Volatile Oil. Determination of Reservoir Fluid Composition From a Non-Representative Fluid Sample

1999 ◽  
Author(s):  
Rafael H. Cobenas ◽  
Marcelo A. Crotti
Keyword(s):  
Volatile Oil ◽  
Fluid Composition ◽  
Fluid Sample ◽  
Reservoir Fluid

New Downhole-Fluid-Analysis Tool for Improved Reservoir Characterization

2008 ◽  
Vol 11 (06) ◽  
pp. 1107-1116 ◽  
Author(s):  
Chengli Dong ◽  
Michael D. O'Keefe ◽  
Hani Elshahawi ◽  
Mohamed Hashem ◽  
Stephen M. Williams ◽  
...  
Keyword(s):  
Real Time ◽  
Hydrocarbon Composition ◽  
Fluid Composition ◽  
Fluid Sample ◽  
Fluid Analysis ◽  
Reservoir Fluid ◽  
Reservoir Conditions ◽  
Fluid Properties ◽  
Fluid Characterization ◽  
Compositional Grading

Summary Downhole fluid analysis (DFA) has emerged as a key technique for characterizing the distribution of reservoir-fluid properties and determining zonal connectivity across the reservoir. Information from profiling the reservoir fluids enables sealing barriers to be proved and compositional grading to be quantified; this information cannot be obtained from conventional wireline logs. The DFA technique has been based largely on optical spectroscopy, which can provide estimates of filtrate contamination, gas/oil ratio (GOR), pH of formation water, and a hydrocarbon composition in four groups: methane (C1), ethane to pentane (C2-5), hexane and heavier hydrocarbons (C6+), and carbon dioxide (CO2). For single-phase assurance, it is possible to detect gas liberation (bubblepoint) or liquid dropout (dewpoint) while pumping reservoir fluid to the wellbore, before filling a sample bottle. In this paper, a new DFA tool is introduced that substantially increases the accuracy of these measurements. The tool uses a grating spectrometer in combination with a filter-array spectrometer. The range of compositional information is extended from four groups to five groups: C1, ethane (C2), propane to pentane (C3-5), C6+, and CO2. These spectrometers, together with improved compositional algorithms, now make possible a quantitative analysis of reservoir fluid with greater accuracy and repeatability. This accuracy enables comparison of fluid properties between wells for the first time, thus extending the application of fluid profiling from a single-well to a multiwall basis. Field-based fluid characterization is now possible. In addition, a new measurement is introduced--in-situ density of reservoir fluid. Measuring this property downhole at reservoir conditions of pressure and temperature provides important advantages over surface measurements. The density sensor is combined in a package that includes the optical spectrometers and measurements of fluid resistivity, pressure, temperature, and fluorescence that all play a vital role in determining the exact nature of the reservoir fluid. Extensive tests at a pressure/volume/temperature (PVT) laboratory are presented to illustrate sensor response in a large number of live-fluid samples. These tests of known fluid compositions were conducted under pressurized and heated conditions to simulate reservoir conditions. In addition, several field examples are presented to illustrate applicability in different environments. Introduction Reservoir-fluid samples collected at the early stage of exploration and development provide vital information for reservoir evaluation and management. Reservoir-fluid properties, such as hydrocarbon composition, GOR, CO2 content, pH, density, viscosity, and PVT behavior are key inputs for surface-facility design and optimization of production strategies. Formation-tester tools have proved to be an effective way to obtain reservoir-fluid samples for PVT analysis. Conventional reservoir-fluid analysis is conducted in a PVT laboratory, and it usually takes a long time (months) before the results become available. Also, miscible contamination of a fluid sample by drilling-mud filtrate reduces the utility of the sample for subsequent fluid analyses. However, the amount of filtrate contamination can be reduced substantially by use of focused-sampling cleanup introduced recently in the next-generation wireline formation testers (O'Keefe et al. 2008). DFA tools provide results in real time and at reservoir conditions. Current DFA techniques use absorption spectroscopy of reservoir fluids in the visible-to-near-infrared (NIR) range. The formation-fluid spectra are obtained in real time, and fluid composition is derived from the spectra on the basis of C1, C2-5, C6+, and CO2; then, GOR of the fluid is estimated from the derived composition (Betancourt et al. 2004; Fujisawa et al. 2002; Dong et al. 2006; Elshahawi et al. 2004; Fujisawa et al. 2008; Mullins et al. 2001; Smits et al. 1995). Additionally, from the differences in absorption spectrum between reservoir fluid and filtrate of oil-based mud (OBM) or water-based mud (WBM), fluid-sample contamination from the drilling fluid is estimated (Mullins et al. 2000; Fadnes et al. 2001). With the DFA technique, reservoir-fluid samples are analyzed before they are taken, and the quality of fluid samples is improved substantially. The sampling process is optimized in terms of where and when to sample and how many samples to take. Reservoir-fluid characterization from fluid-profiling methods often reveals fluid compositional grading in different zones, and it also helps to identify reservoir compartmentalization (Venkataramanan et al. 2008). A next-generation tool has been developed to improve the DFA technique. This DFA tool includes new hardware that provides more-accurate and -detailed spectra, compared to the current DFA tools, and includes new methods of deriving fluid composition and GOR from optical spectroscopy. Furthermore, the new DFA tool includes a vibrating sensor for direct measurement of fluid density and, in certain environments, viscosity. The new DFA tool provides reservoir-fluid characterization that is significantly more accurate and comprehensive compared to the current DFA technology.

Predicting Reservoir Fluid Phase and Volumetric Behavior From Samples Contaminated With Oil-Based Mud

2002 ◽  
Vol 5 (03) ◽  
pp. 197-205 ◽  
Author(s):  
F. Gozalpour ◽  
A. Danesh ◽  
D.-H. Tehrani ◽  
A.C. Todd ◽  
B. Tohidi
Keyword(s):  
Phase Equilibrium ◽  
Phase Behavior ◽  
Volatile Oil ◽  
Fluid Composition ◽  
Fluid Analysis ◽  
Reservoir Fluid ◽  
Different Types ◽  
The Impact ◽  
Mud Filtrate ◽  
Equilibrium Calculations

Summary The impact of sample contamination with oil-based mud filtrate on phase behavior and properties of different types of reservoir fluids, including gas condensate and volatile oil, has been investigated. Two simple methods are used to determine the uncontaminated fluid composition from contaminated samples. The capability of the methods is demonstrated against highly contaminated samples. An equation-of-state (EOS)-based method also has been developed to predict the phase and volumetric properties of the retrieved composition. The method determines the required parameters of the EOS for the uncontaminated fluid using the developed phase-behavior models from contaminated-sample data. The method has been examined against experimental data of different types of reservoir fluids with successful results. Introduction Accurate reservoir fluid composition and properties are essential for reservoir management and development. Reliable reservoir fluid samples are therefore required; however, major challenges can render the fluid analysis limited in value. The reservoir fluid samples for pressure/volume/temperature (PVT) tests can be collected by bottomhole and/or surface sampling techniques as appropriate. During the drilling process, owing to overbalance pressure in the mud column, mud filtrate invades the formation. If an oil-based mud is used in the drilling, it can cause major difficulties in collecting high-quality formation fluid samples. Because the filtrate of oil-based drilling mud is miscible with the formation fluid, it could significantly alter the composition and phase behavior of the reservoir fluid. Even the presence of a small amount of oil-based filtrate in the collected sample could significantly affect the PVT properties of the formation fluid. Oil-based mud is used widely in the petroleum industry. Contamination with oil-based mud filtrate could affect reservoir fluid properties such as saturation pressure, formation volume factor, gas/liquid ratio, and stock-tank liquid density. Because collecting a reservoir fluid sample is expensive, and accurate reservoir fluid properties are needed in reservoir development, it is highly desirable to determine accurate composition and phase behavior for the reservoir fluid from contaminated samples. This study investigates the impact of sample contamination with oil-based mud filtrates on composition and phase behavior properties of different types of reservoir fluids, including volatile oil and gas condensate samples. The samples were purposely contaminated with a known amount of oil-based mud filtrates in the laboratory. The methods developed in this study were then applied to determine the original composition of the reservoir fluid from contaminated samples. The phase behavior of the contaminated samples was also investigated by performing constant composition expansion (CCE) tests at reservoir and surface conditions. The measured experimental data were used to tune EOSs by adjusting their parameters. The determined parameters of EOS tuned to the contaminated samples were used to calculate the parameters of EOS for the uncontaminated sample. EOS EOSs are used extensively to simulate the volumetric behavior and phase equilibrium of petroleum reservoir fluids. Among different types of EOSs, cubic EOSs have enjoyed considerable success in modeling because they are simple and give reliable results in phase equilibrium calculations. Two EOSs, the Valderrama1 modification of the Patel-Teja (VPT) EOS and a modified Peng-Robinson2 (mPR) EOS, were used in this study to perform phase equilibrium calculations. All binary interaction parameters (BIP) in the mixing rule were set to zero, and the temperature dependency of the attractive term was used as the tuning parameter to fit the measured data.3 Extended compositional analyses (up to C20+) of fluids were used in phase equilibrium calculations. The required critical properties of petroleum fractions to calculate parameters of EOS were determined by perturbation expansion correlations.4 The required boiling-point temperatures were calculated from the Riazi- Daubert5 correlation using the molecular weight and specific gravity of petroleum fractions. The Lee-Kesler6 correlation was used to calculate the accentric factor of compounds. Contaminated Reservoir Fluids Hydrocarbon-based fluids (natural or synthetic oils) are generally used in oil-based drilling muds. Because these fluids are soluble in the reservoir fluid, they can render the fluid analysis limited in value. Determination of the original fluid composition from the analysis of a contaminated sample is feasible, but isolating the properties of the reservoir fluid free from contamination is not easily accomplished. Despite the recent improvements in sampling reservoir fluids,7,8 obtaining a contamination-free formation fluid is a major challenge, particularly in openhole wells. Therefore, modeling techniques are required, along with the laboratory studies, to determine the composition and PVT properties of the uncontaminated fluid. We have demonstrated, as have other investigators,9,10 that an exponential relationship exists between the concentration of components in the C8+ portion of real reservoir fluids and the corresponding molecular weights. For example, if the molar concentration of single carbon number groups is plotted against their molecular weights, it will give a straight line on a semilogarithmic scale. Based on this feature of natural fluids, two methods have been developed in this study to retrieve the original composition of reservoir fluid from contaminated samples. The composition of the C8+ portion of contaminated sample is plotted against molecular weight on a semilogarithmic scale. The plotted data will show a departure from the line over the range affected by the contaminants (see Fig. 1). The concentrations of the contaminants are then skimmed from the semilog straight line, presumed to be valid for the uncontaminated reservoir fluid. The fitted line is used to determine the composition of the uncontaminated fluid. The above method, referred to as the Skimming method, gives a reliable composition of the uncontaminated fluid if the contaminant comprises a limited hydrocarbon range. MacMillan et al.11 developed a similar method. They fitted a gamma distribution function to the composition of the C7+ portion of contaminated oil samples, excluding the composition of contaminants from the datafitting procedure.

Determination of Bubble-Point and Dew-Point Pressure Without a Visual Cell

2014 ◽  
Author(s):  
R.. Hosein ◽  
R.. Mayrhoo ◽  
W. D. McCain
Keyword(s):  
Oil And Gas ◽  
Saturation Pressure ◽  
Volatile Oil ◽  
Phase Region ◽  
Gas Condensate ◽  
Dew Point ◽  
Bubble Point ◽  
Point Pressure ◽  
Dew Point Pressure

Abstract Bubble-point and dew-point pressures of oil and gas condensate reservoir fluids are used for planning the production profile of these reservoirs. Usually the best method for determination of these saturation pressures is by visual observation when a Constant Mass Expansion (CME) test is performed on a sample in a high pressure cell fitted with a glass window. In this test the cell pressure is reduced in steps and the pressure at which the first sign of gas bubbles is observed is recorded as bubble-point pressure for the oil samples and the first sign of liquid droplets is recorded as the dew-point pressure for the gas condensate samples. The experimental determination of saturation pressure especially for volatile oil and gas condensate require many small pressure reduction steps which make the observation method tedious, time consuming and expensive. In this study we have extended the Y-function which is often used to smooth out CME data for black oils below the bubble-point to determine saturation pressure of reservoir fluids. We started from the initial measured pressure and volume and by plotting log of the extended Y function which we call the YEXT function, with the corresponding pressure, two straight lines were obtained; one in the single phase region and the other in the two phase region. The point at which these two lines intersect is the saturation pressure. The differences between the saturation pressures determined by our proposed YEXT function method and the observation method was less than ± 4.0 % for the gas condensate, black oil and volatile oil samples studied. This extension of the Y function to determine dew-point and bubble-point pressures was not found elsewhere in the open literature. With this graphical method the determination of saturation pressures is less tedious and time consuming and expensive windowed cells are not required.

High-Pressure Data and Modeling Results for Phase Behavior and Asphaltene Onsets of Gulf of Mexico Oil Mixed With Nitrogen

2014 ◽  
Vol 17 (03) ◽  
pp. 384-395 ◽  
Author(s):  
Odd Steve Hustad ◽  
Na Jia ◽  
Karen Schou Pedersen ◽  
Afzal Memon ◽  
Sukit Leekumjorn
Keyword(s):  
High Pressure ◽  
Gulf Of Mexico ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Positive Impact ◽  
Viscosity Reduction ◽  
Fluid Composition ◽  
Viscosity Data ◽  
Oil Viscosity ◽  
Reservoir Fluid

Summary This paper presents fluid composition, high-pressure pressure/volume/temperature (PVT) measurements, and equation-of-state (EoS) modeling results for a recombined Tahiti oil, Gulf of Mexico (GoM), and for the oil mixed with nitrogen in various concentrations. The data include: Upper and lower asphaltene onset pressures and bubblepoint pressures for the reservoir fluid swelled with nitrogen. At the reservoir conditions of 94 MPa (13,634 psia) and 94°C (201.2°F), asphaltene precipitation is seen after the addition of 27 mol% of nitrogen. Viscosity data for the swelled fluids showing that the addition of nitrogen significantly reduces the oil viscosity. Slimtube runs indicating that the minimum miscibility pressure (MMP) of the oil with nitrogen is significantly higher than estimated from published correlations. The data were modeled with the volume-corrected Soave-Redlich-Kwong (SRK) EoS and the perturbed-chain statistical association fluid theory (PC-SAFT) EoS. Although both equations provide a good match of the PVT properties of the reservoir fluid, PC-SAFT is superior to the SRK EoS for simulating the upper asphaltene onset pressures and the liquid-phase compressibility of the reservoir fluid swelled with nitrogen. Nitrogen-gas flooding is expected to have a positive impact on oil recovery because of its favorable oil-viscosity-reduction and phase behavior effects.

Application of Tracers in Oil-Based Mud for Obtaining High-Quality Fluid Composition in Lean Gas/Condensate Reservoirs

2007 ◽  
Vol 10 (01) ◽  
pp. 5-11 ◽  
Author(s):  
Fathollah Gozalpour ◽  
Ali Danesh ◽  
Adrian Christopher Todd ◽  
Bahman Tohidi
Keyword(s):  
Drilling Fluid ◽  
Gas Condensate ◽  
Fluid Composition ◽  
Drilling Process ◽  
Two Fluids ◽  
The North ◽  
Reservoir Fluid ◽  
Partially Miscible ◽  
The North Sea ◽  
Mud Filtrate

Summary Oil-based drilling fluids are used extensively in drilling activities worldwide. During the drilling process, because of overbalance pressure in the mud column, the filtrate of oil-based mud invades the formation. This hydrocarbon-based filtrate mixes with the formation hydrocarbon, which can cause major difficulties in obtaining a representative reservoir-fluid sample. Despite the recent improvements in sampling, obtaining a contamination-free formation fluid is a major challenge, particularly in openhole wells. Depending on the type and conditions of the reservoir, the oil-based-mud filtrate is totally or partially miscible with the formation fluid. Oil-based-mud filtrate dissolves completely in reservoir oil; therefore, the captured sample contains the true reservoir oil with added filtrate. Gas condensate (lean gas condensate in particular) is often not fully miscible with mud filtrate. In this case, the mass exchange between gas condensate and mud filtrate makes the sample unrepresentative of the reservoir fluid. In this study, the impact of sample contamination with oil-based-mud filtrate on different types of reservoir fluids, including gas condensate and volatile-oil samples, is investigated. Two simple methods are suggested to retrieve the uncontaminated composition from a contaminated sample in which mud filtrate is totally dissolved in the formation fluid (i.e., reservoir-oil samples). A tracer-based technique is also developed to determine the composition of an uncontaminated reservoir-fluid sample from a sample contaminated with oil-based-mud filtrate, particularly for those cases in which the two fluids are partially miscible. The tracers are added to the drilling fluid, with the additional cost to the drilling-mud preparation being negligible. The capability of the developed techniques has been examined against deliberately contaminated reservoir-fluid samples under controlled conditions in the laboratory. The results indicate the reliability of the proposed methods. Introduction Historically, most drilling in the North Sea has used water-based muds; however, drilling certain formations with water-based muds can be difficult, primarily because of the hole instability caused by the swelling of water-absorbing rock. Problems of this type can be greatly alleviated by using mud suspended in an oil (rather than water) base. These oil-based muds also provide better lubrication and achieve significant increases in drilling progress (Davies et al. 1984). In recent years, oil-based drilling fluid has been used extensively in drilling activities in the North Sea. During the drilling process, because of overbalance pressure in the mud column, the mud filtrate invades the reservoir formation. Using an oil-based mud in the drilling, the mud filtrate can mix with the formation fluid. This can cause major difficulties in obtaining high-quality formation-fluid samples. Depending on the type and conditions of the reservoir, the mud filtrate can be totally or partially miscible with the formation fluid. This can alter the composition and phase behavior of the reservoir fluid significantly. Hence, the measured data using the collected formation-fluid samples need to be corrected for the contamination. In this study, contamination of different types of reservoir fluids with oil-based-mud filtrate, where the two fluids are partially or totally miscible, is discussed. Practical decontamination techniques are proposed to retrieve the original fluid composition from contaminated samples.

A Sensitivity Study of Reservoir Performance Using a Compositional Reservoir Simulator

1972 ◽  
Vol 12 (01) ◽  
pp. 3-12
Author(s):  
Edward T.S. Huang
Keyword(s):  
Limited Range ◽  
Measured Data ◽  
Compositional Variation ◽  
Fluid Sample ◽  
Laboratory Measurements ◽  
Reservoir Performance ◽  
Reservoir Fluid ◽  
Liquid Phases ◽  
Fluid Properties ◽  
Compositional Simulator

Abstract Simulation of isothermal fluid flow in a reservoir using a compositional simulator requires fluid properties that are functions of pressure and properties that are functions of pressure and composition. These properties, i.e., K-values, densities and viscosities of both vapor and liquid phases, are usually obtained from general correlations phases, are usually obtained from general correlations or laboratory measurements of a reservoir fluid sample during a differential-depletion experiment in a PVT cell. prediction of fluid properties of complex mixtures using existing correlations is generally subject to great uncertainties. The laboratory measured data that are generally correlated as functions of pressure have validity only over a limited range of compositional variation. The purposes of this paper were (1) to assess, using a linear compositional simulator, the error introduced into calculated reservoir performance by employing fluids with a given range of uncertainties in their physical properties; and (2) to examine the validity of using the physical data correlated in the compositional simulator as functions of pressure rather than functions of both pressure and composition. The gas cycling process was chosen for illustration because composition changes during this process are large and results are affected more than in a depletion-type process. The hypothetical reservoir fluid system considered in this study was a methane-n-butane-n-decane mixture chosen to simulate a volatile oil system. The results of this investigation show for the particular system studied that:(1)the K-values for particular system studied that:(1)the K-values for the lighter components have the most significant effect on the calculated reservoir performance; and(2)simulations using fluid properties that are equivalent to the data measured during a differential depletion experiment reliably predict reservoir performance even under conditions where significant performance even under conditions where significant variations in reservoir fluid composition occur. Introduction A number of papers have recently been published concerning the development of compositional reservoir simulators-the mathematical models that simulate isothermal flow of multiphase, multicomponent fluids in porous media considering mass transfer effects. These models, which properly describe the distribution of each individual component in both vapor and liquid phases and account for pressure and compositional dependence of K-values, phase densities and viscosities, are more rigorous than the conventional simulators. The latter assumes that the heavy component does not exist in the vapor phase. To use the compositional simulator, it is highly desirable that fluid properties, i.e., K-values, densities and viscosities, as functions of pressure and composition, be available. However, for complex reservoir fluid mixtures, this information is rarely available. These fluid properties are usually calculated from published generalized correlations or obtained from laboratory measurements of a reservoir fluid sample by performing differential depletion experiments in a PVT cell. Prediction of fluid properties of complex mixtures using existing correlations is generally subject to great uncertainty. These errors will certainly have effects on the predicted reservoir performance. These effects may predicted reservoir performance. These effects may even be amplified if all the fluid properties are calculated from correlations. Improvement of the correlation predicted data by adjusting these data to match the limited available experimental values for the system of interest can be make. Yet there is no guarantee that the adjusted data will describe reliable fluid behavior in the region away from the matched points. On the other hand, the laboratory measured data, which are expressed as functions of pressure only, have validity over a limited range of pressure only, have validity over a limited range of compositional variation. When compositions of reservoir fluids vary significantly, the reliability of applying the laboratory measured data in the numerical simulation becomes questionable. SPEJ p. 3

Sign in / Sign up

Export Citation Format

Share Document