D.R. Doty

Predicting temperature profiles in a flowing well

In this paper, a simple model suitable for hand calculations is presented to predict temperature profiles in two-phase flowing wells. The model, developed with measured temperature data from 392 wells, assumes that the heat transfer within the wellbore is steady-state. Comparisons between the models predictions and field data indicate that the model is highly accurate within its range of application.

An improved method for gas lift allocation optimization

A new method for finding the optimum gas injection rates for a group of continuous gas lift wells to maximize the total oil production rate is established. The new method uses a quasi-Newton nonlinear optimization technique which is incorporated with the gradient projection method. The method is capable of accommodating restrictions to the gas injection rates. The only requirement for fast convergence s that a reasonable estimate of the gas injection rates must be supplied as an initial point to the optimization method. A method of estimating the gas injection rates is developed for that purpose. A computer program is developed capable of implementing the new optimization method as well as generating the initial estimate of the gas injection rates. This program is then successfully tested on field data under both unlimited and limited gas supply. The new optimization technique demonstrates superior performance, faster convergence, and greater application.

Severe slugging in offshore pipeline riser-pipe systems

Severe slug flow (i.e., terrain-dominated slug flow) was studied in a simulated offshore pipeline riser-pipe system. Severe slug flow is characterized by extremely long liquid slugs generated at the base of the vertical riser. This phenomenon occurs at low gas and liquid flow rates and for negative pipeline inclinations. Slugging in some offshore platforms has required the use of operating procedures that drastically curtail production. Losses in flow capacity up to 50% have been reported. A hydrodynamic model has been developed for severe slug flow. The models predictions agree with experimental data. The model can be used to design new pipeline riser-pipe systems or to adjust the operation of existing systems to prevent the occurrence of severe slug flow. Also, a flow-regime map is presented for predicting the severe slug flow regime, where the boundaries are determined analytically. Finally, additional methods are proposed to prevent the flooding of separation facilities by riser-pipe generated slugs.

An Improved Model for Sucker Rod Pumping

An improved model for predicting the behavior of sucker rod pumping installations is presented. This model incorporates the dynamics of the liquid columns as well as the sucker rod string through a system of partial differential equations. This system of equations is solved by a modified method of characteristics on a digital computer. The model predicts the polished-rod and pump dynamometer cards and incorporates the effects of liquid inertia and viscosity. The model is capable of simulating a wide variety of pumping conditions for which liquid physical properties are important. The information predicted by the model is useful in the design and operation of sucker rod pumping installations.

Casing Heading in Flowing Oil Wells

Casing heading, an unsteady flow in oil wells completed without packers, occurs when both gas and liquid superficial velocities are low. A hydrodynamic model is presented that simulates laboratory data for the cases considered. Results confirm that heading occurs for v /SUB sL/ < 1.0 ft/s and 0.34 ft/s < v /SUB sg/ < 1.0 ft/s, and that choking reestablishes stability.

A Review of Multiphase Flow Through Chokes

Three comprehensive mechanistic models (Ashford and Pierce, 1974; Sachdeva et al., 1986; Perkins, 1990) dealing with multiphase flow through chokes have been studied. Their common purpose is to determine the flow through a choke under both critical and subcritical flow. Except for their initial assumptions, these three models are basically the same and all are based on the energy equation. Performance of the model developed by Perkins (1990) was the best in a comparison test against a database of 1239 points. The other two models deviated up to 40 percent when compared to the Perkins model for large values of choke to pipe size ratios. A very important application of these models is in prediction of the pressure either upstream or downstream of the choke, given the flow rate through the choke and the known pressure. Pressure predictions against the flow direction (i.e., upstream of the choke) were found to be in very good agreement with measurements. Not more than 13 percent average absolute error and 17 percent standard deviation were observed. Similar errors were reported for the downstream pressure predictions for data in the subcritical flow regime. However, downstream pressure predictions for data in the critical flow regime gave large errors, up to 40 percent, which were expected.

Normalization of Nitrogen-Loaded Gas-Lift Valve Performance Data

The flow performance of two nitrogen-loaded gas-lift valves and one combination gas-lift valve was tested under simulated downhole conditions. Set pressures up to 1,500 psig and injection pressures up to 1,650 psig yielded a maximum observed gas flow rate of 3.6 MMscf/D. Pressure-operated valve performance depends on injection pressure for specific production and dome pressures, valve geometry, and other factors. Two performance characteristics, separated by the test-rack opening pressure, are observed; throttling and orifice flows. Semimechanistic models predict throttling and orifice flow performance. Experimental performance characteristics tune the model for any port size used in the Camco R-20 valve.

A Simple Model To Predict Natural Gas Separation Efficiency in Pumped Wells

A new mechanistic model to predict natural separation efficiency in vertical pumped wells has been developed. The model is based on the combined phase momentum equations and a general slip-closure relationship. New drag-coefficient correlations have been developed that correspond to bubbly (i.e., undistorted particle) and churn-turbulent flow regimes. The model indicates that natural separation efficiency depends strongly on geometry, void fraction, and in-situ gas flow rate.

A Mechanistic Model for Predicting Pressure Drop in Vertical Upward Two-Phase Flow

A comprehensive mechanistic model is formulated to predict flow patterns, pressure drop, and liquid holdup in vertical upward two-phase flow. The model identifies five flow patterns: bubble, dispersed bubble, slug, churn, and annular. The flow pattern prediction models are the Ansari et al. (1994) model for dispersed bubble and annular flows, the Chokshi (1994) model for bubbly flow, and a new model for churn flow. Separate hydrodynamic models for each flow pattern are proposed. A new hydrodynamic model for churn flow has been developed, while Chokshis slug flow model has been modified. The Chokshi and Ansari et al. models have been adopted for bubbly and annular flows, respectively. The model is evaluated using the expanded Tulsa University Fluid Flow Projects (TUFFP) well data bank of 2052 well cases covering a wide range of field data. The model is also compared with the Ansari et al., (1994), Chokshi (1994), Hasan and Kabir (1994), Aziz et al. (1972), and Hagedorn and Brown (1964) methods. The comparison results show that the proposed model performs the best and agrees well with the data.

System Analysis for Sucker-Rod Pumping

Pumping free gas in an oil well can significantly decrease the efficiency of a sucker rod pumping installation. Pump placement depth and the use of a down hole gas-liquid separator (gas anchor) found to be significant variables in improving the overall efficiency. A procedure is presented which shows when and by how much the use of a gas anchor improves the efficiency of a sucker rod pumping system. It was found that at lower pump intake pressures the gas anchor usually improves efficiency, while at higher pump intake pressures the use of a gas anchor will produce no positive effect. Also, it was found at elevating the pump to the highest position which still allows for proper pump loading can significantly reduce the operating costs for a sucker rod pumping installation. Finally, a procedure is presented for directly calculating pump volumetric efficiency as well as the required volumetric pump displacement rate.