Technical Field of the Invention [0002] This invention relates to an apparatus and methods for providing treatment chemicals in a subterranean formation. More particularly, the invention relates to methods of ensuring permanent treatment of wells. Such treatments aim at inhibiting the formation of scales.

[0003] Concurrently with production fluids such as crude oil, dissolved salts are typically produced which form mineral deposits or scales such as barium sulfate, strontium sulfate, calcium sulfate and calcium carbonate. These mineral deposits tend to reduce the effective diameter of the production tubing. by pluging them or damageing some valves or other subterranean equipments. Similar problems may occur in injection wells where the injected fluids are typically brines, for instance, constituted by the formerly separated water phase of a produced fluid.

[0004] To alleviate the scale problems, various treatments have been developed that include for instance, injecting into the reservoir a solution comprising a scale inhibitor adsorbed onto the rock and later desorbed during fluid production. Different systems have been developed that provide a relatively slow release of the scale inhibitor. Reference is made for instance to U. S. Patents No. 3,827, 977,4, 602,683, 5,141, 655 and 5,604, 185.

[0005] However, most current scale inhibition treatments are only effective for a limited period.. It is common to repeatedly treat the well every few months. Each treatment requires means-such as pumping equipment and/or coiled tubing injectors-for injecting the inhibitor solution and a preflush or afterflush treatment. Even though each

cleaning job is relatively simple and constitutes a minimal cost for the oil industry, the repetition of the treatments months after months impedes the profitability of the well.

[0006] Another disadvantage of the conventional technologies is that the treatments are often administrated by guesswork. Repeated analysis of the produced fluids are mostly impracticable and hence, would not necessarily provide good information on the fluids present downhole.

[0007] Therefore it would be desirable to provide simpler means to treat wells. In particular, inhibit scale formations and further provide better ways of assessing the effectiveness of the treatment.

Summary of the invention [0008] In one embodiment, the invention relates to a method for treating a subterranean formation comprising providing a container located within the production tubing-or near the bottomhole extremity of the production tubing, said container filled with slowly- released chemicals and comprising at least an opening.

[0009] The invention relates also to a method of replenishing the container comprising fishing the container with a fishing tool connected to a slick line, a wireline or a coiled tubing, refilling the container at the surface and replacing it downhole.

Brief Description of tAze Drawings [0010] Figure 1 shows a container suspended from a hanger with a nipple having a lock profile.

[0011] Figure 2 shows a retreivable container suspended from an anchor.

[0012] Figure 3 shows a container suspended from a hydraulic wireline set with retractable jaws.

Detailed Description of the Preferred Embodiments [0013] In a preferred embodiment, the container consists of a meshed like basket through which the production fluids will flow. The mesh-or other apertures-are preferably of relatively high dimension so that the flow of production fluids is not significantly impeded.

[0014] The container is preferably suspended near the bottomhole extremity of the production tubing so that at least a large fraction of the production fluids are effectively treated before entering the production tubing.

[0015] In one embodiment of the invention, illustrated Figure 1, the container is suspended to a hanger seating in a lock profile of a nipple located within the tubing, near its downhole extremity. Advantageously, most tubings are already equipped with such a nipple. In this Figure 1, the well is schematized by a casing, usually cemented, that ensures zonal isolation and the mechanical integrity of the well. The production fluids are displaced up to the surface through production tubing. In the pay zone, perforations are provided for the formation fluids to enter the wells. Similar configurations may be found with injection wells (even if of course, the flow is inversed from surface to the subterranean formation). The hanger is preferably provided with connection means (here not represented) that allow a secure connection for instance to a slick line or wireline or a coiled tubing though a detent self-locking device, used for locating the basket into the wellbore and retrieving it either at periodical interval or when surface analysis show an increase of the production of scales.

[0016] According to another embodiment,'schematized figure 2, the tubing is provided with an anchor catcher and the container is suspended to that anchor set and retrieved when needed through the use of a coiled tubing, wireline, slickline or similar equipment.

[0017] According to a third embodiment, schematized figure 3, the container is suspended to a hydraulic wireline set comprising retractable jaws. This embodiment makes it possible to adjust the position of the basket at the lower extremity-or inside the tubing-to ensure a better treatment.

[0018] According to fourth embodiment, the container is permanently anchored at the end or into the production tubing and a coiled tubing (slickline or similar) is used to refill

it. This embodiment is usually not preferred since it does not allow one to assess the release rate of chemicals into the well and therefore, the periodicity of the refill operation may not be accurate.

[0019] According to another embodiment, not represented, the container may be introduced into the well by pumping it into the hole (like a pig), and similarly pumping it out. In this later case, the tubing should preferably be equipped with a latch or recess or equivalent means to stop the container in the appropriate location.

[0020] The chemicals to be slowly released may be encapsulated within a polymeric enclosure. The enclosure may consist of any polymer that can degrade over a period of time to release said chemicals and will typically be chosen depending on the release rate desired. Degradation of the polymer can occur, for example, by hydrolysis, solvolysis, melting, or other mechanisms.

[0021] Preferred polymers are selected from the group consisting of homopolymers and copolymers of glycolate and lactate, polycarbonates, polyanhydrides, polyorthoesters, and polyphosphacenes. Most preferably, said polymer is poly (lactic acid-co-glycolic acid).

[0022] The encapsulation may be accomplished by known methods such as double emulsion technique involving the evaporation of a secondary emulsion by freeze drying or other drying method.

[0023] The scale inhibitor may also be delivered under the form of porous ceramic particles such as the ones described in W099/36668 hereby included by reference.

Another method for making porous particles suitable to introduce chemicals into a well is also known from GB2,284, 223 and GB2,298, 440 also included by reference.

[0024] A large variety of scale inhibitors are available commercially. Most of the commercialized scale inhibitors contain several reactive groups (carboxylate and/or phosphonate) which are capable of interacting with polyvalent metal ions to prevent scale deposits. Examples of inhibitors include a polycarboxylate, (homo or copolymer of an ethylenically unsaturated acid monomer such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, fumaric acid, mesoconic acid, citraconic acid and the like),

monoesters of diacids with alkanols, e. g. , having 1-8 carbon atoms, and mixtures thereof.<BR> <P>Monomeric and polymeric phosphonates, e. g. , aminomethylenephosphonates and homopolymers and copolymers of vinylphosphonate. Another class of inhibitors which may be used in practicing the method of this invention are organic phosphate esters such as phosphate esters of polyols and their salts containing one or more 2-hydroxyethyl groups, and hydroxylamine phosphate esters obtained by reacting polyphosphoric acid or phosphorus pentoxide with hydroxylamines such as diethanolamine or triethanolamine.

[0025] Though the invention is preferably used for delivering scale inhibitor, the same equipment-and method of replenishing it-can be used for other type of chemicals. For instance, several containers may be located near distinct perforation areas and a distinct chemical marker (for instance a dye) may be provided in each container. This provides an easy way to identify producing-and non-producing-areas. In that later case, a string of containers may be used, all anchored to one single seat.