James F. Lea

Effect of Gaseous Fluids on Submersible Pump Performance

This paper reports the results of two independent laboratory investigations on the effect of free gas on centrifugal submersible pump performance. One investigation used air and water as the working fluids, with the free gas allowed to escape out an annulus. A separate investigation was conducted using diesel fuel and CO/sub 2/ as the working fluids with intake pressures up to 400 psi. The results of the two investigations agreed in general at low pressures, where gas volumes exceeding 10% by total volume began to cause serious reduction in pump performance curves. Better performance at higher pressures with the same gas volumes present was found to occur at higher pump intake pressures.

Dynamic Analysis of Plunger Lift Operations

Plunger lift is a method of artificial lift that uses a free piston traveling up and down inside the tubing in a cyclic manner. The piston serves to increase the efficiency of lifting liquids in gas/liquid production by reducing liquid fallback through the gas. Presented here is a description of a dynamic model of plunger lift operations that, as opposed to previous methods of analysis, includes calculation of the plunger velocity as the plunger and liquid slug travel up the tubing. Also, an analysis of plunger cycles in a high gas/liquid ratio (GLR) well is presented to indicate the maximum rate of slug buildup and the maximum casing pressure necessary to lift the plunger and accumulated liquids. The information presented allows a more detailed engineering approach to analyzing the performance of a plunger-lifted well.

Gas Solubility in Oil-Based Drilling Fluids: Effects on Kick Detection

This paper describes the effects of gas solubility on the properties of oil-based drilling fluids. Prediction methods for gas solubility in oil-based muds were tested by experimental work and found acceptable. Expansion of discrete mixtures of mud and gas are predicted. A blowout simulation program was written and used to predict the effects of a kick on the surface-observable indicators (pit gain, well flow). While the bulk of this paper emphasizes the properties of oil-based muds, a comparison with water-based muds under similar conditions is made. The general conclusions are applicable to both types of muds. It is concluded that pit gain is the most reliable indicator of a kick during drilling in either oil- or water-based muds. It is recommended that a pitlevel measurement system be designed that will detect a pit gain of less than 5 bbl (0.795 m/sup 3/) in the whole mud system.

Selection of Artificial Lift

Selection of the most economical artificial lift method is necessary for the operator to realize the maximum potential from developing any oil or gas field. Historically the methods used to select the method of lift for a particular field have varied broadly across the industry, including Determining what methods will lift at the desired rates and from the required depths. . Evaluating lists of advantages and disadvantages. . Use of expert systems to both eliminate and select systems. . Evaluation of initial costs, operating costs, production capabilities, etc. using economics as a tool of selection. This paper will highlight some of the methods commonly used for selection and also include some examples of costs and profits over time calculated to the present time as a tool of selection. The operator should consider all of these methods when selecting a method of artificial lift, especially for a large, long-term project.

Downhole Beam Pump Operation: Slippage and Buckling Forces Transmitted to the Rod String

New results for downhole beam or rod pumps from fairly recent test data are in conflict with traditional relationships. Additional testing is in progress. A pump with large plunger/barrel clearance will cause high fluid slippage. A pump with smaller clearance will cause less fluid slippage, but a tighter fit will tend to increase rod buckling at the pump to a greater degree and may lead to advanced pump and rod wear rates. Therefore, it is important to be able to predict pump slippage and also related forces at the downhole pump. Considerations for pump leakage and example calculations are presented using the older and the new pump slippage relationships. A derivation to account for the pump velocity effects on slippage is also presented. The effect of pump clearances on possible rod buckling above the pump is also studied. Further additional possible causes of rod buckling are presented, discussed, and compared. The results will help the reader to decide on sizing pump clearances to provide leakage for pump lubrication without losing too much on pump efficiency. Several ideas on the sources of rod buckling, such as flow through the travelling valve, and the plunger-barrel interaction are presented and compared. A review of methods to combat rod buckling above the pump is presented.

CHAPTER 11 – GAS LIFT

Publisher Summary This chapter gives a complete overview of gas lift. Gas lift is an artificial lift method whereby external gas is injected into the produced flow stream at some depth in the well bore. The additional gas augments the formation gas and reduces the flowing bottom hole pressure, thereby increasing the inflow of produced fluids. For dewatering gas wells, the volume of injected gas is designed so that the combined formation and injected gas will be above the critical rate for the well bore, especially for lower liquid producing gas wells. For higher liquid rates, much of the design procedure may more closely mirror producing oil well gas lift techniques. In addition, gas lift may not lower the flowing pressure as much as an optimized pumping system; there are several advantages of a gas lift system that often make gas lift the artificial lift method of choice. For gas wells in particular, when producing a low amount of liquids, the producing bottom hole pressure with gas lift may compare well with other methods of dewatering. For higher liquid rates, the achievable producing BHP may be higher than pumping techniques. Of all artificial lift methods, gas lift most closely resembles natural flow and has long been recognized as one of the most versatile artificial lift methods. Because of its versatility, gas lift is a good candidate for removing liquids from gas wells under certain conditions. The two fundamental types of gas lifts used in the industry today are continuous flow and intermittent flow, and these are briefly explained in this chapter.

CHAPTER 3 – CRITICAL VELOCITY

Publisher Summary The method of calculating a critical velocity will be shown to be applicable at any point in the well. It should be used in conjunction with methods of nodal analysis if possible. In this chapter, the relatively simple “critical velocity” method is presented to predict the onset of liquid loading. This technique was developed from a substantial accumulation of well data and has been shown to be reasonably accurate for vertical wells. In critical flow concepts, the study explains the transport of liquids in near vertical wells is governed primarily by two complementing physical processes before liquid loading becomes more predominate and other flow regimes such as slug flow and then bubble flow begin. The critical velocity is generally defined as the minimum gas velocity in the production tubing required to move liquid droplets upward. A “velocity string” is often used to reduce the tubing size until the critical velocity is obtained. Lowering the surface pressure (e.g., by compression) also increases velocity. The study explains the Turner droplet model; it is generally believed that the liquids are both lifted in the gas flow as individual particles and transported as a liquid film along the tubing wall by the shear stress at the interface between the gas and the liquid before the onset of severe liquid loading.

CHAPTER 9 – HYDRAULIC PUMPING

Publisher Summary The hydraulic pumping system takes liquid (water or oil) from a liquid reservoir on the surface, puts it through a reciprocating multiplex piston pump or horizontal electrical submersible pump to increase the pressure, and then injects the pressurized liquid (power fluid) down-hole through a tubing string. At the bottom of the injection tubing string, the power fluid is directed into the nozzle of a jet pump or to the hydraulic engine of a piston pump, both of which have been set well below the producing fluid level. The surface injection pressures normally range from approximately 2000 psi up to 4000 psi, with some going up to but rarely above 4500 psi. An electric motor, diesel engine, or gas engine is used to drive the multiplex pump. The fundamental operating principle of subsurface hydraulic pumps is Pascals Law, postulated by Blaise Pascal in 1653. This principle makes it possible to transmit pressure from the surface by means of a liquid-filled tubing string to any given point below the surface. The chapter further highlights applications to dewatering wells-gas and coal bed methane, limitations of other forms of lift, advantages of hydraulic pumping, disadvantages of hydraulic pumping, and different types of operating systems.

CHAPTER 8 – USE OF FOAM TO DELIQUIFY GAS WELLS

Publisher Summary Foams have several applications in oil field operations. They are used as a circulation medium for drilling wells, for well cleanouts, and as fracturing fluids. These applications differ slightly from the application of foam as a means of removing liquid from producing gas wells. The former applications involve generating the foam at the surface with controlled mixing and using only water. In gas well liquid removal applications, the liquid-gas-surfactant mixing must be accomplished down hole and often in the presence of both water and liquid hydrocarbons. The principal benefit of foam as a gas well dewatering method is that liquid is held in the bubble film and exposed to more surface area resulting in less gas slippage and a low-density mixture. In addition, foam is a particular type of gas and liquid emulsion. Gas bubbles are separated from each other in foam by a liquid film. Surface-active agents (surfactants) generally are employed to reduce the surface tension of the liquid to enable more gas-liquid dispersion. The liquid film between bubbles has two surfactant layers back to back with liquid contained between them. This method of tying the liquid and gas together can be effective in removing liquid from low volume gas wells. The application of foam to unloading low rate gas wells generally is governed by two operating limitations, which are economics and the success of foam surfactants in reducing bottom hole pressure. Both limits are defined by comparison to other methods of unloading wells.

Plunger Slippage for Rod-Drawn Plunger Pumps

A theoretical calculation is needed to predict pump slippage in a rod pumped well. Slippage lubricates the pump plunger and barrel and prevents galling of metals. Additionally, an estimate of plunger slippage is necessary to calculate pump efficiency. Historical plunger slippage equations have been proven incorrect by recent testing. A new method for calculating plunger pump slippage in rod pumped wells is introduced. This method involves calculating a velocity profile for an annulus with the inner wall moving parallel to the outer wall. An average velocity is determined for the annular fluid flow, which in turn is used to calculate the fluid slippage. The results are evaluated against the historical field data and compare favorably to recent testing for smaller plunger clearances. Work remains to be done at larger clearances.