Gas Injection Into Fractured Reservoirs Above Bubble Point Pressure

Abstract

Among all enhanced oil recovery (EOR) scenarios, gas injection seems to be promising for implementation in naturally fractured reservoirs. The use of CO₂ has received considerable interest as a method of EOR but a major drawback is its availability and increasing cost. Therefore, an alternative gas like CH₄ or N₂ must be considered to meet the economic considerations. To investigate the efficiency of oil recovery by CO₂, N₂, and CH₄ injection in fractured carbonate rock, a series of experiments was designed. Both miscible and immiscible schemes for gas injection were carried out on a low-permeable outcrop carbonate rock that was surrounded by fracture, established with a novel experimental method. The experiments aimed to investigate the potential of oil recovery by secondary and tertiary gas injection under high-temperature conditions. The matrix block was saturated using a recombined mixture of Iranian live oil, and by pumping water into the annular space, the space between rubber sleeve and outer jacket, high overburden pressure was exerted to obtain the desired homogeneous saturation. Using a back-pressure regulator, the pressure was kept above the bubble point pressure. The inlet was attached to a constant pressure pump injecting gas or water above the bubble point pressure, and the overburden pressure was removed gradually and the inlet fluid inflated the rubber sleeve. The amount of produced water from the annular space was recorded to estimate the distance between the rubber sleeve and sand face. This distance creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery as a function of time was monitored during the experiments. Results from both secondary and tertiary gas injection experiments indicate that CO₂ injection at elevated pressure and temperature is more efficient than N₂ and CH₄ injection.     

An Experimental Investigation of Foam for Gas Mobility Control in a Low-Temperature Fractured Carbonate Reservoir

Abstract

This work concerns the experimental investigation of surfactant alternating CO₂ injection in carbonate rocks. The core samples provided from a low-temperature fractured light oil reservoir, located in southwest Iran. The experiments were designed to observe the effect of CO₂–foam injection on gas mobility and oil recovery at different surfactant concentrations. The core samples were initially saturated with synthetic/field brine, 5,000 ppm, and then flooded with live oil to reach connate water saturation at reservoir condition, 115°F and 1,700 psia. The commercial surfactant used was sodium lauryl sulfate as an anionic surfactant. The results of this work, along with field-scale simulation and/or economic considerations, could be helpful in making reliable decisions about optimum condition of foam-assisted water-alternating-gas (FAWAG) processes.

Core flooding results demonstrated that macroscopic sweep efficiency increased due to foam generation inside the core. In addition, it led to an increase of between 5 and 12% in the recovery factor in comparison to a water-alternating-gas (WAG) process. Furthermore, the value of the critical micellar concentration for the surfactant–oil system was about 2,000 ppm.

After primary and secondary recovery processes, there is a large amount of trapped oil in the reservoir. Extensive research has been directed toward enhancing the recovery of this oil, but limited success has been achieved. FAWAG injection appears to have more applicability to recover the trapped resources. However, little attention has been paid to experimental investigation of FAWAG processes in low-temperature oil reservoirs.     

Prediction of equilibrium water dew point of natural gas in TEG dehydration systems using Bayesian Feedforward Artificial Neural Network (FANN)

Abstract

The aim of this paper is to predict the equilibrium water dew point of natural gas in TEG dehydration process using feedforward artificial neural network (FANN). The ANN model shows a good result as the coefficient of determination of 0.9989 and 0.9976 was obtained for training and testing data respectively with relatively small value of mean square errors of 0.0203 and 0.0221. 0.5% of average absolute deviation percentage was observed which is comparable with the literatures. It clearly shows that FANN gives a good prediction on water dew point of natural gas in TEG dehydration process.     

The Optimization of Underground Gas Storage in a Partially Depleted Gas Reservoir

Abstract

Underground gas storage (UGS) is a well-known technology to supply variable demand of natural gas market. The objective of this work is to demonstrate the feasibility of UGS development in an Iranian depleted fractured gas reservoir. For this purpose, gas demand analysis was conducted based on yearly consumption, and the peak condition demand was obtained. Then, a full-field multicomponent reservoir model was constructed based on the rock and fluid data and comprehensive production history. A modified Peng-Robinson equation was used to estimate the phase behavior of the reservoir and injection fluid. The compositional model was verified through performing history matching on gas production rate.

After history matching, an injection/withdrawal (I/W) schedule was designed to meet necessary gas consumption. Production scenarios were conducted to obtain optimum operating conditions for this reservoir. It was determined that at least four new wells are required to establish successful UGS plan to meet the target withdrawal rate. Results show that the use of a horizontal well is superior to a vertical well because of less water production during storage cycles. Water cut, productivity, and injectivity indexes of individual wells were considered as prominent factors to find the optimum well pattern, using a connection transmissibility factor to compare the capacity of each well to produce. In addition to constant I/W rate, a variable I/W rate scenario was proposed whose result showed no difference compare to the results obtained in previous runs.

Results show that the reservoir under study is capable of providing peak gas consumption during the cold months in Iran.     

Experimental Study of the Effect of Permeability on Foam Diversion

Abstract

Through single-core and dual-core experiments, relations of pressure, liquid flow rate of outlet, and gas-phase saturation to injection fluid volume were researched. Effect of permeability on foam diversion was analyzed. Experiments indicate that gas saturation increases and then decreases with core permeability at the same PV during subsequent liquid injection for single-core experiment. When the permeability ratio is less than 11, liquid flow rate ratio is less than 1. When permeability ratio lies in the range of 11 to 14.9, liquid flow rate ratio is greater than 1, and foam diversion is declining. The best diversion is observed at a permeability ratio of 4.0. The effect of permeability on foam diversion is that capillary pressure is greater in the low-permeability core, and bubbles in foam break easily, which causes cross flow of gas phase.     

An Experimental Study of Secondary WAG Injection in a Low-Temperature Carbonate Reservoir in Different Miscibility Conditions

Abstract

This experimental study is aimed at evaluation of the performance of secondary WAG injection in carbonate cores at different pressures. To do so, a comprehensive series of high-pressure high-temperature (HPHT) core flooding tests are conducted. The fluid system includes reservoir dead and live crude oil, CO₂, and synthetic brine while the chosen porous media consists of a number of fractured carbonate core samples. Parameters such as oil recovery factor, water and oil production rates, and pressure drop along the core are recorded for both dead and live oil. According to results, at first increasing pressure improves the oil recovery, but this improvement after MMP is not as significant as it is before MMP. Also recoveries of dead and live oils at same pressure show different values due to differences in miscibility condition of injected gas. Then as the graphs demonstrate, relative permeability reduction due to hysteresis effect has dominant effect on pressure drop curves. Finally, as the production rate curves show, nearly all of the remained oil after breakthrough is produced as the gas is being produced and almost no oil can be recovered during water production portions.     

Use of Starved Cells of a Klebsiella Pneumoniae Strain Isolated from a Brazilian Oil Reservoir in a Transport and Plugging Experiment in a Sandstone Core

Abstract:

Microbial biomass can be used as a plugging agent in high-permeability zones in applications of microbial enhanced oil recover (MEOR) technology to oil-bearing strata. Laboratory tests have shown that bacterial cells, reduced in size by starvation, can penetrate more deeply when injected into a porous medium and also grow therein after nutrient is added. Transport and growth of Klebsiella pneumoniae starved cells through a Rio Bonito sandstone core (440 mD of permeability) were tested, to verify application to oil recovery. The injection of starved cells followed by nutrient medium at a flow rate of 1.8 mL/h, promoted cell growth in the core, which raised the inner pressure from 0.016 to 13.7 psi, resulting in a 99.9% reduction in the permeability of the core. The biobarrier formed persisted during the injection of 4 pore volumes of brine into the core at a flow rate of 1.8 mL/h, keeping the permeability reduced to 99.7%.     

Experimental Study of Asphaltene Deposition During Different Production Mechanisms

Abstract

Sudden changes in key parameters such as pressure, temperature, and fluid composition may result in asphaltene precipitation and deposition, consequently reducing permeability and porosity as well as well injectivity and productivity. Sandstone cores of an Iranian reservoir were studied under high pressure and temperature. Asphaltene deposition was studied in recycled gas injection, CO₂ injection, and natural depletion experiments. The authors observed that these processes could be ranked for the deposition severity viewpoint in the aforementioned order. Qualitatively investigation of cores indicated nonuniform deposition of precipitated asphaltene along a flooded core and reducing deposition from entering core terminal to the core outlet.     

Performance Evaluation of Cyclic Pressure Pulsing in a Depleted,Naturally Fractured Reservoir with Stripper-Well Production

Abstract

We present a parametric study on the operational parameters of cyclic pressure pulsing with N₂ and CO₂. We aim to develop a better understanding of how operational parameters affect the process performance in a shallow, naturally fractured, and depleted reservoir of the Big Andy Field in eastern Kentucky. The study includes analyses of various design parameters such as the injection rate, lengths of injection and soaking periods, cycle rate limits, and number of cycles. Incremental oil production, peak oil rate, and net present value (NPV) are considered as the performance criteria. Analyses have been performed using a single-well, dual-porosity, compositional reservoir model.     

Wellbore Flow and Heat Transfer of Drilling with Foam

Abstract

On the basis of special physical properties of foam drilling fluid, a mathematical model of foam flow and heat transfer in a wellbore was established, and the solution of the model was proposed. Employing the established mathematical model and its solution, numerical calculation was conducted to analyze the effect of wellbore heat transfer on the hydraulic parameters of foam drilling. The results indicated that foam temperature in a drilling pipe was always lower than that in the annulus and formation temperature. At the bottom of the annulus, foam temperature was lower than the formation temperature, whereas in the top of the annulus, foam temperature was higher than the formation temperature. Most important, with increasing well depth, liquid injection rate, and gas injection rate, the deviation between foam temperature at the bottom of the annulus and formation temperature increased. Wellbore heat transfer not only resulted in increased foam quality in the top of the annulus, bottom pressure, and minimum gas injection rate but also a decrease in the foam quality at the bottom of the annulus and minimum cutting transport velocity. Therefore, the stability and cutting transport capacity of foam decreased. In addition, foam density and Fanning friction coefficient were affected by wellbore heat transfer. Although to a certain extent wellbore heat transfer has an effect on hydraulic parameters of foam drilling, the effect was limited and could be counteracted by increasing gas injection rate and back pressure.     

The Validation of Methods to Calculate Gas-Oil Relative Permeability Using Capillary Pressure Data from South Pars Reservoir

Abstract

Gas-oil relative permeability is the most important parameter in the simulation of fluid flow in the gas condensate reservoirs. Experimental measurements of relative permeability need core samples with regular shape, which is costly and time consuming. On the other hand, experimental data of relative permeability may also have significant error and uncertainty in many cases. One source of uncertainty is that the input to numerical simulator is uncertain and inaccurate; it may be reduced if the number of input parameters is decreased, especially if the parameters with the greatest uncertainty are avoided. It is possible to impose only capillary pressure data, because relative permeability can be predicted consistently using specific models. The present methods, which are based on capillary pressure, considered the porous media as a bundle of capillary tubes; they indeed are mercury flow paths that are filled during the mercury injection capillary pressure test at the certain value of capillary pressure. The authors applied the existing capillary models for relative permeability calculations to a gas condensate reservoir. The tested samples have a wide range of liquid permeability from less than 1 to 18 md. The results of this study show that the Purcell model has the best fit with experimental data for wetting phase (oil), and the differences between measured and model data were almost negligible. The predictions of nonwetting phase (gas) relative permeability were a good agreement with experimental data except for Purcell model. Results reveal that the relative permeability could be computed by using accurate capillary pressure data.     

A Nonlinear Approach to Gas Lift Allocation Optimization With Operational Constraints Using Particle Swarm Optimization and a Penalty Function

Abstract

Most gas lift well networks confront the limitations of both gas supply and compressor capacity. Thus, simultaneously allocating a number of the wells the optimum of the gas injection rate and the gas injection depth to reach the maximum oil production, in addition to satisfying the stated constraints, is the most important effort for the design process. This novel approach is a modification of the conventional allocation optimization problem in which no constraint on the gas injection pressure at the surface was considered. In this work, a model using a particle swarm optimization algorithm and penalty function method is presented to solve the new approach of the allocation optimization problem with high speed and desirable accuracy. Then the model is tested on a group of wells.     

An Experimental Phase Diagram of a Gas Condensate Reservoir

Abstract

Most condensate gas reservoirs discovered in Iran are rich in intermediate components. The phase diagram of these fluids is essential for reservoir management. The available commercial reservoir fluid simulators are not able to find the critical point of some of these condensate gas reservoirs to obtain the phase diagram quality lines. The quality lines of these reservoir's samples can be obtained experimentally using the constant mass expansion test data. The authors present experimental development of gas condensate phase diagram along with retrograde condensation and revaporization characterization during the reservoir depletion process of one Iranian condensate gas reservoir.     

A Novel Approach to the Gas-Lift Allocation Optimization Problem

Abstract

Because compressed gas is a scarce and expensive resource, allocating an optimal amount of gas injection to a group of wells to increase the oil production rate is an important optimization problem in the gas lift operation. In this article, a particle swarm optimization algorithm is employed to assign an optimum gas injection rate for each individual well. Also, a new gas lift performance curve-fit that can reduce the time and volume of the computation is suggested. Finally, the algorithm is tested on five wells in an Iranian oil field.     

Prediction of Asphaltene Precipitation during Pressure Depletion and CO2 Injection for Heavy Crude

Abstract

In this work, a thermodynamic approach is used for modeling the phase behavior of asphaltene precipitation. The precipitated asphaltene phase is represented by an improved solid model, and the oil and gas phases are modeled with an equation of state. The Peng-Robinson equation of state (PR-EOS) was used to perform flash calculations. Then, the onset point and the amount of precipitated asphaltene were predicted. A computer code based on the solid model was developed and used for predicting asphaltene precipitation data reported in the literature as well as the experimental data obtained from high-pressure, high-temperature asphaltene precipitation experiments performed on Sarvak reservoir crude, one of Iranian heavy oil reserves, under pressure depletion and CO₂ injection conditions. The model parameters, obtained from sensitivity analysis, were applied in the thermodynamic model. It has been found that the solid model results describe the experimental data reasonably well under pressure depletion conditions. Also, a significant improvement has been observed in predicting the asphaltene precipitation data under gas injection conditions. In particular, for the maximum value of asphaltene precipitation and for the trend of the curve after the peak point, good agreement was observed, which could not be found in the available literature.     

An Experimental and Numerical Investigation of Solvent Injection to Heavy Oil in Fractured Five-Spot Micromodels

Abstract

In this work a series of solvent injection experiments was conducted on horizontal glass micromodels at several fixed flow rate conditions. The micromodels were initially saturated with heavy crude oil. The produced oil as a function of injected volume of solvents was measured using image analysis of the continuously provided pictures. In order to investigate the macroscopic behavior of the process in different media, several fractured, with constant width, and nonfractured five-spot micromodels were designed and used. The measured data have also been used for verifying and developing a simulation model that was later used for sensitivity analysis of some parameters that affect oil recovery. The results show that when the fracture spacing increased, the oil recovery decreased. In contrast, as the fracture orientation angle (the angle with the mean flow direction) or solvent viscosity increased, the oil recovery increased. A critical value for the ratio of connate water saturation to the oil volume has been found that, beyond that the solvent injection process, loses its efficiency. The final recovery of a water alternating solvent (WAS) injection process in fractured medium in the presence/absence of connate water saturation was considerably greater than that obtained either by continuous solvent injection or water injection alone. Good agreement was observed between experimental and simulation results when a dual permeability model was used in our simulation.     

A Correlation for Prediction of the Required Minimum External Surface Area of Gas Cartridge Filters in the Petroleum Industry

Abstract

A correlation has been derived from analytical solution of mass and momentum balance equations for accurate and rapid estimation of the required minimum external surface area of gas cartridge filters in the petroleum industry. The proposed correlation is especially important for preliminary design of these units regarding gas injection projects and underground storage of natural gas. High replacement costs associated with gas cartridge filters and numerous applications of these units are the most important justifications of the present work. Predicted results and fabricator's recommended data regarding the required minimum external surface area of gas cartridge filters are in good agreement with an average absolute deviation (AAD) of 2.4%.     

A BIOMASS PYROLYSIS GASIFIER APPLICABLE TO RURAL CHINA

Abstract

A pyrolsis gasifier was developed to make use of indigenous biomass to help solve the problem of rural energy shortage particularly in engine fuel shortage and to reduce pollution. First, (low diagram of pyrolysis gasification system was designed and the sketch of gasifier was drawn. Second, the gasifier thus drawn was made. It was successfully operated with corncob feeding. Its performance parameters were measured in an experimental scheme designed according to an orthogonal test table. A variance analysis was taken of the results were taken to sort out the best experimental condition. A run on such conditions gave a heating value of the gas as high as 12.4MJ/nm³ and without tar, a conversion rate about 60.2%, and a total energy efficiency about 59.3%. Regressive analysis of the data that shows the flow rate of the gas is an exponential function of time. Finally, explanations to the analysis results of the data and what problems of the gasiSSfier to be tackled were given.     

An Experimental Investigation of Optimum Capillary Number on Vertical Immiscible Gas Injection in Iranian Carbonate and Sandstone Rocks

Abstract

The authors' purpose was twofold: (a) to study optimum capillary number for acquiring maximum oil recovery in vertical gas-oil displacement using carbonate and sandstone cores, and (b) to investigate the influence of capillary end effect on optimum capillary number extracted from experimental data. Two sets of forced gravity drainage experiments were performed on carbonate and sandstone cores. The first set of experiments comprised seven vertical nitrogen injection tests using carbonate core, which had been saturated by kerosene in ambient laboratory condition. The second set consisted of six vertical nitrogen injection tests using sandstone core that had been saturated by live oil under reservoir temperature and pressure conditions. In each set of experiments, in constant bond number, one optimum capillary number (optimum flow rate) was recorded in which the maximum oil recovery was achieved. When capillary end effect was negligible, optimum capillary number was compatible with critical capillary number, which was calculated in critical gravity drainage velocity while in considerable end effect, optimum capillary number was higher than the calculated critical capillary number. In addition, the residual oil saturation in carbonate rock showed a declining trend when capillary number increased, whereas the minimum of residual oil saturation in sandstone core has been observed in optimum capillary number.     

煤层气欠平衡钻井环空注气工艺优化

充气欠平衡钻井技术是目前煤层气钻井中减少煤储集层伤害、保护煤储集层最常用的措施之一.针对煤层气羽状水平井充气欠平衡钻井技术特征,优选了水平及竖直井筒内多相流动压降计算模型,确立了井底欠压值的求解方法,给出了求解的计算流程.根据沁水盆地煤层气田某羽状水平井实际资料,通过对不同注气压力、不同注气量以及不同钻井液排量条件下井底欠压值的计算和分析得出:当环空注气压力一定时,井底欠压值随钻井液排量的增大而单调递增,且环空注气量越大,井底欠压值越小;在最佳环空注气压力条件下,当钻井液排量一定时,井底欠压值随着环空注气量的增大而单调递减,当环空注气量一定时,井底欠压值随着钻井液排量的增大而增大.利用该计算模型和计算方法,结合现场欠平衡钻井工程要求.可以优化出最佳的环空注气工艺参数.