Otevwemerhuere, J. (Addax Petroleum) | Nwosu, C. (Addax Petroleum) | Olare, J. (Addax Petroleum) | Jefford, Leigh (Addax Petroleum) | Parkins, Steve (Addax Petroleum) | Cavalleri, C. (Schlumberger) | Shrivastava, C. (Schlumberger) | Espinosa, H. (Schlumberger) | Mougang, M. (Schlumberger)
Low resistivity low contrast (LRLC) reservoirs have been successfully produced for many years; however detection and detailed description of their properties and potential would remain a challenge in absence of an exhaustive formation evaluation program. Proper understanding of the geological evolution of such reservoirs to explain their distribution and variations in petrophysical properties is also vital.
Low resistivity pay reservoirs encountered in West Africa are often characterized by variation in resistivity values in vertical and horizontal directions due to fine grains and conductive layers within the coarse grained sands and clearly marked sand-shale laminations. This is accurately solved by tri-axial induction resistivity measurement in combination with high resolution measurements able to define any contributing layer level-by-level through robust anisotropic interpretation methods. However, heterogeneity, mixed clays effect, and complexity in rock texture require new technology and innovative interpretation models in multi-domain approach.
Advances in logging technologies, interpretation software, and analytical methodologies enable better and more refined reservoir models to be fashioned and tweaked as needed on a case-by-case basis. The case study analyzes log responses, implication of heterogeneity and mixed clays content on the generation of LRLC pay reservoirs in deltaic environment offshore Nigeria.
Precise application of advanced log measurements and integration of core data in a common workflow, built around the concepts of evolution of LRLC reservoirs lead to accurate pay quantification. Borehole image interpretation suggests that the low resistivity contrast is attributed to dispersed clays coating around the sand grains in the toe part of a delta front in major coarsening up and feeble fining up sequences. This is also confirmed by variations of elastic properties of the matrix.
Petrophysical logs recorded at high resolution correlate inferring the main causes of LRLC pay are clay content and distribution, and small grain sizes intermingled to the reservoir rock, hence resulting in low resistivity values in all directions and drastically increased irreducible water. The logs based model is confirmed by calibration to core analysis results. The confident results of the study confirm the power of collaboration between petrophysics, rock mechanics and geology in innovative interpretation workflows for enhanced reserves estimate and Producibility prediction in heterogeneous media.
Ndokwu, Chidi (Baker Hughes) | Okowi, Victor (Baker Hughes) | Foekema, Nico (Baker Hughes) | Caudroit, Jerome (Addax Petroleum Development) | Jefford, Leigh (Addax Petroleum Development) | Otevwe, Joseph (Addax Petroleum Development) | Fang, Xiaodong (Addax Petroleum Development) | Idris, Maaji (Addax Petroleum Development)
High-angle or horizontal wells pose many geological challenges that include maintaining well trajectory within a particular horizon in drain sections, detecting stratigraphic positions after passing a discontinuity, and subsurface feature identification. Geo-steering has shown increased value over the years because it uses data from different sources, including borehole imaging, to meet these challenges. Bulk density and gamma ray borehole images can be used to describe the near-wellbore environment, and that description can be analyzed further to explain the near-wellbore structural geology. In this study, structural analysis and zonation of bulk density and gamma ray images were used to detect the fault zone, while a geo-steering application was used to pick the true stratigraphic depth after crossing the fault. Provision of an alternative model to seismic-only interpretations and a better understanding of subsurface structures are the industrial benefits of this study. The Niger delta sedimentary basin of Southern Nigeria is a prograding depositional complex of Cenozoic-aged sand and shales that extends from about longitude 3 - 9 E and latitude 4 30' - 5 20' N. This paper demonstrates the importance of geo-steering, shows the application of geo-steering in a high-angle well drilled in the Niger delta sedimentary basin, and establishes the importance of structural analysis from borehole images in making final geo-steering interpretations. This paper also shows that borehole imaging is an additional and useful source of information in the planning stage of any drilling campaign.