Coiled Tubing Times - oil & gas service, fracturing, coil tubing, well intervention, workover

Journal

Issue 39, March 2012

TECHNOLOGY OF CLEANING GAS WELLS BOTTOMHOLES FROM ARGILLO-ARENACEOUS PLUGS ENSURING CONTROLLED DEPRESSION IN WELL-FORMATION SYSTEM DURING COILED TUBING FOAM WASHING

Technical approach to keeping of preset differential pressure on the formation in the course of well foam washing was developed on the basis of field hydrodynamic research

Theoretic models are known to be unable of simulating all variety of physical and chemical factors influencing the behavior of the required process characteristics. Thus, experimental studies [1] of hydrostatic pressure of foams and air-water mixtures were carried out in comparison with their theoretical analogues. The tests showed that with pressure growth, i.e. as approaching the bottomhole, experimental foam points deviate from the design ones towards increase. Air-water mixtures deviate 1.5-2 times more than foams. It is obvious that in dynamics the effect of deviations towards underrating of the design bottomhole pressure as compared to its actual values becomes more evident.

The authors relate this fact to bubbles channeling. In the conditions of rising current it increases the actual density as compared to the calculated one.

This article suggests using foam as an elastic medium, producing pressure inversion when released in the driven up rising current.

This explains the lower response of wellhead pressure as well depth increases. Since the theory of the pressure inversion effect is not described, it is necessary to specify theoretical model for particular conditions of North Stavropol UGSF by using adaptation method, i.e. by matching the design and field data and by trend analysis.

The adapted model is aimed at specifying the gross coefficient of circulation pressure loss

and dependence

along the ring channel from wellhead to bottomhole.

Regression equations:

Since the mechanism under review can not be correctly described for the time being, the reference mathematical model is specified by a statistically distributed method of regression equation.

The adaptive coefficient l is introduced in order to correct the pressure losses and the linear regressive dependence is found:

Coefficients А, В, С are found based of measurement data.

After is introduced into the reference model, the pressure data calculated at the bottomhole Р_к_, slightly diverge from the measurements, shown in Table 1 as inviscid "r".

Overall losses coefficients in the mode options are calculated from the actual measurement data (Table 1) and linear dependence is drafted:

i - number of the mode.

Numerical values of regression equation coefficients are specified in Table 1.

Calculations of depth pressure distribution were made in the modes (1-6) in the course of foam flow in comparison with pressure distribution in static condition (Figures 1-2).

The behavior of local losses coefficient DР(h) is represented in dimensionless form in Figure 3.

Taking into account comparatively small scattering of the curves , we can find the dependence of the relative average coefficient of losses from the ring channel depth section:

The functions are defined in the same way and compared in various modes; they are shown in Figure 4. By averaging out the obtained functions from dimensionless depth , we can find regressive equations for them. Generalized changes of the relative loss and pressure coefficients along the ring channel are represented in Figure 5.

Calculation of differential pressure for North Stavropol UGSF with limiting formation pressures during the operation (at the end of recovery and pumping)

In order to provide depression in the well-formation system during the removal of argillo-arenaceous plugs with the help of hydrodynamic model adapted for these conditions, the boundary practices for extreme operational conditions were developed: the lowest formation pressure at the end of the recovery practice and the highest formation pressure at the end of pumping.

The calculations were made for the UGSF of the banks of Khadum and green measures. The objective was to define the delivery of fluid and gas in the rising current of the ring channel meeting the requirements of bottomhole cleaning and providing the admissible bottomhole depression.

The admissible bottomhole depression corresponds to the preset limit of formation gas flow rate. The blanket gas recovery rate is limited in such a way, so that during its partial replacement with foam a low efficiency compressor allowing the influx from the formation of no more than 1,000 m³ per day could be used.

The objective was achieved by selecting flow rate conditions and implementing all limiting factors. It required numerous runs of foam wash simulation model with the conditions habitual for argillo-arenaceous plugs removal.

Consider the results of the calculations for North Stavropol UGSF. The calculation of the necessary rising speed of the bottomhole current for destroying and removing the argillo-arenaceous plugs produced the value: V = 0,23 m/с.

Variant I. Green measures banks. End of the recovery practice

The depth of the well is 900 m; the limiting formation pressure Р_f= 5.6 MPa; the delivery of the pumped foam producing liquid Q_пл = 0,0016 m³/s; gas delivery Q_г = 0,074 m³/s; degree of aeration a = 46,25; bottomhole balance, Р_з = 5.6 MPa; bottomhole flow velocity V_з = 0.22 m/s.

The substituting foam is produced by the same amount of the pumped liquid Q_пл = 0,0016m³/s and inflow of formation gas Q_г = 0,09 m³/s, or equal to flow rate q_г = 7.77 thousand m³/d

With the wellhead pressure being Р_w = 0,17 MPa, the bottomhole receives Р_b = 5.47 MPa. The possibility of receiving 7.77 thousand m³/d with the depression of DР = 5,6-5,47 = 0,13 MPa depends on formation productivity .

With the destruction of the sand plug, К_прод progresses and q_г, Р_з R Р_пл principle remains, the wellhead pressure should be increased up to 0.3 MPa to keep the required inflow level q_г necessary for cleaning the bottomhole.

Thus, with the limiting formation pressure of 5.6 MPa and pumped liquid Q_ж = 0,0016m³/s, the gas delivery necessary for cleaning the green measures well should be equal to the well flow rate q_г = 7.77 thousand m³/d and the wellhead pressure should be kept at the level:

With the formation gas flow rate being within the limits of no more than 1 thousand m³/d, a compressor with the output of 5 m³/min is required.

Variant II. Green measures banks. End of pumping operation.

Unlike in the previous variant, the limiting formation pressure was Р_пл = 9.1 MPa. The condition of balance with Q_ж = 0.0016 m³/s is achieved, when Q_г = 0.0107 m³/s and bottomhole velocity V_з = 0,12 m/s. V_з < V_ун , that is why the substituting foam is defined with Q_г = 0.09 m³/s as well as in the previous case, which corresponds to well flow rate of q_г = 7.77 thousand m³/d, Q_ж = 0.002 m³/s, V_з = 0.23, Р_з = 8.56, Р_у = 1 MPa. As the formation productivity increases and the sand plug being destroyed, the pressure on the wellhead should be increased from 1 MPa to 1.2 MPa:

Like in the previous case, a compressor with the output of 5 m³/min is required.

Variant III. The banks of Khadum horizon. End of pumping operation.

The depth of the well is 800 m, limiting formation pressure is 3.3 MPa. The balance on the bottomhole is achieved at Q_ж = 0.001 m³/s, Р_у = 0.1 MPa, Q_г = 0.077 m³/s, V_з = 0,26 m/s, Р_з = 3,3 MPa.

Substitution of the initial foam with the flow of formation gas and creation of depression is achieved by decreasing the delivery of foam producing liquid:

Q_ж = 0.0008m³/s, Р_у = 0.1 MPa, Q_г = 0.077 m³/s, V_з = 0.27 m/s, Р_з = 2.81 MPa.

With formation productivity being increased by cleaned bottomhole and lowered depression Р_з R Р_пл , the critical increase in the wellhead pressure will be Р_у = 0.26 MPa.

As a result, the well flow rate of 6,650 m³/d is possible, when the wellhead pressure is kept within the limits:

With a formation inflow of up to 1,000 m³/d a compressor with output of 4 m³/min is required.

Variant IV. The banks of Khadum horizon. End of recovery operation. The depth of the well is 800 m, the limiting formation pressure is 2.3 MPa.

The balance on the bottomhole is reached when Q_ж = 0.00054 m³/s,

Q_г = 0.07 m³/s, V_з = 0.28 m/s, Р_з = 2.3 MPa, Р_у = 0.1 MPa.

Substitution of the initial foam with the formation gas inflow on the depression is done by means of increasing gas flow:

Q_г = 0.09 m³/s, Q_ж = 0.00054 m³/s, Р_у = 0.1 MPa, V_з = 0.38 m/s, Р_з = 2.1 MPa.

Then, as the formation productivity increases, the critical rise of the wellhead pressure is defined as: Р_у = 0.175 MPa.

In the suggested variant the well flow rate of 7,770 m³/d required to clean the bottomhole is possible with wellhead pressure being kept within the limits of

With the formation gas inflow limited to 1,000 m³/d, a compressor with the output of 5 m³/min is required.

Table 2 shows the optimum technological parameters for underbalanced foam washing of the argillo-arenaceous plugs of the wells of North Stavropol UGSF.

Table 3 shows the results of washing the sand plugs following the recommended technology.

Conclusions

The advanced technology of removing argillo-arenaceous plugs from the well bottomhole in the conditions of the abnormally low pressure provides for cleaning of not only the borehole, but also of the bottomhole zone due to formation gas inflow in the process of circulation.
The field hydrodynamic research of washing the well with foam systems defined boundary conditions for keeping depression in the "well- formation" system in the process of removing argillo-arenaceous plugs with the use of a coiled tubing unit. This objective is achieved by controlling surface pressure and injection rate of foam-forming liquid and compressor-pumped inert gas.
Mathematical model and suitable software are offered for defining pressure and foam velocity in all elements of the circulation system.

REFERENCES

1. Tagirov, K. M. Opening of Productive Oil and Gas Formations with Abnormal Pressure / K. M. Tgirov, A. N. Gnoyevykh, A. N. Lobkin - M. : Nedra, 1996.

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