This report addresses a class of interpretation problems where formation water salinity is highly variable. In such environments, saturation equations that rely on a good knowledge of the water salinity, such as resistivity-based equations (Archie) or thermal neutron capture cross section (Sigma), have a high degree of uncertainty. The problem is further complicated by low-resistivity-pay, where the resistivity contrast between water and pay zones is quite small.
In this case study, the client drilled a vertical well and acquired LWD triple combo and capture cross section (Sigma) while drilling. This data allows us to perform a new formation evaluation approach which simultaneously solves for Sw and Rw, by using both resistivity and Sigma measurements. There's no need for prior knowledge of Rw. This method produces continuous Sw and Rw logs that best honor the input resistivity and Sigma. After drilling was finished, formation pressure tests and fluid samples were subsequently taken to validate LWD interpretation results.
The fluid characterization provided by our Sw log is highly consistent with the fluid samples collected by the wireline tool. Interpreted salinity log also matches lab measurement on the water sample taken from the reservoir.
In many producing fields, it is important to constantly monitor changes in hydrocarbon saturation, fluid contacts and reservoir pore pressure. Typically, as part of the development plan for a field, water is injected into strategically designated formations to sweep residual hydrocarbon and maintain formation pressure. Over the life span of a development field, the physical properties of the injected water can vary significantly depending on its source. As the injected water gradually replaces the original interstitial water in the pore space, the reservoir water salinity becomes a major uncertainty. The challenge of evaluating these water flooded reservoirs, therefore, lies in the unknown water salinity.
