Stratigraphic and structural controls on groundwater salinity variations in the Poso Creek Oil Field, Kern County, California, USA

Groundwater total dissolved solids (TDS) distribution was mapped with a three-dimensional (3D) model, and it was found that TDS variability is largely controlled by stratigraphy and geologic structure. General TDS patterns in the San Joaquin Valley of California (USA) are attributed to predominantly connate water composition and large-scale recharge from the adjacent Sierra Nevada. However, in smaller areas, stratigraphy and faulting play an important role in controlling TDS. Here, the relationship of stratigraphy and structure to TDS concentration was examined at Poso Creek Oil Field, Kern County, California. The TDS model was constructed using produced water TDS samples and borehole geophysics. The model was used to predict TDS concentration at discrete locations in 3D space and used a Gaussian process to interpolate TDS over a volume. In the overlying aquifer, TDS is typically <1,000 mg/L and increases with depth to ~1,200–3,500 mg/L in the hydrocarbon zone below the Macoma claystone—a regionally extensive, fine-grained unit—and reaches ~7,000 mg/L in isolated places. The Macoma claystone creates a vertical TDS gradient in the west where it is thickest, but control decreases to the east where it pinches out and allows freshwater recharge. Previously mapped normal faults were found to exhibit inconsistent control on TDS. In one case, high-density faulting appears to prevent recharge from flushing higher-TDS connate water. Elsewhere, the high-throw segments of a normal fault exhibit variable behavior, in places blocking lower-TDS recharge and in other cases allowing flushing. Importantly, faults apparently have differential control on oil and groundwater.

KARAKTERISTIK FLUIDA PEMBAWA MINERALISASI PIT RAMBA JORING DEPOSIT MARTABE, SUMATRA UTARA

Penelitian ini bertujuan untuk mengetahui evolusi hidrotermal dari fluida pembawa mineralisasi yang terdapat di Pit Ramba Joring, meliputi temperatur, tekanan, densitas dan kedalaman pembentukan mineralisasi. Penelitian ini menggunakan analisis inklusi fluida. Sampel yang dianalisis merupakan sampel yang terindikasi terbentuk bersamaan dengan mineralisasi bijih (syngenetik), pada dua tekstur utama bijih yaitu batuan dengan tekstur silica vuggy yang terbentuk pada kisaran Th 331ºC hingga 394ºC, Tm -15,19ºC hingga -12,94ºC dengan salinitas 14,42 wt% hingga 14,89 wt%. NaCl equivalent dan batuan dengan tekstur brecciated terbentuk pada kisaran Th 287,8°C hingga 398,2°C, Tm -14,89°C hingga -11,76ºC serta salinitas 13,91 wt% hingga 4,87 wt%. NaCl equivalent. Korelasi positif antara temperatur homogenisasi pembentukan mineralisasi dengan salinitas larutan menandakan bahwa proses leaching yang membentuk rongga pada batuan terjadi pada stage awal proses alterasi hidrotermal yang diawali dengan proses leaching membentuk rongga (vuggy silica) yang diikuti proses silisifikasi dengan tekstur brecciated, pada fase ini berlangsung pengendapan mineralisasi. Asal larutan yang menjadi larutan pembentuk endapan hidrotermal merupakan jenis larutan basinal water-seawater dalam bentuk connate water yang masih dipengaruhi oleh air permukaan/meteoric water yang menunjukkan adanya indikasi hubungan dengan magmatic water. Densitas fluida pembentuk bijih logam berkisar pada 0,8 gr/cm3 sampai dengan 0,9 gr/cm3 sedangkan proses pembentukan bijih pada Pit Ramba Joring termasuk dalam isothermal mixing dengan sedikit pengaruh pemanasan/boiling.

Pore Size, Tortuosity, and Permeability From NMR Restricted Diffusion in Organic-Rich Chalks

Permeability estimation is crucial for formation evaluation, but faces challenges when used in low-permeability, unconventional formations. NMR well logging is often used to estimate formation permeability, but in many unconventional formations, the current NMR methods are not adequate. We have developed a new method to estimate permeability using a modified Carman-Kozeny model with pore size, tortuosity, and porosity information inferred from NMR restricted diffusion measurements. In this study, we focus on two low-permeability, organic-rich chalks (0.017 and 0.035 md) with connate water present. They are from the same formation but have different depths, TOC (total organic carbon), and bitumen content. These differences affect pore size, tortuosity, and permeability. The core samples are pressure saturated with two hydrocarbons—high-pressure methane or decane—with connate water present. NMR measurements are conducted under pressure to obtain the restricted diffusivity of the hydrocarbon-bearing pore space. In planning the NMR restricted diffusivity measurements, an optimum series of diffusion-encoding times are chosen for the unipolar stimulated-echo pulse sequence to obtain the correlation between the restricted diffusivity (D) and free diffusion length (LD). By applying the Padé fit to the restricted diffusivity, we can better estimate the diffusive tortuosity (τ) and pore-body size (d) of the hydrocarbon-filled pore space. The estimated pore-body size, tortuosity, and porosity from NMR are then used to predict permeability. We introduce a modified Carman-Kozeny model, which shows advantages over older methods like SDR and Timur-Coates models. The advantages of the new method are shown in organic-rich chalk with complex pore structures and organic matter. This new method can potentially be used for estimating permeability by well-logging and core-log integration.

EFFECT OF CO2 REACH ENVIRONMENT ON THE PETROPHYSICAL PROPERTIES OF ARTIFICIALLY CONSOLIDATED CORE SAMPLES

In order to develop, maintain and deplete reservoirs economically around the globe, various measurements are needed with a high demand on natural core samples. The next stage in the life of every reservoir is a secondary or tertiary method to enhance productivity. However, to tailor the available methods and technologies to the reservoir, several screening processes, feasibility studies and pilot experiments are needed. As an aid to these, like a sensitivity analysis, continuous measurements are set up to study fluid flow, chemical reactions, additional recovery and much more, but for all of these, core samples are needed. The lack and high value of natural core samples yield that the demand cannot be satisfied from this source alone. The aim of the study was to create an artificially consolidated stone core sample, a model material, which can be suitable for being the subject of these experiments, with additional benefits in mass production and reservoir parameter-based quality control. In this article the authors wish to present partial results of a big study, this time with comparing the porosity, permeability, connate water and capillary pressure parameters of the core samples used with different after-cure techniques. The process of compaction was the same, but the overburden pressures and the effect of CO2 rich curing were examined. For this, part of the samples was prone to high CO2 environment for different timespans during the after treatment of the samples. The petrophysical parameters were then measured on all of the groups, including a control group and the CO2 affected cores. The focus was on porosity, permeability, connate water saturation/wettability and capillary pressure measurements and the common features and differences in the yielded pore space’s structure are summarized in this article.

Understanding the Hydrogeochemical Evolution of Groundwater in Coastal Aquifers of Southwest Bangladesh

&lt;p&gt;Groundwater development in coastal aquifers of southwest Bangladesh is challenged by both natural and anthropogenic activities resulting in a landward migration of marine waters and increase in the risk of seawater intrusion. In some cases, infiltration of dissolved evaporite salts in the shallow aquifer and presence of connate water in the deep aquifer (DA) are the sources of groundwater salinity other than modern seawater intrusion. A detail investigation of these sources is imperative for a sustainable development and management of coastal aquifers. This work investigates the hydrogeochemical processes affecting groundwater chemistry by interpreting conventional plots, ionic delta, HFE-diagram, stable isotopes, and geochemical modelling. There are three hydrogeological units delineated in this area: upper shallow aquifer (USA) (&lt;100 m bgl), lower shallow aquifer (LSA) (100-200 m bgl) and DA (below 200 m bgl). The hydrochemistry data reveal that the median values of total dissolved solids of the aquifers have a decreasing trend from top to bottom: USA with 7012 mg/l, LSA with 2622 mg/l and DA with 787 mg/l. Sodium is the dominant cation in all waters. The dominant anion in DA is HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, but in shallow aquifers Cl&lt;sup&gt;-&lt;/sup&gt;. The main water type based on the classification of Stuyfzand (1989) is the brackish to saline NaCl type in the shallow aquifers. The Br&lt;sup&gt;-&lt;/sup&gt;/Cl&lt;sup&gt;-&lt;/sup&gt; ratio and relatively enriched &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values in these NaCl waters suggest an origin derived from evaporate dissolution. Reverse cation exchange during intrusion, replacing Na&lt;sup&gt;+&lt;/sup&gt; with Ca&lt;sup&gt;2+&lt;/sup&gt;, results in CaCl and CaMix water types. These waters infiltrate into the LSA. The water in the DA is mainly fresh NaHCO&lt;sub&gt;3&lt;/sub&gt;+ type which originated by cation exchange from infiltrating fresh water. Ca&lt;sup&gt;2+&lt;/sup&gt; was replaced by Na&lt;sup&gt;+&lt;/sup&gt; due to the cation exchange, the water became undersaturated with respect to calcite and secondary calcite dissolution caused elevated bicarbonate concentrations. Near the present-day shoreline and at larger depths, the NaHCO&lt;sub&gt;3&lt;/sub&gt;+ type water mixes with connate water, increasing salinity. The enriched &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values in the DA suggest an origin in a warmer climate, implying that this water has infiltrated a long time ago, much farther inland, probably during the Holocene climatic optimum. It can be expected that the salinization of the shallow aquifers will continue to increase if evaporite deposition and seasonal flooding occur. For a sustainable use of the groundwater resource in this coastal region and to prevent from even further worsening of water quality in its shallow aquifers, it is advised to develop future exploitations in the DA.&lt;/p&gt;&lt;p&gt;Key words: Groundwater chemistry; Stable isotope; Evaporite; Cation exchange; Calcite dissolution.&lt;/p&gt;

Evaluation of calculated connate water saturation values in case of unconventional rock samples

The present study represents possibilities of calculating the connate water saturation - CWS - values of samples from unconventional reservoirs and how to evaluate the obtained result. CWS is an extremely important property of the reservoir rocks. It basically determines the value of the resource and can also predict production technology difficulties. For the samples included in the measurement program, significant or extremely high CWS values were determined. Analysis of the corrected pore size distribution proved to be the most appropriate method for interpreting CWS values, although, it also shows some correlation with the most frequent pore radius - MFPR - and porosity.