Oil extraction - Saiyu

Japanese: 採油 - さいゆ

The extraction of crude oil from a reservoir.

[Shozo Tanaka]

Oil drain mechanism

The mechanism by which oil flows in an oil layer is called the oil drainage mechanism or oil drive mechanism. In addition to crude oil, natural gas and salt water exist in an oil layer. Natural gas exists as free gas at the top of the oil layer, and as dissolved gas in the crude oil. The part of the oil layer where free gas exists is called the gas cap. Salt water exists around the oil layer. This salt water is called end water. These natural gas and end water are the energy source that pushes oil from the oil layer toward the oil well. When the oil layer development begins and the oil layer pressure decreases, the dissolved gas in the crude oil separates and expands, and the free gas in the gas cap expands and drives the oil toward the oil well. When the former plays the main role, it is called dissolved gas push type oil drainage, and when the latter plays a significant role, it is called gas cap gas push type oil drainage. End water also penetrates into the oil layer as the oil layer pressure decreases, and flows to the oil well along with the oil. This is called water push type oil drainage. Changes in oil production in oil fields are influenced by these oil drainage mechanisms. Oil fields that are subject to water-driven oil discharge have a small decline in oil production and produce oil at a constant rate over a long period of time. There are many water-driven oil fields in the Middle East. Dissolved gas-driven oil discharge declines the fastest.

[Shozo Tanaka]

Secondary Harvesting

Regardless of the above oil drainage mechanisms, if the reservoir pressure is high, the crude oil will flow to the oil well due to the inherent energy of the reservoir, but as the reservoir pressure decreases, the amount of oil produced will decrease, and eventually the oil field will no longer be economically viable. Even at this stage, a large amount of oil remains in the reservoir. Recovery rates vary depending on the oil drainage mechanism, with estimates of 5-30% for the dissolved gas push type, 20-40% for the gas cap gas push type, and 35-70% for the water push type. Secondary recovery is the process of recovering this remaining oil. The previous stage, in which oil is extracted using the reservoir's inherent energy, is called primary recovery.

The most widely used method for secondary recovery is water flooding. Water flooding is a method in which water is injected from the surface into the oil reservoir through a water injection well, maintaining the reservoir pressure and forcing the oil into the oil well by the action of the water. Water flooding has been widely used in the United States since the 1950s and has spread throughout the world. At first, it was used in oil fields where the reservoir pressure had been depleted, but recently, water injection has been actively carried out at the early stage of oil field development to prevent the reservoir pressure from decreasing even when the reservoir pressure is high. Water flooding is also carried out in large oil fields in the former Soviet Union, the Middle East, and the North Sea, and water flooding plans are incorporated into oil field development plans from the beginning. Under such circumstances, the distinction between primary and secondary recovery is meaningless, but the term secondary recovery is still used. In addition to water flooding, there is also gas injection as a secondary recovery method. In this method, natural gas produced from the oil reservoir is injected back into the top of the oil reservoir, preventing the reservoir pressure from decreasing and increasing the recovery rate. In order to prevent the waste of natural gas resources and replenish oil reservoir energy, there is a trend toward banning the release of gas, which is a glamorous sight in oil fields, and making gas injection mandatory.

Even after secondary recovery, a large amount of oil still remains. In an experiment in which oil in an oil reservoir was pushed out with water, the amount of oil remaining in the pores was reduced to 20-30% of the original amount due to the displacement of the water, but it is unlikely that oil would be extracted this efficiently in an actual oil reservoir. This led to the idea of recovering the oil remaining after secondary recovery. This is called tertiary recovery, or increased recovery method.

[Shozo Tanaka]

Third harvest

As oil prices have risen, oil extraction has become economically viable even with higher oil extraction costs, and as a way to increase oil resources, tertiary oil extraction has been actively applied to oil fields and laboratory research has become more common. Tertiary oil extraction methods include thermal oil extraction, chemical flooding, and gas miscible oil extraction. Thermal oil extraction methods include steam injection and fire flooding, which injects air into the oil reservoir to burn the oil and use the resulting energy. Steam injection is widely used in heavy oil reservoirs in California, where the oil reservoir is heated to reduce the viscosity of the oil, and the steam is used to push the oil into the oil well. Chemical flooding is a method of increasing oil production by injecting a surfactant solution or an alkaline solution into the oil reservoir. Gas miscible oil extraction is a method of injecting a hydrocarbon gas or carbon dioxide into the oil reservoir to create an intermediate phase between the oil and gas in which both are well soluble, thereby increasing the oil extraction rate.

[Shozo Tanaka]

Oil well

From the primary to tertiary stage of recovery, oil in the oil reservoir flows into the oil well due to the action of natural gas and water, and is produced through the oil well to the surface. A pipe called casing is inserted into the hole drilled to the oil reservoir, and the surrounding area is hardened with cement. A drilling device called a gun perforator is used to drill holes in the oil reservoir part of the oil well with gunpowder or bullets, connecting the oil well to the oil reservoir. A pipe called tubing is inserted into the oil well, and the oil flows out to the surface through the tubing. The wellhead device of an oil well is also called a Christmas tree. A flow control device called a bean is attached to the wellhead. A bean is a metal rod with a small diameter hole or a special valve for flow control, and it limits the amount of oil produced from the oil well. The oil and gas that flow out of the oil well are separated by a separator. The gas-oil ratio is the amount of gas produced divided by the amount of oil. Crude oil is stored in tanks and natural gas is transported via pipelines to consumption areas.

[Shozo Tanaka]

Oil Restrictions

Oil in the oil reservoir is pushed into the oil well by gas and water, but gas and water are less viscous than oil and flow more easily, so if the outlet of the wellhead is opened wide and the pressure in the oil well is reduced, the gas and water will bypass the oil and flow through the oil reservoir, reducing the oil recovery rate. For this reason, a bean is attached to the wellhead to lower the gas-oil ratio and limit the amount of oil produced. If the wellhead valve is opened wide in an attempt to increase crude oil production in an oil field, oil production will stop early.

[Shozo Tanaka]

Oil Extraction Method

At the beginning of oil field development, when the reservoir pressure is high, the oil well will naturally gush out. As the reservoir pressure decreases, the oil flow becomes intermittent and eventually stops. At this stage, gas lift oil extraction and pump oil extraction are performed. In gas lift oil extraction, compressed gas is injected into the oil well, and the oil is forced out by the expanding force of the gas. In pump oil extraction, an oil extraction pump is attached to the bottom end of the tubing, and the pump's plunger is raised and lowered by a long, thin iron rod called a sucker rod lowered below the surface to extract oil.

[Shozo Tanaka]

Well Stimulation

To increase the oil production of an oil well, it is necessary to increase the permeability of the oil reservoir rocks surrounding the well. Well stimulation methods are used to treat oil wells to increase the production of oil, and the main methods are acid treatment and hydraulic fracturing. Acid treatment is a method of treating limestone oil reservoirs with an acid solution mainly composed of hydrochloric acid, and is widely used in various places, including the Middle East, where there are many limestone oil reservoirs. Hydraulic fracturing is a process in which a well is filled with liquid to apply pressure, creating vertical or horizontal cracks in the strata, which are then filled with gravel. Although not a direct oil-increasing method, gravel filling is also widely used, in which gravel is filled on the outside of the well casing to prevent sand from flowing into the well from the oil reservoir and prevent accidents during oil well production.

[Shozo Tanaka]

Well production capacity

Each oil well has a specific production capacity. In large oil fields in the Middle East, a single well produces more than 1,000 kiloliters of crude oil per day, but some oil wells produce less than 1 kiloliter per day even at the beginning of the field development. The production index is an index that indicates the production capacity of such an oil well. The production index is the amount of oil produced divided by the difference between the pressure of the oil reservoir and the pressure of the oil well. Because there are differences in the production index of oil wells, it is said that while 60 oil wells are sufficient in Mexico to produce about 1 million barrels of oil per day, 6,000 wells are needed in the former Soviet Union and 56,000 wells are needed in the United States.

[Shozo Tanaka]

Well Testing

To test the production capacity of an oil well, a casing is inserted into the well, the well is finished, and then a test oil is run, followed by a period of oil extraction to determine the stable oil production volume. With this method, when the oil production volume is low, losses will occur due to inserting the casing, so a commonly used method is to attach a device called a formation tester to the bottom of the drilling pipe, lower it into the open well, and test the oil production volume. This test is called a drill pipe test, or DST. The oil that is spilled to the surface during the DST is burned with a special burner, and the flames give the impression that the oil well is successful.

[Shozo Tanaka]

Offshore oil production

When an offshore oil field is discovered, oil extraction equipment is installed on a platform, and the crude oil and natural gas produced are transported to land via an undersea pipeline. If the oil field is small, such a large-scale method cannot be carried out economically. Therefore, efforts are being made to reduce costs by using floating production facilities to avoid the need to build a platform, or by completing wells on the seabed. As there is an urgent need to develop marginal oil fields, which are at the limit of profitability, in the future, research into oil extraction equipment for small-scale offshore oil fields is expected to become more active.

[Shozo Tanaka]

Oil mining

Research is being conducted into tunneling, a method of oil extraction in which oil-containing rocks are dry-distilled on the surface by digging tunnels into the oil layer, and the oil that seeps out of the tunnels is pumped up. In Japan, a 9,852-meter tunnel was dug into the oil layer at Higashiyama Oil Field in Niigata Prefecture in 1939 (Showa 14). However, the implementation of tunneling is a future issue.

[Shozo Tanaka]

Oil Well Equipment
©Shogakukan ">

Oil Well Equipment

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

油層から原油を採取することをいう。

［田中正三］

排油機構

油が油層内を流れる機構を排油機構または油の駆動機構という。油層内には原油のほかに天然ガスと塩水が存在する。天然ガスは油層上部に遊離ガスとして、また原油中に溶解ガスとして存在する。遊離ガスの存在する油層部分をガスキャップという。塩水は油層周囲に存在する。この塩水を端水(はすい)という。これら天然ガスと端水が油層から石油井へ向かって油を押し流していくエネルギー源である。油層開発が始まり油層圧力が低下すると、原油中の溶解ガスは分離膨張し、ガスキャップ中の遊離ガスは膨張し油を石油井へ駆動していく。前者が主要な作用をするときは溶解ガス押し型排油、後者の働きが著しいときはガスキャップガス押し型排油という。端水も油層圧力低下とともに油層へ浸入してきて、油を伴って石油井へ流れる。これを水押し型排油という。油田の産油量の変化はこれら排油機構により左右される。水押し排油を受ける油田の産油量は減退が小さく、長期にわたり一定のレートで産油する。中東の油田には水押し型油田が多い。溶解ガス押し型排油はもっとも減退が早い。

［田中正三］

二次採収

以上のどの排油機構であっても、油層圧力が大きければ、原油は油層が本来もつエネルギーにより石油井へ流れていくが、油層圧力の低下とともに産油量は減少し、ついには経済的に油田が維持できなくなる。この段階でも油層には多量の油が残留している。採収率は排油機構により異なり、溶解ガス押し型は5～30％、ガスキャップガス押し型は20～40％、水押し型は35～70％と推定されている。この残留している油を採収しようというのが二次採収である。その前の油層本来のエネルギーで油を採収する段階を一次採収という。

　二次採収でもっとも広く採用されている方法が水攻法である。水攻法は、地表より油層へ水圧入井を通して水を圧入し、油層圧力を維持し、水の作用で油を石油井へ押し流していく方法である。水攻法はアメリカで1950年代から広く採用されるようになり、世界中に広まっていった。初めのうちは油層圧力が枯渇した油田で採用されていたが、最近では油層圧力が高くとも、油層圧力の減退を防ぐため積極的に油田開発の初期の段階で水圧入が実施されるようになった。旧ソ連地域、中東、北海などの大油田でも水攻法が実施されており、油田開発計画のなかには当初から水攻法の計画が織り込まれている。かかる状況下では一次採収と二次採収の区別は意味がないが、二次採収という用語は依然として用いられている。二次採収として水攻法のほかにガス圧入法がある。この方法は油層から産出した天然ガスをふたたび油層頂部に圧入し、油層圧力の減退を防ぎ採収率を増加させる方法である。天然ガス資源の浪費を防ぎ、油層エネルギー補給のため、油田の華やかな景観であるガスの放散燃焼を禁止し、ガス圧入を義務として実施する趨勢(すうせい)にある。

　二次採収を実施してもまだ多量の油が残留する。油層中の油を水で押す実験をすると、水の置換により孔隙(こうげき)中に残る油は、当初の20～30％に下がるが、実際の油層ではこのように効率よく採収されるとは考えられない。そこで二次採収後残留した油の採収が考えられるようになった。これを三次採収あるいは採収率増加法という。

［田中正三］

三次採収

石油価格の上昇により、石油の採収費が高くなっても経済的に採算がとれるようになったことと、石油資源の増加を図る方法として、三次採収の油田への適用と実験室の研究は盛んに行われるようになった。三次採収法として熱採収法、化学攻法およびガスミシブル法がある。熱採収法には水蒸気圧入と、油層に空気を圧入し、油を燃焼させそのエネルギーを利用する火攻法がある。水蒸気圧入は、カリフォルニアの重質油層などに広く適用され、油層を加熱し油の粘度を下げ、水蒸気で油を石油井へ押し流していく。化学攻法は、界面活性剤の溶液やアルカリ溶液を油層へ圧入し増油を図る方法である。ガスミシブル法は、炭化水素系のガスや炭酸ガスを油層へ圧入し、油とガスの間に油やガスの両方ともによく溶解しあう中間相をつくり採収率をあげることを目的としている。

［田中正三］

石油井

一次採収から三次採収まで、油層中の油は天然ガスや水の作用により石油井へ流入し、石油井を通って地表へ産出する。油層まで掘削された孔にケーシングとよばれるパイプが挿入され、その周囲はセメントで固められる。石油井の油層部分は、ガンパーホレータとよばれる穿孔(せんこう)装置で火薬や弾丸で孔をあけ、石油井と油層とを連結する。石油井にはチュービングとよぶパイプを挿入し、油はチュービングを通って地表へ流出する。石油井の坑口装置を別名クリスマスツリーという。坑口にはビーンとよぶ流量調節用の器具を取り付ける。ビーンは金属棒に小口径の孔をあけたものか、流量調節用の特殊なバルブで、石油井の産油量を制限している。石油井から流出した油とガスはセパレーターで分離される。産出したガス量を油量で割った値をガス油比という。原油はタンクに蓄えられ、天然ガスはパイプラインで消費地へ送られる。

［田中正三］

採油制限

油層中の油はガスや水により押し流されて石油井へ流入するが、ガスや水は油より粘性が低く流れやすいため、坑口装置の流出口を広く開き石油井の圧力を低くすると、ガスや水は油を押さずバイパスして油層を流れてしまうので油層の採収率は悪くなる。このため坑口にビーンを取り付け、ガス油比を低くし、産油量を制限して採油が行われている。油田で原油の増産をしようとして、坑口のバルブを広く開くと、油の産出が早期に止まってしまう事態がおこる。

［田中正三］

採油法

油田開発の当初、油層圧力が高いと石油井は自噴する。油層圧力が低下するにつれ、油の流出は間欠的となり、ついには停止する。この段階でガスリフト採油や、ポンプ採油が行われる。ガスリフト採油は石油井へ圧縮ガスを圧入し、ガスの膨張する力で油を流出させる。ポンプ採油は、チュービングの下端に採油ポンプを取り付け、ポンプのプランジャーを地表より降ろしたサッカーロッドとよぶ細長い鉄棒で上下させて採油する。

［田中正三］

坑井刺激法

石油井の産油量を増加させるには、坑井周囲の油層岩石の浸透性を大きくする必要がある。増油のための坑井処理を坑井刺激法といい、酸処理と水圧破砕法がおもな方法である。酸処理は、石灰岩でできている油層を塩酸を主成分とする酸溶液で処理する方法で、石灰岩の油層が多い中東をはじめ各地で広く用いられている。水圧破砕は坑井に液体を満たして圧力を加え、地層に垂直または水平の亀裂(きれつ)をつくり、そこに砂利を充填(じゅうてん)する作業である。直接的な増油法ではないが、坑井のケーシングの外側に砂利を充填し、油層から坑井への砂の流入を防ぎ、石油井の採油中の事故防止をする砂利充填法も広く用いられている方法である。

［田中正三］

坑井の産出能力

石油井はそれぞれ特定の産出能力をもっている。中東の大油田では1本の石油井から1日に1000キロリットル以上の原油を産出しているが、油田開発当初でも産油量が1日1キロリットル以下という石油井もある。かかる石油井の産出能力を表す指数が産出指数である。産出指数は、産油量を油層の圧力と石油井の圧力の差で割った値である。石油井の産出指数に差があるため、1日当り約100万バレルの産油量を得るのに、メキシコでは60本の石油井で足りるが、旧ソ連地域では6000本、アメリカでは5万6000本必要であるともいわれている。

［田中正三］

坑井テスト

石油井の産出能力のテストのためには、坑井にケーシングを挿入し仕上げしてから試油を行い、ある一定期間採油して、安定してとれる産油量を決定する。この方法では、産油量が少ないとき、ケーシング挿入などによる損失を受けるので、掘管(ほりかん)の下端にフォーメーションテスターとよぶ器具を取り付け、裸坑に降ろし、産油量を試験する方法が多く用いられている。このテストを掘管テストあるいはDSTという。DSTで地表へ流出した油は特殊なバーナーで燃やし、その炎は石油井の成功を印象づける。

［田中正三］

海底油田の採油

海底油田が発見されると、プラットホームの上に採油設備が設置され、産出した原油と天然ガスは海底パイプラインで陸上へ送られる。油田規模が小さいと、かかる大規模な方法は経済的に実施できない。そこで、浮遊式の生産施設を用いてプラットホームの建設を省略したり、坑井を海底仕上げするなどの方法でコストの減少を図る。今後マージナル油田とよばれる採算限界油田の開発が急務になっているので、小規模海底油田の採油装置の研究は盛んになると思われる。

［田中正三］