RU2492321C1 - Method to detect gas hydrates in low-temperature rocks - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: standard electric logging of a well is carried out in low-temperature rocks, the area of possible bedding of gas hydrates and hydrate formation is identified in them. In the identified area of low-temperature rocks, on the basis of data of standard electric logging, zones are registered, in which measured values of the apparent electric resistance of low-temperature rocks are equal to at least 15 Ohm.m. Coolant is pumped in the investigated rock interval, afterwards thermometry is realised using highly sensitive thermometers, providing for error of temperature measurements of not more than 0.01°C, and zones are sought for, rock temperature in which, relative to the lowest registered temperature in the identified zone is at least by 0.2-0.5°C lower than the temperature of rocks adjacent to the borders of the detected zones. At the same time the latter zones are considered as zones containing gas hydrates. The area of possible bedding and hydrate formation is the area of rock bedding characterised by availability of thermobaric conditions for gas hydrates existence in rocks.

EFFECT: its higher efficiency by detection of gas hydrate rocks bedded in low-temperature rocks below a foot of permafrost rocks.

3 cl, 1 dwg

Description

The invention relates to the gas and oil industries and can be used, in particular, in identifying gas hydrates in low-temperature rocks (NP) during the construction and operation of wells in NP.

Closest to the proposed is a method for identifying gas hydrate rocks in the permafrost zone, which consists in conducting standard electric logging of the well, performing its thermometry and searching for zones containing gas hydrates (see RF patent No. 2428559, Е21В 36/00, Е21В 47/06, 2011).

The disadvantage of this method is its low efficiency, due to the fact that it does not allow to identify gas hydrate rocks occurring in low-temperature rocks below the bottom of permafrost (IMF).

The technical result, which the proposed method aims to achieve, is to increase its efficiency by identifying gas hydrated rocks occurring in low-temperature rocks below the bottom of the permafrost.

This technical result is achieved due to the fact that in the method for detecting gas hydrates in low-temperature rocks, which consists in conducting standard electric well logging in the geological section of its low-temperature rocks (NP), performing its thermometry and searching for zones containing gas hydrates, standard electric well logging carried out in the NP, lying below the base of permafrost, and before the thermometry of the well in the NP, an area of possible occurrence of gas hydrates and hydrate formation (ZGO) is distinguished, characterized by the thermobaric conditions for the existence of gas hydrates in the rocks, in the selected area of the oilfield, according to the standard electric logging data, zones in which the measured values of the apparent electrical resistance (CES) are at least 15 Ohm · m are recorded and pumped in the coolant well, and after the thermometry, they search for zones, the temperature of the rocks in which, relative to the lowest recorded temperature in the area, is not less than 0.2-0.5 ° C lower than the temperatures of the rocks adjacent to the boundaries of the detected zones while the latter are considered as zones containing gas hydrates, and also due to the fact that the region of occurrence of rocks, characterized by the presence of thermobaric conditions for the existence of gas hydrates in the rocks, is taken as the region of possible occurrence of gas hydrates and hydrate formation and it is defined as the area of occurrence of rocks in the geological section of the well , where the pressure values in the rocks, at temperatures corresponding to these pressures, are equal to hydrostatic or abnormal pressures, which are not lower than the values spring hydrate pressures and corresponding temperature values, not higher than equilibrium, with the corresponding component composition of natural gas in a given area of rock occurrence and, in addition, due to the fact that thermometry in the well is carried out using highly sensitive thermometers (VHF), which provide an error in temperature measurements no more than 0,01 ° C.

The invention is illustrated by the diagram in figure 1, which shows the results of the study of wells in the context of low-temperature rocks lying below the bottom of the permafrost. In figure 1, the positions 1, 2, 3, 4 indicate the data of thermometry for wells No. 301, No. 501, No. 601 and No. 1001; accordingly, position 5 indicates the location of points corresponding to the depth of the bottom of the IMF for each well; position 6 - the area of possible occurrence of gas hydrates and hydrate formation located at certain depths. Position 7 denotes the intervals of depths identified for each well, where the rocks with low temperatures are located (low-temperature anomaly) in the studied section of the reservoir; position 8 - zones of occurrence of rocks with values of CES equal to values not lower than 15 Ohm · m; position 9 - the points where the lowest temperature in the corresponding rock location zones for each well is recorded (this rock temperature is 0.2 ° C-0.5 ° C lower than the temperature of the rocks adjacent to the boundaries of the selected zones; position 10 and 11 indicate the upper (roof) and lower (sole in depth) boundaries of the thermobaric region of possible occurrence of gas hydrates and hydrate formation; position 12 indicates the depth of the permafrost zone (determined by the zero isotherm recorded for the studied important).

For the four examined wells of the northern field, Fig. 1 shows thermograms with the identification of gas hydrates in the oilfield below the bottom of the IMF, which are registered for wells No. 301, No. 501, No. 601 and No. 1001. For well No. 501, thermometry was performed after completion of cement injection for an intermediate column with a diameter of 245 mm 24 hours after the well was idle. For well No. 601, registration was carried out after drilling was completed under a conductor with a diameter of 324 mm. In the case of well No. 301, thermometry was carried out in the depth interval 0-1450 m while monitoring the technical condition of the well after gas extraction from the formation. For well No. 1001, thermometry was carried out in the depth interval 0-1489 m after exposure to coolant rocks, namely, after gas extraction from the bottom while monitoring the technical condition of the well.

The method is implemented as follows. In NP wells, during geophysical studies of the geological section of low-temperature rocks, they carry out its standard electric logging. Electric logging is carried out in NP, lying below the bottom of the permafrost. After that, thermometry of the well is performed. Before the thermometry of the well in the NP, the area of possible occurrence of gas hydrates and hydrate formation is distinguished, characterized by the conditions for the possible existence of gas hydrates in the rocks. According to the standard electric logging data, zones in which the measured values of the apparent electrical resistance of the NPs are equal to at least 15 Ohm · m are recorded in the selected area of the oilfield according to standard electric logging data, and then the coolant is pumped in the well.

Then they search for zones (position 8 in figure 1), the temperature of the rocks in which, relative to the lowest recorded temperature (position 9) in the selected area, is not less than 0.2 ° C-0.5 ° C below the temperature of the rocks, adjacent to the boundaries of the detected zones. These zones are considered as zones containing gas hydrates. In this case, the thermometry performed at the well should be carried out using methods of highly sensitive thermometry, which provides an error in measuring temperatures of not more than 0.01 ° C.

As the coolant pumping in wells, the drilling fluid used in the construction of a well in the wellbore section of the wellhead during drilling, flushing the well before lowering the casing in the wellbore or cement injection when cementing the well in the wellbore section can be considered. Also, as a coolant pumping in wells, fluid selection from the well is used during its testing or in the study of its technical condition.

Thermometry should be carried out after completion of the coolant pumping, for example, cement injection into the well. As a rule, it is carried out no earlier than 10-15 hours after the end of the cement injection. In this case, data on thermometry can be obtained after any pumping of the coolant in the well.

It should be noted that according to the data of the pressures and temperatures of the rocks of the region of possible occurrence of gas hydrates and hydrate formation in the studied section of the well in the specified area, taking into account the obtained data of standard electric logging, zones are recorded in which the measured values of the CES of low-temperature rocks are at least 15 Ohm · m. After that, the thermometry of the well is carried out and a search is carried out for zones where the temperature of the rocks in which, relative to the lowest recorded temperature in the selected zone, is not less than 0.2 ° C-0.5 ° C below the temperature of the rocks adjacent to the boundaries of the detected zones. Such zones are considered as zones containing gas hydrates.

The region of possible occurrence of gas hydrates and hydrate formation is taken to be the region of occurrence of NP, characterized by the thermobaric conditions for the existence of gas hydrates in the rocks. It is defined as the area of the occurrence of the NP in the geological section of the well, where the pressure in the rocks at the corresponding temperature values are equal to hydrostatic or anomalous pressures that are not lower than the equilibrium hydrate pressures and the corresponding temperature values that are not higher than the equilibrium values for a certain component composition natural gas in this area of occurrence of NP.

This area can also be determined by the results of parametric exploratory drilling at the field with registration in the low-temperature section of the well of the corresponding values of reservoir pressures and natural rock temperatures.

Using the effect of increased coolant temperature on the studied rocks in the section of the borehole and recording lower temperatures in zones with increased IES in the rocks after completion of the coolant pumping, it is possible to determine, taking into account the performed thermometry, the presence of a phase transition in the studied rocks during decomposition of gas hydrates and, accordingly, more intense absorption heat in the allocated zones, leading to lower temperatures in these allocated zones. This fact indicates the presence of gas hydrates in these zones.

Consider examples of identifying gas hydrate rocks using thermometry data (see the diagram in figure 1). For the four examined wells of the northern field, Fig. 1 shows thermograms with the detection of gas hydrates in the oilfield below the bottom of the IMF. These thermograms were taken for wells No. 301, No. 501, No. 601 and No. 1001. Moreover, for well No. 501, a thermogram was recorded after the end of cement injection for the intermediate casing of the well with a diameter of 245 mm 24 hours after the end of the cement injection and the downtime of the well (position 2 in FIG. 1). For well No. 601, the thermogram was taken after drilling the wellbore under a conductor with a diameter of 324 mm (position 3 in figure 1). For well No. 301, thermometry was carried out at depths in the range of 0-1450 m during the monitoring of the technical condition of the well after gas extraction from the formation (position 1 in figure 1). For well No. 1001, thermometry was carried out at depths in the range of 0-1489 m also after exposure to coolant rocks (position 4 in FIG. 10).

In the diagram (see Fig. 1), the lowest temperature, marked with position 9 in the selected zones (position 8), corresponds to the depth of the formation with hydrates (it can be in the middle of the selected zone in the case of the same rock hydration in the zone).

The bottom of the IMF, below which the section of the oilfield is examined, is shown in figure 1 for all wells by position 5. The sole of the permafrost zone with lower temperatures in the oilfield is shown by position 12 in figure 1, while for the studied wells it can be traced to depths of 280 m. At a depth of 200 m m of oil temperature for the studied wells varied within the range of 0-2.0 ° C.

If there is a normal hydrostatic reservoir pressure of 2.0 MPa or increased anomalous pressure (above hydrostatic) at a depth of 200 m in wells, for example 2.5 MPa, and deep gas penetrates into them from a depth of 750 m or more, a gas hydrate deposit may exist in them . Studies have shown that in the rocks in the selected ZGO (position 6 in figure 1) at a pressure of, for example, 2.5 MPa and rock temperature minus 2.0 ° C or lower, gas hydrates with a certain composition of natural gas can occur and form. The aforementioned makes it possible to distinguish a depth of 200 m as the upper boundary (roof) of the WGO (position 10). Considering that at a depth of 560 m the temperature of the rocks can vary from 7.5 ° C to 10.2 ° C with the corresponding reservoir equilibrium hydrate formation pressure, it should be considered that gas hydrates can exist in these rocks with a certain methane gas composition. This depth (position 11 in FIG. 1) is taken as the lower boundary (sole) of the ZGO.

As can be seen from the data presented in the diagram of figure 1, studies to identify gas hydrates in the oilfield are carried out in the range of 200 - 560 m. This is lower than the depth of the bottom of the IMF (for wells No. 301 and No. 501, the depth of the sole of the IMF is noted, respectively, at depths 52 m and 48 m (position 5); for well No. 601 - at a depth of 97 m and for well No. 1001 - at a depth of 80 m) for the considered wells within the zone of the ZGO, where, according to thermobaric conditions and taking into account thermometry data, zones with gas hydrates.

Consider the implementation of this method on the example of well No. 301.

For this well, standard electric logging is carried out in the selected area of the ZGO and a zone is recorded in it at depths of 222 m - 247 m with a CES of rocks equal to 16-23 Ohm · m in the zone (position 8 in figure 1). In the future, after the construction of this well is completed, it is monitored for its technical condition with the selection of gas (coolant) from the well from the bottom for 10 hours. After the end of the gas (coolant) extraction, after 7 hours, highly sensitive thermometry is carried out with the separation of the rock interval at depths of 220 m - 240 m, in which a decrease in temperature is recorded. In this depth interval, the lowest temperature (position 9) is observed, equal to 15.6 ° C. Compared to the rock temperature of 15.8 ° C and 16.3 ° C at the zone boundary (position 8), it is 0.2-0.7 ° C lower than the temperature of 15.6 ° C. Thus, we can assume that in the rocks in the allocated zone (position 8), gas hydrates are contained.

When drilling this well in the zone of the VGO, there was an increase in the gas content of methane gas in the drilling fluid during the passage of the above gas hydrate intervals. With further deepening of the well, it was noted, and it also recorded an increase in gas content, that the drilling fluid had an increased positive temperature, which also indicates the presence of gas (gas hydrate) formations in the ZGO. As the studies show, this is due to the decomposition of gas hydrates under the influence of an elevated temperature of the drilling fluid and the release of free gas when drilling in oil wells. The noted factor of increasing the gas content of the drilling fluid during drilling in the VGO also confirms the presence of gas hydrates in the studied section of the oilfield.

For the studied four wells of the rock, taking into account the electric logging data, zones (position 8 in Fig. 1) with high values of IES are highlighted. For well No. 301 in the depth interval 222 m - 247 m KES = 16-23 Ohm · m. For well No. 501 in the depth interval 210 m - 238 m IES = 17-24 Ohm · m. For well No. 601 in the depth interval 218 m - 228 m IES = 15-21 Ohm · m. For well No. 1001 in the depth interval 220 m - 243 m IES = 17-22 Ohm · m. It should be noted that these zones are located inside the depth intervals (position 7) with lower rock temperatures and include rocks (position 9 in figure 1) having the lowest temperature. The rocks of the studied wells in the selected zones (position 8 in figure 1) are considered to contain gas hydrates.

When drilling the above-mentioned wells in the zone of the ZGO, there was an increase in the gas content of methane gas in the drilling fluid during the drilling of depth intervals where there are gas hydrates. With further deepening of the wells, the used drilling fluid was characterized by an increased positive temperature. This indicated the presence of gas strata in the VGO, including gas hydrate.

The points where the minimum temperatures were detected (position 9 in Fig. 1) in the selected low-temperature interval of depths (position 7), as well as the presence of zones (position 8) with CES values of at least 15 Ohm · m, allow reliable isolation of rocks with gas hydrates.

Chilled gas evolved during decomposition of gas hydrates in the identified zones (position 8 in FIG. 1) freely moves up and down in the indicated depth intervals (position 7) and additionally cools the surrounding rocks with the formation of “low temperature funnels” that are clearly traceable in the diagram (position 7 in figure 1) relative to the lowest temperature (position 9 in figure 1) in the low-temperature range of depths.

Using the proposed method allows to increase its efficiency by identifying gas hydrate rocks occurring in low-temperature rocks below the bottom of the permafrost. It also makes it possible to reliably detect gas hydrates and map gas hydrate deposits according to well thermometry data during their construction to deeper productive horizons. In this case, there is no need for special geological exploration-intensive work on the selection and study of cores with gas hydrates.

Claims (3)

1. A method for detecting gas hydrates in low-temperature rocks, which consists in conducting standard electric logging of a well containing geological section of its low-temperature rock (NP), performing its thermometry and searching for zones containing gas hydrates, characterized in that standard electric logging of a well is carried out in NPs lying below the base of permafrost (MMP), and before the thermometry of the well in the NP, an area of possible occurrence of gas hydrates and hydrate formation, characterized by the presence of m in it the thermobaric conditions for the existence of gas hydrates in the rocks, in the selected area of the oilfield according to standard electric logging, zones are recorded in which the measured values of the apparent electrical resistance of the oilfield are at least 15 Ohm · m, and the coolant is pumped in the well, and after thermometry, they search zones, the temperature of the rocks in which, relative to the lowest recorded temperature in the selected zone, is not less than 0.2-0.5 ° C lower than the temperatures of the rocks adjacent to the boundaries of the detected zones, at m is regarded as the last zone containing gas hydrates. 2. The method according to claim 1, characterized in that for the area of possible occurrence of gas hydrates and hydrate formation take the area of occurrence of the NP, characterized by thermobaric conditions for the existence of gas hydrates in the rocks, which is defined as the site of occurrence of the NP in the geological section of the well, where the pressure values in the rocks at the corresponding these pressures, the temperature values are equal to hydrostatic or anomalous pressures that are not lower than the equilibrium hydrate pressures and the corresponding temperature temperature not higher than equilibrium, with a certain component composition of natural gas in a given area of occurrence of NP. 3. The method according to claim 1, characterized in that the thermometry in the well is carried out using highly sensitive thermometers that provide an error in temperature measurements of not more than 0.01 ° C.

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