Evaluation of formation damage due to frac stimulation of a Saudi Arabian clastic reservoir

Several wells in a Central Saudi Arabian reservoir were selected as candidates for frac stimulation to mitigate near-wellbore formation damage induced during drilling. The reservoir is a meander to braided stream complex with reservoir quality controlled by grain size, total clay content, and quartz cementation. Mineralogical findings included the identification of pore filling vermicular and finely crystalline kaolinite that were susceptible to salinity shock. Swelling illite/montmorillonite clays were shown to be present. The presence of chlorite clays necessitated the use of an iron control chemical in conjunction with acid stimulation procedures. A coreflood study highlighted problems with low-salinity treatment brines and a linear polymer (guar gum) used for the step-rate test. It demonstrated that the recommended clay stabilizer was ineffective at low concentrations. The critical salt concentration (CSC) to KCl was found to be significantly higher than that reported by previous researchers. The effect is a consequence of the multiple clay systems, and the synergistic effects of clay juxtaposition in the pore space. CSC values of 10 wt.% KCl were required to fully stabilize the formation clays. Core-based tests using methods employing minutely gradual changes in salinity or pH [Khilar, K.C., Fogler, H.S., 1984. The existence of a critical salt concentration for particle release. J. Coll. Interface Sci. 101, 214–224.] [Khilar, K.C., Vaidya, R.N., Fogler, H.S., 1990. Colloidally induced fines release in porous media. J. Pet. Sci. Eng. 4, 213–221.] are believed to be inappropriate for evaluation of formation damage problems relevant to field scale operations, and CSC values determined by these methods are too low.