Abstract
Water flooding evaluation in development wells is one of the biggest technical challenges in Bohai Bay. Due to the extremely low salinity of formation and injected water, resistivity contrast between hydrocarbon and water sands is low. Moreover, the clay mineral in the rock matrix increases overall conductivity and can lead to misinterpretation of water saturation.
In this report, we present a case study of water flooding evaluation with LWD logs using a novel approach. The distinguishing features of this approach are: 1. It removes the clay effect on porosity and resistivity measurements; 2. It computes an irreducible water saturation to assess what fraction of the pore fluid is movable. The key enabling technology in this approach is LWD nuclear capture spectroscopy. The most valuable inputs provided by spectroscopy are the matrix responses and rock lithology dry weights, particularly the clay. The following are the steps of our workflow:
Derive matrix density and matrix neutron from the element logs and compute matrix-adjusted porosity.
Derive clay dry weight from element logs and compute modern day water saturation Swi with a shaly sand conductivity equation.
Compute irreducible water saturation Swi from rock lithology.
For oil zones with original water at irreducible level, the water flooding in modern day can simply be estimated from Sw – Swi.
Included in this case study are 6 development wells in Bohai Bay logged with LWD natural GR, caliper, neutron-density, propagation resistivity and capture spectroscopy. We used the proposed approach to evaluate water flooding in these wells and our results are consistent with production data.
