Disclosure of Invention
The invention aims to provide a method for solving under-compaction and overpressure in a stratum, which overcomes the defects in the prior art, has the characteristics of rapidness, simplicity, convenience and accuracy, and provides a new method for accurately evaluating the under-compaction and overpressure.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for determining under-compaction overpressure in a stratum comprises the following steps:
step 1: determining a normal compaction section and a under-compaction action section according to the difference change characteristics of the average acoustic time difference, the average stratum density and the average neutron porosity of the mudstone section along with the average burial depth;
step 2: fitting an exponential relation between the sound wave speed of the normal compaction section and the scatter diagram of the vertical effective stress;
and step 3: the vertical effective stress of the under-compaction action section and the change point of the sound wave speed of the under-compaction action section are required to fall on the change trend line of the normal compaction action, so that the vertical effective stress of the under-compaction action section can be solved by substituting the sound wave speed of the under-compaction action section into the exponential relation obtained in the step 2;
and 4, step 4: and solving an overpressure value in the under-compaction action section by combining a relational expression of the vertical effective stress and the overpressure of the under-compaction action section.
Further, step 1 specifically includes:
step 1.1: identifying a mudstone section with the thickness of more than 5m on the logging curve;
step 1.2: reading the average acoustic wave time difference value AC, the average stratum density value DEN1, the average neutron porosity value CNL, the average burial depth H and the average density DEN2 of the stratum above the H burial depth point of the selected mudstone section on a logging curve;
step 1.3: in the same depth coordinate system, a scatter diagram of the relation of the time difference value AC of the average sound wave of the mudstone section with the average buried depth H of the mudstone section, a scatter diagram of the relation of the average stratum density value DEN1 of the mudstone section with the average buried depth H of the mudstone section and a scatter diagram of the relation of the average neutron porosity value CNL of the mudstone section with the average buried depth H of the mudstone section are made, and a normal compaction section and a under-compaction section are determined according to the scatter diagrams.
Further, step 1.3 determines that the normal compaction section and the under-compaction action section are specifically as follows: according to the principle that the porosity and the buried depth of the mudstone in the normal compaction section have index change, the sound wave time difference in well logging, the neutron porosity and the stratum porosity are positively correlated, and the stratum density and the stratum porosity are inversely correlated, the mudstone in the normal compaction section has the rule that the average sound wave time difference AC and the buried depth H are exponentially changed, the average stratum density value DEN1 and the average neutron porosity value CNL are linearly changed along with the average buried depth H, and the normal compaction section is arranged at the relatively shallow part of the buried depth, so that the normal compaction section is comprehensively determined;
and comprehensively determining the under-compaction well section of the well according to the change rules that the under-compaction action section has the average acoustic time difference AC, the average stratum density value DEN1 and the average neutron porosity value CNL are deviated from the corresponding normal compaction sections along with the change of the average burial depth H, the change curves of the under-compaction action section and the normal compaction sections are basically parallel, the average acoustic time difference AC and the average neutron porosity value CNL are deviated to the direction with abnormal large, and the average stratum density value DEN1 is deviated to the direction with abnormal small.
Further,
step 2 specifically comprises: by using the vertical effective stress sigma of the mudstone in the normal compaction section
vMaking a scatter diagram with the sound wave speed V, fitting the exponential relation of the sound wave speed V and the sound wave speed V to obtain an exponential equation with constants a and b, and solving the numerical values of a and b to obtain the vertical effective stress sigma of the mudstone in a normal compaction section and an under-compaction section
v' variation with its acoustic velocity V:
wherein the sound wave velocity V is 1/AC × 1000.
Further, the relation between the vertical effective stress of the under-compaction action section and the overpressure in the step 4 is as follows:
σv’=DEN2×g×H-Pformation of earth
In the normal compaction zone, PFormation of earthEqual to hydrostatic pressure PQuiet(ii) a In the under-compacted zone, PFormation of earthEqual to hydrostatic pressure PQuietAdding the overpressure value delta P of the under-compacted mudstone, namely the delta P of the normal compaction section is 0, so that
PFormation of earth=PQuiet+△P
Wherein, PQuiet=ρWater (W)×g×H,ρWater (W)Is the density of the formation water.
Compared with the prior art, the invention has the following beneficial technical effects:
the method is mainly based on logging series data such as acoustic time difference, stratum density, neutron porosity and the like, and a normal compaction section and an under-compaction action section are determined according to the difference change characteristics of the average acoustic time difference, the average stratum density and the average neutron porosity of a thicker mudstone section along with the buried depth; fitting an exponential relation between the acoustic velocity of the normally compacted mudstone section and a scatter diagram of the vertical effective stress; substituting the sound wave speed of the under-compacted mudstone overpressure section into the obtained exponential relation to obtain the vertical effective stress of the under-compacted overpressure mudstone section; and solving the size of under-compaction pressurization in the overpressure section by combining the relation between the vertical effective stress of the overpressure section and the overpressure. The method can obtain the accurate under-compaction and over-pressure only by using logging information, has the characteristics of rapidness, simplicity, convenience and accuracy, and improves the evaluation efficiency of the under-compaction and over-pressure.
Detailed Description
The invention is described in further detail below:
a method for determining under-compaction overpressure in a formation, comprising the steps of:
step 1: identifying a mudstone section with the thickness of more than 5m on the logging curve;
step 2: reading the average sound wave time difference value AC (mu s/m) and the average stratum density value DEN1 (g/cm) of the selected shale section on the logging curve3) Average neutron porosity value CNL (%), average depth of burial H (m), and average density of formation above H burial point DEN2 (g/cm)3);
And step 3: in the same depth coordinate system, making a scatter diagram of a relation of an average acoustic time difference value AC of a mudstone section along with the buried depth H, a scatter diagram of an average stratum density value DEN1 along with the buried depth H and a scatter diagram of an average neutron porosity value CNL along with the buried depth H, wherein according to the characteristics that the porosity and the buried depth of the mudstone of a normal compaction section have exponential change, and the principles that the acoustic time difference value, the neutron porosity and the stratum porosity are positively correlated and the stratum density and the stratum porosity are inversely correlated in the well are measured, the normal compaction mudstone section has an exponential change of the average acoustic time difference AC and the buried depth H, the average stratum DEN1 has a linear change rule along with the buried depth H and the average neutron porosity value CNL along with the buried depth H, and the other normal compaction section is required to appear at a part with relatively shallow buried depth, so as to comprehensively determine the normal compaction section; according to the characteristic that the mudstone at the under-compaction action section has abnormally high porosity, the mudstone at the under-compaction action section has the variation trend that the average acoustic time difference AC, the average stratum density value DEN1 and the average neutron porosity value CNL deviate from the corresponding normal compaction section along with the variation of the burial depth H, the average acoustic time difference AC and the average neutron porosity value CNL deviate towards the abnormally large direction, the average stratum density value DEN1 deviates towards the abnormally small direction, and the variation rule that the variation curve of the average acoustic time difference AC and the average neutron porosity value CNL is basically parallel to the variation curve of the corresponding normal compaction section is provided, and the well section with the characteristics is the under-compaction action section of the mudstone, as shown in figure 2;
a transition zone is generally arranged between the normal compaction section and the under-compaction action section, the transition zone is characterized in that the average acoustic wave time difference AC, the average stratum density value DEN1 and the average neutron porosity value CNL deviate from the variation trend of the corresponding normal compaction section along with the variation of the average burial depth H, the average acoustic wave time difference AC and the average neutron porosity value CNL deviate towards an abnormally large direction, the average stratum density value DEN1 deviates towards an abnormally small direction, and the variation curve of the average stratum density value DEN1 is not parallel to the variation curve of the normal compaction section.
And 4, step 4: establishing a vertical effective stress sigmav' associated with average depth of burial H, average density of overburden DEN2, gravitational acceleration g, and formation pressure PFormation of earthMathematical relation of
Formula 1, sigmav’=DEN2×g×H-PFormation of earth
In the normal compaction zone, PFormation of earthEqual to hydrostatic pressure PQuiet(ii) a In the under-compacted zone, PFormation of earthEqual to hydrostatic pressure PQuietPlus the overpressure value DeltaP of the poorly compacted mudstone, so
Formula 2, PFormation of earth=PQuiet+△P
Wherein, PQuiet=ρWater (W)×g×H,ρWater (W)Where Δ P for the normal compacted section is 0, is the density of the formation water;
and 5: calculating the sonic velocity V (Km/s) of the mudstone section
Formula 3, V1/AC 1000
Step 6: the variation of the vertical effective stress of the normal compaction section and the sound wave speed thereof follows the exponential variation of the normal compaction, and the relation point of the vertical effective stress of the under-compaction action section and the sound wave speed thereof also falls on the variation trend line of the normal compaction action, wherein the vertical effective stress sigma of the mudstone of the normal compaction section and the under-compaction action sectionv' variation with its acoustic velocity v
In the formula (4), the first and second groups,
where a and b are constants which pass the obtained vertical effective stress σ of the normal compacted zone
v' is obtained by fitting this exponential relationship to the value of its acoustic velocity v.
And 7: by using the vertical effective stress sigma of the mudstone in the normal compaction sectionv' fitting an exponential relation between the sound wave velocity V and the sound wave velocity V to obtain an exponential equation with a constant a and a constant b, thereby obtaining a numerical value a and a numerical value b, as shown in FIG. 3.
And 8: substituting the acoustic velocity value (point c in figure 3) of the shale of the under-compaction action section into the obtained exponential equation to solve the vertical effective stress value sigma of the under-compaction action sectionv', using equation 1 to solve for formation pressure P in the under-compacted zoneFormation of earthThen, an overpressure value delta P of the under-compaction action section is solved by using the formula 2, the overpressure value is the overpressure value of the mudstone of the under-compaction action section in the stratum solved at this time,as shown in fig. 3, this overpressure value is also equal to the effective stress difference between points c and d in fig. 3.
The following is further illustrated with reference to specific examples:
taking an X well in a certain area of the Quaszechne basin as an example, a specific technical method for solving the overpressure of the under-compaction effect in the stratum is described as follows:
the first step is as follows: arranging the logging curves of the X well, selecting natural gamma, natural potential, acoustic time difference, stratum density, neutron porosity, well diameter, deep induction resistance and other series in the logging curves, guiding the selected logging series curves into a single well of Carbon software, identifying mudstone sections with the thickness of more than 5m on a single well diagram of the X well, and reading the average acoustic time difference value AC (mu s/m) and the average stratum density value DEN1 (g/cm) of each mudstone section3) Average neutron porosity value CNL (%), average depth of burial H (m), and average density of formation above H burial point DEN2 (g/cm)3)。
The second step is that: in an Excel table, in the same depth coordinate system, making a scatter diagram of the relation of the average acoustic wave time difference value AC with the buried depth H of an X well mudstone section, a scatter diagram of the relation of the average stratum density value DEN1 with the buried depth H, a scatter diagram of the relation of the average neutron porosity value CNL with the buried depth H, and according to the difference change rule of a typical logging series of a normal compaction section and a lack compaction section with the buried depth, namely the mudstone of the normal compaction section has the characteristic of abnormally high porosity according to the change rule that the average acoustic wave time difference AC and the buried depth H are exponentially changed, the average stratum DEN1 with the buried depth H and the average neutron porosity value CNL with the buried depth H are linearly changed, and the mudstone of the lack compaction section has the characteristic of abnormally high porosity and shows the change trend that the average acoustic wave time difference AC, the average stratum density value DEN1 and the average neutron porosity value CNL deviate from the corresponding normal compaction sections with the change of the buried depth H, and the change curves of the average, the X well is judged to be a normal compaction section above the depth of 3204m, an under-compaction section at the depth of 3750-4158m and a transition zone at the depth of 3204-3750m (figure 4).
The third step: calculating the vertical effective stress sigma of the mudstone section of the X well by using the
formula 1 and the formula 2
v', the acoustic velocity V of the mudstone section of the X well is calculated by using the
formula 3, and the positive is made in the Excel tableVertical effective stress sigma of normal compaction section
v' a change in acoustic velocity v thereof, fitting an exponential relationship (fig. 5),
equation 5,
the fourth step: substituting the acoustic velocity value of the under-compaction action section into the obtained exponential equation to solve the vertical effective stress value of the under-compaction action section, solving the formation pressure of the under-compaction action section by using formula 1, and solving the overpressure value (figure 5) of the under-compaction action section by using formula 2, wherein the overpressure value is the magnitude of the under-compaction action overpressure in the formation which is solved at this time, and is shown in table 1.
TABLE 1 calculation data sheet for under-compaction and overpressure in X well formation
The method is mainly based on logging series data such as acoustic time difference, stratum density, neutron porosity and the like, and a normal compaction section and an under-compaction action section are determined according to the difference change characteristics of the average acoustic time difference, the average stratum density and the average neutron porosity of a thicker mudstone section along with the buried depth; fitting an exponential relation between the acoustic velocity of the normally compacted mudstone section and a scatter diagram of the vertical effective stress; substituting the acoustic wave speed of the mudstone at the under-compaction action section into the obtained exponential relation to obtain the vertical effective stress of the mudstone at the under-compaction action section; and solving the magnitude of the undercompaction overpressure in the section by combining the relation between the vertical effective stress and the overpressure of the mudstone at the undercompaction section. The method can obtain the accurate under-compaction and overpressure values only by using logging information, has the characteristics of rapidness, simplicity, convenience and accuracy, and improves the evaluation efficiency of the under-compaction and overpressure values.