Fluid Characterization with LWD Resistivity and Capture Cross Section Enhances Understanding of Horizontal Well Production – A Case Study in a Siliciclastic Brownfield

In this case study, we examine oil wells in a brownfield, where water production is a major concern. The target reservoirs consist of several thin sandstone beds with excellent lateral extension. As a result, the operator generally chooses to produce from long horizontal sections. In order to better understand production and make informed decision about future development plans, it is critical for the operator to accurately characterize fluids in the field through well logging. The challenge in formation evaluation lies in the uncertainty in formation salinity, because the injected water makes formation water salinity highly variable. In such environments, saturation equations that rely on a good knowledge of the water salinity, such as resistivity-based equations (Archie) or ther mal neutron capture cross section (Sigma), cannot fully capture the variability in fluids distribution in the reservoir. The problem is further complicated by low-resistivity-pay, where the resistivity contrast between water and pay zones is quite small.

A recently proposed workflow can solve for water salinity at each depth level by simultaneously inverting for water saturation (S_w) and water resistivity (R_w) from resistivity and Sigma logs. This method does not require a priori knowledge of water salinity. One can further compute an irreducible water saturation (S_wi) from lithological dry weights, which in turn are derived from geochemical logs. The amount of water that's movable in the pore space is then given by S_w – S_wi. We can use S_w – S_wi as a qualitative indicator of water floods distribution and use it to understand fluids production of horizontal wells and design the next phase of development to target bypassed hydrocarbon.

In this report, we studied the LWD logs and production history of 3 horizontal wells in a brownfield, where formation salinity is highly variable. It is found that water production is directly related to S_w – S_wi. We also show that resistivity-based saturation equations can give misleading interpretation of the fluid type due to variable salinity. Adding Sigma into the interpretation model accounts for salinity changes and gives results that are consistent with production history.