CN117784278B - Prediction method and prediction system for dense sandstone gas dessert - Google Patents

Abstract

The invention discloses a prediction method and a prediction system for a dense sandstone gas dessert, and relates to the technical field of oil-gas geological exploration. Comprising the following steps: drawing an intersection chart of cumulative gas yield, monthly gas yield and year and month of production of the tight sandstone gas well, judging the decreasing trend of the cumulative gas yield of the single well according to the intersection chart, and calculating the cumulative gas yield of the single well of the tight sandstone gas according to the decreasing trend; acquiring static control factors of compact sandstone gas, and creating an enrichment coefficient EF; and drawing a prediction intersection diagram, and obtaining a lower limit value of dense sandstone gas enrichment according to the prediction intersection diagram, wherein the lower limit value is a dense sandstone gas dessert value. According to the method, the characteristics of the tight sandstone gas reservoir are taken as a starting point, a prediction intersection chart is drawn, and the lower limit value of the enrichment of the tight sandstone gas is obtained through the prediction intersection chart, namely the value of the dessert of the tight sandstone gas, so that the method can replace the unimpeded flow of the tight sandstone gas well to reflect the enrichment degree of the tight sandstone, and the detection of the dessert of the tight sandstone gas is more accurate.

Description

A prediction method and prediction system for tight sandstone gas sweet spots

Technical Field

The invention relates to the technical field of oil and gas geological exploration, and in particular to a prediction method and a prediction system for tight sandstone gas sweet spots.

Background technique

With the continuous development of unconventional oil and gas geological theory and the advancement of exploration and development technology, the prediction of tight sandstone gas sweet spots has become the key and core of unconventional natural gas geology, and is also the weak link in the research of natural gas accumulation, enrichment and target prediction.

The exploration and development of tight sandstone gas in most gas fields in China has entered a situation of "development as the main factor and exploration as the auxiliary factor". The main controlling factors for evaluating tight sandstone gas sweet spots have gradually shifted from six major factors to reservoir-based. The existing technology uses enrichment main controlling factors, multivariate linear regression, reservoir double sweet spot evaluation parameters, and seismic attribute prediction based on machine learning to calculate the unimpeded flow rate of tight sandstone gas wells, thereby evaluating tight sandstone gas sweet spots.

The defects of the above-mentioned existing technologies are: the tight sandstone gas reservoir has the characteristics of "low porosity and permeability - dense, and must be fracturing and transformed during development", the unimpeded flow rate of tight sandstone gas wells in the usual sense cannot reflect the enrichment degree of tight sandstone, and the evaluation of tight sandstone gas sweet spots is not accurate enough.

Summary of the invention

Based on this, it is necessary to provide a prediction method and prediction system for tight sandstone gas sweet spots in response to the above technical problems.

The embodiment of the present invention provides a method for predicting tight sandstone gas sweet spots, comprising:

Draw a cross-plot of the cumulative gas production, monthly gas production and production year and month of tight sandstone gas wells.

According to the intersection diagram, the decreasing trend of the cumulative gas production of a single well is identified, and the cumulative gas production of a single well of tight sandstone gas is calculated through the decreasing trend.

Draw the predicted cross-plots of the cumulative gas production of a single well and porosity Φ , single-layer sand body thickness H , and enrichment factor EF _, as well as the predicted cross-plots of the single-layer sand body thickness H and gas saturation Sg ;

Draw the predicted cross-plots of the cumulative gas production of a single well, porosity Φ , gas saturation Sg _, single-layer sand body thickness H , and enrichment factor EF respectively;

The lower limit value of the tight sandstone gas enrichment is obtained according to all the predicted cross-plots, and the lower limit value is the tight sandstone gas sweet spot value.

In addition, the method of identifying the decreasing trend of the cumulative gas production of a single well according to the cross-plot includes:

When the cumulative gas production and monthly gas production of tight sandstone gas wells present a linear relationship on the cross-plot, the decreasing trend of the cumulative gas production of a single well is exponential decrease;

When the cumulative gas production and monthly gas production of tight sandstone gas wells present a semi-logarithmic linear relationship on the cross-plot, the decreasing trend of the cumulative gas production of a single well is a harmonic decrease;

When the cumulative gas production and monthly gas production of tight sandstone gas wells are neither in a linear relationship nor in a semi-logarithmic linear relationship on the intersection diagram, the decreasing trend of the cumulative gas production of a single well is a hyperbolic decrease.

In addition, the single well cumulative gas production of tight sandstone gas predicted by the decreasing trend includes:

When the decreasing trend is harmonic decreasing and the decreasing index n=1, then:

Q / _Qi = (D / _Di ) ⁿ ;

When the decreasing trend is exponential, the decreasing exponential , then:

Q / _Qi = (D / _Di ) ⁿ

Wherein, Q is the cumulative gas production of a single well, Qi _is the initial production in the decline stage, D is the production decline rate, _Di is the initial instantaneous decline rate, n is the decline index, and i is a positive integer;

When the decreasing trend is hyperbolic, 1＜decreasing exponent n＜∞,

Q = _Qi /{1+( _Di /n)×t} ⁿ

Among them, Q is the cumulative gas production of a single well, _Qi is the initial production in the decline stage, _Di is the initial instantaneous decline rate, t is the production time in the decline stage, n is the decline index, and i is a positive integer.

In addition, the static control factors specifically include:

Porosity Φ: The porosity Φ of a single sand body is calculated using the combined logging parameters of natural gamma GR, neutron logging CNL, acoustic logging AC and density logging DEN;

Gas saturation S _g : Calculate the water saturation model using Archie's formula and obtain the gas saturation S _g :

S _w ⁿ = abR _w /R _t Ф ^m

_Sg = 1 - _Sw

Among them, _Sw is water saturation, a and b are lithology coefficients, _RW is formation water resistivity, _Rt is formation resistivity, Ф is porosity, m is cementation coefficient, and n is saturation index;

Single-layer sand body thickness H: The thickness of a single-layer sand body is determined based on the porosity Φ and gas saturation Sg _of the single-layer sand body.

In addition, the enrichment factor EF is:

EF = H × Φ × S _g

Among them, H is the thickness of a single sand layer, Φ is the porosity, _{and Sg} is the gas saturation.

In addition, the lower limit values for obtaining tight sandstone gas enrichment include:

By intersecting the cumulative gas production of a single well with the thickness H of a single sand body, an evaluation standard for the thickness of a single sand body in terms of the richness of tight sandstone gas reservoirs is established.

Through the intersection of single well cumulative gas production and porosity Φ, the evaluation standard of tight sandstone gas enrichment and porosity is established;

Through the intersection _of single-layer sand body thickness H and gas saturation Sg , the evaluation standard of tight sandstone gas richness and gas saturation is established;

Through the intersection of single well cumulative gas production and enrichment factor EF, the evaluation standard of tight sandstone gas enrichment and enrichment factor is established;

The lower limit of tight sandstone gas enrichment is determined based on the single-layer sand body thickness evaluation standard, porosity evaluation standard, gas saturation evaluation standard and enrichment coefficient evaluation standard.

In addition, a prediction system for tight sandstone gas sweet spots includes:

The drawing module is used to draw the intersection diagram of the cumulative gas production, monthly gas production and production year and month of tight sandstone gas wells;

The identification module is used to identify the decreasing trend of the cumulative gas production of a single well according to the intersection diagram, and calculate the cumulative gas production of a single well of tight sandstone gas through the decreasing trend;

A combined module is used to obtain the static control factors of tight sandstone gas, which include porosity Φ , gas saturation Sg and single-layer sand body thickness _H. All static control factors are combined to create an enrichment factor EF ;

The secondary drawing module is used to draw the predicted cross-plot of the cumulative gas production of a single well and the porosity Φ , the thickness of a single sand body H , and the enrichment factor EF , as well as the predicted cross-plot of the thickness of a single sand body H and _the gas saturation Sg ;

The inference module is used to obtain the lower limit value of the tight sandstone gas enrichment according to all the predicted intersection diagrams, and the lower limit value is the tight sandstone gas sweet spot value.

Compared with the prior art, the above-mentioned prediction method and prediction system for tight sandstone gas sweet spots provided by the embodiment of the present invention have the following beneficial effects:

The present invention draws a cross-plot of the cumulative gas production, monthly gas production and production year and month of a tight sandstone gas well, and identifies the decreasing trend of the cumulative gas production of a single well according to the cross-plot; calculates the cumulative gas production of a single well of tight sandstone gas according _to the decreasing trend; obtains static control factors of tight sandstone gas, the static control factors include: porosity Φ , gas saturation Sg and single-layer sand body thickness H , and creates an enrichment factor EF by combining all static control factors; draws predicted cross-plots of the cumulative gas production of a single well with porosity Φ , single-layer sand body thickness H , and enrichment factor EF, and predicted cross-plots of _the single-layer sand body thickness H and gas saturation Sg ; and obtains a lower limit value of the enrichment of tight sandstone gas according to the predicted cross-plot, which is the sweet spot value of tight sandstone gas. Compared with the existing technology, based on the characteristics of tight sandstone gas reservoirs, the predicted intersection diagrams of single well cumulative gas production and porosity Φ , single-layer sand body thickness H , and enrichment factor EF , as well as the predicted intersection diagram of single-layer sand body thickness H and gas saturation Sg were drawn _respectively ; the lower limit value of tight sandstone gas enrichment was obtained through the predicted intersection diagram, and the lower limit value was the tight sandstone gas sweet spot value.

The above scheme can replace the open flow rate of tight sandstone gas wells to reflect the enrichment degree of tight sandstone, and the detection of tight sandstone gas sweet spots is more accurate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic flow chart of a method for predicting tight sandstone gas sweet spots provided in one embodiment;

FIG2 is a hyperbolic decreasing trend diagram of a method for predicting tight sandstone gas sweet spots provided in one embodiment;

FIG. 3 is a prediction cross-plot of a method for predicting tight sandstone gas sweet spots provided in an embodiment.

Detailed ways

In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

In one embodiment, a method for predicting tight sandstone gas sweet spots is provided, as shown in FIG1 , and the method includes:

1. Draw a cross-plot of the cumulative gas production, monthly gas production and production year and month of tight sandstone gas wells.

2. According to the intersection diagram, the decreasing trend of cumulative gas production is identified, and the cumulative gas production of a single well of tight sandstone gas is calculated based on the decreasing trend.

2.1. Identify the decreasing trend of the cumulative gas production of a single tight sandstone gas well based on the intersection diagram.

(1) When the cumulative gas production and monthly gas production of tight sandstone gas wells present a linear relationship on the intersection diagram, the decreasing trend of the cumulative gas production of a single well is an exponential decrease.

(2) When the cumulative gas production and monthly gas production of tight sandstone gas wells present a semi-logarithmic linear relationship on the cross-plot, the decreasing trend of the cumulative gas production of a single well is a harmonic decrease.

(3) If neither of the above two characteristics is obvious, the decreasing trend of the cumulative gas production of a single well is a hyperbolic decrease.

The upper left corner of Figure 2 is the intersection diagram of single well Daji 6-7, the upper right corner of Figure 2 is the intersection diagram of single well Daji 7-8, the lower left corner of Figure 2 is the intersection diagram of single well Daji 5-4 to 4, and the lower right corner of Figure 2 is the intersection diagram of single well 1-7 to 6.

2.2. Calculate the cumulative gas production value of a tight sandstone gas well by using the decline trend of single well production corrected by production data;

The most commonly used method is the law of declining returns proposed by Arps:

Q / _Qi = (D / _Di ) ⁿ

Where, Q = gas production, 10 ⁴ m ³ /d; _Qi = initial production in the decline phase, 10 ⁴ m ³ /d; D = production decline rate, 1/month; _Di = initial instantaneous decline rate; n = decline index; i is a positive integer. Based on the existing drilling and gas test results, including but not limited to gas production ( Q ), initial production in the decline phase ( Qi ₎ and test production time (t). Then, the production decline rate (D) is calculated by gas production ( Q )/test production time (t), and the initial instantaneous decline rate ( Di ) is calculated by initial production in the decline phase ( Qi ₎ /test production time ( _t ).

The decreasing trend is divided into three types: exponential decreasing, hyperbolic decreasing and harmonic decreasing;

When the decreasing trend is exponential, the monthly output and cumulative output are in a linear relationship on the ordinary coordinate system, and the decreasing exponent n →∞;

When the decreasing trend is harmonic decreasing, the monthly output and cumulative output are in a semi-logarithmic linear relationship, and the decreasing index n = 1;

When the decreasing trend is hyperbolic, 1＜decreasing exponent n＜∞:

Q = _Qi /{1+( _Di /n)×t} ⁿ

Where, Q = gas production, 10 ⁴ m ³ /d; _Qi = initial production in the decline phase, 10 ⁴ m ³ /d; _Di = initial instantaneous decline rate, 1/month; t is the production time in the decline phase; n = decline index; i is a positive integer.

Table 1 Comparison between the predicted and actual cumulative gas production of a typical single well in a certain block of tight sandstone gas in the seventh year

The hyperbolic decline law is applied to calculate the cumulative gas production in the seventh year. As shown in Table 1, through the typical wells with a long production time in the Daning-Jixian block, it is found that the cumulative gas production predicted by the hyperbolic decline is highly consistent with the actual production data, which further verifies that the decline of the tight gas in the Upper Paleozoic in this study area conforms to the hyperbolic decline law, and this decline law conforms to the dynamic trend of tight gas production in the study area.

3. Obtain the static control factors of tight sandstone gas, which include porosity Φ, gas saturation Sg _and single-layer sand body thickness H. Combine all the static control factors to create the enrichment factor EF.

3.1. Optimize the static control factors of tight sandstone gas, such as single-layer sand body thickness (H), porosity (Φ) and gas saturation ( S _g ) and other related parameters.

(1) Porosity Φ

The calculation of reservoir porosity by logging data mainly relies on three porosity logging methods: neutron, density, and sonic. There is no essential difference in the formation porosity reflected by these three porosity logging methods in homogeneous clastic rock formations. Considering that the tight sandstone has strong vertical and horizontal heterogeneity, which may affect the porosity value reflected by a single factor, the logging parameter combination of natural gamma logging parameter GR, neutron logging parameter CNL, sonic logging parameter AC, and density logging parameter DEN is selected to calculate the porosity. Based on the constraints of the gas test results of ^1000m3 and ^3000m3 production of a single well, the interpretation combination parameters of the single-layer sand body of the gas layer of He8+Shan2+Benxi drilling in Daning-Jixian block are read respectively. According to the statistical analysis results, the intersection relationships between the natural gamma logging parameters GR, neutron logging parameters CNL, acoustic logging parameters AC and density logging parameters DEN and the porosity Φ corresponding to the production of ^1000m3 and ^3000m3 were respectively selected, and a model of the combined logging parameters of the natural gamma logging parameters GR, neutron logging parameters CNL, acoustic logging parameters AC, density logging parameters DEN and the porosity Φ was established.

The porosity of the single-layer sand body in the study area was fully interpreted according to the model: the calculation of reservoir porosity by logging data mainly relies on three porosity logging methods: neutron, density, and acoustic wave. There is no essential difference in the formation porosity reflected by these three porosity logging methods in homogeneous clastic rock formations. Considering that the tight sandstone has strong vertical and horizontal heterogeneity, it may affect the porosity value reflected by a single factor. Therefore, the sandstone is evaluated based on the natural gamma logging parameter GR and the neutron logging parameter CNL. After that, the porosity is calculated by the combination of the acoustic logging parameter AC and the density logging parameter DEN. The average value of the two calculated values is taken as the calculated porosity value of the sandstone.

The porosity of mudstone is calculated using Wyllie's acoustic time difference equation (Wyllie et al., 1956):

Φ _AC = ( Δt-Δt _ma ) / ( Δt _f - Δt _ma ) (1)

Where, Δt is the acoustic wave propagation time, Δt _f is the acoustic wave propagation time of the pore fluid, Δt _ma is the acoustic wave propagation time of the matrix (or particles), and Φ _AC represents the porosity of the sandstone at the burial depth z.

The mudstone porosity calculated by density logging is determined by the following formula (Dasgupta et al., 2016):

Φ _DEN = ( ρ _ma - ρ _b ) / ( ρ _ma - ρ _f ) (2)

Where ρma is the sandstone matrix density, ρb is the sandstone volume density obtained _from density logging, _and ρf _is the pore fluid density.

(2) Gas saturation S _g

For low permeability sandstone gas-bearing reservoirs, the rock Archie parameter characteristics were analyzed through rock resistivity experiments, and the water saturation model was established using the Archie formula. The reliability of water saturation was verified by high-pressure mercury injection experiments and nuclear magnetic resonance experiments combined with the actual geological background of tight gas accumulation in the study area.

The rock resistivity experiment refers to the standard "SY T 5385-2007 Laboratory Measurement and Calculation Method of Rock Resistivity Parameters". Using the Archie formula, the formation factor ( F ), resistivity increase rate ( I ), cementation index ( m ) and saturation index ( n ) are calculated in turn. For multiple rock samples from the same formation, the cross plot of formation factor ( F ) and porosity ( Φ ) and the cross plot of resistivity increase rate ( I ) and water saturation ( S _w ) are plotted in double logarithmic coordinates. The formation factor ( F ) and porosity ( Φ ) curves, and the resistivity increase rate ( I ) and water saturation ( S _w ) curves are fitted with a power function to obtain the lithology coefficients ( a and b ) and cementation index ( m ), saturation index ( n ). The Archie formula is:

Among them, Sw is water saturation, a and b are lithology coefficients , RW is formation water resistivity, Rt is formation resistivity, Ф is _porosity , _m is cementation coefficient, _and n is saturation index.

(3) Single-layer sand body thickness H

The thickness of the single-layer sand body is determined according to the porosity Φ and gas saturation Sg _of the single-layer sand body. Specifically, based on the interpretation of the gas layer under the constraints of single-well production of ^1000m3 and ^3000m3 , the interpretation combination parameters of the single-layer sand body in the gas layer of He8+Shan2+Benxi in the Daning-Jixian block are selected as the natural gamma logging parameter GR, neutron logging parameter CNL, acoustic logging parameter AC, density logging parameter DEN, porosity Φ, and gas saturation Sg _. The single-layer sand body thickness is interpreted according to the logging combination parameters. Based on the gas test results of existing wells, that is, according to the measured constraints of single-well production of ^1000m3 and ^3000m3 , the interpretation combination parameters of the single-layer sand body in the gas layer of He8+Shan2+Benxi in the Daning-Jixian block are read respectively. According to the statistical analysis results, the lower limits of the natural gamma logging parameter GR, neutron logging parameter CNL, acoustic logging parameter AC, density logging parameter DEN, porosity Φ and gas saturation Sg logging parameters corresponding to the production of ^1000m3 and ^3000m3 are respectively selected, and a logging combination parameter model of the natural gamma logging parameter GR, neutron logging parameter CNL, acoustic logging parameter AC, density logging parameter DEN, porosity Φ and gas saturation Sg is _established to interpret the thickness of the single-layer _sand body gas layer in the study area, as shown in Example 1.

Example 1

Selection of combined interpretation parameters for single-layer sand bodies in He8+Shan2+Benxi gas layers in Daning-Jixian block

When the daily production of a single well is 1000m ³ : the acoustic logging parameter AC≥195μs/m, the resistivity value M2RX≥70Ω.m, the neutron logging parameter CNL≤18.0%, the density logging parameter DEN≤2.67g/cm ³ , and the natural gamma logging parameter GR≤92API.

When the daily production of a single well is 3000m ³ : the acoustic logging parameter AC≥195μs/m, the resistivity value M2RX≥70Ω.m, the neutron logging parameter CNL≤16.0%, the density logging parameter DEN≤2.64g/cm ³ , and the natural gamma logging parameter GR≤90API.

The electrical property values of the Shanxi Formation Shan 2 Section of the Upper Paleozoic Permian in the Daning-Jixian block are constrained by the daily gas production of a single well for 90 days as follows:

When the daily production of a single well is ^1000m3 : gas saturation Sg ≥66%, porosity calculated by logging Φ _≥4.1 %, acoustic logging parameter AC≥195μs/m, neutron logging parameter CNL≤19.5%.

When the daily production of a single well is ^3000m3 : gas saturation Sg ≥75%, porosity calculated by logging Φ _≥5.5 %, acoustic logging parameter AC≥195μs/m, neutron logging parameter CNL≤19.5%.

3.2. Combining the above static parameters, the enrichment factor (EF) is established, that is, EF = H × Φ × S _g , which is used to evaluate the tight sandstone gas sweet spot.

4. Draw the predicted cross-plots of the cumulative gas production of a single well and porosity Φ , gas saturation S _g , and enrichment factor EF , as well as the cross-plots of the thickness H of a single sand body and gas saturation S _g .

According to the predicted cross-plot, the lower limit of tight sandstone gas enrichment is obtained, which is the sweet spot of tight sandstone gas, as shown in Figure 3:

4.1. By intersecting the cumulative gas production of a single well with the thickness of a single sand body (H), an evaluation standard for the thickness of a single sand body in terms of the richness of a tight sandstone gas reservoir is established (as shown in Figure 3 (a)).

4.2. By intersecting the single-layer sand body thickness (H) and gas saturation ( S _g ), the evaluation standard of tight sandstone gas richness and gas saturation is established (as shown in Figure 3 (b));

4.3. By intersecting the cumulative gas production of a single well with the porosity (Φ), the richness and porosity evaluation standard of tight sandstone gas reservoirs is established (as shown in Figure 3 (c));

4.4. By intersecting the cumulative gas production of a single well with the enrichment factor (EF), an evaluation standard for the enrichment factor of tight sandstone gas reservoirs is established (as shown in Figure 3 (d)).

4.5. The lower limit of the tight sandstone gas enrichment is obtained according to all the prediction cross-plots. The lower limit is the sweet spot of tight sandstone gas, as shown in Table 2. In the table, the cumulative gas production of Class I is 0.077×10 ⁸ m ³ in 7 years, which is equivalent to 3000m ³ per day per well in 7 years, and the cumulative gas production of Class II is 0.026×10 ⁸ m ³ in 7 years, which is equivalent to 1000m ³ per day per well in 7 years. The above is the lower limit of industrial-scale gas layers in a certain block of PetroChina.

Table 2 Evaluation criteria for sweet spots in key strata of tight sandstone gas in a block of the Ordos Basin

In one embodiment, a prediction system for tight sandstone gas sweet spots is provided, comprising:

The drawing module is used to draw the intersection diagram of the cumulative gas production, monthly gas production and production year and month of tight sandstone gas wells.

The identification module is used to identify the decreasing trend of the cumulative gas production of a single well according to the intersection diagram, and calculate the cumulative gas production of a single well of tight sandstone gas through the decreasing trend.

The combined module is used to obtain the static control factors of tight sandstone gas and to create the enrichment factor EF by combining all the static control factors.

The secondary drawing module is used to draw the predicted cross-plot of the cumulative gas production of a single well and the porosity Φ , the single-layer sand body thickness H , and the enrichment factor EF , as well as the predicted cross-plot of the single-layer sand body thickness H and _the gas saturation Sg .

The inference module is used to obtain the lower limit value of the tight sandstone gas enrichment according to all the predicted intersection diagrams, and the lower limit value is the tight sandstone gas sweet spot value.

The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims (5)

1. A method for predicting a dense sandstone gas dessert, comprising:

drawing an intersection diagram of cumulative gas yield, monthly gas yield and year and month of production of the dense sandstone gas well,

Judging the decreasing trend of the cumulative gas yield of the single well according to the intersection graph, and calculating the cumulative gas yield of the single well of the tight sandstone gas according to the decreasing trend;

acquiring static control factors of the dense sandstone gas, wherein the static control factors comprise: the porosity phi, the gas saturation S _g and the single-layer sand thickness H are combined, and all static control factors are combined to create an enrichment coefficient EF;

Respectively drawing a prediction intersection chart of the cumulative gas yield and the porosity phi of a single well, the thickness H of a single-layer sand body and the enrichment coefficient EF, and a prediction intersection chart of the thickness H of the single-layer sand body and the saturation S _g of gas;

Obtaining a lower limit value of the enrichment degree of the dense sandstone gas according to all the predicted intersection graphs, wherein the lower limit value is a dense sandstone gas dessert value;

wherein,

The step-down trend of the cumulative gas production of the single well is judged according to the intersection diagram, and the step-down trend comprises the following steps:

When the cumulative gas yield of the compact sandstone gas well and the monthly gas yield are in a linear relation on an intersection chart, the decreasing trend of the cumulative gas yield of the single well is an exponential decrease;

when the cumulative gas yield of the compact sandstone gas well and the monthly gas yield are in a semilogarithmic straight line relationship on an intersection chart, the decreasing trend of the cumulative gas yield of the single well is in harmony decrease;

When the cumulative gas yield of the compact sandstone gas well and the monthly gas yield are in neither a straight line relation nor a semilogarithmic straight line relation on an intersection graph, the decreasing trend of the cumulative gas yield of the single well is hyperbolic decreasing;

The single well cumulative gas yield of the compact sandstone gas is calculated through the decreasing trend, and the method comprises the following steps:

when the decreasing trend is the harmonic decreasing, the decreasing index n=1, there is:

Q/Q_i＝(D/D_i)ⁿ

When the decreasing trend is decreasing, the decreasing index n → infinity is:

Q/Q_i＝(D/D_i)ⁿ

Wherein, Q is the cumulative gas production of a single well, Q _i is the initial yield in the decreasing stage, D is the yield decreasing rate, D _i is the initial instantaneous decreasing rate, n is the decreasing index, and i is a positive integer;

When the decreasing trend is a hyperbolic decrease, the decreasing index n < ++1 is:

Q＝Q_i/{1+(D_i/n)×t}ⁿ

wherein, Q is the cumulative gas production of a single well, Q _i is the initial yield of the decreasing stage, D _i is the initial instantaneous decreasing rate, t is the production time of the decreasing stage, n is the decreasing index, and i is a positive integer.

2. The method for predicting a dense sandstone gas dessert of claim 1, wherein said static control factors specifically comprise:

porosity Φ: calculating the porosity phi of the single-layer sand body by using a logging combination parameter natural gamma parameter GR, a neutron logging parameter CNL, an acoustic logging parameter AC and a density logging parameter DEN;

gas saturation S _g: calculating a water saturation model through an Archie formula, and solving for the gas saturation S _g:

S_w ⁿ＝abR_w/R_tФ^m

S_g＝1-S_w

Wherein S _w is water saturation, a and b are lithology coefficients, R _W is formation water resistivity, R _t is formation resistivity, phi is porosity, m is cementing coefficient, and n is saturation index;

thickness H of single-layer sand body: the thickness of the single-layer sand body is determined according to the porosity phi and the gas saturation S _g of the single-layer sand body.

3. The method of claim 2, wherein the enrichment factor EF is:

EF＝H×Φ×S_g

wherein H is the thickness of a single-layer sand body, phi is the porosity, and S _g is the gas saturation.

4. A method of predicting a dense sandstone gas dessert as claimed in claim 3, wherein said obtaining a lower limit of dense sandstone gas enrichment comprises:

establishing a single-layer sand thickness evaluation standard of the dense sandstone gas enrichment degree through intersection of the single-well cumulative gas yield and the single-layer sand thickness H;

Establishing a dense sandstone gas enrichment porosity evaluation standard through intersection of single well cumulative gas yield and porosity phi;

Establishing a dense sandstone gas enrichment degree gas saturation evaluation standard through intersection of the single-layer sand thickness H and the gas saturation degree S _g;

Establishing a dense sandstone gas enrichment degree enrichment coefficient evaluation standard through intersection of the accumulated gas yield of a single well and an enrichment coefficient EF;

And determining the lower limit value of the gas enrichment degree of the compact sandstone according to the single-layer sand thickness evaluation standard, the porosity evaluation standard, the gas saturation evaluation standard and the enrichment coefficient evaluation standard.

5. A system for predicting a dense sandstone gas dessert, comprising:

the drawing module is used for drawing an intersection chart of cumulative gas yield, monthly gas yield and production year and month of the tight sandstone gas well;

the judging and identifying module is used for judging the decreasing trend of the cumulative gas yield of the single well according to the intersection graph, and calculating the cumulative gas yield of the single well of the tight sandstone gas according to the decreasing trend;

the simultaneous module is used for acquiring static control factors of the tight sandstone gas, wherein the static control factors comprise: the porosity phi, the gas saturation S _g and the single-layer sand thickness H are combined, and all static control factors are combined to create an enrichment coefficient EF;

The secondary drawing module is used for respectively drawing a prediction intersection chart of the cumulative gas production and the porosity phi of the single well, the thickness H of the single-layer sand body and the enrichment coefficient EF, and a prediction intersection chart of the thickness H of the single-layer sand body and the gas saturation S _g;

The inference module is used for obtaining a lower limit value of the enrichment degree of the dense sandstone gas according to all the prediction intersection graphs, wherein the lower limit value is a dense sandstone gas dessert value;

wherein,

The step-down trend of the cumulative gas production of the single well is judged according to the intersection diagram, and the step-down trend comprises the following steps:

When the cumulative gas yield of the compact sandstone gas well and the monthly gas yield are in a linear relation on an intersection chart, the decreasing trend of the cumulative gas yield of the single well is an exponential decrease;

when the cumulative gas yield of the compact sandstone gas well and the monthly gas yield are in a semilogarithmic straight line relationship on an intersection chart, the decreasing trend of the cumulative gas yield of the single well is in harmony decrease;

When the cumulative gas yield of the compact sandstone gas well and the monthly gas yield are in neither a straight line relation nor a semilogarithmic straight line relation on an intersection graph, the decreasing trend of the cumulative gas yield of the single well is hyperbolic decreasing;

The single well cumulative gas yield of the compact sandstone gas is calculated through the decreasing trend, and the method comprises the following steps:

when the decreasing trend is the harmonic decreasing, the decreasing index n=1, there is:

Q/Q_i＝(D/D_i)ⁿ

When the decreasing trend is decreasing, the decreasing index n → infinity is:

Q/Q_i＝(D/D_i)ⁿ

Wherein, Q is the cumulative gas production of a single well, Q _i is the initial yield in the decreasing stage, D is the yield decreasing rate, D _i is the initial instantaneous decreasing rate, n is the decreasing index, and i is a positive integer;

When the decreasing trend is a hyperbolic decrease, the decreasing index n < ++1 is:

Q＝Q_i/{1+(D_i/n)×t}ⁿ

wherein, Q is the cumulative gas production of a single well, Q _i is the initial yield of the decreasing stage, D _i is the initial instantaneous decreasing rate, t is the production time of the decreasing stage, n is the decreasing index, and i is a positive integer.