In order to understand the performance of waterflooding in nano-darcy permeability, naturally fractured shale reservoirs, an integrated study of spontaneous imbibition has been performed. In this study, oil recovery during spontaneous imbibition in naturally fractured shale samples was improved using different water formulations. Different water solutions were formulated by adding different amounts of HCl and NaOH to either distilled water or 30 wt% KCl-base brine solution. Eight water formulations, distilled water, 30 wt% KCl brine low pH HCl solutions (0.39, 0.4, and 0.57), and high pH alkaline solutions (11.8, 11.9, and 13) were examined to recover oil from shale rock samples. Outcrop core samples from Mancos shale were used in this study. The samples were 2.54 to 3.81 cm in diameter and 0.762 to 5.08 cm in length. Firstly, we studied the porosity of the used samples using CT Scanning. The average porosity was 2.7% for the used samples. Secondly, we studied the rock stability and spontaneous imbibtion of different Mancos samples in distilled water, 30 wt% KCl, low pH solutions and high pH alkaline solutions. Mancos samples were very sensitive to distilled water which resulted in whole samples damage due to sever clay swelling. During the spontaneous imbibition, the maximum oil recovery was 59% using distilled water due to whole sample damage. The low and high pH solutions also improved oil recovery from Mancos samples up to 53% due to mineral dissolution and with more stability conditions compared with distilled water. In addition to mineral dissolution, rock wettability was also altered to more water-wet conditions using low and high pH solutions that has been confirmed by the measured contact angles on the used Mancos samples, where the measured contact angles were less compared to rock initial values. The high and low pH solution affected the rock hardness differently. The low pH solutions lowered Mancos rock hardness up to 34%, while the high pH solutions strengthened the rock up to 143% more.