Formation evaluation can become complex when the invading mud-filtrate properties are unusual, variable or unknown like in sodium potassium (Na/K) formate water base mud (WBM) environments. In these situations, computed reservoir properties are adversely affected and become strongly dependent on the formation invasion status.
The Permian age reservoir discussed in this paper, consists of highly unconsolidated heterogeneous sandstone sequences, saturated with condensate rich gas. From a drilling engineering perspective, the shales are often unstable, requiring high mud overbalance to maintain hole stability in wells with high inclinations, which resulted in recurrent differential sticking incidents. The use of formate based drilling fluids in this field, gained acceptance over time, primarily to minimize drilling problems.
The downside of formate muds, however, is that log data interpretation encounters serious challenges because of the uncertain petrophysical properties of the mud, affecting log measurements in two ways. The first are those effects related to the mud present inside the borehole and surrounding the tool, or so-called environmental effects. The second are those related to the invading mud-filtrate present inside the formation, resulting in pessimistic porosity, mineralogy and permeability estimates.
This paper shows how Na/K formate WBM filtrate effects can be identified and eliminated using Logging-While-Drilling (LWD) time-lapse data acquisition and analysis to provide time-independent logs in a manner that renders the logs immune to various mud-filtrate effects. These logs, together with a corresponding new petrophysical model, make it possible to do away with the mud-filtrate petrophysical properties, and to solve for porosity, mineralogy and fluid saturations from standalone nuclear measurements, irrespective of the formation invasion status.
Moreover, the results demonstrate how valuable LWD time-lapse data acquisition can be, and that data acquired while drilling – especially resistivity data in this instance – are important to validate this novel formation evaluation interpretation approach.
Volumetric Formation Evaluation (FE) describes the composition of underground formations in terms of volumetric percentages of minerals and fluids present. This starts by assigning different petrophysical properties – also called endpoints – to the minerals and fluids present. These endpoints describe what different measurements would read if only one mineral or only one fluid was present. The next step is to select a forward model – made up of so called mixing-laws – to describe what different measurements would read when the minerals and fluids are mixed in different volumetric percentages. Finally, actual log measurements are acquired and the forward model inverted to solve for the actual volumetric percentages of minerals and fluids present.