure that the pressure of ground water in the annular void 24 between the liner 16 and well 10 is kept substantially uniform along the length of the well. Preferably, the orifices 20 ensure that the adjacent ground water pressure is substantially uniform along the distance D of the well 10 due to the predetermined required head losses across the orifices 20 and resulting uniform rate inflow.

[0070] Offsetting the pressure losses to achieve substantially uniform adjacent ground water pressure substantially mitigates the risk of saline upwelling occurring, in particular, near the discharge end 40. The uniformity of pressure, in turn, provides for a uniform rate of areal fluid inflow along the elongated length of the liner body 18.

[0071] The person skilled in hydraulics will appreciate that pressure loss (h) through an orifice in a fluid-carrying pipe varies as flow squared (Q.sup.2) and that hydraulic friction and velocity losses occurring along the pipe are also closely related to Q.sup.2. Because of this relationship, the orifices 20 may be calculated such that the head loses of entry into the liner 16 substantially match the cumulative head loses at any point along the liner 16.

[0072] The amount of hydraulic friction calculated as part of determining and selecting the shape, size and spacing of the orifices 20 may be calculated using known and applicable hydraulic mathematical methods such as, for example, the Hazen and Williams or Colebrook-White equations taking into account a plurality of input parameters.

[0073] The input parameters may include adopted design flow, the liner's 16 diameter and length D and roughness coefficients for the liner 16. The roughness coefficients that are utilised, preferably, take into account one or more spatial and/or temporal adjustments for the liner 16 such as, for example, variations in roughness that occur due to the long-term effects of corrosion.

[0074] Generally, the hydraulic friction calculations that are performed will be based on the principle that net cumulative hydraulic friction in a well, assuming substantially uniform flow, is approximately 33.3% of the friction that would apply if the design flow occurred over the total length of the well. For conduits having uniform inflow rates and increasing lateral flows, total cumulative head losses may, therefore, be calculated as:

0.333Lh.sub.f

where L is the effective liner length and h.sub.f is head loss in meters per unit length of conduit based on the design flow of the horizontal well.

[0075] It will be appreciated that the hydraulic friction values that are calculated using these methods are estimates only and contain a degree of inherent error (perfect friction coefficients being the reserve of the pure mathematician). Error safety margin corrections within appropriate hydrological engineering tolerances are, therefore, also taken into account when determining the shape, size and spacing of the orifices 20 in the present invention. Error safety margins of between 10% and 15% may, for example, be added into the calculations.

[0076] The orifices 20 are, further, selected to mitigate pressure losses that would occur as a result of variable velocity head occurring along the liner 16.

[0077] Velocity head (V.sub.h) is, preferably, calculated using the formula:

V.sub.h=V.sup.2/2g

where V is the liner flow velocity (meters/second) and g represents the acceleration due to gravity (meters/second.sup.2).

[0078] A person skilled in hydraulics will further appreciate that, when total cumulative fluvial head losses are known, due to friction and velocity head, a proportion of these losses (typically, 5% to 10%) provide a basis for limiting inflow rates of ground water at the upstream end 62 of the well 10 and ensuring that inflow rates will be as determined along the total well length. The determination of the shape, size and spacing of the orifices 20, therefore, also takes into account adopted initial entry loss occurring at the upstream end 62.

[0079] As illustrated in FIGS. 1 and 2, the orifices 20 are located consistently about the circumferential surface of the liner 16 and the spacing of the orifices 20, in general, increases towards the discharge end 40 of the well screen body 14.

[0080] The diameter of each of the orifices 20 is, preferably, selected such that between two and four orifices are provided for each meter length of the liner 16 at the upstream end 62. The spacing increases towards the discharge end 40.

[0081] The adopted initial entry loss that is taken into account when selecting the shape, size and spacing of the orifices 20 is critical to ensure that controlled rates of inflow are achieved at the upstream end 62 of the liner 16. The adopted initial entry loss is also used to help determine the required diameter of the orifices 20, and to limit the number of orifices 20 that must be present in the liner 16.

[0082] It will further be appreciated that the geological composition and properties of the strata and aquifer 34 surrounding the well 10 may not be homogenous. The permeability (and related hydraulic conductivity) of the strata and the water table level may vary along the elongated course of the well. The shape, size and spacing of the orifices 20 that are calculated may take these variations into account and ensure that uniform pressure and required rates of fluvial inflow are achieved along the liner 16 notwithstanding such geological variations.

[0083] FIG. 3 shows a profile of a 500 m long horizontal well showing alternative locations for collection locations 68, 70.1 and 70.2. Collection locations influence the cumulative head loss in liner pipes and may permit utilisation of more economical (smaller diameter) well screens and liner pipes.

[0084] Drilling profiles 66 and 80 indicate differences that may result due to smaller diameter well screens. The curved sections of the profiles, may be abandoned after completion of the horizontal section 82 or, alternatively, the original access to the horizontal well may be utilised for installation of a collection pump.

[0085] FIG. 3 shows hydrogeological data for a horizontal well parallel to a coastline. Indicated data includes sea level 78, initial water level 76, invert of screen 82 and underlying aquatard 72.

[0086] FIG. 4 shows a graph 84 that illustrates the relationship between cumulative pressure loss, shown on the y axis 86, and distance along the elongated length of a 500 metre long horizontal well, shown on the x axis 88.

[0087] The values depicted in the graph 84 were calculated based on an assumed well effective length D of 500 metres. If a collection sump were located centrally within the well installation (such as, for example, sump location 70.1 in FIG. 3), then the effective length would be halved, and the diameters of the well screen 12 and liner 16 could be reduced accordingly.

[0088] The line 90 represents total cumulative pressure loss, including taking into account friction and velocity head, that is experienced at any point along the well section. The dotted line 92 represents friction at any point along the well section. The adopted initial entry loss into the liner is indicated by reference numeral 94.

[0089] As indicated by reference numeral 96, the graph values may be used as a basis for calculating the relative pressure loss and the size, shape and spacing of orifices 20 of a liner 16 for a well for each 5 or 10% section of the liner 16. Each section of the liner 16 will comprise substantially common orifice spacing. Flow corrections in order to conform to the shape of the cumulative pressure loss curve 90 would adjust automatically to lateral flows in the annular space 24 inside the well.

[0090] The number of orifices that may be required for each section of liner 16 is calculated using mathematical formulas and/or algorithms, as appropriate. The following formula may, by way of example, be used (using units commonly used in the United States) as part of these calculations:

Q=20Cd.sup.2h.sup.5

where Q is flow (in gpm), C is an appropriate co-efficient, d is the diameter of each the orifices (in inches) and h is head loss (in feet). A typical value that will be given for co-efficient C is 0.61. Where, however, entry holes are drilled into a HDPE pipe, the wall thickness of the pipe may exceed the diameter of each orifice. In these cases, a value of 0.7 may be given to co-efficient C, and the opening would be classified as a square edge nozzle.

[0091] It is principally envisaged that the liner 16 that is the subject of the present invention will be used to improve the performance and efficiency of horizontal well installations used for extracting ground water. In particular, the liner 16 will significantly improve the extraction of ground water from freshwater coastal aquifers residing above saline-water wedges. It will be appreciated, however, that the liner 16 may also be used to improve other types of fluid-based horizontal well installations such as, for example, oil wells.

[0092] The liner 16 may, further, be used to improve the performance and efficiency of horizontal injection-type wells. Horizontal wells may be drilled in order to facilitate the injection, re-charge, disposal or in-ground treatment of fluids such as excess water or effluents, into geological strata. Such types of wells would, similarly, be provided with a perforated screen or casing that extends along the well's longitudinal length and governs the discharge and outflow of fluid from the well into the adjacent strata.

[0093] The same principles and issues described above for extraction wells become similarly manifest during the operation of injection wells, albeit in reverse. This includes accumulated hydraulic friction that acts along the elongated course of a horizontal injection well. The present liner 16 may, therefore, be inserted inside an injection well in order to provide uniform pressure in the space between the liner 16 and injection screen. The liner 16 will substantially control the rate of fluid discharge through the well screen into the surrounding strata.

[0094] Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.