Студопедия  
Главная страница | Контакты | Случайная страница

АвтомобилиАстрономияБиологияГеографияДом и садДругие языкиДругоеИнформатика
ИсторияКультураЛитератураЛогикаМатематикаМедицинаМеталлургияМеханика
ОбразованиеОхрана трудаПедагогикаПолитикаПравоПсихологияРелигияРиторика
СоциологияСпортСтроительствоТехнологияТуризмФизикаФилософияФинансы
ХимияЧерчениеЭкологияЭкономикаЭлектроника

Reservoir Rocks and Hydrocarbon Migration

 

Wells drilled into source rocks do not yield much oil because kerogen can’t flow easily from the rock into the well. Thus, any organic matter in an oil shale remains trapped among the grains and can’t move easily. So to obtain oil, companies drill instead into reservoir rocks, rocks that contain (or could contain) an abundant amount of easily accessible oil and gas, meaning hydrocarbons that can be extracted out of the ground.

To be a reservoir rock, a body of rock must have space in which the oil or gas can reside, and must have channels through which the oil or gas can move. The space can be in the form of open spaces, or pores, between clastic grains (which exist because the grains didn’t fit together tightly and because cement didn’t fill all the spaces during cementation) or in the form of cracks and fractures that developed after the rock formed. In some cases, groundwater passing through rock dissolves minerals and creates new space.

Porosity refers to the amount of open space in a rock (Fig. 6). Oil or gas can fill porosity just as water fills the holes in a sponge. Not all rocks have the same porosity. For example, shale typically has a porosity of 10%, while sandstone has a porosity of 35%. That means that about a third of a block of sandstone actually consists of open space.

FIGURE 6.14.7 The porosity and permeability of a sedimentary rock depend on the character of the rock. For example, poorly cemented sandstone can have high porosity and permeability, whereas well-cemented sandstone does not. Shale tends to be impermeable and to have low porosity, and the porosity in limestone is commonly due to the presence of cracks. Rocks with high porosity and permeability make the best reservoir rocks. Not only can such rocks hold a lot of oil, but the oil can also flow relatively easily and thus can be pumped out efficiently.

Permeability refers to the degree to which pore spaces connect to each other. Even if a rock has high porosity, it is not necessarily permeable (Fig. 6). In a permeable rock, the holes and cracks (pores) are linked, so a fluid is able to flow slowly through the rock, following a tortuous pathway.

Keeping the concepts of porosity and permeability in mind, we can see that a poorly cemented sandstone makes a good reservoir rock, because it is both porous and permeable. A highly fractured rock can be porous and permeable, even if there is no pore space between individual grains. The greater the porosity, the greater the capacity of a reservoir rock to hold oil; and the greater the rock’s permeability, the easier it is for the oil to be extracted.

In an oil well, which is simply a hole drilled into the ground to where it penetrates reservoir rock, oil flows from the permeable reservoir rock into the well and then up to the ground surface. If the oil in the rock is under natural pressure, it may move by itself, but usually producers must set up a pump literally to suck the oil up and out of the hole.

To fill the pores of a reservoir rock, oil and gas must first migrate (move) from the source rock into a reservoir rock, which they will do over millions of years of geologic time (Fig. 7). Why do hydrocarbons migrate? Oil and gas are less dense than water, so they try to rise toward the Earth’s surface to get above groundwater, just as salad oil rises above the vinegar in a bottle of salad dressing. Natural gas, being less dense, ends up floating above oil. In other words, buoyancy drives oil and gas upward. Typically, a hydrocarbon system must have a good migration pathway, such as a set of permeable fractures, in order for large volumes of hydrocarbons to move.

FIGURE 7. Initially, oil resides in the source rock. Oil gradually migrates out of the source rock and rises into the overlying water-saturated reservoir rock. Oil rises because it is buoyant relative to water—it tries to float on water. The oil is trapped beneath a seal rock. If natural gas exists, it floats to the top of the oil.




Дата добавления: 2015-09-12; просмотров: 242 | Поможем написать вашу работу | Нарушение авторских прав

ENERGY RESOURCES | SOURCES OF ENERGY IN THE EARTH SYSTEM | What Are Oil and Gas? | Types of Oil and Gas Traps | Task 1. Skim through the text and discuss the main stages of the American oil industry development. | THE MODERN SEARCH FOR OIL | Task 2. Read the text and discuss the following questions. | CAN OTHER ENERGY SOURCES MEET THE NEED? |


lektsii.net - Лекции.Нет - 2014-2024 год. (0.006 сек.) Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав