Effects of interactions in natural gas/water/rock system on hydrocarbon migration and accumulation

The study of natural gas accumulation process in tight formation has become the focus of the petroleum industry. One of the priorities is the effects of interactions in natural gas/water/rock system on hydrocarbon migration and accumulation process. On the macroscopic scale, we investigate the interactions in natural gas/water/rock system by formation fluorescence test and production data analysis. One the microscopic scale, the mechanisms are revealed by mathematical analysis and experimental methods considering the variation of geological temperature and pressure. The effects of interactions in natural gas/water/rock system are also simulated by numerical simulation. The results are visualized and quantified. A novel semi-analytical method based on a physical experiment is proposed to calculate the temperature- and pressure-dependent contact angle and interface tension which reflect the interactions in the natural gas–water–rock system. This semi-analytical is embedded in the numerical simulation during the simulation of the natural gas charging process. The results indicate that with the increase of geological temperature and pressure, the contact angle will increase and the interface tension between natural gas and water will decrease. The capillary resistance in the formation will be reduced. Since the decrease of capillary resistance, the natural gas can be charged into smaller pores, so that the actual charging threshold is lower than the one originally obtained under present reservoir conditions. After considering the temperature and pressure during the accumulation process, some sand bodies that were thought not to be charged may have natural gas accumulate.

A Temperature Operating Window Concept for Application of Nonionic Surfactants for EOR in Unconventional Shale Reservoirs

Surfactant performance is a function of its hydrophobic tail, and hydrophilic head in combination with crude oil composition, brine salinity, rock composition, and reservoir temperature. Specifically, for nonionic surfactants, temperature is a dominant variable due to the nature of the ethylene oxide (EO) groups in the hydrophilic head known as the cloud point temperature. This study aims to highlight the existence of temperature operating window for nonionic surfactants to optimize oil recovery during EOR applications in unconventional reservoirs. Two nonylphenol (NP) ethoxylated nonionic surfactants with different EO head groups were investigated in this study. A medium and light grade crude oil were utilized for this study. Core plugs from a carbonate-rich outcrop and a quartz-rich outcrop were used for imbibition experiments. Interfacial tension and contact angle measurements were performed to investigate the effect of temperature on the surfactant interaction in an oil/brine and oil/brine/rock system respectively. Finally, a series of spontaneous imbibition experiments was performed on three temperatures selected based on the cloud point of each surfactant in order to construct a temperature operating window for each surfactant. Both nonionic surfactants were observed to improve oil recovery from the two oil-wet oil/rock system tested in this study. The improvement was observed on both final recovery and rate of spontaneous imbibition. However, it was observed that each nonionic surfactant has its optimum temperature operating window relative to the cloud point of that surfactant. For both nonionic surfactants tested in this study, this window begins from the cloud point of the surfactant up to 25°F above the cloud point. Below this operating window, the surfactant showed subpar performance in increasing oil recovery. This behavior is caused by the thermodynamic equilibrium of the surfactant at this temperature which drives the molecule to be more soluble in the aqueous-phase as opposed to partitioning at the interface. Above the operating window, surfactant performance was also inferior. Although for this condition, the behavior is caused by the preference of the surfactant molecule to be in the oleic-phase rather than the aqueous-phase. One important conclusion is the surfactant achieved its optimum performance when it positions itself on the oil/water interface, and this configuration is achieved when the temperature of the system is in the operating window mentioned above. Additionally, it was also observed that the 25°F operating window varies based on the characteristic of the crude oil. A surfactant study is generally performed on a single basin, with a single crude oil on a single reservoir temperature or even on a proxy model at room temperature. This study aims to highlight the importance of applying the correct reservoir temperature when investigating nonionic surfactant behavior. Furthermore, this study aims to introduce a temperature operating window concept for nonionic surfactants. This work demonstrates that there is not a "one size fits all" surfactant design.

THE FORMATION MECHANISMS OF COMPOSITION OF DRINKING GROUNDWATER OF THE Kyiv DEPOSIT (ON THE EXAMPLE OF THE OBOLON WATER INTAKE STRUCTURE)

This paper presents the results of the assessment of interactions in the water-rock system using an integrated approach including the balance method and the method of geochemical (thermodynamic) modelling. Assessment is carried out for conditions of Cenomanian-Callovian and Bajocian aquifers within the Obolon groundwater intake structure in Kyiv. The results obtained demonstrate that groundwater of the Cenomanian-Callovian and Bajocian aquifers within the Obolon groundwater intake structure differ in chemical composition, physicochemical conditions, and especially in the formation of water composition due to the interactions in the water-rock system. This paper proposes division of water into groups, taking into account both the features of chemical composition and its formation process. The water group characterized by anomalous ratio of chlorine and sodium is distinguished, as well as the possible formation mechanism of this water composition is proposed. The chemical composition of the waters of both aquifers meets the requirements of Ukrainian legislation for drinking water quality (GSanPiN 2.2.4-171-10). Groundwater quality of the Cenomanian-Callovian complex is shown to be higher than that of the Bajocian aquifer. For both aquifers, the water of higher quality is the one with cationic composition determined largely by ion exchange. The ion exchange processes can be controlled to a certain extent by regulating the water withdrawal from the wells, and hence the water quality can be regulated in this way as well. Another way to regulate water quality could be the mixing of water from two aquifers during water treatment, which would, on the one hand, compensate the insufficient water quality of individual aquifers and, on the other hand, provide for continuous well operation contributing to the maintenance of more or less stable physicochemical processes. However, these hypotheses require further detailed consideration and, if confirmed, a detailed justification of their feasibility.

Structural features and formation processes of a complex hydrogeochemical section in the Baikal rift zone

The purpose of the work is to study the effect of organic matter on the formation of ion-salt and gas composition of nitrogen-methane and methane thermal water occurring in the sedimentary rocks of deep horizons of artesian basins. The object of research is the Tunka intermountain artesian basin of the Baikal rift zone and the Tungor gas and oil field of the Okhotsk-Sakhalin basin, in the deep horizons of which soda (inversion) low- and high-mineralized groundwater is common. The study combines the results of the traditional study of the composition of natural solutions and the quantitative research of physical and chemical interactions in the “water – rock” system conducted using the Selector software package according to the degree of the hydrogeochemical process, which was set by the value of the rock/water ratio. Chemically pure water and rocks of medium chemical composition were used in interaction. With the use of physicochemical modeling the formation of thermal water composition in sedimentary rocks depending on the interaction degree between water and rock and the amount of organic matter was unravelled. As a result, it was determined that the organic matter present in the rock has the dominant influence on the intensity of the hydrogeochemical process determining the amount of mineralization, the ratio of components, and the amount of methane, nitrogen, and carbon dioxide produced. The correspondent compositions of the model and natural solutions showed the possibility to form low- and high-mineralized sodium bicarbonate groundwater of different gas-saturation degree in the conditions of deep horizons of sedimentary basins due to the internal reserves of the “water – rock” system not involving any components from external sources.

Evaluation of Cement-Casing & Cement-Rock Bond Integrity During Well Operations

This paper presents the results of laboratory static and dynamic tests on casing-cement-rock systems exposed to axial loads under ambient conditions. A new testing method has been developed. The casing-cement-rock system mostly fails due to tension and shear stresses. In various applications such as HPHT, deep-water, (steam) injection or geothermal wells, the cement-casing bond is exposed to cyclic thermomechanical loads resulting in casing elongation, contraction, expansion and subsequently in cyclic radial and axial stresses at the cement-casing-rock system. Cement is a brittle material which can fail when subjected to repeated application of stresses lesser in magnitude than the statically determined strength. A novel atmospheric test cell has been designed and constructed. In order to achieve the fatigue limits of the cement-casing bond, a set of testing procedures has been established. Several tests are conducted to evaluate de-bonding. The focus on de-bonding is achieved by allowing the casing to move through the test while preventing any cement movement. Thus, when a force is applied in the axial z-direction - either the casing is pulled out (tension) or pushed down (compression) - the casing has enough space to move in both directions. The advantage of this testing method is that different stress ratios can be applied during the test.

Stability of "Support-Surrounding Rock" System for Fully Mechanized Longwall Mining in Steeply Dipping Coal Seams

The key to the safe and efficient longwall mining of steeply dipping seams lies in the stability control of the "support-surrounding rock" system. This paper analyzes the difficulty of controlling the stability of the support during the longwall mining process of steeply dipping coal seams in terms of the characteristics of the non-uniform filled-in gob using a combination of physical test, theoretical analysis and field measurements. Considering the floor as an elastic foundation, we built a "support-surrounding rock" mechanical model based on data obtained on "support-surrounding rock" systems in different regions and the laws of support motion under different load conditions. Our findings are summarized as follows. First, depending on the angle of the coal seam, the caving gangue will roll (slide) downward along the incline, resulting in the formation of a non-uniform filling zone in the deep gob in which the lower, middle, and upper sections are filled, half-filled, and empty, respectively. In addition, an inverted triangular hollow surface is formed on the floor of the gob in the middle and upper sections behind the support. Furthermore, as the angle of the coal seam, length of the working face, and mining height increase, the characteristics of the non-uniform filled-in gob are enhanced. Second, we found that, as a result of support by the gangue, the "support-surrounding rock" system is relatively stable in the lower part of the working face while, in the middle and upper sections of the working face, the contact method and loading characteristics of the support are more complicated, making stability control difficult. Third, the magnitude and direction of the load, action point, and mining height all affect the stability of the support to varying degrees, with the tangential load and action position of the roof load having the most significant impacts on the stability of the support. Under loading by the roof, rotation and subsidence of the support inevitably occur, with gradually increasing amplitude and effects on the inter-support and sliding forces. Finally, we found that it is advisable in the process of moving the support to adopt "sliding advance of support" measures and to apply a "down-up" removal order to ensure overall stability. These research results provide reference and guidance of significance to field practice production.