CN102183532A - Method for improving computed tomography (CT) measured fluid saturation precision - Google Patents

Abstract

The invention relates to a method for improving computed tomography (CT) measured fluid saturation precision. The method comprises the following steps of: a, performing CT scanning on a dry core sample inside and outside a core holder respectively to acquire CT values; b, performing CT scanning on a core of complete saturated brine inside and outside the core holder respectively to acquire CT values; c, making bound water by using the core in the holder, measuring the CT value of the core in a bound water state, and calculating the saturation of the bound water according to the water or liquid outlet volume after the bound water is made completely; d, performing CT scanning on brine for saturation, oil for experiments and peripheral air respectively to acquire CT values, wherein the CT value of the air is a constant value in any medium; e, calculating the CT value of the brine and the CT value of the oil in the core holder by using formulas III and V; and f, calculating the oily water saturation in the displacement process by using the formula V according to the corrected brine and oil CT values acquired in the step e, wherein the relative error of the saturation is within 1 percent.

Description

A kind of method that improves CT measurement fluid saturation precision

Technical field

The present invention relates to a kind of method of testing rock core fluid saturation, be specifically related to the method that a kind of CT accurately measures fluid saturation.

Background technology

The CT technology is as measuring technology conventional in the core analysis, is widely used in the aspects such as measurement, flowing experiment research of nonuniformity mensuration, core sample selection, the crack quantitative test of core description, rock core, online saturation degree.And utilize CT that the saturation degree of fluid in the rock is measured, become the most general in petroleum industry, one of the most effective experimental technique of CT technology.

The ultimate principle of CT scan rock is: the X-ray beam that CT machine internal X-ray pipe produces from a plurality of directions along object certain selected tomography aspect shine, the X ray amount that sees through by mensuration, organize the absorption coefficient of constituent parts volume through calculating this aspect after the digitizing, these absorption coefficients can constitute different character matrixs; Carry out digital-to-analog conversion by high-speed computer in the machine, can show on screen or take photos, the image of reconstruction can also provide each pixel X ray attenuation coefficient, uses the CT value representation usually.

For a fault surface of rock, the CT value that image provides is the mean value of each volume element, that is:

CT _dry＝(1-Φ)CT _grain+ΦCT _air (1)

CT _DryThe CT value of-dried rock fault surface

CT _GrainThe CT value of-rock skeleton

CT _AirThe CT value of-air

The factor of porosity of Φ-rock, %

Want the saturation degree of fluid in the computing rock, must be earlier with rock 100% saturated a kind of fluid.As elder generation's 100% saturation water, then obtain:

CT _waterwet＝(1-Φ)CT _grain+ΦCT _water (2)

CT _WaterwetThe CT value of fault surface behind-rock 100% saturation water

CT _GrainThe CT value of-rock skeleton

CT _WaterThe CT value of-water

The factor of porosity of Φ-rock, %

And the factor of porosity of rock also can calculate by (1) (2) two formulas:

$\mathrm{\Φ}=\frac{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{dry}}}{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{air}}}---\left(3\right)$

The CT value of a certain moment rock fault surface is in the displacement process:

CT _x＝(1-Φ)CT _grain+Φ(S _wCT _water+S _oCT _oil) (4)

CT _xThe CT value of-a certain moment rock fault surface

CT _GrainThe CT value of-rock skeleton

CT _WaterThe CT value of-water

CT _OilThe CT value of-oil

The factor of porosity of Φ-rock, %

S _w-water saturation, %

S _o-oil saturation, %

According to S _w+ S _o=1 reaches (1) (2) (4) formula can calculate moisture, oil saturation:

$S_o=\frac{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_x}{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{dry}}}\·\frac{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{air}}}{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{oil}}}$

$S_w=1-\frac{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_x}{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{dry}}}\·\frac{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{air}}}{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{oil}}}---\left(5\right)$

As can be seen from the above equation, calculate the CT value that the two-phase fluid saturation degree need be known air, water and oil.But under normal conditions, rock core all is to experimentize in core holding unit, and because ray hardened influence, it is different that water is directly exposed to airborne CT value with oil in the core holding unit neutralization; Oil and the CT value of water in core holding unit are difficult to again obtain, and all application are exposed to the CT value of airborne water and oil and calculate, and can bring certain error to the result of calculation of saturation degree.

Adopt another kind of method in some research work, be about to dry behind rock 100% saturation water, 100% saturated oil again obtains:

CT _oilwet＝(1-Φ)CT _grain+ΦCT _oil (6)

CT _OilwetThe CT value of fault surface behind-rock 100% saturated oil

CT _GrainThe CT value of-rock skeleton

CT _OilThe CT value of-oil

The factor of porosity of Φ-rock, %

Again moisture, oil saturation are calculated:

$S_o=\frac{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_x}{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{oilwet}}}$

$S_w=\frac{{\mathrm{CT}}_x-{\mathrm{CT}}_{\mathrm{oilwet}}}{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{oilwet}}}---\left(7\right)$

Avoid oil, water CT value to change the error of bringing with the method, also improved the degree of accuracy of measuring.But common rock core displacement test all needs to make irreducible water, and the method makes corresponding experimentation too loaded down with trivial details, and carrying out needs twice also oven dry of saturated liquid before the displacement test, and the core sample more intense to susceptibility also can cause certain injury.

Summary of the invention

The objective of the invention is to propose a kind of simple experimental method of testing that can improve CT measurement fluid saturation precision.

A kind of method that CT measures the fluid saturation precision that improves of the present invention, it comprises the steps:

A. after the core sample oven dry, carry out CT scan, obtain the CT value CT of this dried rock core fault surface _{Outside the dry}Again this rock core is packed in the core holding unit and condition that displacement process is same under and under the same CT condition rock core is scanned, obtain the CT value CT of dried rock core fault surface _{Dry In}

B. with behind this rock core 100% saturated brine, in the inside and outside CT scan of carrying out of core holding unit, experiment condition and step a are in full accord respectively, the outer CT value CT of core holding unit behind acquisition rock core 100% saturated brine _{Outside the waterwet}With the CT value CT in the core holding unit _{In the waterwet}

C. rock core utilizes oily expelling water to make irreducible water in clamper, measures the CT value of core sample under the irreducible water state; And, made behind the irreducible water according to water outlet, fluid volume irreducible water saturation S in endpiece metering water outlet, oil pump capacity _wCalculate;

D. carry out CT scan with oil and peripheral air and obtain CT value CT with salt solution, experiment saturated respectively _{Outside the water}, CT _{Outside the oil}And CT _AirWherein, the CT value of air all is definite value in any medium;

E. utilize formula III that the CT value of salt solution in the core holding unit is calculated CT _{Water In}

F. pass through the CT of dried core sample in the core holding unit _{In the dry}, 100% saturated brine the CT of core sample _{In the waterwet}, the CT of core sample under the irreducible water state _{In the x}, the irreducible water saturation S that obtains of step c _wAnd the CT of salt solution in core holding unit that obtain of step e _{In the water}, utilize formula V that the CT value of oil in the core holding unit is calculated CT _{In the oil}

$S_w=1-\frac{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_x}{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{dry}}}\·\frac{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{air}}}{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{oil}}}---\left(V\right)$

G. corrected salt solution and the oily CT value that obtains according to step e, step f utilizes formula V that the profit saturation degree in the displacement process is calculated.

Beneficial effect of the present invention is, final irreducible water saturation was easy to by the volumetric method accurate measurement when irreducible water was made in this method utilization, by this irreducible water saturation, and core sample scans the rock porosity value that obtains outside clamper, CT value to water and oil utilizes formula convection cell saturation degree to calculate after proofreading and correct again, can improve the degree of accuracy of measurement greatly, make the relative error of saturation degree in 1%.On the other hand, this method only needs a saturated liquid, has reduced operation steps compared with the prior art.

Description of drawings

Fig. 1 is that oil-water is to permeability curve figure.

Embodiment

Embodiment 1 utilizes the CT scan technology accurately to measure profit two phase saturations and to make profit relative

Permeability curve

(1) sample: the I4D of Daqing oil field Portugal well sandstone core sample 32#.

(2) instrument: GE LightSpeed 8 helical layer Medical CT scanners; The carbon fiber core holding unit; Quzix SP-5000 high-pressure metering pump.

(3) CT scan condition: scanning voltage: 100kV, sweep current: 100mA, scan mode: axial scan, scanning bed thickness: 1.25mm, each scan slice width of cloth number: 49 width of cloth.

(4) experimentation:

1. after core sample oven dry is weighed and measured factor of porosity and air permeability, be placed on core holding unit and scan outward, and the CT value of writing scan direction, position and each scan slice;

2. during core sample is packed core holding unit into, add confined pressure to 5MPa, under the direction of scanning with step 1 rock core is scanned with distilled water, and the CT value of writing scan position and each scan slice;

3. the NaI aqueous solution of rock core vacuum saturated 5% is 24 hours, takes out to claim weight in wet base and buoyant weight, calculates volume of voids and degree of saturation.Degree of saturation is being carried out next step experiment again more than 99%;

Core sample respectively outside core holding unit and in the core holding unit (adding the 5MPa confined pressure) scan, direction of scanning, position respectively with step 1,2 in full accord, write down the CT value of each scan slice;

5. rock core utilizes oily expelling water to make irreducible water in clamper, and in endpiece metering water outlet, oil pump capacity, according to water outlet, fluid volume irreducible water saturation is calculated after having made irreducible water;

6. the core sample (in clamper) of having made irreducible water is scanned, direction of scanning, position and step 2 are in full accord, write down the CT value of each scan slice;

7. 5%NaI solution, the experiment of saturated usefulness are carried out CT scan with oil and peripheral air and obtain the CT value;

8. oil and 5%NaI solution are injected rock sample by the ratio of setting, treat mobile stablizing after, core sample (in clamper) is scanned, direction of scanning, position and step 2 are in full accord, write down the CT value of each scan slice; Write down two ends pressure reduction simultaneously;

9. change profit and inject ratio, repeating step 8 injects than finishing experiment until last profit;

10. utilize formula (III) and (V) each profit is injected than the profit two-phase saturation computation of each section down, and average and be this profit and inject saturation degree than the interior profit two-phase of following rock core; Utilize Darcy formula that the relative permeability of profit two-phase is calculated, and draw permeability-ratio curve as shown in Figure 1.

Experimental data and result are as shown in table 1.

Table 1

Reference examples 1 utilizes volumetric method, conventional CT method and the inventive method to measure profit two phase saturations

(1) sample: two of the I4D of Daqing oil field Portugal well sandstone core samples.2# sample length 6.31cm wherein, diameter 2.541cm, factor of porosity 19.8%, air permeability 154mD; 34# sample length 5.717cm, diameter 2.538cm, factor of porosity 22.5%, air permeability 642mD.

(2) instrument: GE LightSpeed 8 helical layer Medical CT scanners; The carbon fiber core holding unit; Quzix SP-5000 high-pressure metering pump.

(3) CT scan condition: scanning voltage: 100kV, sweep current: 100mA, scan mode: axial scan, scanning bed thickness: 1.25mm.

(4) experimentation:

1. after the core sample oven dry, be placed on core holding unit and scan outward, and the CT value of writing scan direction, position and each scan slice;

2. during core sample is packed core holding unit into, add confined pressure to 5MPa, under the direction of scanning with step 1 rock core is scanned with distilled water, and the CT value of writing scan position and each scan slice;

3. the NaI aqueous solution of rock core vacuum saturated 5% is 24 hours, takes out to claim weight in wet base and buoyant weight, calculates volume of voids and degree of saturation.Degree of saturation is being carried out next step experiment again more than 99%;

Core sample respectively outside core holding unit and in the core holding unit (adding the 5MPa confined pressure) scan, direction of scanning, position respectively with step 1,2 in full accord, write down the CT value of each scan slice;

5. rock core utilizes oily expelling water to make irreducible water in clamper, and in endpiece metering water outlet, oil pump capacity, according to water outlet, fluid volume irreducible water saturation is calculated after having made irreducible water;

6. the core sample (in clamper) of having made irreducible water is scanned, direction of scanning, position and step 2 are in full accord, write down the CT value of each scan slice;

7. 5%NaI solution, the experiment of saturated usefulness are carried out CT scan with oil and peripheral air and obtain the CT value;

8. with the flow velocity injection rock sample of 5%NaI solution with 0.5ml/min, 0.5min back termination of pumping, the volume of oil and water scans core sample (in clamper) simultaneously in the recorded stream fluid, direction of scanning, position and step 2 are in full accord, write down the CT value of each scan slice.

9. turn on pump again, every 0.5min stops pump No. one time, and repeating step 8 is tested until no longer fuel-displaced end.

10. utilize formula (III) and (V) to the profit two-phase saturation computation of each section after the termination of pumping each time, and average and be the saturation degree of profit two-phase in this moment rock core; Calculate profit two phase saturations according to volumetric method simultaneously, computing formula is as follows:

$S_w=\frac{\mathrm{vt}+S_{\mathrm{wi}}{V}_p-\mathrm{\Δ}{V}_w}{V_p}\×100\%$

S _w-water saturation, %

The v-injection rate, ml/min

The t-injection length, min

S _Wi-irreducible water saturation, %

V _p-volume of voids, ml

Δ V _w-accumulative total water outlet volume, ml

It is as shown in table 2 to utilize volumetric method, conventional CT method (referring to the background technology part) and the inventive method to measure profit two phase saturation results, with the volumetric method measurement result is standard, conventional CT method measuring error is 0.5～3.6%, and the measuring error of the inventive method only is 0～1.5%, as seen, method measuring accuracy of the present invention is higher.

Table 2

Claims (1)

1. one kind is improved the method that CT measures the fluid saturation precision, it is characterized in that it comprises the steps:

A. after the core sample oven dry, carry out CT scan, obtain the CT value CT of this dried rock core fault surface _{Outside the dry}Again this rock core is packed in the core holding unit and condition that displacement process is same under and under the same CT condition rock core is scanned, obtain the CT value CT of dried rock core fault surface _{Dry In}

B. with behind this rock core 100% saturated brine, in the inside and outside CT scan of carrying out of core holding unit, experiment condition and step a are in full accord respectively, the outer CT value CT of core holding unit behind acquisition rock core 100% saturated brine _{Outside the waterwet}With the CT value CT in the core holding unit _{In the waterwet}

C. rock core utilizes oily expelling water to make irreducible water in clamper, measures the CT value of core sample under the irreducible water state; And, made behind the irreducible water according to water outlet, fluid volume irreducible water saturation S in endpiece metering water outlet, oil pump capacity _wCalculate;

D. carry out CT scan with oil and peripheral air and obtain CT value CT with salt solution, experiment saturated respectively _{Outside the water}, CT _{Outside the oil}And CT _AirWherein, the CT value of air all is definite value in any medium;

E. utilize formula III that the CT value of salt solution in the core holding unit is calculated CT _{Water In}

F. pass through the CT of dried core sample in the core holding unit _{In the dry}, 100% saturated brine the CT of core sample _{In the waterwet}, the CT of core sample under the irreducible water state _{In the x}, the irreducible water saturation S that obtains of step c _wAnd the CT of salt solution in core holding unit that obtain of step e _{In the water}, utilize formula V that the CT value of oil in the core holding unit is calculated CT _{In the oil}

$S_w=1-\frac{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_x}{{\mathrm{CT}}_{\mathrm{waterwet}}-{\mathrm{CT}}_{\mathrm{dry}}}\·\frac{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{air}}}{{\mathrm{CT}}_{\mathrm{water}}-{\mathrm{CT}}_{\mathrm{oil}}}---\left(V\right)$

G. corrected salt solution and the oily CT value that obtains according to step e, step f utilizes formula V that the profit saturation degree in the displacement process is calculated.