книги / Методы и технологии добычи нефти и газа

where – dynamic viscosity of fluid.

Later Darcy carried out multiple experiments on a pipe filled with different rocks by pumping fluids of different viscosity through them. As a result of these experiments, Darcy discovered that different fluids move in different rocks at a different speed. Into equation (1), he inserted proportionality factor K, later called by him as “permeability coefficient”. Equation (1) assumed the form: V = K ∆P / µ L , where L is the

pipe length.

Since the fluid speed in the pipe can be determined through a known method, having divided the volume of flowing liquid by the pipe cross-section area, then

V = Q/F = K ∆P / L or Q/F = K ∆P / L. From the last equation, we find the per-

In an oil formation, fluid is driven from the pool outline to the well by means of different kinds of energy. Several types of drive can be distinguished. The most efficient is a water drive, when the oil-bearing formation crops out at an elevation higher than that of the well location (see fig. 1). If in the upper part of the oil-bearing formation there is some free gas called a gas cap, then oil is driven toward the well due to gas expansion, i.e., under the gas pressure acting on the oil part of the formation. Practically all oil deposits contain a certain amount of gas dissolved in the oil; in some deposits, it is smaller, in the other – greater. With the pressure in the bottomhole zone decreasing below the saturation pressure (the pressure at which the first gas bubbles begin emitting from the oil), gas forces out oil towards the bottomhole. Such a drive is called the dissolved gas drive. Energy is also stored in the elastic system of rock, oil, and underlying water. As the pressure in the bottomhole zone decreases, the rock, oil, and water are expanding, thereby increasing their volume and forcing out the oil toward the bottomhole. This phenomenon is called an elastic drive. Sometimes, the elastic energy of formation water drive system reaches great magnitudes. If to symbolize the specific elastic capacity of a deposit by β , then the total elastic capacity of the deposit at a pressure drop in it ∆ P on the average will be

40

equal to ∆ Vf = βV ∆ P, where V is the volume of the deposit. The value of specific elastic capacity β is called the elastic capacity coefficient. It depends on elasticity of rock, fluid, and on porosity: β= m βf + δr , where m is the porosity factor of rock. If the kinds of energy listed above are not available in the formation, then the energy of gravity will be acting, i.e., gravity drive. However, in practice the action of only one drive is hardly possible. Depending on conditions, different drives will be acting simultaneously.

Scientist Dupuye, whom we mentioned above, substantiated the theory of fluid inflow from the pool outline to the well (see fig. 2). By transforming equation

(2) relative to Q (volume of fluid): Q = K F ∆ P/µL. By inserting the indices according to fig. 2, we obtain Q = K 2 πr h ∆ P / x dP / dr; from this equation, we find dP = Q µ / K 2 π h x dr/r. After taking the logarithm of both halves of the equality, we shall obtain a differential equation of fluid inflow towards the well: Q = 2 πK h (Ppool – Pbh) µln Rk/rc.

Thus, with the help of Darcy’s rectilinear law and Dupuye’s differential equation we have substantiated theoretically the movement of fluid over an oil formation towards a bottomhole.

The oil is to rise from the bottomhole to the wellhead at the expense of intrinsic energy of the formation of by the energy applied from the surface, i.e., by flowing or by lifting.

Flowing can take place at the expense of a hydrostatic column of fluid in the well and at the expense of dissolved gas; in the first instance, flowing is called artesian on condition that Pwh ≥ Psat. Flowing at the expense of gas dissolved in oil is called a gas-lift flowing, which can take place under two conditions: the first condition, when the saturation pressure is less than the bottomhole pressure, but greater than the wellhead one (see fig. 3, Pwh – wellhead pressure, Psat – saturation pressure,

Pbh – bottomhole pressure).

41

2. INCREASING THE PRODUCTIVE CAPACITY OF WELLS

Productive capacity of wells depends on a great number of factors and measures. First of all, it depends on formation characteristics, formation-pressure maintenance system, initial or secondary drilling-in of oilor gas-bearing formation, etc. In close-grained collectors, the inflow of fluid and gas to wells is very small, in spite of large depression. Therefore, first of all, it is necessary to increase the permeability of rocks, making up an oilor gas-bearing formation. In good collectors, there is a great probability of natural permeability decrease at initial and secondary drilling-in of oiland gas-bearing formations. There is a sufficiently great number of stimulation methods, but they are not always efficient. Among these are chemical, physical, biological, and combination methods.

Chemical methods are based on chemical action of acids on rocks and cement, binding these rocks. For carbonate rocks, treatment with hydrochloric acid is very efficient. As a result of reaction, the existing passages are increased and new passages are formed. In addition to hydrochloric acid, other acids such as hydrofluoric, sulfanic, and acetic are employed.

Of wide application at oil and gas fields are physical methods. Among these are: mechanical, thermal, wave, and others. At present, the most efficient method of acting on the bottomhole zone is a hydraulic fracturing of formation. On the average, productive capacity of wells at the Russian oil fields has increased by 10 times thanks to this method. But this method is very expensive. At the chair, there is a computer program for carrying out a hydraulic fracturing of formation and for hydrochloric acid treatment of the bottomhole zone. During practical training classes, you will be able to carry out these operations by yourself. Good results were obtained by drilling side holes from the worked-out wells. A method of secondary drilling-in of oil-bearing formations by use of flexible perforators with penetration into the formation by 15 to 20 m is being introduced. Unfortunately, the drilling and operating enterprises very seldom employ nowadays a hydraulic jet perforating which gives good results as compared with other methods of secondary drilling-in of oiland gas-bearing formations.

42

At oil fields with highly viscous oils it is expedient to employ thermal treatment of the bottomhole zone. It is possible to pump different heat carriers and electric heaters into the formation. Undeservingly, a very efficient method of thermal- gas-chemical treatment worked out by the Perm specialists was neglected.

A wave or vibration method of formation treatment has not found a wide application either. Oscillations of low-frequency band (from 20 to 300 Hz) in combination with jet pumps can produce good results both in development of new wells and in case of wells with degrading bottomhole zone.

The oil producing enterprises widely employ now different solvents for dissolving solid sediments in pores.

Unfortunately, biological treatment of oil-bearing formations for increasing the oil recovery ratio is not employed. In different countries, a great number of species of bacteria have been obtained, decreasing the viscosity of oil. Bacteria can be introduced into the formation through the formation-pressure maintenance system or directly into the oil-producing wells. In the latter instance, bacteria help to prevent deposition of paraffin in the producing string.

In practice, the above-said methods should be employed in combination.

Of great importance for increasing the productive capacity of wells is the proper selection of oil production equipment. This applies first of all to wells with a low production rate.

43

Fig. 1. Diagram of sheet deposit

44

Fig. 2. Fluid inflow

Fig. 3. Type of flowing wells:

1.Artesian flowing

2.Gas-lift flowing at Pwh < Psat, Pbh > Psat

3.Gas-lift flowing at Pbh , Psat

45

Ivan R. Yushkov

ENHANCED OIL RECOVERY

1. INTRODUCTION

In consequence of oil wells operation not all oil resources are recovered, but only part of them. During oil fields development final oil recovery makes 0.4–0.5 of oil accumulations in carbonate reservoirs and 0.4–0.8 – in terrigenous reservoirs. This effect is achieved under the most favorable conditions (poor oil viscosity, high permeability, beds homogeneity, flooding pattern management, dense well spacing etc.).

At least 0.1 (10 %) enhanced oil recovery can lead to significant oil production increase and significant economic performances increase. It can have great influence on producing fields, where there are systems of oil gathering and treatment, reservoir-pressure maintenance, roads, electric power lines, communication systems etc.

One of the ways to increase oil recovery factor is application of new methods of advanced recovery.

Experience of implementation of methods of advanced recovery shows that their efficiency depends on adequate selection of the method for specific conditions of the field. The factors are divided into three main groups:

-reservoir characterizations (oil viscosity, in-place permeability, seam depth, its thickness, homogeneity, oil saturation, reservoir pressure etc.);

-technological parameters (injected agent, its concentration, bank dimensions, well spacing, mining system etc.);

-engineering data (provision of facilities, equipment, their quality, availability and disposition of sources of raw materials (agents), well stock condition, climate conditions etc.).

46

Some specific criteria of methods of advanced recovery were developed on the basis of laboratory investigations, pilot tests and field trials. They are classically divided into three groups:

-physicochemical characteristics (water flooding with drilling mud surfactant, water flooding with polymer viscosifiers, acid, alkalies and other agents injection etc.);

-methods of miscible displacement (use of carbon dioxide (СО2), hydrocarbon gas, water-gas mixture, micellar solutions etc.);

-hot-wire methods (hot water injection, steam injection, wet in-situ burning).

2. PHYSICOCHEMICAL METHODS OF ENHANCED

OIL RECOVERY

2.1. Water and drilling mud surfactant solutions injection

Water and drilling mud surfactant solutions, which are injected into the reservoir, have multilateral influence on physicochemical properties of reservoir systems. Even their small concentration lead to significant water boundary tension decrease on the boundary of oil and hard surface (σ), in the result oil is swept from porous medium more thoroughly. Drilling mud surfactant facilitates oil globule fracturing, surrounded by water, decrease pressure differential for liquid filtration in porous medium, improve water detergent properties.

Drilling mud surfactant influence on pores surface wettability by reservoir fluids: decrease of wettability angle (θ), intensity of water capillary penetration into oil saturation rock. As a result oil droplets, which are adhered to rock, are washed out.

Operating procedure: drilling mud surfactant concentration in the injected water solution makes 0.05 %; volume of solution bank makes – 50–100 %. It is expected that oil production will be increased 10–15 % (5–10 %) in the consequence of solution injection.

Drilling mud surfactant solutions were injected in Arlan, Tuymazin, Romashkin, Shagirtsko-Gozhanskoe and other oil fields.

47

The process is monitored both in production and injection wells. Concentration of solution is measured during injection into injection and production wells. Input profile is measured in injection and production wells. Water encroachment into the gas and oil and other parameters are also measured.

2.2. Injection of water polymer solutions

The essence of polymer flooding method is to balance oil mobility and driving medium displacement agent to increase the area of stimulation of formation. Injection of polymer solutions into productive formations changes hydrodynamic performances of the development target. It causes activation of interlayers, which don’t participate in the process during common flooding. Mechanism of polymer solutions operation consists in water (driving medium displacement agent) mobility decrease.

Flow pattern of water polymer solutions in porous medium can be different. At that, it as well as flow resistance factor is defined by injection speed, polymer concentration in the solution, temperature and filtration characteristics of rocks. Polymer adsorption in porous medium contributes to the efficiency upgrading of the method.

Processes of cation-exchange and physicochemical characteristics of the surface also have significant influence.

Liquid mobility (oil and water) in reservoir conditions is conditionally expressed by relation of permeability to phase for a particular liquid and its viscosity. Oil production depends greatly on oil and water mobility M ratio.

М=Кв/µв : Кн/µн,

where К – permeability to phase of water and oil relatively; µ – dynamic viscosity of water and oil relatively.

We can’t influence on permeability to phase at this development stage, though it changes during the development operations. Increase in water encroachment into well production causes increase in water permeability to phase and decrease in oil permeability to phase. The formula can be modified:

М=µн/µв

48