RU2013526C1 - Method for temporary isolation of a gas-bearing stratum - Google Patents

Abstract

FIELD: oil and gas producing industry. SUBSTANCE: temperature is determined for well reservoir conditions. The well is provided with a bottom-hole choke. A hydrate forming gas is injected into the formation, the gas being compressed to the pressure higher than those of the formation. The gas injected through the bottom-hole choke. The gas flow rate is determined from a dependance presented in the description. The gas flow rate ensures the bottom-hole temperature reduction below the hydrate forming temperature. Then the pressure applied to the wellhead is increased. The gas injection is stopped. The bottom-hole area is filled with a foam and the borehole - with the well killing fluid. EFFECT: enlarged operating capabilities.

Description

 The invention relates to the oil and gas industry, in particular to repair work carried out in gas wells, and can be used for temporary blockage of the reservoir during various technological operations in the process of overhaul or underground repair of gas wells.

 A known method of temporary isolation of highly permeable zones in the well by injection into the reservoir [1].

 The disadvantage of this method is the clogging of the pore channels of the reservoir as a result of the chemical interaction of several components injected into the reservoir separately. In this case, uncontrolled absorption of the components by the sections of the formation with different permeability occurs. In addition, the method requires acid treatment after repair work in order to restore the permeability of the formation.

 A known method of temporary blockage of perforation channels in the well with water-soluble salt [2].

 The disadvantage of this method is that after repair, the salt is dissolved by pumping water through perforation channels, which limits its scope in productive formations, the rocks of which contain clay material.

 A known method of isolating a gas reservoir, which includes starting a well in hydrate formation mode and injecting before this a portion of water into the bottomhole zone [3].

 The disadvantage of this method is the possibility of using it to isolate formations with thermodynamic conditions close to the equilibrium hydrate formation conditions, which is typical for gas fields located in permafrost zones. In addition, the method provides for injection of water into the formation, which negatively affects the permeability of productive rocks having clay material.

 The closest in technical essence to the proposed is a method of isolation of mine workings from the influx of groundwater [4]. The method involves injecting a hydrate former into a water-bearing rock at a pressure higher than the reservoir pressure, and then a sharp increase in pressure to a value higher than the phase transformation of the water-gas mixture into a solid.

 The disadvantage of this method is that a sharp increase in gas pressure is technically difficult due to its compressibility. In addition, high pressure can lead to hydraulic fracturing of rock formations, i.e., the amount of overpressure in the well is limited by the value of rock strength.

 The aim of the invention is to simplify the process and increase its efficiency.

 This goal is achieved by the fact that in the method of temporary isolation of the gas-bearing formation, which includes injecting gas into the reservoir at a pressure higher than the reservoir, the well is equipped with a downhole fitting before injection and injection is carried out at a constant pressure with a flow rate that reduces the temperature at the bottom below the hydration temperature until wellhead pressure, then the bottomhole zone of the well is filled with foam, and the wellbore is filled with killing fluid.

, где Q - расход газа, м³/с;
d - диаметр штуцера, м;
μ - молекулярный вес газа;
С_P - удельная теплоемкость газа при Р= = const, Дж/кг ˙град;
Δ Т - разность температур до и после дросселя, ^оС;
P₁- давление закачиваемого газа, Па;
Р₂ - пластовое давление, Па;
γ =

, где С_v - удельная теплоемкость при v = сonst.The gas flow rate is determined by the formula
Q = 0.785d

where Q is the gas flow, m ³ / s;
d is the diameter of the fitting, m;
μ is the molecular weight of the gas;
With _P is the specific heat of the gas at P = const, J / kg ˙ deg;
Δ T is the temperature difference before and after the throttle, ^о С;
P ₁ - pressure of the injected gas, Pa;
P ₂ - reservoir pressure, Pa;
γ =

where C _v is the specific heat at v = const.

 A comparative analysis of the proposed solution with the prototype shows that the proposed method differs from the known one in that for the implementation of the process, the temperature at the bottom is reduced to the temperature of hydrate formation, for which the process of throttling natural gas through the bottom hole is used. As soon as the hydrate formation process is completed (a temporary blocking screen is formed), which is fixed by increasing pressure at the wellhead, the gas supply is stopped.

 The analysis showed that it is known to use the throttling process for low-temperature gas separation (Yu. P. Korotaev. Exploitation of gas fields. M.: Nedra, 1975, p. 320). However, the artificial creation of hydrates by throttling natural gas according to known sources has not been identified. The method satisfies the criterion of "significant differences".

, где С_P - удельная теплоемкость газа при Р= = соnst, Дж/кг ˙ град;
μ - молекулярный вес газа;
Δ Т - разность температур до и после дросселя, ^оС;
Р₁ - давление закачиваемого газа, Па;
Р₂ - пластовое давление, Па;
γ =

, где С_V - удельная теплоемкость газа при V = const. Расход газа при дросселировании
Q = V˙S, где S - площадь сечения дросселя
S =

= 0,785 d², где d - диаметр дросселя.The essence of the invention lies in the fact that when the flow of gas under high pressure into the space with low pressure through a narrow hole, there is a sharp decrease in gas temperature (Joule-Thomson effect). To implement the method, the shoe (lower part) of the tubing string (tubing) is pre-equipped with a downhole fitting. With downhole throttling, the rate of outflow of compressed gas through the tubing shoe can physically be represented as the outflow of gas from a cylinder through a small hole or nozzle. Considering the flow to be steady and laminar and neglecting the gas velocity in the cylinder, the downhole throttle velocity is determined from the equation
v =

where С _P is the specific heat of the gas at Р = const, J / kg ˙ deg;
μ is the molecular weight of the gas;
Δ T is the temperature difference before and after the throttle, ^о С;
P ₁ - pressure of the injected gas, Pa;
P ₂ - reservoir pressure, Pa;
γ =

where C _V is the specific heat of gas at V = const. Throttle flow rate
Q = V˙S, where S is the throttle sectional area
S =

= 0.785 d ² , where d is the diameter of the throttle.

.The equation for determining the required gas flow rate (injection rate) at which the Joule-Thomson integral effect is observed, i.e., cooling the bottom of the well, has the form
Q = 0.785d

.

 At a certain gas flow rate, gas hydrates are formed in the near-well zone of the well, which create a temporary insulating layer. As soon as the pressure at the wellhead begins to increase, gas injection is stopped and foam is injected into the bottom of the filter zone as a heat-insulating material between the hydration screen and the filling fluid, which fill the well after the foam. After repair work, the well is mastered by aeration, the hydration screen is destroyed by known methods (pressure reduction, methanol injection, gas temperature increase) and transferred to operation.

 PRI me R. The initial data for the calculation.

Formation pressure P ₂ MPa 1.5
Wellhead pressure P _y , MPa 1.7
The thickness of the productive area h, m 10
The temperature of the injected gas T ₁ ^about 20
The reservoir temperature T _pl , ^about With 50
Molecular Weight of Gas μ 16
Specific heat capacity of gas С _P , J / kg ˙ city 33.26 ˙10 ³
The ratio of beats specific heat γ 1,309
Injected gas pressure P ₁ 5.0 MPa
Throttle diameter d, m 0.1
Diameter of production casing D, m 0.14
The well is equipped with a downhole fitting with a diameter of 0.1 m.

 1. For reservoir conditions, the temperature of hydrate formation is determined, for which they use the schedule of equilibrium conditions (Makogon Yu. F. Gas hydrates, prevention of their formation and use. M.: Nedra, 1985, p. 19).

For well conditions, hydrate formation temperature T _g.o. = 7 ° ^C. It was established experimentally that the maximum rate of hydrate formation is observed when supercooling about 7 ° ^C. Thus, the temperature after the reactor should be about 0 ^o C, t. E. Δ T = 20 ° ^C

,
Q = 0,785·0,01

=
= 0,785·0,01

= 0,785·0,1

=
= 0,785·0,01·144,2 = 11,32 м³/c.The gas flow rate is determined by the formula
Q = 0.785d

,
Q = 0.785 · 0.01

=
= 0.785

= 0.785 · 0.1

=
= 0.785 · 0.01 · 144.2 = 11.32 m ³ / s.

Gas is pumped at a rate of 11.32 m ³ / s until the pressure at the mouth becomes equal to P _y = 1.8 MPa. The gas injection is stopped and foam is supplied to the bottomhole zone in the amount of Q _p = 0.785D ² ˙h
Q _p = 0.785 ˙ 0.142 ˙ 10 = 0.154 m ³ . Then the wellbore is filled with jamming fluid and proceed with repair work.

 The proposed method allows to prepare the well for repair work on natural gas when it is injected into the well, while the prototype requires the injection of a special gas - hydrate former. In addition, the implementation of the prototype method requires sealing the wellhead after increasing pressure, which is feasible for rock formations. The proposed method does not require the development of special equipment.

Claims (2)

 1. A METHOD FOR TEMPORARY INSULATION OF A GAS-BASED LAYER, comprising injecting a hydrate generator gas into a formation under a pressure higher than the formation pressure, characterized in that the gas is injected by throttling through the bottomhole fitting with a flow rate that decreases the temperature at the bottom of the well below the temperature of hydrate formation and increases the pressure at the wellhead , followed by filling the bottom hole of the well with foam, and the wellbore with a kill fluid. 2. The method according to p. 1, characterized in that the gas flow rate is determined by the formula
C sub p cdot DELTA ≈T left [{1 - left ({P sub 2 over P sub 1}
right) sup down 200 {{gamma -1} over gamma}} ≈≈ back 300 right
]} ≈ back 250 down 50 right) sup down 90 {1/2} back 80,}>
where Q is the gas flow, m ³ / s;
d is the diameter of the fitting, m;
μ is the molecular weight of the gas;
C _p - specific heat of gas at P = const, J / kg · deg;
Δ T is the temperature difference before and after the nozzle, ^o C;
P ₁ - pressure of the injected gas, Pa;
P ₂ - reservoir pressure, Pa;
γ = C _p / C _v ,
where C _v is the specific heat at V = const.