Hydrolyzable resins typified by polylactic acid are excellent in biodegradability and are now under study as substitutes for various plastics in various application fields from the viewpoint of environmental improvement, and some of them have been put to practical use.
In recent years, use of the resins as additives to be added to drilling fluids used to extract underground resources has been proposed (refer to Patent Document 1).
For example, a winze drilling method called “hydraulic fracturing method” is now widely employed to extract underground resources. In this drilling method, a high pressure is applied to a drilling fluid filled in a winze to form cracks (fractures) in the vicinity of the winze so as to improve permeability (ease of the fluid's flow) in the vicinity of the winze and to expand the effective sectional area through which a resource such as oil or gas flows into the winze, thereby increasing the productivity of the winze. This drilling fluid is also called “fracturing fluid”, and a viscous fluid such as gel-like gasoline was used. However, an aqueous dispersion prepared by dissolving or dispersing polymer particles in water has recently been used due to the development of shale gas produced from shale strata existent at relatively shallow sites in consideration of an influence on the environment. A hydrolyzable resin such as polylactic acid is proposed as this polymer.
That is, polylactic acid is a substance which exhibits hydrolyzabibility and biodegradability and is decomposed by water or an enzyme contained in the ground even if it remains in the ground and therefore does not exert a bad influence on the environment. Water used as a dispersion medium can be considered to have almost no influence on the environment as compared with gasoline.
When a drilling fluid containing hydrolyzable resin particles such as polylactic acid dispersed in water is filled in a winze and pressurized, the resin particles permeate an area in the vicinity of the winze and become a sealing material for the formed fractures to temporarily block a flow channel of a resource such as gas or oil effectively. To form new fractures, the fluid is also used as a diverting agent for switching a flow channel of a fracturing fluid. Further, since the fluid is hydrolyzed in the winze and disappears, the hydrolyzable particles do not need to be removed in a post-step, thereby making it possible to carry out the drilling of the winze efficiently.
By the way, a large number of studies have been made on the heat resistance and mechanical properties of a hydrolyzable resin such as polylactic acid but almost no studies are made on its particle shape.
For example, Patent Document 2 discloses a biodegradable resin composition (polylactic acid composition) comprising polylactic acid having a D-isomer content of 2 mass % or less and lamellar silicate. However, Patent Document 2 is aimed to improve the heat resistance and mechanical properties of this polylactic acid composition and does not investigate its particle shape at all.
Patent Document 3 discloses a powder comprising polylactic acid having a crystallinity of 30% or more. This polylactic acid powder has such high mechanical grindability that it can be particulated. However, Patent Document 3 does not investigate its particle shape as well.
Further, the inventors of the present invention proposed a polylactic acid composition which comprises low crystalline or amorphous polylactic acid having a crystallinity of 40% or less as a matrix and has a dispersion structure that an organically modified polysaccharide or swollen or expanded layered silicate salt is dispersed as a grinding accelerator in the matrix (Japanese Patent Application No. 2014-47835).
This polylactic acid composition has advantages that it has excellent mechanical grindability and is easily particulated at a low cost to be usable as an additive for drilling fluids. That is, since an agent to be added to a liquid, such as a drilling fluid which is used in a large quantity is used in a large quantity as a matter of course, the possibility of preparing it as fine particles which are easy to be injected into a liquid such as water and easy to handle at the time of injection at a low cost brings about a great benefit in terms of cost.
Although the mechanical grindability of particles used as an additive for drilling fluids is studied in the above previous application, the particle shape is not investigated at all as well.