Hamid Reza Nasriani

Challenges of Fluid Phase Behavior Modeling in Iranian Retrograde Gas Condensate Reservoirs

Production from gas condensate reservoirs requires precise determination of reservoir fluid properties along with their positive impact on real reservoir performance evaluation and fluid in place volume calculation. This fact is particularly important because liquid drop out phenomena occurs as a result of pressure drop due to fluid production and condensate remains in reservoir [Mohebzadeh, 2005. In this study the challenges and problems of fluid phase behavior simulation in southern Iranian retrograde gas condensate reservoirs is discussed. Most of the problems and challenges are inaccuracy of equation of states near the critical point, special conditions and produced fluid flow constraints, sampling pressure and reservoir pressure during the sampling process, problems related to the constant composition expansion and constant volume depletion experiments, inefficiency in plus fraction characterization, problems arising from lack of the swelling test, and finally C7+ characterization.

Gas Injection for Enhancement of Condensate Recovery in a Gas Condensate Reservoir

Gas condensate reservoirs suffer losses in well productivity due to near wellbore condensate dropout when the flowing bottom-hole pressure declines below the dew point pressure. Pressure maintenance and gas cycling are the common practices used in the oil and gas field to alleviate this problem and develop gas condensate reservoirs. The injection of dry gas into a retrograde gas condensate reservoir helps in vaporizing the condensate and increases its dew point. This article investigates the situation of one of the reservoirs located in southern Iran in Zagros area. First, based on the reservoir composition, the phase diagram has been plotted. Peng–Robinson equation of state for the equilibrium calculations and Lee–Kesler characterization of heavy fractions are used in this software. After that the effect of nitrogen, pure methane, a composition of ethane and methane, and carbon dioxide injections on reservoir recovery has been investigated and compared to a natural depletion scheme. The full system, including two separators and a stock tank, are simulated simultaneously and the effect of each type of injection on the liquid and gas production is investigated. Full mixing has been assumed in all of the injections studied. By comparing between the results it is concluded that in an injection process, required injection rate to maintain reservoir pressure above dew point pressure and avoid liquid formation in the reservoir for pure methane, a composition of ethane and methane, pure nitrogen and carbon dioxide, and liquid recovery in all cases are investigated. According to the results, with increasing ethane mole percent in the injecting gas a lower injecting rate for the same liquid recovery is needed. Because with increasing heavy components mole percent in injecting gas, average molecular weight of injecting gas, and reservoir gas becomes closer and there will be a better mixing between them and, therefore, liquid recovery will be improved. Thus, a composition of ethane and methane with more ethane mole percent is better than others.

A Thorough Investigation of Clean-up Efficiency of Hydraulic Fractured Wells Using Statistical Approaches

Hydraulic fracturing is considered as one of the most effective stimulation techniques to improve recovery especially from unconventional low permeability reservoirs. However this promising stimulation technique sometimes does not respond as expected. Significant amount of work has been dedicated to this topic with ineffective fracturing fluid (FF) clean-up considered as one of the main reasons for this underperformance. However there are still great deals of uncertainties in this area primarily due to large number of parameters affecting FF invasion and its back flow. This work presents results of 10 different sets of numerical simulations consisting of injection, soaking and production periods for 40960 runs. Each set consists of 4096 runs and investigates the simultaneous impact of 12 pertinent parameters (fracture permeability, matrix permeability (km), end points and exponents of Corey gas and FF relative permeability curves in both matrix and fracture and matrix capillary pressure (Pcm) (depending on interfacial tension (IFT), km and pore size index). Two-level full factorial experimental design and linear response surface statistical approaches were used to sample the variables domain, covering a wide practical range determined with the support of our 11 industrial sponsors, and generate output response. Results indicate that improvement in FF mobility inside the fracture is the major factor affecting FF cleanup efficiency. In line with this finding, maintaining high Pcm, by retaining high IFT, results in cleaner fracture (lower gas production loss, GPL). That is, increasing IFT retains FF within the matrix and allows more gas to flow freely inside the fracture. This was confirmed by the corresponding saturation map of FF distribution. The effect of Pcm was more pronounced when drawdown was very low and/or soaking time was extended. At very low drawdown and when km was reduced, in a set within its variation range, the effect of the resultant increase of Pcm on GPL was more pronounced than that of the resultant decrease in FF mobility. Generally when injected FF volume increased, larger GPL was observed and reduction of GPL (cleanup) was also slower. As fracture length decreased, cleanup was faster and the effect of fracture pertinent parameters on GPL, compared to those of matrix, decreased. This works findings allows better evaluation of benefits of this costly operation leading to an optimized design and more efficient ways to improve its performance. For instance, sometimes aiming for a longer fracture, due to its FF poor cleanup performance, is not practically attractive and use of IFT reducing agents to produce more FF during the back flow period, would not have the intended impact to bring back its performance to the desired ideal level. Copyright © 2014, Society of Petroleum Engineers.

A Study of Hydraulic Fracturing Clean-up Efficiency in Unconventional Gas Reservoirs Using Statistical Approaches

Hydraulic fracturing is widely used to improve well productivity especially in unconventional reservoirs. This costly operation, however, sometimes underperforms. One of the main reasons for this poor performance is poor clean-up efficiency of injected fracturing fluid (FF). In this work, a parametric study of FF clean-up efficiency of hydraulic fractured vertical wells was performed with 49152 simulations (in 12 sets) consisting of injection, soaking and production periods. Due to the large number of required simulations, that were conducted using a commercial reservoir simulator, a developed computer code was used to automatically read input data, run simulations and creates output data. In each set (consisting of 4096 runs), simultaneous impacts of 12 parameters (fracture permeability, matrix permeability and capillary pressure, end points and exponents of Corey gas and FF relative permeability curve in both matrix and fracture)were studied. To sample the variables domain and analyse results, two-level full factorial experimental design and linear surface model describing dependency of gas production loss (GPL), compared to 100% clean-up, to pertinent parameters at three production periods (10, 30 and 365 days) were considered and supported by the tornado charts of fitted equations, frequency of simulations with given GPL and FF saturation maps. Results indicate that generally parameters controlling FF mobility within fracture had greatest impact on GPL reduction. However in sets with very low matrix permeability especially when applied pressure drop during production is low, the effect of fluid mobility in the matrix on GPL is more pronounced, in other words, it is important how gas and FF flow within matrix rather than how fast fracture is cleaned. In tighter gas formations, generally more GPL and slower clean-up was observed. The effect of matrix capillary pressure on GPL reduction was more pronounced when drawdown was very low and/or soaking time was extended. This observation was more profound in tighter formations, i.e. for these formations, the effect of a change in drawdown and/or soaking time on matrix capillary pressure and GPL was more pronounced. These findings can be used to make better decisions on the performance and optimised design of hydraulic fracturing, which is a costly but widely used stimulation technique for unconventional low permeability gas reservoirs.

An integrated study of cleanup efficiency of short hydraulic fractured vertical wells using response surface methodology

In tight gas reservoirs, gas well production is impaired after Hydraulic fracturing, that is mostly due to fracturing fluid (FF) invasion into matrix and fracture and poor clean-up efficiency. The scope of this study is to investigate the clean-up efficiency in short hydraulic fracture vertical wells and observe how the effect of pertinent parameters on gas production loss (GPL) changes with the hydraulic fracture length. The impact of 12 parameters including fracture permeability, matrix permeability, End point and Exponent of Corey gas and FF relative permeability curve in both matrix and fracture, and Interfacial Tension and Pore Size Index (capillary pressure) have been studied by developing a computer code. Interactive linear surface model describing the dependency of GPL to the pertinent parameters was used. The results indicate that as fracture length decreased the effect of fracture parameters (fracture permeability and End point and Exponent of Corey gas and FF relative permeability curve in fracture) on GPL decreased and the effect of those relevant parameters in matrix on GPL increased. The effect of capillary pressure in reducing GPL is less pronounced in shorter fractures. In shorter fractures, faster fracture clean-up was observed compare to the one for longer fracture.

A Comparison of Clean-Up Efficiency of Multiple Fractured Horizontal Wells and Hydraulically Fractured Vertical Wells in Tight Gas Reservoirs

Hydraulic fracturing is a well-established stimulation technique especially for unconventional reservoirs. However, sometimes its performance is less than expected due to poor cleanup of the injected fracture fluid. There are publications studying the clean-up efficiency of hydraulic fractures in vertical wells, however, there is little information in this area for multiple fractured horizontal wells, MFHWs. This paper compares the fracture cleanup efficiency of a hydraulically fractured vertical well (VW) with that of multiple fractured horizontal wells (MFHWs) using numerical simulations and statistical approaches. In these simulations, twelve parameters related to fracture and matrix relative permeability and matrix capillary pressure were varied simultaneously based on the two-level full factorial experimental design statistical method. Gas production loss (GPL), as the response term, was calculated and input into Response Surface Model. In each set, the correlation between parameters and GPL was established and compared to investigate the post-fracturing cleanup efficiency. Results show that the direction of impact of parameters for VW and MFHW are similar except for the matrix permeability, Km. This difference is attributed to the flow geometry and how the well has been completed. This observation underlines that in MFHWs, the cleanup mechanism and the sequences of the importance of pertinent parameters could be different from those of the VWs. Results also indicate that matrix Book’s type capillary pressure pertinent parameters are more important in the MFHW sets whilst Corey type relative permeability parameters are more important in VWs. These observations suggest that having a higher capillary pressure is more important in MFHWs because it results in more FF imbibition into the matrix and consequently less resistance to gas flow within the fracture. Several MFHW models with different number of fractures were constructed, which demonstrated that although the impact on well productivity is significant, its impact on clean-up performance was minimal. These results improve our understanding of VWs and MFHWs clean-up and design of such costly operations.

Optimizing volumetric sweep efficiency in water flooding by streamline simulation

ABSTRACTEarly shutting time in production wells due to water production performs an important role to determine production efficiency and useful life of the reservoir.In this study, in order to postpone the shut-in time of producing wells, increase oil displacement, and enhance production efficiency, production and injection wells capabilities with respect to their position in the reservoir were studied by using the concept of streamline.In the oil reservoirs, increasing injection flow rate does not necessarily enhance oil displacement and recovery. Therefore, suitable injection rates according to injection and production wells position have to be optimized. Also, production wells flow rate can affect sweep efficiency optimization extremely and increase the efficiency of injection wells. In this study, according to the position of production and injection wells and water production rates resulting from injection wells, four scenarios with different injection and production rates were investigated. This op...

Choke Performance in High-rate Gas Condensate Wells Under Subcritical Flow Condition

Flow through a surface choke can be described as either critical or subcritical. Most of the correlations available to petroleum engineers are for critical flow but in a lot of high rate gas/condensate wells subcritical flow occurs in large choke sizes. Field data of 15 high rate wells producing from 10 gas condensate reservoirs located in Iran were used to extend two main approaches for high rate wells: a plotting technique proposed by Al-Attar to describe the subcritical flow behavior of gas condensates through wellhead chokes and a general Gilbert-type formula obtained by nonlinear regression of data points for high rate gas condensate wells. The present study extends the work of Al-Attar for high rate gas condensate wells flowing through different large choke sizes between 40/64 and 192/64 in. under subcritical flow conditions, in order to develop individual empirical correlations to be applied to predict gas flow rates under a wide range of flow conditions usually encountered during the flow of gas condensates through wellhead chokes in high rate wells .Also, a new Gilbert-type correlation was generated for high rate Iranian gas condensate wells using nonlinear regression based on the same data points. A comparison between these two approaches is done with five different errors parameters. According to five different errors analysis, the results of gas flow rates calculated by the general formula were compared with those calculated by the individual choke size formulae, and the latter were found to be more accurate. The results of this study could be considered in the design and implementation of deliverability tests, pressure transient tests, well control, and long-term well production of high rate gas condensates wells.