Slimtube measurement is one of the standard experimental techniques used for determining the minimum miscibility pressure (MMP) of an oil and injection gas system prior to the initiation of an enhanced oil recovery (EOR) project. It is preferred because it involves actual fluid displacement in a porous medium. However, the specific criterion for determining the cut-off point during the measurement is not uniquely agreed upon in the literature. Different criteria have been proposed by researchers and this has been one of the setbacks of using Slimtube measurements. The most commonly used criterion is the 1.2 PV criterion, which uses the recovery after injecting 1.2 pore volumes of the displacing gas as the cut-off. However, experimental observations show that even at supercritical condition, the volume of a gas is a strong function of the experimental pressure. Therefore, there is a need to develop an alternative means of determining the MMP that is not subject to particular pore volumes injected during Slimtube measurements. This work presents different means of determining the MMP, based entirely on recovery and the particular displacement phenomenon. In this approach, two new parameters are defined - the instantaneous recovery rate (IRR) and the oil recovery rate (ORR). The maximum values for these parameters for each experiment are used as the cut-off value. This new criteria was used in analyzing nine experimental data using oil from the Permian Basin. The results were compared with MMP prediction based on maximum recovery from each of the runs and the results were found to be in agreement. These new criteria will provide consistent cut-off point for experimental runs because Slimtube measurements take a long time to complete. The new procedure ensures that adequate data have been gathered during each experimental run, sufficient for a consistent experimental analysis.

This study systematically analyzes the dark shale samples of the Silurian Qusaiba Formation from the Tayma Quadrangle outcrop section in the northwest of Saudi Arabia, and assesses the source and nature of its organic matter and the main sedimentary environmental conditions during accumulation of organic matter. The Qusaiba shale samples are characterized by total organic carbon (TOC) and sulfur contents with total values in the range of 0.87–1.76 wt. % and 0.59–4.64 wt. %, respectively, indicating a marine setting ranging from dysoxic to anoxic environmental conditions. The biomarkers are characterized by a relatively low Pr/Ph ratio between 0.50 and 1.24. The abundance of tricyclic terpanes and high C₂₇ and C₂₉ regular steranes equated to C₂₈ regular sterane, providing evidence that the organic matter derived primarily from marine organisms, including algal and other aquatic organic matter and some terrigenous land plants, and was deposited under dysoxic to anoxic environmental conditions. The significant low oxygen environmental conditions may contribute to preservation of organic matter during deposition. The inorganic geochemical indications suggest that the Qusaiba shales were accumulated in a warm–humid climate and with low salinity stratification conditions of the water columns.

Equations of state (EoS) are used to simulate laboratory pressure, volume and temperature (PVT) experiments. Each PVT experiment mimics a form of phase behavior progression for the reservoir fluid from the reservoir and ultimately to the surface facilities. The use of a characterized fluid sample in compositional reservoir simulation study is precluded by a comparison and regression of EoS simulations to PVT laboratory data. In this study, EoS phase behavior software was applied to match laboratory PVT data of several reservoir fluid samples from offshore Niger Delta. The paper presents the fluid characterization routines, simulations, and regression capabilities of the software which was used for the determination of EoS parameters needed to match experimental data. Four EoS models are available through the generalized Martin's equation in the software used. This enabled the use of Redlich-Kwong (RK), Soave-Redlich-Kwong (SRK), Peng-Robinson (PR) and Zudkevitch-Joffe (ZJ) EoS. Two 3-parameter extensions of the PR EoS are also available, one based on a Peneloux et al. volume shift, the other being an implementation of the Schmidt-Wenzel (SW) two-parameter PR EoS. The SRK EoS similarly has a three-parameter extension. This gives a total of seven different EoS models. The predictive capability, strength and weaknesses of these models were subsequently compared and analyzed. Effect of the degree of input fluid characterization and pseudodization on EoS PVT experimental modeling was investigated i.e. characterization to C7+, C12+, C20+, C30+, on the simulation output for each of the EoS models available in the software. The results showed varying degree of simulated experimental data congruence depending on the PVT experiment being modeled. With regression, both the PR and ZJ EoS gave comparable and generally good agreement with laboratory data. Without regression or significant tuning of EoS parameters, neither EoS adequately predicts observed reservoir fluid PVT behavior. This work further elucidates the succinct effect of fluid models used in compositional reservoir simulations which forms the basis for different production scenario and realizations.

The current study investigated the effectiveness of the chemical weathering conditions of the outcrop sections of the Silurian Qusaiba Formation in the Tayma area of Northwestern Saudi Arabia, and their implications for the oxidation and leaching of organic matter within the shale units. The shale units' organic matter characteristics, mineralogy, and elemental geochemistry were studied using total organic carbon (TOC) content, Rock-Eval pyrolysis, X-ray diffraction, and X-ray fluorescence spectrometer methods. The analyzed Qusaiba shale units have moderate to high TOC and sulfur contents in the range of 0.64-1.46 wt % and 0.59-4.64 wt %, respectively, resulting in S/TOC ratio between 0.56 and 3.94 and indicates the marine environment for the studied Qusaiba shale units. The analyzed Qusaiba shale samples have intensive chemical weathering in the source area, as demonstrated by the comparatively high kaolinite, illite, and chlorite clay minerals of up to 60 wt %. The strong weathering of silicates (primarily K-feldspars) and the degradation of the mica in warm, humid climate conditions are confirmed by the high values of the chemical index of alteration (CIA), plagioclase index of alteration, and chemical index of weathering in the range of 72.63-78.62%, 92.72-99.53%, and 95.15-99.69%, respectively. Therefore, the chemical weathering has strongly affected the concentration of organic matter content in the Qusaiba shale units and its petroleum generation potential, as implied by the high amounts of oxygenation organic matter (Type IV) and the low hydrogen index values of up to 50 mg of hydrocarbon (HC)/g of TOC. The oxygenation condition of the outcrop sections was confirmed by the high oxygen index values of up to 242 mg carbon dioxide (CO₂)/g TOC and low values of relative hydrocarbon index in the range of 0.09-0.58 mg HC/g TOC. Based on the results highlighted in this study, the weathering conditions of the outcrop sections during the mid-Carboniferous Hercynian deformation erosion event strongly affected the organic matter in the Silurian Qusaiba shale units, which were oxidized and leached. Thus, their characteristics are not suitable for the HC generation potential.