Gamal Ragab Gaafar

Ascendancy of Continuous Profiles of Grain-Size Distribution for Depositional Environment Studies

Knowledge of structure, composition and texture, the three fundamental attributes of sedimentary deposits, is necessary for a robust analysis and interpretation of depositional environment. In the absence of core information; borehole images, “elemental-capture spectroscopy” and NMR logs all contribute to understanding the structure, composition and grain-size attributes. Grain-size is considered the most important textural parameter because it reflects the processes and energy levels active at the time of deposition. Early studies show that depositional processes and environment can be inferred from size distributions of Krumbein (1934) and Sahu (1964). These inferences are based on deductions and experimental data and deliver acceptable accuracy, although Krumbein (1941) shows other textural (e.g. shape, roundness, roughness) and compositional information enhances certainty). Building on early work, we present a methodology and examples where NMR derived grain-size distributions are used to infer depositional environment. In contrast to spot-core analysis, NMR provides a continuous along hole profile of grain-size-distributions. This study is the first to utilize a continuous grain-size-distribution profile. The study was conducted blindfolded, without initial reference to core studies, to test the robustness of the methodology. By using statistical parameters from grain-size distributions that are characteristic of depositional environments, and applying Sahu (1964) linear-discrimination functions, depositional systems for two wells were inferred to be shallow marine-to-fluvial deltaic. Linear-discriminate analysis of geostatistical variables showed bimodal distributions of sediments dominated by fine-grain sands and silts. The studied sandstones were concluded to be mainly fine grained, moderately to poorly sorted, fine skewed, mesokurtic, leptokurtic occasionally platykurtic in nature. Our conclusion was “shallow-marine deposition” and was compared and confirmed with core-driven studies as “shallow marine”. In summary, profiles of grain-size-distributions from NMR logs provide important information about changes in depositional energy level from which we infer depositional setting and reservoir quality. A precise depositional environment interpretation is crucial for optimal/economical field development.

Keys to Success in Determination of In-Situ Petrophysical Properties of Unconsolidated Sands by Traced-Coring: A Case Study

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 be corrected for it or be prevented. 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. AAPG Datapages/Search and Discovery Article #90294 ©2017 Asia Pacific Region, The Third AAPG/EAGE/MGS Oil and Gas Conference, Yangon, Myanmar, February 22-24, 2017

Quantification of Clay Mineral and Log Response Toward Reservoir Rock Properties

Clay minerals are fine grained which compose of complex aluminum silicate with definite crystalline structure. They are divided into four major important groups which are kaolinite, illite, montmorillonite (smectite), and chlorite. The effect of clay minerals on formation evaluation and reservoir performance depends on its morphology, cation exchange capacity, and swelling properties. The occurrence of clay minerals leads to inaccurate values of porosity, water saturation, and permeability. In addition, the impacts of clay minerals during drilling, water injection, and acid stimulation are investigated as it leads to formation damage near wellbore or deep into formation. The current study aims at investigating the effect of clay minerals on log response and reservoir characteristics and to compare its impact on reservoir performance against reported works. Methodology used in the present research involves log interpretation, clay mineral characterization and to analyze the effect of clay on water relative permeability, water saturation, and capillary pressure curve. Results were discussed and benchmarked against selected literatures. Based on the effects of clay minerals, there are reduction in water relative permeability due to fine migration and swelling of clays. In addition, it was found that accurate value of water saturation can be obtained by using Waxman–Smits model. Moreover, it is shown that capillary pressure curve is reflected by the heterogeneity and bimodality of the reservoir. In this project, it is shown that the effect of clay minerals on reservoir can lead to the inaccuracy of determining reservoir characterization and its effect on reservoir productivity.