RU2521091C1 - Bubble-point pressure determination method - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: bubble-point pressure determination method includes measurement of bottomhole pressure at different oil production rates and registration of bottomhole pressure change curves upon returning the well to production in linear and nonlinear oil influx modes above or below bubble point pressure. At that wellhead pressure change curves and dynamic level changes in annular space are recorded additionally. By measurement results average density is calculated at each moment of time for the mixture column at annular space and the curve of average density changes in time at annular space is plotted. The free gas release is fixed when a gas separator releases it to the annular space. The value of bubble point pressure is determined by comparison of the mixture density change curve with pressure changes at pump suction at a certain period of time.

EFFECT: improvement of accuracy measurement of bubble point pressure.

1 tbl, 1 ex, 3 dwg

Description

The invention relates to the oil and gas industry and can find application for fields in which the achievement of a profitable flow rate is possible only when the bottomhole pressure is lower than the saturation pressure.

A known method for determining the pressure of oil saturation with gas, including the simultaneous measurement of pressure in a working well and flow temperature, as well as studies of changes in composition along the wellbore in the area of the perforation interval, and measure the value of the working part of the uncovered power in the absence of flow, established, for example, by thermometry, and at the time when the maximum operating power is reached, the saturation pressure is determined from the pressure diagram at that point in depth where the change in the width of the fluctuation track is greater or equal to two values of the width of the fluctuation track of the compositional change diagram in the downstream section from the beginning of the increase in fluctuation.

Its disadvantage is that the method provides for determining the pressure of saturation of oil with gas only in gushing wells [RF Patent No. 2017951, "Method for determining the saturation of oil with gas", stated 05/28/1991; publ. 08/15/1994 - analogue].

The closest in technical essence to the present invention is a method for determining the pressure of saturation of oil with gas, including measuring bottomhole pressures at various oil rates and recording curves of changes in bottomhole pressure after starting a well in linear and non-linear modes of oil flow [RF Patent No. 1260510, “Method for determining saturation oil gas in the well, "stated. 03/08/1985; publ. 09/30/1986 - prototype].

The disadvantage of this method is the complexity of determining the transition point, due to repeated starts and stops of wells with different flow rates, which is not acceptable in the field, the result is a decrease in measurement accuracy. In addition, in this method, it is not the saturation pressure that is determined, but the pressure at the beginning of the curvature of the indicator diagram that is different from it.

The invention solves the problem of increasing the accuracy of determining the pressure of saturation of oil with gas in the field.

The problem is solved in that the change in the average density of the mixture in the annulus in time characterizes the processes of changing the composition of the fluid in the annulus. According to the invention, the appearance of free gas in the annulus causes a decrease in the average density of the mixture. By comparing the curve of the density change with the change in pressure at the pump inlet at this time, we can determine the value of the saturation pressure. This method of determining saturation pressure is very sensitive. A gas separator below the pump intake releases gas released from the oil into the annulus, which immediately leads to a decrease in the density of the mixture in the annulus.

The essential features of the method are:

1. Determination of the pressure of oil saturation with gas in the well.

2. Continuous bottomhole pressure measurement at various oil flow rates.

3. Registration of the bottomhole pressure change curves after the start-up of the well in linear and non-linear modes of oil flow above and below the saturation pressure.

4. Additional recording of wellhead pressure and dynamic level changes in the annulus.

5. Calculation according to measurements for each instant of time of the average density of the mixture column in the annulus for each instant of time.

6. The construction of the curve of the change in the average density of the mixture in the annulus in time.

7. Fixing the appearance of released free gas, causing a change in the average density of the mixture, which throws a gas separator below the pump intake into the annulus.

8. Determination of the pressure of saturation of oil with gas by comparing the curve of the density change of the mixture with the pressure change at the pump inlet at the moment.

Signs 1-3 are common with the prototype of the essential features, and signs 4-8 are the distinctive essential features of the invention.

SUMMARY OF THE INVENTION

As you know, the pressure of saturation of oil with gas is usually determined in laboratories by deep oil samples. However, in field practice, there are situations when laboratory data are not available or oil samples are still being studied, and at the stage where the engineer is in the process of performing work, it is already necessary to know the saturation pressure. In this regard, a method was developed for determining the pressure of oil saturation with gas under field conditions, including measuring bottomhole pressures at various oil production rates and recording downhole pressure change curves after starting a well in linear and non-linear modes of oil flow above and below the saturation pressure, as well as recording the change curves wellhead pressure and dynamic level in the annulus.

The change in the average density of the mixture in the annulus characterizes the processes of changing the composition of the fluid in the annulus in time and is calculated by the formula

$${\rho }_{\mathrm{from}m}=\frac{p_{PR.n\mathrm{but}\mathrm{from}}-R_{s\mathrm{but}tR}}{\left(N_{PR.n\mathrm{but}\mathrm{from}}-GNR\right)g}\left(\mathrm{one}\right)$$

where: p _pr.nas and p _shut - pressure, respectively, at the intake of the pump and wellhead annulus;

H _pr.nas - the depth of the pressure sensor at the pump _inlet ;

PLR - oil level depth in the annulus;

g is the acceleration of gravity.

The appearance of free gas in the annulus causes a decrease in average density. By comparing the curve of the density change with the change in pressure at the pump inlet at this time, it is possible to determine the value of the saturation pressure. A gas separator below the pump intake releases gas released from the oil into the annulus, which immediately leads to a decrease in the density of the mixture in the annulus.

The technical effect consists in the fact that the proposed method for determining the saturation pressure is very sensitive and can improve the measurement accuracy.

The invention is illustrated by graphs, where:

figure 1-3 shows the correlation between the average density of the mixture in the annulus determined by the formula (1) and the pressure at the pump inlet for wells 1, 2, 3.

The method is as follows.

The method for determining the pressure of oil saturation with gas in the well includes measuring bottomhole pressure with depth instruments mounted in the submersible pump housing at various oil flow rates and recording downhole pressure change curves after starting a well in linear and non-linear modes of oil flow above and below the saturation pressure. Additionally, with the help of electronic sensors, registration of wellhead pressure and dynamic level changes in the annulus is recorded.

According to the technology, throughout the entire research period, measurements are constantly made of the pressure at the pump out and at the intake of the pump, the level in the annulus, the buffer and annular pressures at the wellhead, the flow rate at the metering unit, and at the surface. Most of the parameters are registered by the computer and displayed on the display for visual inspection.

Power supply of the submersible pump installation and pressure and temperature sensors installed at the reception is carried out through a frequency converter, which allows you to smoothly select the operating frequencies of the pump installation, thereby changing its pressure characteristics, and, thus, set different modes of operation of the well.

According to the measurements at each time point, the average density of the mixture column in the annulus is calculated by the formula (1), a curve is built of the average density of the gas-liquid mixture in the annulus when the pressure at the pump inlet during the operation of the well changes (Figs. 1, 2, 3) and fix the appearance of released free gas, causing a change in the average density of the mixture, which throws a gas separator below the pump intake into the annulus. On all graphs, the pressure is quite unambiguously determined, below which the density of the mixture in the annulus becomes lower than the density of oil. The pressure at which the density of the mixture becomes lower than the density of oil, i.e. free gas begins to stand out, is the saturation pressure.

An example of a specific implementation.

Example 1

As an example, data are presented on the study of three wells of the Priobskoye oil field, revealing formations with different filtration characteristics.

During research, bottomhole pressure and temperature at the pump inlet were recorded with a special sensor. At the wellhead, buffer and annular pressures were recorded, and the flow rate of the well at a metering unit.

Density calculations were performed for the entire period of research for each well. In the graphs presented, the pressure is quite unambiguously determined, below which the density of the mixture in the annulus becomes lower than the density of the oil. This decrease in density of the mixture occurs due to the appearance of free gas released from the oil. The pressure at which the density of the mixture becomes lower than the density of oil, i.e. free gas begins to stand out, is the saturation pressure. The results of determining the saturation pressure for different wells are shown in the table. For all wells, they are in a rather narrow range of 124-127.4 at.

Table Three well saturation pressure results Well numbers Pn, at one 3 one 127.4 2 126.0 3 124.0

Claims (1)

A method for determining oil saturation pressure with gas in a well, including measuring bottomhole pressures at various oil production rates and recording downhole pressure change curves after starting a well in linear and nonlinear oil flow regimes above and below the saturation pressure, characterized in that the wellhead pressure change curves are additionally recorded and the dynamic level in the annulus, according to measurements for each instant of time, the average density of the mixture column in the annulus is calculated, t the curve of the change in the average density of the mixture in the annulus over time and record the appearance of released free gas that causes a change in the average density of the mixture, which throws the gas separator below the pump intake into the annulus, determine the pressure of saturation of oil with gas by comparing the curve of the change in the density of the mixture with pressure at the pump inlet at a given time.

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