An artificial lake surrounded by a grass embankment, with high chain-link fencing around all four sides to keep small boys from swimming in it on hot summer days. This, to the average person, is a reservoir—a place where water is stored in vast quantities for future use. The image is doubtless at least partly responsible for the erroneous impression that oil in its natural state is stored in large underground pools, abysmal, cavelike, their walls dripping with black, oily slime.
While municipal and industrial water reservoirs are clearly visible entities, nobody has ever seen an underground oil reservoir. It is possible, however, to look at and hold in the hand a piece of an oil reservoir. Go out to Saudi Arabia's Ghawar field, far in the desert, or to offshore Safaniya, in the Arabian Gulf, where the Arabian American Oil Company (Aramco) is now drilling. Occasionally the derricks working at these sites bring up long cylindrical samples of the rock the drilling bit encounters a mile or more below the surface. Examine these cores carefully. They tell a lot about what an oil reservoir really is.
First of all, the cylinders look as solid as the stone front of an office building. Viewed through a magnifying glass, they turn out in most cases to be made of millions of sand-like grains. Often their surface, in the language of the trade, "bleeds." If the wetness exuding from inside feels slippery there is a good reason, for these slender columns of rock, only inches in diameter, contain minute quantities of petroleum.
From its nine producing fields, spread over great distances across eastern Saudi Arabia and out into the Gulf, Aramco's crude oil production has been averaging around two million barrels a day. That oil moves from microscopic spaces in porous sandstone and limestone, where it had been resting inert for millennia, to the bottoms of wells and then up thousands of feet to the top. What is more, the oil moves the whole distance from its original habitat to the wellhead without having to be pumped. Taken together, the oil reservoirs in the company's nine producing fields, measured around their outer periphery, cover an area of about 1,300 square miles. Spotted strategically in these fields are about 300 producing wells, each with a well bore no more than seven inches across.
The control of fluid movement in these reservoirs is a formidable challenge to reservoir engineers. They must control the amount of petroleum taken out of vast areas underground, i.e., oil reservoirs, in order to maintain a balance between withdrawal rates and energy pushing the oil to the surface. They have to devise in producing zones means of maintaining pressure, the force which moves all Saudi Arabian oil through and out of a reservoir. They must employ techniques most likely to produce maximum quantities of crude oil with the greatest possible efficiency and economy. In the oil business, as in any other industry founded on technology, a resounding scientific success may turn out to be a thumping failure economically if costs and designs do not receive equal attention.
Reservoir engineering, then, deals with the occurrence and movement of fluids in reservoirs and the development and operation of these reservoirs for maximum economic recovery of oil, gas, or both. Like everybody dealing with an applied science, its practitioners are part physicist and part chemist, able to find their way through the loftier branches of mathematics and feel at home in many fields of engineering related to their specialty. In addition, reservoir engineers must have a firm grounding in geology. Their basic working vocabulary includes such words and phrases as porosity, permeability, viscosity, conformance, capillary forces, interfacial tension, connate water, and the kind of line drives never seen on a baseball field.
The petroleum industry is little more than a century old, its recognized beginnings dating from August, 1859, when "Colonel" Edwin Drake's famed wildcat in Titusville, Pennsylvania, struck oil at 69½ feet. During the first half of the industry's span it was the drillers who held the day, and petroleum production was to a large degree a hit or miss proposition. But the more imaginative oil pioneers sought reasons for noticeable drops in yields, tried unsuccessful gas vacuum pumps to "pull" the oil out of the ground, then experimented with injections of gas and air through adjoining wells into production pay zones to "push" oil to the surface.
Around 1914, drillers on the rigs were joined by petroleum geologists, assigned to analyze samples of subterranean strata as they were brought up by core barrels, and the era of systematic petroleum technology slowly emerged. Drilling bits continued to go deeper, drilling operations became increasingly expensive and the demand for petroleum soared. In the beginning oilmen assumed that when oil stopped coming the sources simply had dried up. They had no idea how much oil was being left behind in the reservoir. To recover these untapped reserves the industry had to know much more about what went on deep in the ground in the vicinity of producing wells. Out of this need grew the science of reservoir engineering, which took shape during the 1930's, came to maturity after World War II, has now gone into partnership with the computer, and may be destined to figure in plans for peaceful uses of nuclear energy.
The aim of reservoir engineers was to place oil production on a scientific basis, and as the initial step they drew on geologists' theories of how petroleum was formed in the first place and their knowledge of formations holding it in the earth today.
Although nobody knows for certain, it is believed that crude oil has as its origin microscopic marine plants and animals which settled and mixed with sand and mud at the bottom of ancient seas. Rivers flowing into the seas carried more organic remains, which covered earlier layers of plankton and marine life until the enormous weight of the accumulated sediments compressed them into porous rock strata. Heat, pressure and bacterial action transformed the organic remains, sealed from decay by successive layers settling on top of them, into hydrocarbon compounds; in other words, petroleum. The newly-formed oil and gas started to bubble its way out of mud and silt and float upward through tiny pore channels. Most of these compounds kept on moving until they were eventually lost in the atmosphere. But some oil, as we shall see, did stay behind.
Meanwhile events of momentous import were happening on top of and beneath the earth's surface. The seas gradually receded, over the space of millions of years, and continents assumed their now-familiar shapes. Internal earth forces caused the outer crust to rise and fall, creating visible mountains and valleys on top, and contours and cracks in the nether regions. The oil being sought and produced by companies such as Aramco accumulated in geological "traps"—layers of porous rock covered by layers of impervious rock—made by buckling and folding of the earth.
Like the hydrocarbons which got away, oil and gas, together with salt water, migrated slowly in the direction of the surface through intricately-connected pore channels. But these compounds had been stopped by clay, shales, chalk and various kinds of dense rock layers overhead, which acted as sealing blankets. The hydrocarbons, being lighter than water, rose farthest. The lighter gas in turn separated out from the heavier oil. Each, contained in porous rock, ultimately came to rest at its own level: the gas on top, oil in the middle and water underneath.
Oil and gas at rest deep in the ground, however, are not what interest petroleum men. It is only when these hydrocarbons are in motion, destined ultimately to rise inside a well bore, that they become commercially attractive. Quite reasonably then, reservoir engineers devote much study to those conditions underground which make movement of oil and gas in storage there possible.
The physical characteristics of reservoir rock which make it a natural storage place for oil and gas are linked to those qualities which permit hydrocarbons to move through the same rock: porosity and permeability. Both can be traced to the fact that sand grains, of which sandstone reservoir rocks are built, and fragments of carbonate material, making up limestone beds, are irregular—they never quite fit together properly. There are tiny voids between the grains and the fragments, occupying up to one-third of the volume of the rock, and it is these voids that contain the underground oil, and give the rock itself the quality of porosity. When the voids in reservoir rock are interconnected and continuous, hydrocarbons under pressure can travel through them, and the rock is said to have permeability. The ease with which fluids flow through successions of pore spaces depends, of course, on the arrangement of the spaces themselves. As in a pipeline, the larger and straighter they are the more efficient channels they make for fluids such as oil.
No where are the lengths to which engineers go in all analyzing oil reservoirs better illustrated than in their detailed study of reservoir rocks for indications of porosity and permeability in oil reservoirs. These two all-important characteristics depend on size and shape of the grains, the manner in which nature has stacked them—evenly, as in a brick wall, or haphazardly as in a rock pile—and the quantities of clay and other materials which cement the sand grains together. All these factors determine the volume and direction of pore space between the grains, and it is these spaces, of course, which are of vital concern to the oilman.
Just as water or oil will not flow through pipe without some kind of force behind them, so fluids require force (pressure) to move through permeable rock. A property of fluids which influences their ability to flow is viscosity: the cohesive forces between the molecules in lube oil, for example, are greater than they are in gasoline. For this reason, lube oil is more difficult to pour. Through mid-19th-century experiments with water niters a Frenchman named Henri Darcy derived a formula to predict how much water would flow through these filters. Oilmen have adapted this formula to measure the ease of fluid movement through rock. Now a reservoir engineer who knows (1) the thickness of a producing interval in reservoir rock, (2) the degree of effective permeability of that rock (stated in darcy units or, more commonly, in millidarcies), (3) the fluid properties within the reservoir, and (4) the difference between pressure in pounds per square inch inside a well and out in the reservoir, has a good indication of how many barrels of oil that well is capable of producing in a day.
In order to predict fluid movements in its reservoirs Aramco has ways of testing reservoir rock for porosity and permeability under laboratory conditions. The samples of rock brought to the surface by core barrels at drilling sites are placed in carefully-labeled trays. They are then carried to the company's Oil Operations Laboratory in Dhahran, where plugs one inch in diameter and about 1½ inches long are cut out. Next the plugs are washed in a flask containing a cleaning solvent to remove all oil and foreign matter in them and then are thoroughly dried in a vacuum oven heated to 230° Fahrenheit.
An apparatus called a porosimeter finds the ratio of the combined volume of all pores in a sample core to its total bulk by holding the clean plug in a small airtight vault and having a charge of nitrogen pushed into it under pressure. Porosity is determined by measuring the amount of the gas entering the pore volume of the core.
Nitrogen under pressure is also used to help discover in the lab the permeability of a core plug. A sample of reservoir rock is placed in the metal sleeve of a permeameter into which hoses have been attached at either end. Valves regulate the flow of nitrogen coursing through the length of the plug. Gauge readings reveal the pressure differential across the plug, and the rate of the flow of gas is given on a calibrated flowmeter.
Too many people base their knowledge of oil production on old movies in which an oil well spouts to the sky, while handsome operators leap with joy as they wipe jet-black petroleum from grinning faces. In the past few decades the oil industry has learned a great deal about forces deep inside a reservoir, and the rare blowouts that do develop nowadays are strictly accidental. Their occurrence, far from being good news, means only tragic waste.
Yet the picture of a "runaway well" does illustrate a most important fact about oil reservoirs: that petroleum in the ground is under enormous pressure and that, when the ground is punctured deep enough and at the right place, that pressure is released, carrying oil up with it. The analogy of the freshly-opened bottle of soda pop is often used to demonstrate basic principles of reservoir forces. Remove a bottle cap and some of the drink pours out uncontrolled. Most of the soda, however, remains in the bottle. As everybody knows, it is the gas still bubbling up through the drink that gives the liquid its big initial spurt.
Likewise, it is stored energy in reservoir rock, including that of the gas associated with crude oil, that drives the oil through a reservoir and up a well. And, as in the case of soda pop, that force tends to dissipate rapidly after the "lid" has been taken off. Oilmen have been continually investigating sources of reservoir energy in order to find the most efficient and economical ways of maintaining the force, or at least controlling its decline.
Approximately nine out of ten oil wells in the United States have huge, ungainly "walking" beam pumps which tilt back and forth seesaw fashion to "lift" petroleum out of the ground mechanically. In contrast, many wells throughout the world, including all those in Saudi Arabia, have nothing more on top of them than a motionless series of valves and fittings about 10 feet high, called a control head, or "Christmas tree." So far no pumps are necessary over Aramco's wells because ever since its first oil well went into production in Dhahran 27 years ago company engineers have maintained a balance between natural withdrawals and available reservoir energy. Pumping must be resorted to at so many oil fields elsewhere because, among other reasons, they were developed before oilmen knew about conservation made possible by sound reservoir engineering practice, and oil field energy was irrevocably lost.
The energy which drives oil to and up a well bore has been stored in compression in all reservoir fluids and in the reservoir rock itself. Production specialists learned the hard way that if pressure in a reservoir is to be kept up, oil taken out of pore spaces must be replaced by water or gas. Further, for maximum recovery, the displacing fluids ought to move through the greatest number of these oil-filled voids. If sufficient replacement does not occur naturally, then water, gas or some other pressure-maintenance agency has to be injected into the reservoir through wells often drilled for this sole purpose.
Because of its relative weight, gas exists in reservoirs above an oil deposit and the water below it. What is more, gas and water generally move through permeable pore spaces more easily than oil does, and without proper management can arrive at the well bore first. Proper balance between well rate and the number of wells producing can encourage oil to move evenly through a producing interval, the desirable goal. As gas permeates downward from above and water upward from below, oil well producing rates are reduced in order to avoid serious trouble. Suction set up by too large a flow can cause water in the lower extremities to form a cone right under the well, with the danger that the water cone could eventually penetrate the well bore and severely restrict oil production.
Assuming that a newly-drilled well has struck a productive formation, oilmen have several ways of making reasonably certain that the hole will produce petroleum instead of water or gas. The most logical method after the well has been drilled is, of course, to control the rate of oil flow at the wellhead by proper adjustment of valves on the "Christmas tree." During the final drilling operation oilmen acquire a good idea how far down an oil-producing formation is likely to be encountered. They always try to complete the well between two impermeable structures to insure production from a single reservoir and at times avoid water or gas entry. Such an ideal situation is not always met, however, but in any case drillers must know what type of oil drive they can expect to meet as their bits grind downward.
There are three, rated according to the relative efficiency with which each kind of drive pushes oil. The least efficient, usually, is a dissolved-gas drive. Pressure in this case comes only from gas dissolved in the oil present in reservoir rock, which seeks to expand as oil is produced.
When there is more gas than can dissolve in oil at temperature and pressure conditions existing in a reservoir, and this gas bubbles to the top of a deposit, it creates conditions for a gas cap drive. The layer of gas up inside the impermeable trap pushes down on the oil to aid in its recovery.
A third type of recovery mechanism, often more efficient than the first two, is known as a water drive. Salt water, originally from ancient seas, migrated into porous channels of rock below and outside the relatively small limits of a structural feature containing oil. As the reservoir is produced the water moves into the oil-bearing rock and flushes petroleum ahead of it.
It happens that most oil-producing reservoirs do not fall into any one of these neat categories but have some combination of the three mechanisms as their driving force. Petroleum in Aramco reservoirs, for example, is moved both by water influx and dissolved-gas drives as primary sources of energy.
Natural phenomena of the kind that are learned in every high school physics class can be made to work on the side of the oil producer who takes advantage of them. As one instance, forces of gravity acting on gas, oil and water, which causes them to separate out from each other because of their differences in density, supplement pressure drives to help increase oil recovery when production rates are properly controlled. As another, capillary action, the reason behind a blotter's ability to absorb ink, aids in the removal of petroleum from smaller, low-permeability pore channels in reservoirs where a water drive is present by drawing oil-displacing water through these tiny channels, just as spaces between granules of a sugar lump soak up coffee.
It has been a long time, however, since oilmen relied entirely on the energy existing in nature to drive petroleum out of the ground. They discovered techniques which could get results similar to those obtained from natural reservoir drives if their introduction made economic sense. They began injecting gas into some reservoirs and flooding others with water. At first they used these secondary-recovery methods to restore reservoirs to life after they had become depleted and indigenous energies were too weak to produce oil by themselves. Later, just as a farmer uses chemical fertilizer to enrich soil, petroleum engineers started injection of water or gas early in a reservoir's life to supplement primary drives before these natural forces had dissipated into uselessness.
For some time Aramco has been maintaining pressure in the reservoirs of its Abqaiq field and at 'Ain Dar, the northern extension of the big Ghawar field, by use of water and gas. Aquifers (underground water-bearing formations) supply the liquid for water flooding, which is either injected at the surface or permeates down into the structure by gravity. The gas injected has been separated out from the oil produced by Aramco in installations called gas-oil separator plants. At 'Ain Dar the gas so recovered is collected and compressed in several stages up to 2,100 pounds per square inch in a plant designed to compress and put back into the ground 160 million cubic feet of gas per day. The entire injection complex installed to achieve this end cost $30 million.
Nearly everybody remembers yesterday's "big game" movies set in some distant place such as India in which lines of native beaters advanced noisily through covering bush to drive a tiger into a trap. The beaters were stationed in close formation so the animal could not escape through the human net. In a similar way, a secondary-recovery "sweep" of water or gas should pass through every rock pore in an oil-bearing structure, leaving no part of the reservoir untouched.
Trying for this optimum objective requires careful planning and study to locate water injection wells in precisely the right sort of pattern. Some such wells are placed in rectangular five-spot, seven-spot or nine-spot patterns, with the injection well in the middle. A line-drive operation is one in which injection wells are placed in rows. Sometimes results of reservoir research recommend locating water injection wells around the edges, and gas injection wells at the crest of a known producing formation. Aramco's pressure-maintenance wells have all been drilled in this so-called contour pattern.
Even with the most scientific placement of injection wells, however, oilmen are still falling short of the maximum desired recovery goal with water flooding and gas repressuring. The industry is constantly experimenting with new and often rather startling techniques for driving oil in rock pores up to the surface.
In reservoirs known to contain heavy oils, thermal recovery methods are beginning to gain considerable acceptance. Essentially, this technique calls for heating a reservoir to something like 600° Fahrenheit to make the oil less viscous and able to flow more easily. Early experiments used hot water or steam as the heating element. In one currently-accepted application of the thermal principle, oil in a producing formation is burned, the fire sustained by blowing compressed air into the structure. The heat not only lowers the viscosity of the oil but forms condensed water vapors and vaporized oils, which together make up a composite water and gas drive.
The oil industry is now even thinking about stimulating production in stubborn petroleum reservoirs by nuclear-explosive fracturing of deep underground structures. The first published report about this possibility appeared in an oil trade magazine in December, 1963. The nuclear explosion as envisioned by petroleum scientists would produce a huge subterranean cavity whose roof would most likely collapse, creating in the fractured rock overhead what would amount to a large-diameter well bore into which reservoir fluids would drain.
All through the life of an oil reservoir, from the moment the decision is made to drill through the entire period it is being produced, the subterranean source of oil is tested, examined and analyzed like some giant, slumbering patient. Geologists studying its structures and environment can derive some idea of its extent, but nobody can be really certain that a reservoir contains oil before that particular reservoir is drilled. Still, the true potentialities of the structures remain unknown until it has been on production for some time and carefully-recorded observations are made of its performance. A history of its yield of oil, gas and water and of its pressure and temperature measurements over an extended period can reveal much about how long a reservoir will produce and give its production potential in total barrels.
By close observations of all its reservoirs Aramco knows at any given time how much oil remains within its fields capable of being produced in the future. Through such means, plus new discoveries, the company is able to say that total proven reserves of its liquid hydrocarbons are climbing steadily upward. Three years ago Aramco stated that it had 45 billion barrels of proven reserves. It is now estimated that it has 59.2 billion barrels.
At least once a year, sometimes quarterly and sometimes semi-annually, everyone of Aramco's producing wells can expect a visit from trained teams of Saudi Arabs assigned out of Abqaiq's Bottom Hole Test Unit. The men send down precision instruments housed in stainless steel cylinders which resemble skinny versions of naval torpedoes. These explosionless "bombs" measure the pressure and temperature at certain specified intervals in reservoirs pierced by the wells. The company also has a number of observation wells where such tests can be conducted without having to shut down production. Charted and graphed, information gathered by the measurement teams gives clear preliminary indications of reservoir performance.
But much more than a rough profile is needed before Aramco is ready to commit capital expenditures of the magnitude required to develop and expand oil fields. It costs an average of 1,300,000 to drill a well on land in Saudi Arabia. Including the platform that is required offshore, a new well in the Arabian Gulf can cost the company up to 1,500,000. Pipelines and necessary supporting facilities are tens and hundreds of times as expensive. With so much real money riding on the decisions of production engineers, truly detailed data are required to determine whether a reservoir's potential justifies, for example, drilling new wells—and where. Then there is the question of how many wells are required to control reservoir pressure distribution. There are so many permutations involved in such decisions these days that oilmen have to develop new formulas and turn to electronic aids to help them arrive at what, hopefully, are the right answers. Data provided by pressure and temperature "bombs," core and fluid samples, porosimeters and permeameters are converted into mathematical representations. The coded numbers are then punched into cards which can be "read" by computers.
The programed information collected in the field and in the laboratory is run through such complex computer programs or electronic equipment as the simulator, which handles mathematical representations of pressure performances and fluid movements; the network analyser, consisting of a series of resistors and condensers able to duplicate electrically variations of pressure performance and fluid movement; and the linear programing model, into which are plugged mathematical codes standing for facilities performances, pressure performances and cost factors for each variable under study.
The names Spindletop, El Dorado and Signal Hill call to mind the romance and lustiness of petroleum's earlier days, when the stakes were climbing and chance played a major role. The cool, scientific approach to production challenges which characterizes the oil industry today has perhaps diminished some of the old excitement. But the more the odds favor the producer the greater the benefits accruing to users of petroleum, and that, in this day and age, includes nearly everybody.
Brainerd S. Bates, a graduate of Brown University, is a former editor of Aramco World and a writer for Aramco's Public Relations Department.
