Productivity Of Gas Condensate Reservoirs Research Articles

Phase behavior of gas condensate in porous media using real-time computed tomography scanning

The phase behavior of gas condensate in reservoir formations differs from that in pressure–volume–temperature (PVT) cells because it is influenced by porous media in the reservoir formations. Sandstone was used as a sample to investigate the influence of porous media on the phase behavior of the gas condensate. The pore structure was first analyzed using computed tomography (CT) scanning, digital core technology, and a pore network model. The sandstone core sample was then saturated with gas condensate for the pressure depletion experiment. After each pressure-depletion state was stable, real-time CT scanning was performed on the sample. The scanning results of the sample were reconstructed into three-dimensional grayscale images, and the gas condensate and condensate liquid were segmented based on gray value discrepancy to dynamically characterize the phase behavior of the gas condensate in porous media. Pore network models of the condensate liquid ganglia under different pressures were built to calculate the characteristic parameters, including the average radius, coordination number, and tortuosity, and to analyze the changing mechanism caused by the phase behavior change of the gas condensate. Four types of condensate liquid (clustered, branched, membranous, and droplet ganglia) were then classified by shape factor and Euler number to investigate their morphological changes dynamically and elaborately. The results show that the dew point pressure of the gas condensate in porous media is 12.7 MPa, which is 0.7 MPa higher than 12.0 MPa in PVT cells. The average radius, volume, and coordination number of the condensate liquid ganglia increased when the system pressure was between the dew point pressure (12.7 MPa) and the pressure for the maximum liquid dropout, Pmax (10.0 MPa), and decreased when it was below Pmax. The volume proportion of clustered ganglia was the highest, followed by branched, membranous, and droplet ganglia. This study provides crucial experimental evidence for the phase behavior changing process of gas condensate in porous media during the depletion production of gas condensate reservoirs.

Achieving the Super Gas-Wetting Alteration by Functionalized Nano-Silica for Improving Fluid Flowing Capacity in Gas Condensate Reservoirs

It is well-known that the production of gas-condensate reservoirs is significantly affected by the liquid condensation near the wellbore region. Gas-wetting alteration can be one of the most effective approaches to alleviate condensate accumulation and improve liquid distribution. However, gas well deliverability is still limited because the wettability of cores is altered only from liquid-wetting to intermediate gas-wetting by using traditional chemical stimulation. To solve this bottleneck problem, herein, we developed a fluorine-functionalized nanosilica to achieve super gas-wetting alteration, increasing the contact angles of water and n-hexadecane on the treated core surface from 23 and 0° to 157 and 145°, respectively. The surface free energy reduces rapidly from 67.97 to 0.23 mN/m. The super gas-wetting adsorption layer on the core surface formed by functionalized nanosilica not only increases the surface roughness but also reduces the surface free energy. The core flooding confirms that the required pressure for displacement is apparently reduced. Meanwhile, the core permeability can be dramatically restored after the super gas-wetting alteration. The microscopic visualization is employed to further understand the impact of fluorine-functionalized nanosilica on the fluid flow behavior and mechanism in porous media. The oil saturation in the micromodel decreases sharply from 48.75 to 7.84%, eliminating the "water locking effect" and "Jiamin effect", which indicates that the added functional nanosilica effectively improves fluid flow capacity and may contribute to production in the gas condensate reservoirs. In addition, this work reveals the fluid flow behavior and mechanism in the reservoir in detail, which will expand the better application of this material to many oilfields and other mining engineering systems.

Investigating the applicability and effects of hydraulic fracturing technology on well productivity in one of the Iranian gas condensate reservoirs

Hydraulic fracturing is an important method utilized to enhance oil and gas recovery from reservoirs, especially for low-permeability formations. It has been broadly used in gas reservoirs, especially tight formations. Gas condensate reservoirs (GCR) experience a noticeable loss of productivity after achieving dew point pressure and condensate drop out formation around the wellbore, and could be suitable candidates for hydraulic fracturing operations. The aim of this study is to determine the effect of hydraulic fracturing on well productivity of GCR. A single well reservoir model corresponding to the GCR in the South of Iran and another model with similar conditions for hydraulic fracturing were created and validated. Reservoir model simulation was performed using a commercial compositional simulator. Sensitivity analysis was performed on fracture geometry parameters including the fracture length and width, flow parameters such as positive coupling and inertia effect, and reservoir properties namely permeability and fluid condensate gas ratio (CGR). The results demonstrated that hydraulic fracturing in GCR, especially in this case study, noticeably increases the productivity and production plateau time as well as decreases condensate formation, which increasing the fracture length and width intensifies this effect. Moreover, the optimized fracture length was obtained. It was found that the negative inertia effect in low hydraulic fracture widths and positive coupling in high fracture widths are dominant. Hydraulic fracturing is most effective in low-permeability and high-CGR conditions. These results pave the hydraulic fracturing road not only from industrial viewpoint, but also as a guideline for field-scale projects.

Simulation-assisted gas tracer test study of Tarim Yaha gas-condensate reservoir in China

To enhance oil recovery and optimize a production plan, injected water or gas fluid should be monitored and tracked, which can be realized by gas tracer technology. Tracer test technology has become an effective method for evaluating the flow condition of reservoirs and estimate reservoir properties. Gas tracers, such as chemical and radioactive materials, have been used to identify well-to-well connectivity, reservoir heterogeneity, high-permeability pay zones, sweep efficiency, and residual oil saturation. Moreover, tracer tests have been used to evaluate well interference among multiple hydraulically fractured wells and qualitatively predict possible fracture geometries. Therefore, the accurate quantification of tracer transport is of great importance. However, theoretical study of gas tracers in a condensate gas reservoir has not been thoroughly conducted to date. Current tracer transport models mostly focus on the gas tracer flow in oil reservoirs and cannot be directly applied to condensate gas reservoirs. To the best of our knowledge, the application of a gas tracer to recycle gas injection in gas condensate reservoirs has not been thoroughly studied. In this paper, we propose a mathematical model to quantitatively describe the gas tracer transport mechanism in condensate gas reservoirs. Our mathematical model is based on the mass conservation law and is numerically solved via the Integral Finite Difference method. The results predicted by our model have been compared to and verified by real data collected from the field. The model has been used to optimize the recovery of the Yaha Condensate Gas Field in the Tarim Basin, China. With the assistance of our tracer transport model, the pressure drawdown speed of the Yaha field has been successfully slowed from 0.8 MPa/year to 0.5 MPa/year. Generally, this study not only provides key insights into the transport mechanism of gas tracers but also presents the application of numerical tools to guide the production of gas condensate reservoirs. The technique introduced in this study could be potentially extended to condensate oil production in other fields.

Investigating the effects of water vaporization on the production of gas condensate reservoirs

Well productivity is an important issue in the development of most low and mediumpermeability gas-condensate reservoirs. However, accurate forecasts of productivity can be difficult because of the need to understand and account for the complex processes that occur in the near-well region. When the well pressure falls below the dew-point, a region of high liquid saturation builds up around the well, impairing the flow of gas and reducing the well productivity. It is essential to take account of this condensate-blockage effect when calculating well productivity. Most of the pressure drop in a gas condensate well occurs close to the well walls. Because of the increase in pressure drop and the increase in flow rate, two additional phenomena get involved in gas flow control: The effect of positive coupling (due to increased capillary number) and the effect of inertia (due to non-Darcy flow). In many gas-condensate wells, the final result of these two parameters is improving well productivity, reducing the impairment caused by condensate blockage. Another phenomenon that can take place in high temperature gas reservoir is water vaporization.Pressure drop near well wall causes molar content of water in gas-phase to increase, therefore connate water starts to vaporize near the well walls. This change in connate water saturation near the well wall can influence well productivity by changing saturation of fluids near well walls. The effect of water vaporization on well productivity of gas and gas - condensate reservoirs are investigated by considering a single well one dimensional radial model simulation. The simulations show that water vaporization increases productivity of well by increasing gas saturation and relative permeability near the well walls and improving the mobility of gas; and this effect is stronger in rich gas condensate reservoir than the lean ones.Keywords: Well, Gas, Pressure Drop, Vapor pressure of water

EFFECT OF NON-DARCY FLOW COEFFICIENT VARIATION DUE TO WATER VAPORIZATION ON WELL PRODUCTIVITY OF GAS CONDENSATE RESERVOIRS

Well productivity of gas condensate reservoirs is highly affected by near-wellbore phenomena. Inertial force resulting from convective acceleration of fluid particles in the medium, as well as viscous force, determines the flow of gas through porous media at high velocity. Pressure drop builds up the molar content of water in gas by water vaporization in the near-wellbore region, which means a drop in connate water saturation. Given that the inertial force is a function of the non-Darcy coefficient, β, which itself depends upon connate water saturation, this can ultimately lessen the non-Darcy component of the pressure drop and therefore inertial forces, leading to improvement of well deliverability. Currently, no physically relevant model takes into account the non-Darcian flow coefficient variation due to this phenomenon. This paper utilizes a single-well compositional simulation to exhibit how water vaporization could compensate for the effect of inertia on well productivity of gas condensate reservoirs.

The Effect of Water Vaporization on the Well Productivity of Gas Condensate Reservoirs

Well productivity is a crucial concern in predicting the performance of many gas condensate reservoirs. Accurate forecasting of gas-condensate well productivity usually requires fine-grid simulation to model the formation of the liquid bank and to account for high-velocity phenomena and changes in relative permeability at high capillary numbers. At early stages of production, the pressure drop in the near-wellbore region builds up the molar content of water in gas. This, in turn, results in water vaporization, and hence a drop in the connate water saturation. When water vaporization occurs, pores that were formerly filled by connate water now becomes occupied by gas and condensate only. This causes a change in gas and condensate saturations near the wellbore that can affect the well productivity. No lucid numerical simulation approach considering this effect is suggested in the literature. The authors investigated the effects of water vaporization on the near-wellbore region by compositional simulator of GEM. Enhancement of gas well productivity is observed.

Wettability alteration to intermediate gas-wetting in low-permeability gas-condensate reservoirs

Gas-condensate reservoirs experience significant productivity losses as reservoir pressure drops below the dew point due to condensate accumulation and water blocking and the subsequent reduction in gas relative permeability. One potential way to overcome this problem is to alter reservoir wettability to gas-wetting to reduce condensate accumulation and water blocking in the near wellbore and maintain high productivity. The major goal of this work was to study the mobility of the gas and liquid phase (both water and oil) before and after wettability alteration from strong liquid-wetting to intermediate gas-wetting. For this purpose, in addition to relative permeability measurements, we also conducted various other tests to demonstrate that liquid mobility can be improved significantly due to wettability alteration. As wettability modifiers, fluorinated polymers are capable of delivering a good level of oil and water repellency to the rock surface, making it intermediate gas-wet and alleviating such liquid blockage under high temperature. The contact angle analyses, capillary rise, flow tests, and imbibition spontaneous tests were used to estimate the effect of treatments on wettability. Experimental results demonstrated that the fluorinated polymers can alter the wettability of cores from strong liquid-wetting to gas-wetting, which could decrease the amount of water invaded and resided in the gas formation. Core flood test results demonstrated that the relative permeabilities of both the gas and the liquid phases were increased significantly after the wettability alteration to preferential gas-wetness. The residual liquid saturation was decreased, and the gas production was enhanced greatly due to the wettability alteration. These results imply that gas well deliverability may increase substantially when wettability is altered to intermediate gas-wetting. Efficiency in the extraction of natural gas is important to improve the productivity of gas-condensate reservoirs where liquid accumulates, which is beneficial for the economic and environmental considerations of the oil and gas industry.

Improving productivity and increasing Qatar reserves

At Texas A&M University at Qatar, the faculty, research staff, and students of the Petroleum Engineering Program are currently involved in and also planning for several research projects with four main objectives: improving productivity in Qatar's oil and gas fields; increasing the petroleum reserves of Qatar; developing a strong base of Qatari professionals; and preserving the environment. One of our major projects is directed at enhancing the productivity of gas-condensate reservoirs which are commonly reduced due to condensate blocking. Our research team is tackling the problem on various fronts including a comprehensive experimental study on the wettability changes of rocks that can enhance liquid mobility and gas productivity; a simulation study to identify the critical parameters that need to be considered when trying to optimize well productivity; and a fundamental study that is targeting analytical modeling of multi-phase flow and the stability of various states of wettability at reservoir conditions. Funding for the various aspects of this work is supported by the Qatar National Research Fund's National Priorities Research Program (NPRP), RasGas, and Schlumberger. Another active research area related to productivity enhancement deals with acid stimulation in carbonate reservoirs, which applies to almost every well in Qatar. Part of the work is aimed at enhancing the recovery of spent acid to speed the clean up process and improve gas productivity after stimulation. Another part deals with the development of an acid-jetting process as an improved and more effective stimulation process. More work is in the initial stages that targets optimizing the stimulation treatments in Qatar. We are also actively working on developing models using artificial intelligence techniques to optimize applications of horizontal wells in gas condensate reservoirs with uncertain geological properties. Our work on CO2 addresses two objectives: carbon sequestration and CO2 injection for improved oil recovery. We are conducting experimental, modeling, and simulation work to achieve these objectives.

An Analysis of the Pressure Transient Behavior of Gas-condensate Reservoirs

During production of gas-condensate reservoirs, reservoir pressure is isothermally depleted beyond the point where the fluid phase envelope is crossed, which results in condensate drop out and a decreased gas permeability and well deliverability. Analysis of this type of reservoir based on traditional single-phase pressure transient behavior is no longer feasible. Our goal is to continue to explore the applicability of Boltzmann transformation techniques that could enable the prediction of reservoir performance and account for productivity losses present in such reservoirs, and from which the construction of potential production type-curves could be suggested.

Overview: Production Operations (March 2007)

Welcome to the Production Operations feature once again. Some of the issues that production-operations engineers must address in their daily work include multiphase flow in production systems, well-productivity changes, and production-performance testing and analysis. These issues often are compounded when compositional changes occur in the produced reservoir fluids, as is commonly encountered in the production of gas-condensate reservoirs. This year's Production Operations feature includes some very interesting research and some practical applications of the measurement and evaluation of well performance in multiphase-flow operations. There is significant need for a better understanding of the complex phenomena associated with multiphase flow in production systems, including the metering of fluid flow, the effects that multiphase flow and reservoir-fluid-compositional changes have on well productivity, and the overall production-system performance under multi-phase-and nonlinear-flow conditions. With improved multiphase-flow metering and testing, improved wellbore multiphase-flow modeling, and more-realistic well-completion-efficiency evaluation, the production-operations engineer can make better-informed decisions about a well's performance and its potential for improvement. The articles presented in this issue highlight some of the advances being made to improve the measurement and evaluation of the production performance of wells in oil and gas-condensate reservoirs. Production Operations additional reading available at the SPE eLibrary: www.spe.org SPE 103255 "Optimizing the Productivity of Gas-Condensate Wells" by C. Shi, SPE, Stanford U., et al. SPE 102231 "Modeling Nonisothermal Rapid Multiphase Flow in Wells Under Nonequilibrium Conditions" by G. Michel, U. of Oklahoma, et al. SPE 103222 "Rate Allocation Using Permanent Downhole Pressures" by M. McCracken, SPE, ExxonMobil Upstream Research, et al.