Controlling factors and predictions of well-connected pore-throat volumes in tight oil sandstones

Abstract

Oil production from tight oil sandstone reservoirs has achieved success with the aid of hydraulic fracturing and horizontal well drilling technologies. However, approximately 90% of the oil in these reservoirs remains in place, and reservoir evaluation methods are essential for effective enhanced oil recovery. Well-connected pore throats constitute the connections between fractures and micropores in tight oil reservoirs and lead to high oil recovery levels when using enhanced oil recovery (EOR) methods. In this study, geological factors, such as reservoir quality, lithofacies, and oil charging extent, have been investigated, and their relationships with the well-connected pore-throat volume (Sapex) is discussed. Multiple linear regression and support vector machine (SVM) methods were used for Sapex predictions. The results show that the Sapex values increase as the porosity/permeability increase and then show a decrease. For the flow zone indicator (FZI), Sapex first decreases as the FZI increases and then a slight increase. High Sapex values can be present in nearly all lithofacies, except within some small-scale bedding structures. Increased oil saturations may increase the Sapex, but some rocks with no oil shows still have high Sapex values. High Sapex values are mainly present in two cases: (1) Rocks located close to source rocks with high acidic dissolution levels resulting in secondary porosity and authigenic clay contents. (2) Rocks located far from source rocks that have no oil shows and high contents of chlorite cement, which mainly indicate alkaline diagenetic environments. Predictions of Sapex using multiple linear regression and SVM methods indicate that SVM regression is a good method because of the two high Sapex rock types. The production data from wells with different Sapex values show that the Sapex has a large effect on fluid flow properties.