Gas Shale Ultrasonic Velocity Evolution Induced By Spontaneous Imbibition Under Uniaxial Stress

Abstract:

With strong spontaneous capillary imbibitions, shale gas is significantly different from conventional reservoirs. Water is widely adopted in hydraulic fracturing. In order to better understand the imbibition process, wave velocity evolution caused by spontaneous imbibition is studied through experimental investigation.

One shale sample comes from an outcrop located in Chongqing named as Cls. The other is YC8, 833.33 meters’ depth of Yucan-8 gas well. All samples were cored and polished to cylinders of 25mm in diameter and 50mm in length. After sample preparation, they are dried at 90ºC temperature in an oven-drying for 24 hours.Firstly, samples are saturated with distilled water and n-decane for 24 hours and 48hours respectively, for comparison with the dry samples. After the long-time imbibition,a uniaxial test is conducted at a constant stress rate of 2MPa/minute up to 30MPa. The compressional wave velocities are measured along the longitudinal direction with a classic ultrasonic pulse transmission technique. Arrival times are auto-picked using waveform cross-correlation method. The results are as follows:

1 It is found that significant velocity evolution difference exists between the two shale samples. Water imbibition makes the velocities of Cls lower than that of the dry one. However YC8 shale samples have opposite properties. Theses could not be explained by Gassmann equation and hence needs further research.

2 Stress sensitivity of water saturated Cls sample is larger but in the same order compared with that of the dry one. Maybe It is the response of induced cracks by the water-clay interaction. As to YC8,the stress sensitivities of dry and saturated are nearly the same.

3 n-decane saturation experiment is also conducted on two shale samples. The velocities of saturated Cls shale are larger than that of the dry ones which is different with the water saturation condition. As to YC8, the results are almost the same as water saturated condition.

4 The velocity of Cls shale is caused by physical and chemical effects, but that of YC8 shale only physical effect. Gassmann effect can not represent all the physical effects. Fluid saturation effects can be divided into microscale effect and Gassmann effect, micro-scale effect is relevant to surface area,initial saturation, micro-wettability, capillary force.