Effects of Underbalance and Effective Stress on Perforation Damage in Weak Sandstone: Initial Results

Abstract

Abstract Single-shot laboratory flow tests were performed on six Castlegate sandstone cores to determine the effect of perforating a weak sandstone (approximately 1600-psi mean unconfined uniaxial compressive strength). Experiments were performed at two effective stress levels and various underbalances. The Castlegate was perforated under simulated downhole conditions as described in API RP 43, 5th Edition. Radial flow tests were performed immediately after perforating to establish perforation productivity. These tests gave no information on the distribution of permeability damage; therefore, a radial flow permeameter was used to measure the permeability distribution. As anticipated, the total perforation length in the Castlegate sandstone was greater than in stronger sandstones; however, the shape of the perforations, the amount of debris left inside the perforation tunnel and the effect on productivity due to the level of underbalance were unexpected. Although the number of experiments was limited, the following observations were made: 1)At all conditions the rock around the initial perforation tunnel failed;2)At low effective confining stress, the failed rock was expelled into the "wellbore," creating a large, asymmetric cavity/tunnel;3)At higher effective confining stress, the failed rock remained in the perforation, filling the tunnel;4)Perforation productivity appeared to decrease at higher underbalances; and5)Experimental artifacts may affect measured productivities when testing softer sandstones.