RU2235858C2 - Method for preventing gas migration along behind-column space of oil and gas wells, as well as following intercolumn gas manifestations and gas springs on their mouths - Google Patents

Abstract

FIELD: oil and gas extractive industry.

SUBSTANCE: method includes fixing casing strings by cementation with raising of cement solution of normal thickness up to height of 50 meters from ceiling of gas bed. Above cement ring behind-column space of well is filled with stable argillaceous solution up to well mouth with specific weight generating hydrostatic pressure above cement ring higher than bed pressure and not losing properties concerning generation and preservation of hydrostatic pressure higher than gas pressure in productive bed and in permeable higher bed branches for all period of well operation. Argillaceous solution is made based on exhaust argillaceous solution with slurry with admixture of powder-like polyacrylamide in amount of 0.5-1% for conversion of argillaceous solution to viscoelastic condition for all period of well operation under sub-arctic conditions. Well range in long time frozen rocks is filled by non-freezing solution from well mouth with following control in behind-column space at level of non-freezing solution. In case of lowering of level additional feeding of non-freezing solution of respective thickness is performed for providing for hydrostatic pressure in behind-column space higher than bed pressure above cement ring.

EFFECT: higher effectiveness of method.

Description

The invention relates to the oil and gas industry and can be used in the construction of wells to improve their quality and reduce gas losses in annular space during the development of gas, oil fields with overlying gas reservoirs.

Experience in the construction of gas wells on the Cenomanian deposits of the north of the Tyumen region provides for the following well design:

a conductor with a diameter of 299 mm is lowered to a depth of 550 m and cemented with the rise of cement to the mouth;

production casing with a diameter of 168 mm is lowered to a depth of 1400 meters and cemented with the rise of cement to the mouth.

To provide cement exit at the mouth during cementing of casing strings in the interval from the mouth to a depth of 200 m above the reservoir, such lightening additives to the cement mortar as expanded vermiculite, and more recently, glass microspheres, are used.

If the cement slurry does not reach the mouth during cementing, counter-cementing of the casing strings from the mouth is provided.

However, when constructing wells using the indicated technology, for example, at the Zapolyarnoye oil and gas condensate field, when testing the annulus of the well for tightness according to safety rules in the oil and gas production of clause 2.10.6 at 106 kg / cm ^2, the test pressure decreases to 40-50 kg / cm ² in 30 minutes and even up to 10 kg / cm ² . Thus, the leakage of the cement ring between the string and the conductor 550 meters long was established in all tested wells.

After putting wells in gas fields in the north of the Tyumen region into commercial development, intercolumn gas manifestations and griffins at the mouth of more than 50% of production wells were revealed. According to the results of special field studies of the annulus, hydrodynamic and geophysical studies in the wells, gas leakages were detected through the threaded joints of the casing strings of the wells and along the cement ring cemented to the mouth of the conductors and casing strings of the wells. Gas flows along the cement ring and gas accumulations between the conductor and the casing were also recorded. It was established that the gas enters behind the cemented casing and migrates up the cement ring from the reservoir into the permeable layers of the upper part of the well section, forming technogenic gas accumulations. When gas reaches the mouth along the cement ring, intercolumn gas manifestations and griffins at the wellhead are observed.

It should be noted that during pressure testing of the conductor’s cement ring in accordance with the safety rules in NGP p. 2.10.4 after drilling the cement cup and deepening into the rocks by 1-3 meters under its shoe, the pressure drop drops from 15 to 2-9 kg / cm. The pressure of the crimping according to the safety rules in NGP p. 2.10.4 is determined by the need to ensure tightness under the shoe of the column when closing the wellhead during open flowing. The gas pressure under the shoe of the conductor will be more than 120 kg / cm, and the pressure of the pressure should be: 120-1.20 × 0.98 × 550 m = 55 kg / cm ²

Thus, gas from the formation migrates along an unpressurized cement ring, saturates permeable rock layers under the conductor shoe, and appears at the mouth in the form of intercolumn gas manifestations and griffins behind the conductor. How much gas migrates along the leaky cement ring is almost impossible to estimate. However, in all background measurements of temperature along the well’s bore, taken after prolonged shutdown of the completed wells in anticipation of completion of the gas treatment facility, there is a decrease in T ° of the wellbore from the bottom to a depth of 800-1000 m relative to the Earth’s geothermal gradient by 4-6 °, the difference is higher in the section T ° begins to decrease and in the interval of the shoe of the conductor is leveled. This is due to the throttle effect of reducing T ° of the gas migrating along the leaky cement ring to 800-1000 meters. Above, the gas begins to partially migrate to the host rocks and T ° wells are gradually aligned with the direct geothermal gradient of the Earth.

A known method of preventing gas migration through the annular space of oil and gas wells, including mounting casing strings by cementing in permafrost conditions (see, for example, RF patent No. 2109909, 04/27/1998).

The disadvantage of this method is the low efficiency of preventing gas migration.

In many gas and gas condensate fields of the world, wells with intercolumn gas manifestations, griffins, casing flows have been identified. Boreholes with cemented casing up to the mouth in the axial direction are rigidly connected cement stone with rocks of the section opened by them, regularly repeated protrusions of the casing couplings rigidly mounted in the cement stone, and a drilled directional borehole uneven in diameter, and the cement stone does not create hydrostatic pressure.

Gas can penetrate from the formation into the cement ring behind the casing and migrate in it only if the pressure in the gas-bearing formation exceeds the pressure behind the column, which can only be hydrostatic, and there is free space for gas migration. Hydrostatic pressure behind the casing can only be created by a column of liquid or fluid, but not stone. Gas migration begins with OZC as a result of a decrease in the hydrostatic pressure created by the column of cement mortar when it hardens. The gradient of hydrostatic pressure decreases first to the pressure created by the fluid mixing cement and becomes equal to zero at the end of the setting of cement, since the cement stone has no fluidity. And laboratory studies have shown that cement stone has a smaller volume than the initial volume of cement mortar when injected into the well by an average of 6% for various types of cement mortars, including lightweight ones. In the OZC process, channels are formed in the cement stone due to the emergence of gas with decreasing hydrostatic pressure of the hardening cement, cracks form as a result of water loss and volumetric shrinkage of the solution. Poor removal of the clay crust, which, as it were, dries out due to water leaving to form a cement stone, forms an unfilled annular space with a permeability of several Darcy. Even before the completion of the WLC, gas from the reservoir begins to migrate along the incompletely formed cement ring, forming channels to the permeable layers and up to the formation of annular and annular gas manifestations. The cement stone cracks and collapses as a result of testing the casing of the wells for leaks by overpressure according to the existing RD, which is carried out after the OZZ.

In addition, according to petrophysical studies of cement stone samples performed by Geomen LLC Permneftegeofizika OJSC prepared using lightweight glass microspheres, the latter has a porosity of about 40% and a permeability of 1 fm ² . Cement stone, lightened with expanded vermiculite, has a porosity of about 50% and a permeability of about 5 fm ² .

Thus, the migration of gas from the reservoir beyond the casing of the well, the formation of technogenic accumulations of gas, intercolumn gas and griffins at the wellhead are inevitable when cementing the casing to the wellhead with lightweight solutions and cross-cementing.

The technical result of the invention is to increase the efficiency of the method.

The required technical result is achieved by the fact that the method of preventing gas migration through the annular space of oil and gas wells, as well as subsequent intercolumn gas manifestations and gas griffins at their mouths, includes cementing casing strings with normal density cement slurry up to a height of 50 meters from the roof of the gas reservoir for rigid fixing of the bottom of the column in the interval of the reservoir, and above the cement ring, the annulus of the well is filled with stable clay p solution to the wellhead with a specific gravity that creates hydrostatic pressure above the cement ring above the reservoir and does not lose its properties to create and hold hydrostatic pressure above the gas pressure in the reservoir and in permeable overlying layers for the entire period of operation of the well, while the mud is prepared at the drilling pumping it into a well based on a spent clay solution with sludge with the addition of powdered polyacrylamide in an amount of 0.5-1% to transfer a clay solution and in the viscoelastic state for the entire period of operation of the well in the conditions of the Arctic, the interval of the perennially frozen rocks is filled with an ice-free solution from the wellhead with subsequent control in the annular space of the level of an antifreeze solution, with a decrease in which non-freezing solution of the appropriate density is pumped to provide hydrostatic pressure in the annulus above the reservoir over the cement ring.

This technology will make it possible to qualitatively isolate not only gas-bearing formations and reduce the likelihood of intercolumn gas occurrences with griffins in the wells, but also to dampen the effect on the columns of both natural and man-made deformation processes, and temperature changes and for permafrost with their characteristic reverse frost penetration. This technology will allow to reduce cement consumption for well construction and to get rid of spent clay solutions that accumulate and are buried in the body of dumping of well sites after well construction, creating additional difficulties during operation. It will become possible to carry out repair work on replacing leaky casing pipes with new ones and reliably eliminate wells by first removing uncemented casing pipes and filling the mine with mixtures based on natural materials.

Claims (1)

A method for preventing gas migration through the annular space of oil and gas wells, as well as subsequent intercolumn gas displays and gas griffins at their mouths, including attaching casing strings with cementing and raising cement slurry of normal density to a height of 50 m from the roof of the gas reservoir to firmly fix the bottom of the column to the interval of the reservoir, and above the cement ring, the annulus of the well is filled with a stable clay solution to the wellhead with a specific gravity that creates a guide the residual pressure above the cement ring is above the reservoir and does not lose its properties to create and hold hydrostatic pressure above the gas pressure in the reservoir and in permeable overlying strata for the entire period of the well’s operation, while the mud is prepared at the drilling site before it is pumped into the well based on the spent mud with sludge with the addition of powdered polyacrylamide in an amount of 0.5-1% to transfer the clay solution into a viscoelastic state for the entire period of operation Azhinov in Arctic conditions, a borehole interval permafrost filled with antifreeze solution wellhead, followed by the control level in the annulus freezing solution, which produce at lower pumping freezing the solution an appropriate density to ensure hydrostatic pressure in the annulus above the formation of the cement sheath.