Gas condensate reservoirs: Characterization and calculation of dew-point pressure

A large data bank of more than 700 gas-condensate samples collected from literature and experiments was established.On this basis,empirical correlations and equations of state commonly used to calculate dew-point pressure(DPP)were evaluated.A new model for estimating DPP was proposed.All the empirical correlations and the Peng-Robinson state equation were compared,and sensitivity of parameters was analyzed.The current standards used to identify gas condensate were evaluated and found to be not accurate enough.The Peng-Robinson state equation has no unique solution and is affected by multiple factors such as the characterization of C7+components and the splitting scheme.The Nemeth-Kennedy correlation has the highest accuracy when applied to the data bank established in this study,followed by Elsharkawy correlation and Godwin correlation.While Shokir correlation cannot be used for samples without C7+components,it is therefore the lowest in accuracy.The newly proposed model has an average absolute error,root mean square error and coefficient of determination of 7.5%,588,and 0.87,respectively,and is better than the above four correlations statistically.The proposed model proved to be more accurate and valid when compared to experimental results and simulation with the Peng-Robinson state equation.

Economic Evaluation of Hydraulic Fracturing in a Gas Condensate Reservoir Operating below Dewpoint

Hydraulic fracturing is among the approaches used to optimize production from a gas condensate reservoir. A detailed economic analysis is required to evaluate the profitability and feasibility of hydraulic fracturing as an optimization option in a gas condensate reservoir operating below dewpoint. The objective of this research is to evaluate the economic benefits derivable from the use of hydraulic fracturing to improve gas and liquid recovery from a gas condensate reservoir operating below dewpoint. This research considers the use of four profit indicators to ascertain the profitability of hydraulic fracturing in a gas condensate reservoir operating below dewpoint by increasing the fracture halflength, fracture width and fracture permeability. The production data of the reservoir was obtained and the economic calculations done on excel spreadsheet and plots generated. The four profit indicators considered in the research are Net Present Value, Payout, Discounted Cash Flow Rate of Return and Profit per Dollar Invested. The economic justification was done by carrying out a comparative economic analysis from the result obtained when the reservoir of study was unfractured with that obtained when the reservoir was fractured at various fracture parameters. The economic analysis was done considering a royalty and tax rate of 18.5% and 30% respectively and a gas price of $2/MSCF and condensate price of $30/bbl. This is done so as to find out if the additional cost of hydraulic fracturing can be offset by the recovery from the reservoir when its pressure declined below dewpoint. The result obtained showed that the additional recovery due to hydraulic fracturing by increasing the fracture halflength, fracture width and fracture permeability was not enough to justify the application of hydraulic fracturing when the reservoir pressure declined below dewpoint.

Numerical Simulation of Oil and Gas Two-Phase Flow in Deep Condensate Gas Reservoirs in Bohai Buried Hills

The BZ19-6 gas field is characterized by high temperature and high pressure (HTHP), high condensate content, little difference between the formation pressure and dew point pressure, and large amount of reverse condensate liquid. During the early stage of depletion development, the production gas-oil ratio (GOR) and production capacity remain relatively stable, which is inconsistent with the conventional reverse condensate seepage law. In view of the static and dynamic conflict in development and production, indoor high-temperature and high-pressure PVT experiment was carried out to reveal the mist-like condensation phenomenon of fluids in the BZ19-6 formation. And the seepage characteristics of condensate gas reservoirs with various degrees of depletion under the condition of HTHP were analyzed based on production performance. The change rule of fluid phase state was analyzed in response to the characterization difficulties of the seepage mechanism. The fluid state was described using the miscible mechanism. And the interphase permeability interpolation coefficient was introduced based on interfacial tension. By doing so, the accurate characterization of the “single-phase flow of condensate gas-near-miscible mist-like quasi single-phase flow-oil-gas two-phase flow” during the development process was achieved. Then the accurate fitting of key indicators for oilfield development was completed, and the distribution law of formation pressure and the law of condensate oil precipitation under different reservoir conditions are obtained. Based on research results, the regulation strategy of variable flow rate production was developed. Currently, the work system has been optimized for 11 wells, achieving a “zero increase” in the GOS of the gas field and an annual oil increase of 22,000 cubic meters.

Experimental and modeling study of CO_(2)-Improved gas recovery in gas condensate reservoir

This paper presents the effectiveness of the CO_(2) injection process at different periods during gascondensate reservoir development.Taking a real gas-condensate reservoir located in China's east region as an example,first,we conducted experiments of constant composition expansion(CCE),constant volume depletion(CVD),saturation pressure determination,and single flash.Next,a series of water/CO2 flooding experiments were been investigated,including water flooding at present pressure 15 MPa,CO_(2) flooding at 25.53 MPa,15 MPa,which repents initial pressure and present pressure respectively.Finally,the core flooding numerical model was constructed using a generalized equation-of-state model reservoir simulator(GEM)to reveal miscible flooding mechanism and the seepage flow characteristics in the condensate gas reservoir with CO2 injection.A desirable agreement achieved in experimental results and predicted pressure volume temperature(PVT)properties by the modified equation of state(EOS)in the CVD and CCE tests indicated that the proposed recombination method can successfully produce a fluid with the same phase behavior of initial reservoir fluid with an acceptable accuracy.The modeling results confirm the experimental results,and both methods indicate that significant productivity loss can occur in retrograde gas condensate reservoirs when the flowing bottom-hole pressure falls below dew point pressure.Moreover,the results show that CO_(2) treatment can improve gas productivity by a factor of about 1.39 compared with the water flooding mode.These results may help reservoir engineers and specialists to restore the lost productivity of gas condensate.

Developing a K-value equation for predict dew point pressure of gas condensate reservoirs at high pressure

This paper proposed a new empirical K-value equation is developed to calculate dew pressure for gas condensate reservoirs.This equation is applicable in the wide ranges of composition,temperature,and pressure by considering the effect of composition via two equations for normal boiling point and critical temperature of the mixture.The range of dew pressure,temperature,heptane plus mole fraction,methane mole fraction,N2 mole fraction,CO2 mole fraction,and H2S mole fraction are fallen into 2666.7e9655 Psia,40e350.87
F,0.0021e0.213,0.3344e0.9668,0e0.4322,0e0.0864,and 0e0.942 respectively.As an important point,the proposed equation has any adjustable parameters,in addition,this equation indicates that in order to predict of dew pressure of gas condensate reservoirs,trial and error was not needed and therefore,computational speed increases beyond the accuracy.Moreover,the accuracy is validated by comparing against the experimental data of 81 gas condensate reservoirs samples from published literature and the results of Wilson,Whitson,and Ghafoori equations.Compared to the experimental data,the absolute average deviations of dew pressure calculations for the proposed equation,Wilson,Whitson,and Ghafoori were 7.6%,97.6%,99.4%,and 94.9%respectively.

Change of phase state during multi-cycle injection and production process of condensate gas reservoir based underground gas storage

Based on the differences in production mode and operation process between gas storage and gas reservoir,we established a phase balance test procedure and a theoretical simulation model of phase balance during multi-cycles of injection and production of underground gas storage(UGS)rebuilt from condensate gas reservoir to study the phase characteristics of produced and remaining fluids during multi-cycles of injection and production.Take condensate reservoir gas storage as example,the composition of produced fluid and remaining fluid,phase state of remaining fluid,retrograde condensate saturation and condensate recovery degree in the process of multi-cycles of injection-production were studied through multi-cycle injection-production experiment and phase equilibrium theory simulation.The injected gas could greatly improve the recovery of condensate oil in the gas reservoir,and the condensate oil recovery increased by 42% after 5 cycles of injection and production;the injected gas had significant evaporative and extraction effects on the condensate,especially during the first two cycles;the condensate oil saturation of the formation decreased with the increase of injection-production cycles,and the condensate oil saturation after multi-cycles of injection-production was almost 0;the storage capacity increased by about 7.5% after multi-cycles of injection and production,and the cumulative gas injection volume in the 5 th cycle increased by about 25%compared with that in the 1 st cycle.

Hydrocarbon Charging and Accumulation Process of the Large Bozhong19-6 Condensate Gas Reservoirs in the Southwestern Bozhong Sub-Basin,Bohai Bay Basin,China

The Bozhong19-6(BZ19-6)condensate gas reservoirs,located in the southwestern Bozhong sub-basin,Bohai Bay Basin,China,were paleo-oil reservoirs in the geological past and have undergone at least three successive hydrocarbon charging events.The hydrocarbon migration and accumulation process of“early oil and late gas”has occurred in the current reservoirs.At the end of the sedimentation of the Guantao Formation(N_(1)g,∼12 Ma),the reservoirs began to fill with first stage low-moderate mature crude oil.At the late stage of the Lower Minghuazhen Formation(N_(1)ml)(∼6.7 Ma),the reservoirs were largely charged with second stage high mature crude oil.Since the deposition of the upper Minghuazhen Formation(N_(2)m^(u),∼5.1 Ma),the paleo-oil reservoirs were transformed into shallow Neogene reservoirs due to the reactivation of basement faults.From the late stage of the N_(2)m^(u)to the present day(∼2.8–0 Ma),the reservoirs were rapidly filled by natural gas within a short period.In addition,analysis of the formation of the reservoir bitumen and the conspicuous loss of the lower molecular weight n-alkanes in the crude oil reveal that the injection of a large amount of gas in the late stage caused gas flushing of the early charged oil.

Study on gas-liquid relative permeability experiments of fractured-porous reservoirs

For carbonate reservoirs,gas-water and oil-gas relative permeability curve are of importance for parameters calculation in oil-field development,dynamic analysis and numerical reservoir simulation.The oil-gas/gas-water relative permeability curves were measured with unstable method under normal temperature and low pressure,using fifteen artificial fractured cores.The shape and change characteristics of gas/water and oil/gas relative permeability curve were mainly analyzed.The results showed that for the relative permeability curve of water driving gas.The saturation of equal permeability point,the saturation range of two-phase flow region and the displacement efficiency has a good power-function relation with core permeability.In addition,the saturation of equal permeability point,the saturation range of two-phase flow region and the displacement efficiency will decrease with the increasing of core permeability.However,for oil-gas relative permeability curves,the saturation of equal permeability point,the saturation range of two-phase flow region and the displacement efficiency will increase with the increasing of core permeability.It should be noted that the gas relative permeability presents a straight line in the normalized relative permeability curve,which indicates that the main flow is dominated by fracture with low displacement efficiency.The research results provide theory basis for fracture-vuggy carbonate reservoir development and new research idea for the further study of this kind of reservoir.

Super gas wet and gas wet rock surface: State of the art evaluation through contact angle analysis

Recently,super gas wet and gas wet surfaces have been extensively attended in petroleum industry,as supported by the increasing number of publications in the last decade related to wettability alteration in gas condensate reservoirs.In many cases,contact angle measurement has been employed to assess the wettability alteration.Even though contact angle measurement seems to be a straightforward approach,there exist many misuses of this technique and consequently misinterpretation of the corresponding results.In this regard,a critical inspection of the most recent updated concepts and the intervening parameters in the contact angle based wettability evaluation of liquid-solid-gas systems could aid to provide some remediation to alleviate this problem.To this end,this work presents a survey on the accurate terms and rigorous protocols based on the community of surface science and chemistry.As a preliminary step,advancing,receding,static,and the most stable contact angle terminology are defined.The study is followed by the definition of the contact angle hysteresis effect.The application of surface free energy in the selection of the best gas wet agent is then analyzed.Afterward,the impact of the size-dependent behavior of drop on contact angle is discussed.Finally,a sessile drop experiment is explained to achieve the defined parameters.For future contributions to petroleum industry journals,like this journal,this work could offer an easy use of the conceptual framework for analyzing the results and comparative evaluations in chemical wettability modifier agents.