Mohan Kelkar

An Application of Geostatistics and Fractal Geometry for Reservoir Characterization

This paper presents an application of geostatistics and fractal geometry concepts for 2D characterization of rock properties (k and {phi}) in a dolomitic, layered-cake reservoir. The results indicate that lack of closely spaced data yield effectively random distributions of properties. Further, incorporation of geology reduces uncertainties in fractal interpolation of wellbore properties.

Pipe-Diameter Effect on Liquid Loading in Vertical Gas Wells

The effect of pipe diameter on liquid-loading initiation has been investigated experimentally with pipes having internal diameters of 5.1(2-) and 10.2-cm (4-in.). Two-phase-flow parameters, such as pressure gradient and liquid holdup, were measured. Flow characteristics were determined by visual observation with a high-speed video camera. Critical gas-flow rate for liquid-loading initiation was identified, and comparisons between the two pipe diameters were presented. The critical superficial-gas velocity corresponding to the minimum pressure gradient was found to be faster for the smaller diameter. When the comparison was carried out in terms of mass-flow rates, critical flow rate for liquid loading in a 5.1-cm (2-in.) pipe was less than that in a 10.2-cm (4-in.) pipe. This verifies the use of velocity strings to extend the production life of the gas wells. Additionally, comparison of the data with available mechanistic-models prediction showed significant discrepancies. Possible reasons for these discrepancies are discussed.

Predictions of Two-Phase Critical Flow Boundary and Mass Flow Rate Across Chokes

The objective of this study is to develop and validate a theoretical slip model for two-phase flow through chokes. As opposed to the models (Sachdeva et al. 1986; Perkins 1993) used currently by the industry, the present model accounts for slippage between the liquid and gas phases as they pass through the choke. The theoretical basis of the model is a 1D balance equation of mass, momentum, and energy with the assumptions of constant quality and incompressible liquid phase. The present slip model is capable of predicting the critical-subcritical-flow boundary and the critical and subcritical mass-flow rates. A model-validation study demonstrated the capability of the slip models to predict the critical-flow boundary with an average error and standard deviation of 5.2% and 15.5%, respectively. Furthermore, in a laboratory validation, the present slip model predicted the mass-flow rate with an average error of 2.7% and 12.5% standard deviation. Compared with field data, the present slip model predicted the mass-flow rate with 1.4% average percent error and 15% standard deviation. Compared to existing no-slip models (Sachdeva et al. 1986; Perkins1993) used commonly by the industry, the present slipmodel predictions outperformed their predictions in the average percent error in both laboratory and field validation and in the standard deviation in the laboratory validation only. This validation result indicates the importance of the slippage phenomenon.

Permeability Upscaling for Near-Wellbore Heterogeneities

Reservoir simulations are limited to large scale grid blocks due to prohibitive computational costs of fine grid simulations. Rock properties, such as permeability, are measured on smaller scale than coarse scale simulation grid blocks. Therefore, the properties defined on a smaller scale are upscaled to a coarser scale. Few prior studies on permeability upscaling paid special attention to the problem of the radial flow in the vicinity of a wellbore. This study presents an analytical method to calculate effective permeability for a coarse-grid well-block from fine-grid permeabilities. The method utilizes serial and parallel averaging procedures modified for radial flow. The method is validated with numerical simulations of primary and secondary recovery processes involving 2- and 3-D systems. The results of coarse grid simulations with the permeabilities upscaled through the new well-block approach agree well with the results of the fine grid simulations with initial permeability distributions.

Conditional simulation method for reservoir description using spatial and well-performance constraints

This paper describes a conditional simulation technique that constrains areal permeability fields to typical statistical information and indirectly to waterflood well performance. Near-well effective permeability and reservoir connectivity characteristics are used as indirect well-performance constraints. Results are validated by examining simulated waterflood performance of a five-spot pattern. This technique can be used to reduce the uncertainty of future well performance significantly. Additionally, the effect of alternative operating scenarios, such as infill drilling, can be evaluated more realistically.

Gas well production optimization using dynamic nodal analysis

This work presents a numerical algorithm that permits the production optimization of gas wells using the concept of dynamic nodal analysis. By combining the desirable features of nodal analysis, material balance technique and decline curve analysis, the method is able to match the historical performance of the well data. It is also able to predict the future performance of the gas well under the existing condition as well as altered conditions. The proposed technique, which has several advantages over the classical nodal analysis, can be used for the selection of the timing and capacity of surface compressor, the evaluation of the economic viability of a well stimulation, and the understanding of the effect of individual production component on the productivity of a gas well over the life of that well.

Exploratory Data Analysis of Production Data

Production data represent a source of information about the ongoing dynamic flow process in the reservoir. A proper analysis of these data might therefore give an indication of the relationships governing the fluid flow. Exploratory data analysis is one tool which can be used to extract the required information and to establish a statistical significance to the results. This paper presents a simple approach to examine the interwell communication and interference of a mature waterflood in order to identify and rank areas of potential improvement. Using this as a screening process we can identify areas with the best potential for further and more detailed studies. In addition some of the problems and limitations associated with analysis of production data are discussed.

Wavelet Sensitivity Study On Inversion Using Heuristic Combinatorial Algorithms

The wavelet determination is important for seismic inversion. This study investigates the wavelet sensitivity for inversion using heuristic combinatorial algorithms in the time domain. The results conclude that for inversion using heuristic combinatorial algorithms, precise knowledge of the shape of the wavelet (i. e., frequency, phase and time interval) is not necessary, even though these do help the inversion. Moreover, the frequency band of the wavelet for the new inversion strategy needs to be wider than the frequency band of the true wavelet. The phase and the length have less impact lead to a on the inversion results than the frequency, and mainly time shift of the inverted results.

A Numerical Study of Viscous Instabilities: Effect of Controlling Parameters and Scaling Considerations

Utilization of an accurate finite element simulator with a very fine 2D grid to study viscous instabilities. The parameters controlling unstable displacements are the permeability variance, the size of heterogeneity (scale length), the mobility ratio, and the dimensionless transverse Peclet number. The instabilities increase with increases in the permeability variance, the scale length, and the mobility ratio. With increased instability, the recovery decreases and the breakthrough time decreases. The effects of viscous instabilities can be scaled within the range of parameters investigated.

Exploitation and Optimization of Reservoir Performance in Hunton Formation, Oklahoma

West Carney Field produces from Hunton Formation. All the wells produce oil, water and gas. The main objective of this study is to understand the unique behavior observed in the field. This behavior includes: (1) Decrease in WOR over time; (2) Decrease in GOR at initial stages; (3) High decline rates of oil and gas; and (4) strong hydrodynamic connectivity between wells. This report specifically addresses two issues relevant to our understanding of the West Carney reservoir. By using core and log data as well as fluorescence information, we demonstrate that our hypothesis of how the reservoir is formed is consistent with these observations. Namely, oil migrated in water wet reservoir, over time, oil changed the wettability of some part of the reservoir, oil eventually leaked to upper formations prompting re-introduction of water into reservoir. Because of change in wettability, different pore size distributions responded differently to water influx. This hypothesis is consistent with fluorescence and porosity data, as we explain it in this quarterly report. The second issue deals with how to best calculate connected oil volume in the reservoir. The log data does not necessarily provide us with relevant information regarding oil in place. However, we have developed a new material balance technique to calculate the connected oil volume based on observed pressure and production data. By using the technique to four different fields producing from Hunton formation, we demonstrate that the technique can be successfully applied to calculate the connected oil in place.