TECHNICAL FIELD

The present invention relates to a filling material for underground treatment and a method for filling a well wall, and particularly to a filling material for underground treatment used during construction of a muddy water drilling method, and a method for filing a well wall using the filling material for underground treatment.

BACKGROUND ART

Hydrocarbon resources (hereinafter, collectively referred to as “petroleum or the like”) such as petroleum (including shale oil and the like) or natural gas (including shale gas and the like) have been mined and produced through wells (oil well or gas well, hereinafter, collectively referred to as “well”) having porous and permeable underground layers.

The laying of the well (hereinafter, also referred to as “well drilling”) is performed, for example, by the following process. That is, after steps including a drilling step of, using a drill, drilling a well bore in a direction vertical to the ground surface and, if necessary, further in a horizontal direction, a casing step of preventing collapse of the well bore and preventing fluid leakage through an inner wall of the well bore (well wall), a cementing step of finishing the well wall, a perforation step of perforating the well wall to recover petroleum or the like in a production layer (a layer producing petroleum or the like, for example, a shale layer producing shale gas, etc.), and completing steps including a fracturing step of injecting a high-pressure fluid into the perforation to form and expand a fracture in the well wall, the laying of the well is completed. Rehabilitation of the well that has already been laid is implemented through almost the same process as laying new a well.

In the well laying process, various fluids are used, and various types of water-based fluids, oil-based fluids, and emulsions are used.

In the drilling step, muddy water (also referred to as “drilling fluid”) is circulated to remove drilling wastes from the bottom (tip) of the well bore and around the drill to raise the drilling wastes to the ground, or to cool the drill and its surroundings to provide lubrication, or to prevent eruptions by reducing underground pressure. At this time, when the muddy water escapes through the well wall during drilling or through fractures in the well wall, the well wall and the well bore collapse and unexpected formation fluid intrusion occurs, which is dangerous. Therefore, in order to prevent the muddy water from escaping from the well wall, the muddy water is mixed with a lost circulation material (LCM) for filling the well wall.

As the lost circulation material (LCM), various inorganic substances and organic substances are used, for example, fibrous materials such as cedar skin, sugarcane fiber, and mineral fiber, granules such as limestone, marble, wood, walnut shells, cotton nut shells, corn cobs, and synthetic resins, and flakes such as mica flakes, resin film flakes, small cellophane flakes.

For example, there has been proposed a lost circulation material which is to be charged into drill holes and trenches during construction of the muddy water drilling method, the lost circulation material is formed by molding a mixture of (A) an inorganic fine powder and (B) a water-absorbent resin into a desired shape (see Patent Literature 1), and a lost circulation material which is made by mixing a highly water-absorbent synthetic resin material with a fibrous material such as pulp cotton, cotton, waste paper, waste pulp, asbestos and cotton waste (see Patent Literature 2).

In the perforation step or the fracturing step, a closed space is sequentially formed in the well bores using a pro-arranged filling member such as a frac plug or a frac sleeve, the well wall is perforated by a high-pressure fluid injected into the closed space, and further fractures are formed (perforations may be made using gunpowder). Therefore, when the high-pressure fluid escapes from the well wall, a desired fluid pressure is not obtained, resulting in the inability to form the desired perforation or fracture. Thus, it is necessary to temporarily fill the well wall. Resin particles or the like may be used as the filling material. Further, a fracturing method of sequentially repeating fracturing and temporary filling without using a filling member such as a plug is also adopted. In addition, a method in which a fracture that has been opened is temporarily filled to collect petroleum or the like again by fracturing a different place is also adopted.

It is also necessary to temporarily fill the fractures formed in the production layer before a flow back in which a fracturing fluid is refluxed to the ground, thereby preventing outflow of petroleum or the like and improving productivity. In order to start production of petroleum or the like, in the case of injecting the high-pressure fluid to remove obstacles or the like remaining in the well, when the high-pressure fluid escapes from the well wall, the desired fluid pressure cannot be obtained, and the obstacles cannot be removed. Loss of the function of filling the well wall before installation of equipment is completed can result in unexpected formation fluid intrusion. The filling material is sometimes called a bridge material, and calcium carbonate or the like is often used.

Therefore, well treatment fluids such as the muddy water (drilling fluid), the cementing fluid, the perforation fluid, the fracturing fluid or the completing fluid used in the above steps are compounded with various compounding agents and a temporary plug for temporarily filling the well wall, such as a lost circulation material (LCM) and a diverting agent. Further, prior to the flow of these well treatment fluids into the well bore, a temporary filling fluid containing a temporary plug is flowed into the well bore.

It is desirable that these temporary plugs be removed from the well wall when starting production of petroleum or the like so as not to hinder leaching of petroleum or the like. For this purpose, a liquid containing a material having a dissolving effect on the temporary plug such as an acidic substance or an alkaline substance may be injected into the well bore.

In the well treatment fluids such as the muddy water (drilling fluid), the cementing fluid, the perforation fluid, the fracturing fluid or the completing fluid, when a degradable material that decomposes after a predetermined period of time is used as a compounding agent to be added for various purposes, eliminating the need for recovery or disposal has led to reductions in well drilling costs and process shortening.

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-2002-161268

Patent Literature 2: JP-A-2004-99677

SUMMARY OF INVENTION

Technical Problem

However, a conventional filling material for underground treatment is likely to have low specific gravity and may float in the muddy water for drilling, so that during the filling, the muddy water must be continuously stirred, and there is a problem that the workability of filling the drilled part is poor. In addition, when a polymer material having a high water absorption property is used, the swelling property is too high, and the viscosity of the muddy water for drilling is too high, and similarly, there is a problem that the filling workability is deteriorated. In the muddy water drilling method, during the drilling, the muddy water for drilling continues to be supplied, and therefore, when the filling workability of the muddy water for drilling deteriorates, the capacity of producing crude oil and gas is reduced.

Therefore, in order to solve the above problems, the present invention provides a filling material for underground treatment, which has excellent swelling performance, is effective in preventing phenomena such as lost circulation, and has improved workability of filling a well drilled part with the filling material.

Solution to Problem

As a result of intensive studies, the present inventors have found that the above problems can be solved by using a polyvinyl alcohol-based resin exhibiting a specific swelling ratio when measured under specific swelling conditions. Thus, the present invention has been completed.

That is, the present invention is characterized in the following (1) to (7).

(1) A filling material for underground treatment containing at least one polyvinyl alcohol-based resin of a modified polyvinyl alcohol-based resin and an unmodified polyvinyl alcohol, wherein a swelling ratio of the polyvinyl alcohol-based resin after immersion in water at a temperature of 80° C. for 30 minutes is 250% to 900%.

(2) The filling material for underground treatment according to the above (1), wherein the modified polyvinyl alcohol-based resin is a polyvinyl alcohol-based resin (A) containing a carboxyl group.

(3) The filling material for underground treatment according to the above (1), wherein the modified polyvinyl alcohol-based resin is a crosslinked product (A1) of a polyvinyl alcohol-based resin (A) containing a carboxyl group by heat treatment.

(4) The filling material for underground treatment according to the above (1), wherein the modified polyvinyl alcohol-based resin is a crosslinked product (A2) of a polyvinyl alcohol-based resin (A) containing a carboxyl group with crosslinker.

(5) The filling material for underground treatment according to any one of the above (1) to (4), which is for drilling.

(6) An underground treatment method including: incorporating the filling material for underground treatment according to any one of the above (1) to (5) into a liquid to be flown into a well formed underground.

(7) A method for filling a well wall, including: incorporating the filling material for underground treatment according to any one of the above (1) to (5) into muddy water near the ground where lost circulation is likely to occur.

Advantageous Effects of Invention

According to the filling material for underground treatment of the present invention, the water absorption property is high, so that the polyvinyl alcohol-based resin swollen by water absorption can effectively close gaps such as fractures formed in wells such as drilling holes and trenches, and the dispersibility in muddy water is excellent, so that the workability of filling the drilled part is improved. Therefore, the filling material for underground treatment of the present invention is effective for preventing a lost circulation phenomenon and plugging.

