| dc.description.abstract | Geomechanics testing is an important component of reservoir geomechanics studies for improving drilling, and stimulation operations. Usually the cores for geomechanical tests are exposed to non-native fluid and air prior to tests. Drilling experience and modeling work has shown that shales can be sensitive while exposed to water and air. Therefore, a good understanding of the impact of the exposure to atmosphere and fluids on the shale geomechanical properties is necessary. To understand the impact, a study was conducted to examine changes in shale characteristics because of exposure to different fluids. To facilitate investigation of this complex physico-chemical process, dynamic properties were used. Nine plugs including seven 1-inch diameter 2-inch long samples and two 4-inches diameter samples were cored from the Fluffy Kitten 16 State VDW 2 well. As shale can be susceptible to swelling and deterioration when exposed to non-native water, a new method was used to core the samples to minimize the impact of exposure. The coring process and the exposure time during the preparation process was carefully recorded.
After plug preparations, one sample was preserved in 7% KCl solution, one sample was preserved in decane, one sample was preserved in 11.5 lb/gal NaCl-based drilling mud, six samples including two 4-inches samples and four 1-inch samples were wrapped by cling wraps without using any fluids for perservation. Dynamic measurements were conducted on these two 4-inches samples and four 1-inch samples while they were taken out of the preservation condition under an axial load applied by the MTS (Material Test System) 810. Samples’ weights, lab temperature and relative humidity were measured before each test. After the test, the samples which were previously in a specific kind of fluid (KCl, decane, drilling fluid) were put back into preservation condition while the other air-dry samples were left in the room environment to get exposed. The weights of the samples increased due to the penetration of the preservation fluid through the pre-existing cracks while preserved in the fluids. While the weights of samples which were preserved in fluids stabilized, they were taken out of the preservation fluids and were tested in the same way as the samples which were not contacted with any fluids as described above. The exposure time of these plugs to air and/or fluids during testing were recorded. The time intervals between two tests were gradually increased from 12 hours to 24 hours, and then 48 hours, 96 hours, etc., since the moisture change tends to be less active over time. Decane was used on some plugs, and since decane may have some impact on the rock dynamic properties, one sample was preserved in a jar full of decane to clarify its impact on shale samples. To determine the impact of drilling mud on the mechanical properties of shale which is crucial to well-stability, one 1-inch shale sample was preserved in a jar full of 11.5 lb/gal NaCl-based drilling mud. The change in dynamic mechanical properties was compared.
It was observed all the samples were losing weight because of the loss of the moisture at the start of exposure. However, their weights fluctuated with the change of relative humidity which determined whether the vapor pressure was bigger than the capillary pressure. The weight loss included two kinds of fluids for the samples which were previously preserved in the fluid (decane, drilling mud, KCl solution) during the exposure: 1. Fluids which entered the samples through the pre-existing cracks; 2. Free water in the shale. P-wave velocity increased due to the absorption of preserving fluid as P-wave travels faster in liquid than in air. S-wave velocity was not affected by the absorption of preserving fluid since it cannot travel through liquid and air. It is also observed that P-wave velocity and S-wave velocity increased due to the contraction of samples with the loss of the native moisture content. The measured shale dynamic Young’s modulus increased while losing moisture content without air trapped because of the suction pressure developed. While exposing for a longer time, fractures localized as the air pressure built up while it was trapped inside shale and due to the structure imbalance caused by the movement of moisture content as sudden drop of P-wave velocity, S-wave velocity, Young’s modulus, Poisson’s ratio were observed during exposure. The dynamic signal would be attenuated due to the growth of cracks. It would also significantly decrease due to the localization of fractures.
In this research, it provides evidence that the shale moisture content impacts its dynamic properties. The capillary pressure and vapor pressure equilibrium control shale moisture movement while is contacted with air. The absorption of moisture occurs while the vapor pressure is higher than the capillary pressure between the shale interlayers. The loss of moisture content will also happen while the capillary pressure is higher than the vapor pressure. Due to the loss of native moisture content if the exposure is not excessive, the attraction force between the interlayer will increase. On one hand, this force (if not excessive) can act as a confining effect which tends to strengthen the shale. On the other hand, this force will make shale shrink and can desiccate. Poisson’s ratio did not show a major variation as the P-wave velocity and S-wave velocity increased at a similar rate due to the contraction of shale during the loss of native moisture content at the start of exposure. The penetration of preserving fluid increased shale’s dynamic Poisson’s ratio. P-wave velocity increases if the absorption of preserving fluid happened as P-wave travels faster in liquid than in air. S-wave velocity is not affected by the absorption of preserving fluid since it cannot travel through liquid and air. If the exposure is excessive, the cracks will grow. The growth of the micro-cracks tends to attenuate dynamic signals. While exposure for a longer time causes fractures to localize due to the capillary pressure (air trapped inside shale due to the alteration of absorption and loss of moisture content). The formation of fractures significantly decreases the dynamic P-wave velocity and S-wave velocity. | en_US |
