Optimizing Hydrocarbon Volume Analysis in Heterogeneous Reservoirs in the Niger Delta

Abstract

An intelligent Petrophysical evaluation is essential to optimize development and production in difficult environments in the Niger Delta because of the challenges in reserve estimation and development strategy. The areas are typically characterized by unconsolidated and laminated sandstones and fluvio-marine or lagoonal shales, where accurate estimation of hydrocarbon saturation and consequently volumes has continued to be a challenge in the evaluation of these shaly sand sequences. Most shaly-sand formations have poorly developed sand packages containing laminations of fine grained rock which cause resistivity values to vary in vertical and horizontal directions (Rv & Rh) masking the resistivity contrast when using conventional induction resistivity measurement. The use of a tri-axial multi-array induction tool overcomes the limitation of a standard induction measurement to account for properties of thin beds and complex dipping formations. The challenge of interpreting low resistivity contrast sands lies on determining the actual resistivity of coarse grained components and addressing the volumes of fine grained components in the formation which in the past have been a very cumbersome process. However, the application of advanced interpretation workflows to determine the fractional volume of fine grained rock alongside resistivity of coarse grained rock using matrix corrected porosity, vertical and horizontal resistivity measurements enables a quick and easy approach towards estimation of hydrocarbon volumes. This paper presents an integrated approach for accurate hydrocarbon volumes estimation using triaxial induction measurements of horizontal and vertical resistivity, formation dips and azimuth,, elemental yields from the nuclear spectroscopy tool to quantify mineral volumes with traditional environmentally corrected neutron-density measurements and validation with fluid samples. Comparison of results with conventional approach show over 50% increase in net pay which points to the implication that a good percentage of wells drilled in the past and analyzed using conventional methods either by-passed or under-estimated hydrocarbon reserves.