New Era of Formation Evaluation While Drilling of Complex Reservoirs in Saudi Arabia

Abstract

Within the Saudi Aramco organization, complex reservoir formation evaluation is regarded as a key discipline - fulfilling a critical role in maximizing production and facilitating accurate estimation of reserves. The timely acquisition of high quality and detailed petrophysical properties of the various reservoirs in Saudi Arabia is central to this role. Towards this end, Saudi Aramco Reservoir Description Division (RDD) introduced a new-generation multi-functional Logging While Drilling (LWD) technology into Saudi Arabia, for evaluation of on-shore complex carbonate as well as offshore clastic reservoirs. The traditional LWD measurements of propagation resistivity, gamma ray, density and neutron porosity along with advanced gamma ray capture spectroscopy measurements were compacted into a single collar. The electronic pulsed neutron source required for the spectroscopy measurements also provides a much improved neutron porosity measurement. This technology has been used in a number of Saudi Aramco horizontal wells in carbonate and clastic reservoirs. The data from these wells has been comprehensively evaluated. The results of the evaluation of the data acquired by the new tool and conclusions about the tool's introduction into Saudi Arabia are summarized in this paper. Introduction The geological complexity of the Khuff formation has made formation evaluation very challenging and new logging while drilling measurements are needed to properly evaluate the reservoir in real-time. Newly developed multifunctional logging while drilling tool has been run to date in four wells in Khuff reservoir and the data from these wells has been comprehensively evaluated. The new tool's exclusive capability to perform capture gamma ray spectroscopy and "Sigma" measurements is also examined. The incorporation of these advance measurements into petrophysical evaluations significantly improves the accuracy of mineral volume and saturation computations. Estimation of calcite, dolomite, anhydrite and heavy minerals fractions in the formation assisted in identifying better porosity development and producible reservoir intervals. In this paper examples of advance formation evaluation in Khuff-C reservoir are presented, these examples clearly show the significant contributions this new technology provides for the understanding of complex Khuff-C carbonate reservoir. Further to this, an example from offshore clastics; Wara member sandstone, Mouddud carbonate and Safaniya member sandstone is discussed for real-time petrophysical evaluation and detailed analysis for different clay minerals present. Geological Setting The Khuff formation is the earliest major carbonate sedimentation of the Ghawar field. The formation is deposited in late Permian during transgression of Tethys Sea over the Arabian Plate. Figure-1 shows generalized stratigraphic column.