книги / Организация закачки воды. Система поддержания пластового давления

УДК622.276: 532 + 622.279](075.8) ББК 33.361 + 33.362]я73

Ю 963

Рецензент:

д-р. техн. наук, профессор Г.П. Хижняк

(Пермский национальный исследовательский политехнический университет)

Юшков, И.Р.

Ю 963 Организация закачки воды. Система поддержания пластового давле-

ния : учеб. пособие / И.Р. Юшков, В.Д. Гребнев. – 2-е изд., стереотип. –

Пермь: Изд-во Перм. нац. исслед. политехн. ун-та, 2021. – 44 с. – (На англ. языке).

ISBN 978-5-398-02633-7

Излагаются следующие темы: энергетическое состояние объекта разработки, технологии поддержания пластового давления, подготовка воды для системы подержания пластового давления, оборудование систем поддержания пластового давления и др. Рассчитано для студентов направления 21.03.01 «Нефтегазовое дело», обучающихся в Пермском национальном исследовательском политехническом университете на английском языке.

The following topics are discussed: reservoir pressure, technologies for maintaining reservoir pressure, water preparation for the maintenance system, equipment for maintenance systems, etc. The textbook is destined for the students studying on the educational program 21.03.01 «Oil and Gas Engineering» in English.

УДК 622.276:532 + 622.279](075.8) ББК 33.361 + 33.362]я73

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ing to the formula. The exact assessment of the formation pressure is made by means of depth gages. If the density of liquid or gas in the well is known thus the formation pressure can be calculated.

If a well is drilled in a water zone and the wellbore is filled with water, the bottomhole pressure (the gate is closed) equals to the formation pressure and calculated through:

where Рform and Рhead are formation pressure and wellhead pressure, Pa.

If the well gate is closed the water then will overflow onto the surface, i.e. the well will flow (gush). In the well where the liquid level does not reach the well-

head, the formation pressure is

where Н1 is the liquid column in the well, m.

Formation pressure in the reservoir is usually related to one certain plane surface. This plane surface is usually a sea level or some conditional surface – initial position of the oil and water contact in the reservoir under development. Formation pressure related to this conditional plane surface is called a reduced for-

mation pressure.

If the formation pressure of wells 1, 2, and 3 (fig. 12) is correspondingly equal to Р1, Р2, and Р3, their reduced pressure (in Pa) related to the initial level of the oil and water contact equals to

where x1, x2 and x3 – are the distances from bottomholes to the level of oil and water contact, m; ρo и ρw – density of oil and water, kg/m3; g – acceleration of gravity.

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The changes in formation pressure are scrupulously registered during the exploitation of oil and gas fields. On the basis of factual dimensions of the wells, quarterly isobar charts are produced and weighted average reservoir pressure is assessed. The pressure in sampling and water pumping zones is assessed. Based on the analysis of formation pressure dynamics, the decisions on field development control are made.

Oil reservoir conditions

Depending on the type of energy transferring the liquid or gas to production wells, one can differentiate between various oil reservoir conditions: water drive,

gas drive, dissolved gas drive, gravity drive.

According to experimental and statistical production data, oil recovery fac-

tors depending on reservoir conditions may reach the following values:

Considering the influence of the elastic dilation of liquids and formation on the reservoir operation, one should also consider elastic drive and elastic water drive.

There are often oil deposits that reveal different driving forces simultaneously. Draining conditions of these deposits are called combination drives.

Gas deposit conditions/ Water drive condition

The main source of the reservoir energy under this gas deposit conditions is the pressure of the edge (bottom) water. The conditions for water drive in gas deposits are similar to those conditions in oil deposits.

When the volume of extracted gas is equal to the volume of inflowing water, the formation pressure does not fall and the gas extraction is followed by gradual increase in gas-water contacting.

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If the gas extraction dynamics is increased, the balance between the volumes of extracted gas and water inflowing to the formation can be disturbed; besides the deposit may develop an elastic water drive and gas drive alongside with water drive. Water drive of the gas reservoir is a rare occasion.

Elastic gas-water drive condition

The main source of the reservoir energy under this condition is the elastic water and formation forces as well as forces of the expanding gas.

The influence of elastic water and formation forces is not revealed in the gas deposits at once as far as the deposit formation pressure insignificantly decreases at the initial gas extraction activities. Continuous stable gas extraction leads to the decrease in formation pressure of both the deposit and surrounding lands of the waterbearing reservoir parts. This results in conditions for revealing of elastic water and formation forces. The influence of these forces is directed onto the deposits. Formation waters after penetrating the deposits occupy the emptied formation volume.

Gas drive condition

In the deposits of gas condition, the gas extraction is fulfilled by means of the pressure of extending gas. That is why the gas condition is also called an extending gas condition. This condition is revealed in deposits timed for fully sealed traps that resulted from depositional constraints and tectonic screening. Usually these are small deposits.

The characteristics of the gas drive condition is the formation pressure decrease that is in direct proportion to the gas extraction because the gas drive deposits have no external sources for maintaining of the deposit formation pressure.

Gas recovery factor of the gas and gas-condensate reservoirs is usually bigger than oil recovery factor. Unlike the oil, gases poorly interact with the porous medium surface and they have little viscosity (hundred times and less than viscosity of hi-gravity oil); the compressed gas always has some energy store efficient for

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filtering in the porous medium due to its high elasticity; besides the formation pressure can decrease down to the values close to atmospheric ones. That is why gas recovery of the gas deposits under the gas drive conditions may reach 0,90–0,95, under the water drive conditions – 0,6–0,85. Under the water drive conditions, gas recovery is lower as far as there appears some constraint of the gas resulted from outrunning motions of inflowing and edge waters.

Naturally, the best gas deposits recovery can be obtained through decreasing the formation pressure down the minimum value. That is why, based on the technical and economical considerations, gas development is stopped with the wellhead pressure values above atmospheric ones. In order to calculate the gas resources the wellhead pressure is assumed to be equal to 0,1 MPa.

Liquid and gas influx

During the gas or oil field development the oil or gas radially inflows the wells. The thickness of formation being steady and its structure being homogenous, the rate of liquid (or gas) percolation directed to the well is continuously increasing reaching the maximum on the wellbore walls provided the gas discharge is constant.

When the rated increase, the flow resistance is also increased. Consequently, when the units of liquid (or gas) move to the well, the units of energy consumption per units of distance and related differential pressure per distance (pressure gradients) also constantly increase.

To define the correlation between the well production and differential pressure around the well one can apply Dupuis formula for radial stable influx of the homogenous liquid into the well.

Q= 2πkh(Pпл − Pзаб)

µln Rk

rc

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2. TECHNOLOGICAL DEVELOPMENT

OF OIL DEPOSITS MAINTAINING FORMATION PRESSURE

Natural reservoir energy does not commonly provide high rates of oil withdrawal from the field. Despite the effective water drive draining condition during the field development process the formation pressures start decreasing thus indicating to depletion of reservoir energy. The explanation is that the volume of the inflowing water is usually less than the volume of extracted formation fluids.

Some methods for maintaining of the formation pressure through pumping the water or gas into the producing formation were applied during oil field development activities. Gas should be pumped into the gas cap maintaining the deposit drive condition or the gas cap should be artificially created in the formations where the inclination exceeds 10–15°. Water can be pumped into the formation beyond the oil-drainage boundary, into the boundary zones and into the formations within boundaries. In some cases it is rational to apply the simultaneous force on the formation: pumping both gas and water.

Working substance can be pumped into the formation at the initial stage of the field development. If the elastic reservoir is big, the pumping can be started on a later stage as well.

During edge water flooding, water is pumped into the formation through special injection wells located beyond the oil-drainage boundary along the perimeter. Production oil wells are located within the oil-drainage boundary in rows which are parallel to the boundary.

The most favourable objects for the edge water flooding are the formations composed of homogenous sands or sandstones with good penetrability and without formation damages. Edge water flooding in the formations composed of limestone is not always beneficial as far as some areas may not communicate with the rest of the system of channeling and fractures.

In high-viscosity oil production the process of pumping the water into the formation can also be inefficient because water (which is less viscous comparing

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to the oil) will outrun the oil in motion rushing to some wells and thus untimely flooding them.

In boundary water flooding, maintenance and restoration of the reservoir energy balance is fulfilled through direct pumping of water into the oil-saturated formation area.

Formation pressure maintaining systems

Since the beginning of the oil industry till the 40-s of the twentieth century, the oil deposits were developed till the depletion drive condition where they withdrew not more than 25 % of the initial deposits. There was a rare water drive condition. Withdrawal of the residual oil stock was conducted by means of the so-called secondary oil recovery – pumping of the air and hot air-gas mixture, vacuum process and others.

Since the 40-s there started a qualitatively new development stage of oil production technology – intensive introduction of flooding on both energy-depleted (secondary oil recovery) and maiden (primary oil recovery) fields.

Introduction of flooding methods has a rather long history of challenging two opposing opinions. The oil field development practice in Absheron Peninsula widely demonstrates that water found in the well is undesirable and always results in decrease in oil output, accumulation of different mineral salts in the piping, necessity to lift large amounts of water, etc. That is why a number of specialists had a negative approach to pumping of water into the oil formations.

In the USA they also vacillated to introduce flooding methods for most oil fields and reduced to apply water pumping for secondary oil recovery only.

The research to justify the formation pressure maintenance methods became significant in connection with designing of Tuimazinskoye oil field in Bashkiria (Volgo-Uralskaya oil-and-gas province). Successful implementation of edge water flooding at this field on a wide industry level promoted the introduction of water methods to other gas-and-oil regions of the country. Due to the accessibility of water, relatively simple pumping and high efficiency of displacement of oil with

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