The introduction of formate-based drilling muds successfully addressed drilling challenges related to barite-weighted muds. The muds exhibited peculiar petrophysical properties that adversely affected log interpretation. First, the mud present inside the borehole and surrounding the tool required different environmental corrections; secondly invading mud-filtrate present inside the formation was difficult to account for. Because of the higher density, lower hydrogen index, and high gamma-ray readings associated with Na/K formate-based drilling fluids, for example, petrophysical analysis typically resulted in inaccurate mineralogy and pessimistic porosity and permeability estimates. Such estimates were also strongly dependent on the extent of invasion by the mud. Two new approaches were developed to address these long-standing challenges in gas-bearing siliciclastic and carbonate sequences. Logging-while-drilling (LWD) time-lapse data acquisition makes it possible to track changes in log measurements between a first (drill) pass and a second (wipe) pass as mud filtrate invades the formation. In a first approach, these changes reflect the contrast in petrophysical properties between the mud filtrate and the displaced native formation fluids, and can be used to estimate the unknown petrophysical properties of such mud filtrate. In a second, geometric approach, the different measurements are considered to represent different axes in measurement space, and then the axes are rotated to reduce the number of rotated measurements affected by invasion to just one. This measurement is then discarded, and the remaining rotated measurements are used. In all, up to six different petrophysical models from two different wells were compared. The results indicate a step change in petrophysical analysis in case of Na/K formate-based drilling fluids. They demonstrate how it is possible to build a robust but remarkably simple petrophysical model using only rotated nuclear measurements, with all of the following characteristics. The model is extremely stable as compared to more complex models. It requires neither knowledge of the Na/K formate mud-filtrate characteristics, nor knowledge of its volume. It does not require resistivity input, but consistently reproduces similar saturations when compared with models using resistivity as input. Data from the drill- and the wipe-pass produced identical results, independent of formation invasion.

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.