查看更多>>摘要：The issue of borehole stability has attracted great attention worldwide but serious downhole problems still exist. The influences of various factors on borehole stability have been extensively studied in the literature. It is usually believed the lower limit of safe borehole pressure window is determined by collapse failure while the upper limit is determined by fracture failure of rocks. However, it is proved in this paper the upper limit can also be decided by collapse failure. A new model of determining the safe borehole pressure window is built and the new upper limit based on shear failure criteria is proposed, therefore, a more precise safe borehole pressure window can be determined. The effects of borehole orientation, structural weak planes, in-situ stress and pore pressure are then thoroughly analyzed. The results show that the two limits defined by shear failure criteria are resulted from principal stress alteration and usually featured by change of failure mode. The upper limit of shear failure criteria is quite critical in determining safe mud pressure under various conditions. The increase of mud pressure within boreholes may not alleviate collapse failures. The Mohr-Coulomb criterion confines a narrower window while the Drucker-Prager criterion predict a wider one. For determining the safe borehole pressure window, a uniform insitu stress and a low pore pressure will make it more necessary to consider the upper limit of shear failure criteria. The model built and conclusions drawn can provide new guidance for mud weight selection and well trajectory design in drilling practice.
查看更多>>摘要：Ultrasonic vibration technology as a new type of auxiliary lithotripsy technology, a lot of research on this technology. However, at present, the effect of rock temperature on ultrasonic vibration of rock breaking has not been studied. Therefore, with the help of numerical simulation software, nuclear magnetic resonance detection and penetrant testing methods, it is necessary to complete the numerical simulation and laboratory testing of granite at different temperatures under ultrasonic vibration with granite as the research object. The main results are as follows:1. Under ultrasonic vibration, the number of rock cracks will increase with increasing temperature. And the fracture azimuth intensity in the rock model is 90 degrees concentrated. 2. As the temperature increases, the accumulated thermal damage will promote the large number of expansion and connection of certain internal cracks, resulting in the macroscopic destruction of the rock sample and the increase in crack volume. 3. With the increase of temperature, the increase of porosity of rock samples under ultrasonic vibration has a staged characteristic. And 150 degrees C is the threshold. The significance of this research is to provide theoretical and technical support for the application of ultrasonic vibration technology in the future hard rock geological exploration and deep resource mining.
查看更多>>摘要：When oilfield development enters the middle and late stages, the crude oil-water mixture (COWM) produced from oil wells often has a high water cut and cannot form a stable emulsion. The traditional viscosity model of an emulsion is not suitable for this unstable mixture. Mastering the viscosity characteristics of the unstable crude oil-water mixture with high water cut (UCMH) for pipeline transportation of such mixtures is important. The apparent viscosities of UCMH (water cut of mixture ranging from 0.70 to 0.90) for 16 crude oils were determined via the stirring viscometric method. The results showed that the apparent viscosity of UCMH decreased with increasing shear rate, water cut of mixture, and temperature, demonstrating the property of shear thinning. The apparent viscosity of UCMH did not depend on the content of single surfactants such as resins, asphaltenes, waxes, and mechanical impurities but was closely related to the content of the above-mentioned combinatorial surfactants. The parameter exergy loss rate of shear that describes energy consumption was determined to characterize the external shear action on UCMH, which can fully reflect the essence of the mechanical energy consumption in the emulsifying process under different shearing conditions. A new viscosity model for UCMH was established by the quantitative characterization of the energy consumption and crude oil physical properties (i.e., saturate content, aromatic content, surfactant content, acid value, and crude oil viscosity). In light of the statistical results of 336 groups of validation experimental data from four crude oils, the average relative deviation between the apparent viscosity calculated by the new model and the measured apparent viscosity of UCMH was 5.3%, which is considerably better than the traditional emulsion viscosity models, that is, the Einstein, Vand, Taylor, and Richardson models.
查看更多>>摘要：Acid stimulation is one of the most common chemical approaches to overcome the reduction of crude oil pro-duction due to clogging of oil formation passages. Nonetheless, in this process, the formation of stable and tight acid in crude oil emulsions may lead to further unsolicited formation damage. In this study, the stability of dispersed HCl droplets with concentration of 15 and 28 wt % in a crude oil was characterized by means of the interaction energy theory. Similar experiments were repeated and compared with distilled water. The results showed that water droplets always tend to coalesce in short period of time, while acid droplets resist. This is due to the acid-base interaction energy, which is the most prevailing component among various interaction energies which would profoundly influence the stability of acid in crude oil emulsions. In case of using HCl solution as aqueous phase in the emulsion, this interaction between acid droplets overcomes the others, especially the attractive interactions. The most stable acid in crude oil emulsion would be formed in distances between the droplets shorter than 4 nm because acid-base interactions are short-ranged. For the investigated crude oil, it was also observed that the emulsion with HCl 28 wt % solution was more stable than that for HCl 15 wt %. Bottle tests also agreed qualitatively with the results of this theoretical approach. The practical implications of these results would be that overdosed acidizing with excessive injection rates may lead to unwanted formation damage due to formation of tight acid in crude oil emulsions. Finally, the presented results would serve as a guideline for comparing acids of different concentrations.
查看更多>>摘要：Surfactant alternating gas/CO2 (SAG) flooding is a promising enhanced oil recovery (EOR) technology, which has the potential to improve oil production beyond primary and secondary water flooding methods. However, the performance of SAG flooding is restricted due to instability under high salinity reservoir conditions and excessive surfactant adsorption. One convenient and relatively low-cost option to overcome the limitations and improve the performance of SAG flooding is the use of low salinity water (LSW). In this study, an innovative and effective approach in the form of a low salinity surfactant (LSS) system was developed to suitably enhance the efficacy of immiscible SAG flooding in sandstone reservoir cores. The impact of LSS solutions on various mechanisms responsible for EOR including surface and interfacial tension (IFT), wettability alteration, CO2-foam stability, and surfactant adsorption were investigated. The initial IFT measurements revealed that oil-water IFT could be lowered using low salinity formation water (FW), however a more effective reduction was possible only when surfactants were introduced in such a system to form LSS solutions. Similarly, reductions in the contact angle (from initial 122 degrees to 45 degrees) were achieved with LSS solutions which demonstrated favorable wettability alteration of the reservoir rock from oil-wet to water-wet conditions. Foam stability experiments revealed that the half-life of CO2-foams generated with LSS solutions was appreciably higher, thus indicating the positive impact of low salinity water on foam stability. Additionally, anionic LSS solutions displayed reduced surfactant adsorption behavior onto sandstone cores, whereas for cationic LSS solutions the adsorption values were comparatively higher and exhibited a reverse adsorption trend with decreasing salinity. Finally, core flooding experiments illustrated that significantly higher residual oil could be mobilized and recovered through the tertiary immiscible low salinity surfactant alternating gas/CO2 (LSSAG) flooding. Nearly 23% original oil in place (OOIP) was produced by employing anionic LSS solution as the chemical slug during the LSSAG flooding. However, under similar experimental conditions, LSSAG flooding with cationic LSS solution recovered only about 16 %OOIP despite its comparatively better wettability alteration and IFT performance. In summary, using anionic LSS solutions constitute an effective means to suitably improve the performance of immiscible SAG flooding in sandstone reservoir cores.
查看更多>>摘要：The correct use of temporary plugging agents for chemical diverting fracturing technology is the key to stimulate reservoir volume be expanded in the oil and gas industry. By injecting the temporary plugging agents into the initial fracture to form a plugging body with higher strength, so as to force the subsequent flow pressure to divert to other positions to open more new fractures, and then remove the plugging. Based on this background, this paper divides the action mechanism of the temporary plugging agent into four parts, which are introduced respectively. It can be found that the plugging structure can be divided into single structure and composite structure. Faced with development of harsh reservoirs, the composite plugging structure has the highest success rate, but there have always been problems such as complex combination optimization, higher material cost, and difficult flow back. The supercharging mechanism essence is to increase the fracture restart pressure. The diverting mechanism is related to stress redirection, which depends on the superposition of stress states around the fracture, but the difficulty is that there is no suitable method for accurate stress analysis. The mechanism of plugging removal mainly includes three functions: dissolution, degradation and breaker, but each has its own limitations. In addition, through the investigation of new temporary plugging agent materials, it is found that the temperature-controlled temporary plugging agent is in line with the future development trend. To sum up, the findings of this paper will help to better and quickly understand the overall action mechanism and main control factors of temporary plugging agent for diverting fracturing, and also provide theoretical guidance and enlightening suggestions for the performance optimization and future development of temporary plugging agent.
查看更多>>摘要：274 crude oils pertaining to the groups of extra light (gas condensates), light, medium, heavy, and extra heavy crude oils were characterized by true boiling point distillation, specific gravity and kinematic viscosity at 21.11 and 37.78 degrees C. Eight published regression empirical methods were examined for their capability of accurately predicting the crude oil viscosity. Among them the model of Kotzakoulakis and George was found to provide the lowest average absolute relative error (AARE) of 24.0% with AARE of 21.5% for the crude oils containing < 30 wt % vacuum residue (VR) and AARE of 37.2% for the crude oils having > 30 wt% VR. The model of Aboul-Seoud and Moharam exhibited the lowest AARE (16.3%) for the crude oils with < 30 wt% VR. A new nonlinear regression model was developed that predicted the viscosity of the 274 crude oils with AARE of 19.5%, with AARE of 14.9% for the crude oils containing < 30 wt% VR, and AARE of 42.0% for the crude oils having > 30 wt% VR. Another model based on the artificial neural network (ANN) technique was developed. The ANN model predicted the viscosity of the 274 crude oils with AARE of 44.3%, with AARE of 50.2% for the crude oils with < 30 wt% VR, and AARE of 13.9% for the crude oils containing > 30 wt% VR. The combination of predicting the viscosity of crude oils having < 30 wt% VR by the new nonlinear regression model with the predicting of viscosity of crude oils with > 30 wt% VR by the ANN model provides of AARE of 14.9% of viscosity prediction for the entire data base of 274 crude oils.
查看更多>>摘要：The ultimate recovery factor in tight and shale resources is limited and is usually in the range of 5-10%. Although high-intensity fracturing and refracturing can increase recovery, an enormous amount of oil will remain in place, hence the desirability of enhanced oil recovery methods. Many of the shale reservoirs are oil-wet with negligible water uptake. By altering the wettability, water can spontaneously imbibe into the formation matrix, creating a counter-current flow that forces the oil out. Here, we assess the likelihood of increased oil recovery by modifying the fracturing-fluid composition (salinity and ion concentration) that transforms the formation wettability into a more water-wet state.& nbsp;Oil wetting of tight formations is usually controlled by the adhesion of oil droplets on the surface of clay minerals. When clay minerals are not predominant, the oil attached to carbonate minerals can significantly control rock wettability. In this study, we first identify the primary reactions that define the initial wettability of the rock depending on the formation mineralogy, formation water composition, and oil type. Second, we build a geochemical model considering the surface complexation of various minerals to mimic the wettability state of the reservoir. Third, we validate our method on zeta-potential, contact angle, and imbibition data from a previously published study using different fluids with different salinities. Finally, we present a mechanistic approach to model the wettability-alteration impact on spontaneous imbibition and compute the incremental oil recovery contributed by different fracturing fluid compositions.& nbsp;Based on the studied case, the oil adhesion to clay can be reduced by tuning the fracturing fluid salinity. The ionic concentrations of 2.5 wt % of NaCl and 5.0 wt % of CaCl2 induced the smallest contact angles of 44.7 and 51.2. We observe that further brine dilution increases the contact angle. For example, distilled water shows the most oil-wet condition with a contact angle to 93.4. We argue that the main factors that maximize water wetting for the reported optimum salinities are the contrast in the rock and oil surface potential and the sodium con-centration. Spontaneous-imbibition simulations indicate that the low salinity fluids promote a change in water-oil capillary pressure, leading to increased water uptake in the cores and improved oil recovery compared to distilled water. The agreement between the developed model and experimental data implies that the wettability in shale and tight formations can be quantitatively predicted and regulated.
查看更多>>摘要：In this paper, a super hydrophobic/oleophobic material CS-1 with super hydrophobic/oleophobic properties was successfully prepared with nano silica as the main raw material and 1H, 1H, 2H, 2H-perfluorooctyl trichlorosilane as modifier. The structures of synthesized product were characterized by the infrared spectroscopy (FTIR). The contact angle of super hydrophobic/oleophobic material CS-1 was measured by contact angle measuring instrument, and its hydrophobic/oleophobic properties were analyzed systematically. The micro morphology of super hydrophobic/oleophobic material CS-1 was determined by scanning electron microscope. The results of wetting properties show that the maximum water phase contact angle of hydrophobic/ oleophobic material CS-1 is 159.58 degrees and the maximum oil phase contact angle is 135.63 degrees. Based on triple action of constructing micro-nanostructure on rock surface, reducing gas-water surface tension and oil-water interfacial tension, the optimized formula of the unlocking agent 1.5 wt% super hydrophobic/oleophobic material CS-1 + 0.06 wt% NFS surfactant +0.09 wt% C3 Gemini surfactant was screened by single factor method. Finally, the performance of unlocking agent is comprehensively evaluated by surface energy spectrometer, laser particle size analyzer, automatic contact angle tester and core unlocking equipment. The results show that the water phase contact angles of the core treated with the unlocking agent are greater than 150 degrees and the oil phase contact angles are greater than 130 degrees in acidic, alkaline and high temperature environments. The maximum recovery rate of water locking damage gas measurement permeability is 91.1%, and the maximum recovery rate of oil locking damage gas measurement permeability is 67.4%. It shows that the unlocking agent can effectively improve the gas logging permeability of the core and change the wettability of the core surface, so as to relieve the liquid locking of the condensate gas reservoir. It has a good application prospect in the middle and late stage of condensate gas reservoir development.
查看更多>>摘要：The accurate estimation of production is the bottleneck technique that constraints the efficient development of oil and gas fields. However, such multivariate and asymmetric reservoir parameters and highly nonlinear fluid flow behavior stake a stringent claim for precise production forecast, which makes semi-analytical modeling and numerical simulation techniques expose challenges. Based on the applications of data modeling methods in the prediction of oil and gas production, this paper proposes the procedures of data-driven models for multivariate oil field data with small samples. In addition, the strengths, weaknesses and limitations of widely used data driven models and their combination models are analyzed in detail, and the experiences and lessons in oil and gas production prediction are summarized based on the applications of data-driven models in oilfield cases. Furthermore, the data modeling method for flow equations with complex boundary and mechanism will be a challenge and future direction to make production predictions more quickly and accurately.
NSTL
Elsevier