Fundamentals of Finding and Producing Oil & Gas in the Illinois Basin
Hydrocarbons – crude oil and natural gas – are found in certain layers of rock that are usually buried deep beneath the surface of the earth. In order for a rock layer to qualify as a good source of hydrocarbons, it must meet several criteria.
Characteristics of Reservoir Rock
Figure 1: Porosity of Rock
For one thing, good reservoir rocks (a reservoir is a formation that contains hydrocarbons) have porosity. Porosity is a measure of the openings in a rock, openings in which petroleum can exist. Even though a reservoir rock looks solid to the naked eye, a microscopic examination reveals the existence of tiny openings in the rock. These openings are called pores. Thus a rock with pores is said to be porous and is said to have porosity (Figure 1).
Another characteristic of reservoir rock is that it must be permeable. That is, the pores of the rock must be connected together so that hydrocarbons can move from one pore to another (Figure 2). Unless hydrocarbons can move and flow from pore to pore, the hydrocarbons remain locked in place and cannot flow into a well. In addition to porosity and permeability reservoir rocks must also exist in a very special way. To understand how, it is necessary to cross the time barrier and take an imaginary trip back into the very ancient past.
Figure 2: Permeability of Rock
Imagine standing on the shore of an ancient sea, millions of years ago. A small distance from the shore, perhaps a dinosaur crashes through a jungle of leafy tree ferns, while in the air, flying reptiles dive and soar after giant dragonflies. In contrast to the hustle and bustle on land and in the air, the surface of the sea appears very quiet. Yet, the quiet surface condition is deceptive. A look below the surface reveals that life and death occur constantly in the blue depths of the sea. Countless millions of tiny microscopic organisms eat, are eaten and die. As they die, their small remains fall as a constant rain of organic matter that accumulates in enormous quantities on the sea floor. There, the remains are mixed in with the ooze and sand that form the ocean bottom.
As the countless millennia march inexorably by, layer upon layer of sediments build up. Those buried the deepest undergo a transition; they are transformed into rock. Also, another transition occurs: changed by heat, by the tremendous weight and pressure of the overlying sediments, and by forces that even today are not fully understood, the organic material in the rock becomes petroleum. But the story is not over.
For, while petroleum was being formed, cataclysmic events were occurring elsewhere. Great earthquakes opened huge cracks, or faults, in the earth’s crust. Layers of rock were folded upward and downward. Molten rock thrust its way upward, displacing surrounding solid beds into a variety of shapes. Vast blocks of earth were shoved upward, dropped downward or moved laterally. Some formations were exposed to wind and water erosion and then once again buried. Gulfs and inlets were surrounded by land, and the resulting inland seas were left to evaporate in the relentless sun. Earth’s very shape had been changed.
Meanwhile, the newly born hydrocarbons lay cradled in their source rocks. But as the great weight of the overlying rocks and sediments pushed downward, the petroleum was forced out of its birthplace. It began to migrate. Seeping through cracks and fissures, oozing through minute connections between the rock grains, petroleum began a journey upward. Indeed, some of it eventually reached the surface where it collected in large pools of tar, there to lie in wait for unsuspecting beasts to stumble into its sticky trap. However, some petroleum did not reach the surface. Instead, its upward migration was stopped by an impervious or impermeable layer of rock. It lay trapped far beneath the surface. It is this petroleum that today’s oilmen seek.
Types of Petroleum Traps
Anticline Trap
Geologists have classified petroleum traps into two basic types: structural traps and stratigraphic traps. Structural traps are traps that are formed because of a deformation in the rock layer that contains the hydrocarbons. Two common examples of structural traps are fault traps and anticlines.
An anticline is an upward fold in the layers of rock, much like an arch in a building. Petroleum migrates into the highest part of the fold, and its escape is prevented by an overlying bed of impermeable rock (A).
A fault trap occurs when the formations on either side of the fault have been moved into a position that prevents further migration of petroleum. For example, an impermeable formation on one side of the fault may have moved opposite the petroleum-bearing formation on the other side of the fault. Further migration of petroleum is prevented by the impermeable layer (B).
Fault Trap
Stratigraphic Trap
Stratigraphic traps are traps that result when the reservoir bed is sealed by other beds or by a change in porosity or permeability within the reservoir bed itself. There are many different kinds of stratigraphic traps. In one type, a tilted or inclined layer of petroleum-bearing rock is cutoff or truncated by an essentially horizontal, impermeable rock layer (C).
Or sometimes a petroleum-bearing formation pinches out; that is, the formation is gradually cut off by an overlying layer. Another stratigraphic trap occurs when a porous and permeable reservoir bed is surrounded by impermeable rock. Still another type occurs when there is a change in porosity and permeability in the reservoir itself. The upper reaches of the reservoir may be impermeable and nonporous, while the lower part is permeable and porous and contains hydrocarbons.
SECURING LEASES
Once a likely area has been selected, the right to drill must be secured before drilling can begin. Securing the right to drill usually involves leasing the mineral rights of the desired property from the owner. The owner may be the owner of all interest in the land, or just the mineral rights. As payment for the right to drill for and extract the oil and gas, the owner will usually be paid a sum call a “lease bonus” or a “hole bonus” for every well drilled on the leased land. He will also retain a royalty on the production, if any, of the leased property. The royalty is the right to receive a certain portion of the production of property, without sharing in the costs incurred in producing the oil, such as drilling, completion, equipping and operating or production costs. The costs are borne by the holder of the right to drill and extract the mineral, which right is usually referred to as the working interest. In many cases the procurement of the lease from the land owner is accomplished by a lease broker who will, in turn, offer and then assign the lease to an operator such as New Spirit, Inc. New Spirit, Inc. is very selective in choosing leases for drilling.
DRILLING
Drilling Rig
Once an area has been selected and the right to drill thereon has been obtained, actual drilling may begin. The most common method of drilling in use today is rotary drilling. Rotary drilling operates on the principle of boring a hole by continuous turning of a bit. The bit is the most important tool. The rest of the rig (a derrick and attendant machinery) is designed to make it effective. While bits vary in design and purpose, one common type consists of a housing and three interlocking movable wheels with sharp teeth, looking something like a cluster of gears. The bit, which is hollow and very heavy, is attached to the drill stem, composed of hollow lengths of pipe leading to the surface. As the hole gets deeper, more lengths of pipe can be added at the top. Almost as important as the bit is the drilling fluid. Although known in the industry as mud, it is actually a prepared chemical compound. The drilling mud is circulated continuously down the drill pipe, through the bit, into the hole and upwards between the hole and the pipe to a surface pit, where it is purified and recycled. The flow of mud removes the cuttings from the hole without removal of the bit, lubricates and cools the bit in the hole, and prevents a blow out which could result if the bit punctured a high pressure formation. (See the drilling rig to the right.)
The cuttings, which are carried up by the drilling mud, are usually continuously tested by the petroleum geologist in order to determine the presence of oil.
DRILLING TO TOTAL DEPTH
Drill Head
The final part of the hole is what the operating company hopes will be the production hole. But before long, the formation of interest (the pay zone, the oil sand, or the formation that is supposed to contain hydrocarbons) is penetrated by the hole. It is now time for a big decision. The question is: Does this well contain enough oil or gas to make it worthwhile to run the final production string of casing and complete the well?
EVALUATING FORMATIONS
Cuttings
Examining Cuttings – To help the operator make his decision, several techniques have been developed. One thing that helps indicate whether hydrocarbons have been trapped is a thorough examination of the cuttings brought up by the bit. The mud logger or geologist catches cuttings at the flow ditch and by using a microscope or ultraviolet light can see whether oil is in the cuttings.
Well Logging – Another valuable technique is well logging. A logging company is called to the well while the crew trips out all the drill string. Using a portable laboratory, truck-mounted for land rigs, the well loggers lower devices called logging tools into the well on wireline. The tools are lowered all the way to bottom and then reeled slowly back up. As the tools come back up the hole, they are able to measure the properties of the formations they pass. Electric logs measure and record natural and induced electricity in formations. Some logs ping formations with sound and measure and record sound reactions. Radioactivity logs measure and record the effects of natural and induced radiation in the formations. These are only a few of many types of logs available. Since all the logging tools make a record, which resembles a graph or an electrocardiogram (EKG), the records, or logs can be studied and interpreted by an experienced geologist or engineer to indicate not only the existence of oil or gas, but also how much may be there. Computers have made the interpretation of logs much easier.
Coring – In addition to these tests, formation core samples are sometimes taken. Two methods of obtaining cores are frequently used. In one, an assembly called a “core barrel” is made up on the drill string and run to the bottom of the hole. As the core barrel is rotated, it cuts a cylindrical core a few inches in diameter that is received in a tube above the core-cutting bit. A complete round trip is required for each core taken. The second is a sidewall sampler in which a small explosive charge is fired to ram a small cylinder into the wall of the hole. When the tool is pulled out of the hole, the small core samples come out with the tool. Up to thirty of the small samples can be taken at any desired depth. Either type of core can be examined in a laboratory and may reveal much about the nature of the reservoir.