Abstract
In this paper, we examine the evaluation of a low resistivity pay siliciclastic reservoir in Bohai Bay, China. The reservoir consists of fine to silt sized sand grains with dispersed illite and smectite minerals filling the pore throats. Due to the large surface area associated with the small grains and the clay minerals, a significant amount of irreducible water is bound to the rock surface, dramatically lowering the resistivity of the pay zone. This phenomenon makes open hole log interpretation quite challenging because pay zone resistivity can sometimes read very close to that in water zone. Wireline formation tester shows oil gradient in sands where open hole logs indicate as wet and shaly. However, these sands were excluded from the initial reserves estimation so that uncertainty is minimized.
In order to confirm hydrocarbon production potential and grow reserves in the low-resistivity sands, the operator perforated these sand intervals and acquired production logs. The borehole fluid density and water holdup from production logs clearly indicate that there's hydrocarbon production coming from the low resistivity zones. Detailed analysis revealed that these sands contribute 46 cubic meters of oil per day with no water production. This result confirms our theory that the low resistivity is caused by bound water trapped in clay minerals. These rocks are actually saturated with oil with almost no free water. Based on this finding, we made a few adjustments to our petrophysics model. We modified the amount of bound water associated with clay so that there's no free water in the zone. Also, we adjusted the parameters used in the shaly sand saturation equation to better account for the conductivity contributed by clay.
With this improved interpretation model, we made production predictions for a few offset wells based on their open hole logs. Additional perforations were added in the low resistivity sands of these wells according to log interpretations. Production history of these wells are highly consistent with our predictions and shows that the low resistivity sands produce good quality oil with almost no water. These newly discovered pay zones increased proven reserves by about 430 thousand tons.
Production logging is typically a production diagnosis tool to improve the understanding of reservoir petrophysics. The novelty of our idea is that we use production logging to provide the ground truth of reservoir fluids in the low resistivity pay and use this information to improve our petrophysics model. The improve model allows us to accurately predict the production from low-resistivity pay sands in other wells, without the need for further production logging.
