Keys to Success in Determination of In-Situ Petrophysical Properties of Unconsolidated Sands by Traced-Coring: A Case Study
Authors: Zulkipli Siti Najmi , Mehmet Altunbay Michael , Gaafar Gamal Ragab , Shah Jamari M. 
Published: October 2017

Objectives of obtaining in-situ values of water saturation, formation water salinity, true formation resistivity (Rt) and SCAL data by core analysis can only be achieved if extraneous fluid invasion is kept at a controlled level and corrected for it or be prevented. The impossibility of zero invasion of cores by mud-filtrate makes the traced-coring a compelling method. Application of liquid based tracers such as tritium and deuterium oxide (D20) to determine the amount of fluid invasion is highly recommended in the event of critical in-situ formation properties need to be determined from core. This study presents a set of key factors for controlling invasion of core by extraneous fluids, best practices in quantifying the fluid invasion, handling core at the surface, and suggests types of analyses, specifically, for unconsolidated formations. A comparison of petrophysical parameters determined from traced-core against the results of LWD log interpretation of the same interval is also presented to assess the success/failure of the recommended practices.

The main use of core-driven parameters has dual functionalities. They are used for calibrating LWD data and also are used to form a statistical database for static modeling. Calibration of LWD data with properly obtained core parameters could minimize uncertainties in calculated petrophysical parameters and establish a ground-truth in petrophysical work especially in water saturation (Sw) calculations. In our case study, good agreements are observed between log derived and core measured water saturations and salinity values extracted from the core against salinity from petrophysical study.

Proper time management, core preservation technique, prompt logistical arrangements and well-site core plugging are seen as the main driving factors for a successful coring job. Comparison of fluid invasion profiles between core plugs drilled at well-site and plugs drilled later in the lab are presented to demonstrate and emphasize the importance of time-factor which constitutes the main challenge in the case study and in general. The lack of data from uncontaminated core may result in significant financial losses that may manifest itself as bypassed productive zones, erroneously determined as wet or no-production (dry) intervals, wrong completions or incorrect quantification of actual and recoverable hydrocarbons. Some of these problems are associated with lack or mismanagement of uncertainties in calculation procedures/algorithms; therefore, can be alleviated or lessened with representative and accurate core data. In addition, analyses results based on the representative core could promote better understanding of reservoir behavior and catalyze more refined reservoir management strategy.

The experience acquired in this study revealed and ranked the importance of timing of the events and the procedural steps to obtain minimally invaded core plugs in a traced-core operation. Time is the most critical factor to prevent post-drill fluid-invasion and fluid re-distribution within a core which adversely impact core analysis results. Therefore, the optimum time allowed between the coring and laboratory tests, core transportation strategy, corresponding contamination of core as a function of time, recommended tests, selection of tracers and quick calculation of required tracer volume are the outputs that are elaborated in this paper. This study also highlights potential challenges in coring unconsolidated formations and serves a mitigation plan for lessening invasion of core by providing a set of recommendations for best practices.