Detailed core characterization is often overlooked in the sampling process for core analysis measurements. Random core sampling is usually performed and the selected plugs are not associated with rock types or the reservoir heterogeneity. The objective of this study is to obtain representative samples for direct simulation of petrophysical and fluid flow properties in complex rock types. A robust sampling strategy was followed in reservoir cores from two successive heterogeneous carbonate and siliciclastic formations in the Raudhatain field in Kuwait. The sample selection criteria were based on statistical distribution of litho-types in the cores to ensure optimum characterization of the main reservoir units. The litho-types were identified based on porosity and mineralogy variations along the core lengths utilizing advanced dual-energy X-ray CT scanning. High resolution micro-CT imaging and subsequent segmentation provided 3D representation of the pore space and geometric fabric of the core samples. Primary drainage and imbibition processes were simulated in numerical experiments using a pore-scale simulator by the Lattice Boltzmann Method. Capillary pressure (Pc) and relative permeability (Kr) curves together with water and oil distributions were investigated for complex geometries by the different rock types. The dual energy CT density was compared with wireline log and provided accurate calibrations to the downhole logs. The different rock types gave distinct capillary and flow properties that can be linked to the rock structure and pore type of the samples. The Lattice Boltzmann based pore-level fluid calculations provided realistic fluid distributions in the 3D rock volume, which are consistent with pore-scale physical phenomena. This characterization method by the dual energy CT eliminates sampling bias and allows for each cored litho-type to be equally represented in the plugs acquired for subsequent petrophysical and fluid flow analyses. It also provides accurate calibration tool for downhole logs. The digital analysis gave reliable SCAL data with improved understanding of the pore-level events and proved its effectiveness in providing advanced interpretations at multiple scales in relatively short timeframes.

Operators commonly adopt waterflooding as a secondary recovery method to maintain reservoir pressure and displace remaining oil for production enhancement. Effluent and seawater have been injected into the Upper Burgan formation, which contains multiple layers of sand reservoirs, in the North Kuwait Raudhatain field. Well-based surveillance to understand post-waterflooding hydrocarbon distribution is essential for new perforation additions. Formation saturation monitoring for cased wells is widely performed with pulsed neutron well logging techniques. Pulsed neutron well logging provides time-based thermal neutron capture cross-section (i.e., sigma log) and energy-based element-specific ratios (i.e., carbon/oxygen (C/O) logs). Formation water salinity must be known and high to use sigma data to quantify formation fluids. When formation water salinity becomes a variable due to effluent and seawater injection, sigma log-based saturation analysis is not applicable. A salinity-independent measurement that distinguishes between oil and water is required; consequently, a C/O log must be used to obtain saturation profiles in mixed-water salinity reservoirs. The Upper Burgan formation’s initial water salinity in the Raudhatain field is high (i.e., approximately 220-240 kppm NaCl equivalent); thus, water saturation computation was performed with a sigma log. After the injection of effluent and seawater (mixed-water salinity ranges from 50 kppm to 170 kppm) was started, formation oil volumes must be evaluated using C/O logging. A well-specific Monte Carlo Neutron Particle (MCNP) model and two-detector-balanced C/O data sets were combined to compute oil saturation. We demonstrate multi-well case examples delineating well-based formation saturation profiles in post-injection reservoir conditions. A comparison of sigma- and C/O-based saturation analyses revealed water-flooded zones. Time-lapse sigma data sets highlighted how the water injection impacted thermal neutron capture cross-section measurements. Additionally, multi-detector, time-based nuclear attributes were used to evaluate formation properties and the presence of hydrocarbon-bearing sands. Following pulsed neutron log interpretation, subsequent add-perforation activities were performed; consequently, by-passed or remaining hydrocarbon was successfully produced. Evaluation of current formation fluid distribution in areas of the field where mixed-water salinity exists is challenging. Integrating sigma, C/O, and auxiliary pulsed neutron logs determined the remaining formation oil distribution and volume. The optimized perforation strategy to maximize oil production from existing wellbores was executed.

Detailed reservoir core characterization is often overlooked in the sampling process for core analysis measurements. Random core sampling is usually performed and the selected plugs are often not associated with proper rock types or the reservoir heterogeneity. This leads to unrepresentative selection of the core samples and raises questions about the effectiveness of the core data in reservoir models and their calibrations.In this study, a robust sampling strategy was followed in reservoir cores from two heterogeneous formations in Kuwait. The objective of the core sampling was to obtain representative plugs for petrophysical and fluid flow analyses that would support dynamic model predictability. The sample selection criteria were based on statistical distribution of litho-types in the core to ensure optimum characterization of the main reservoir units. The litho-types were identified based on porosity and mineralogy variations along core lengths utilizing advanced dual-energy X-ray CT scanning. This technique provided high resolution data at 0.5mm spacing to detect all variations in the core. Different porosity cutoffs were assigned for each mineralogy, which resulted in precise identification of the lithotypes in the core.

Real-time azimuthal acoustic measurements were introduced recently in the logging-while-drilling (LWD) industry. For the first time, this technology was used as part of the bottomhole assembly (BHA) to acquire information related to principal stress orientations in the deltaic to marine Zubair clastic sequence of onshore Kuwait. A deviated 8.5-in. hole section of the well was planned through sand-shale sublayers with a borehole inclination ranging from 46 to 88°. This section is characterized by time sensitive borehole deterioration and significant variations in pore pressure. These factors result in severe hole instability and ultimately stuck pipe events and require relatively high mud weights to maintain wellbore stability. LWD azimuthal acoustic technology, free from chemical sources, was used for the first time both in drilling and wipe modes to facilitate time-lapse field stress and wellbore stability analysis. Principal stress orientations were identified from three different sources, including borehole breakouts from azimuthal acoustic caliper, density image, and acoustic anisotropy evaluation. The results were then compared with the existing offset well data and an existing geomechanical 3D model. Variations in observed stress orientation, seismic reflection pattern, and pressure history in offset wells were used to map a fault that is responsible for bypassed oil and for the occurrence of tar and gas. The interpretation was extended to other low throw strike-slip faults; additional fault compartments were identified that could affect the pressure maintenance scheme of the field. This paper discusses the planning, design, and use of LWD azimuthal acoustic technology in this case history well. It also describes the viability, integrity, and reliability of the interpreted results and their use in a detailed geological interpretation in terms of stress orientation, fault trapping, and areal fluid variation. The optimization of real-time drilling operations and petrophysical data acquisition requirements are also investigated to improve future field development and overall reservoir management strategies.