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

摘要

In this case study, we examine oil wells in a brownfield, where water production is a major concern. Therntarget reservoirs consist of several thin sandstone beds with excellent lateral extension. As a result, thernoperator generally chooses to produce from long horizontal sections. In order to better understandrnproduction and make informed decision about future development plans, it is critical for the operator tornaccurately characterize fluids in the field through well logging. The challenge in formation evaluation liesrnin the uncertainty in formation salinity, because the injected water makes formation water salinity highlyrnvariable. In such environments, saturation equations that rely on a good knowledge of the water salinity,rnsuch as resistivity-based equations (Archie) or ther mal neutron capture cross section (Sigma), cannot fullyrncapture the variability in fluids distribution in the reservoir. The problem is further complicated byrnlow-resistivity-pay, where the resistivity contrast between water and pay zones is quite small.rnA recently proposed workflow can solve for water salinity at each depth level by simultaneouslyrninverting for water saturation (Sw) and water resistivity (Rw) from resistivity and Sigma logs. This methodrndoes not require a priori knowledge of water salinity. One can further compute an irreducible waterrnsaturation (Swi) from lithological dry weights, which in turn are derived from geochemical logs. Thernamount of water that’s movable in the pore space is then given by Sw – Swi. We can use Sw – Swi as arnqualitative indicator of water floods distribution and use it to understand fluids production of horizontalrnwells and design the next phase of development to target bypassed hydrocarbon.rnIn this report, we studied the LWD logs and production history of 3 horizontal wells in a brownfield,rnwhere formation salinity is highly variable. It is found that water production is directly related to Sw – Swi.rnWe also show that resistivity-based saturation equations can give misleading interpretation of the fluidrntype due to variable salinity. Adding Sigma into the interpretation model accounts for salinity changes andrngives results that are consistent with production history.