Logging with joint ultrasound and X-ray technologies

1. A method for evaluating a cement sheath located outside of a casing in a wellbore, comprising:

determining a density map of said cement sheath using a nuclear logging technique;

providing an acoustic impedance value of said cement sheath, the acoustic impedance value being provided by:

providing an acoustic model of said cement sheath;

calculating a simulated acoustic impedance of said cement sheath from said acoustic model; and

using said simulated acoustic impedance as said acoustic impedance value; and

determining a map of the speed of sound in said cement sheath using said density map and said simulated acoustic impedance.

2. The method of claim 1 further comprising:

emitting a X-ray from a X-ray source disposed within said wellbore;

measuring a backscatter of said X-ray by a X-ray detector disposed within said wellbore; and

determining said density map from said backscatter.

3. The method of claim 2 wherein:

said X-ray is characterized by an energy level ranging between 100 keV and 350 keV.

4. The method of claim 1 wherein:

said ultrasonic logging technique is a pulse-echo technique.

5. The method of claim 1 , wherein:

said acoustic model approximates said casing and said cement sheath as flat plates and assumes a vertically-oriented plane wave incident signal; or

the method further comprises determining a measured acoustic impedance of said cement sheath using an ultrasonic logging technique, and validating said acoustic model by comparing said measured acoustic impedance to said simulated acoustic impedance.

6. A method for providing an inversion model to predict properties of a cement sheath located outside of a casing in a wellbore, comprising:

providing an acoustic model of said cement sheath as a function of acoustic impedance, wherein said acoustic model approximates said casing and said cement sheath as flat plates and assumes a vertically-oriented plane wave incident signal;

determining a density map of said cement sheath using a nuclear logging technique; and

inverting said acoustic model using said density map and a simulated acoustic impedance to determine a map of the speed of sound in said cement sheath.

7. The method of claim 6 further comprising:

emitting a X-ray from a X-ray source disposed within said wellbore;

measuring a backscatter of said X-ray by a X-ray detector disposed within said wellbore; and

determining said density map from said backscatter.

8. The method of claim 7 wherein:

said X-ray is characterized by an energy level ranging between 100 keV and 350 keV.

9. The method of claim 6 further comprising:

determining a measured acoustic impedance of said cement sheath using an ultrasonic logging technique; and

validating said acoustic model by comparing said measured acoustic impedance to a simulated acoustic impedance calculated using said acoustic model.

10. The method of claim 9 wherein:

said ultrasonic logging technique is a pulse-echo technique; or

the method further comprises transmitting an ultrasonic wave by an ultrasonic transmitter disposed within said wellbore, measuring a reflected ultrasonic wave by an ultrasonic receiver disposed within said wellbore, and determining said measured acoustic impedance from said transmitted and reflected ultrasonic waves.

11. A system for evaluating a cement sheath located outside of a casing in a wellbore, comprising:

a downhole logging tool arranged for determining a density map of said cement sheath using a nuclear logging technique; and

a processor system arranged for determining a map of the speed of sound in said cement sheath using said density map and an acoustic impedance value of said cement, wherein:

said processor system includes an acoustic model of said cement sheath; and

said processor is operable to calculate a simulated acoustic impedance of said cement sheath from said acoustic model and use said simulated acoustic impedance as said acoustic impedance value.

12. The system of claim 11 wherein said downhole logging tool comprises:

a X-ray source; and

a X-ray detector disposed so as to detect a backscatter from said X-ray source;

whereby said density map may be determined from said backscatter.

13. The system of claim 12 wherein:

said X-ray source is operable to produce X-rays an energy level ranging between 100 keV and 350 keV.

14. The system of claim 11 wherein:

said acoustic model approximates said casing and said cement sheath as flat plates and assumes a vertically-oriented plane wave incident signal.