Downhole Gas Separators Research Articles

A Gas Separation Model for a Downhole Gravitational Separator

Summary Pumping systems reduce their efficiency and lifetime when working with free gas at the pump inlet. Downhole gas separators are usually installed upstream of the pump in oil production wells to avoid free gas, but they cannot handle high void fraction. Studies on the so-called inverted-shroud gravitational separator indicate that it is possible to achieve gas separation efficiencies (GSEs) higher than 97%, even in the presence of very high void fraction. Gas separation inside this kind of separator works in two stages. The most critical stage is related to gas entrainment caused by a plunging free-surface flow in an annular channel formed by the production tubing and the separator itself, where strong kinetic energy dissipation takes place. Studies regarding this phenomenon in inverted-shroud separators are scanty. In this study, we present a semianalytical model supported by an empirical turbulent energy-dissipation correlation. The proposed correlation is based on dimensional analysis. New experimental data of the statistical distributions of bubble diameters during the gas entrainment process are presented. We measured bubbles diameters using a high-speed video camera and an edge detection algorithm. Data of both bubble diameters and GSE were used to adjust and validate the proposed model. The model establishes a transition boundary that separates a region of total gas separation (TGS) from a region of partial gas separation (PGS). In other words, it provides the operating envelope of the separator. The results can be used as a starting point for the design of inverted-shroud separators for field applications.

The optimization approach of casing gas assisted rod pumping system

The free gas evolved in the pump chamber results in a low rod pump working efficiency, and it can even lead to a failure. A common and effective solution is to install a downhole gas separator before fluid entering the chamber, which can divert the free gas to the annulus. If we can re-inject the diverted gas back to the tubing at a shallower depth above the pump, the flowing gas is then re-combined with the liquid and decreases fluid density. Consequently, the injected gas also creates additional lifting drive for the liquid. A new technology based on this concept has been developed and called Casing Gas Assisted Rod Pumping (CGARP). This paper firstly presents an analytical model to optimize the overall lifting performance and minimize the operating expenditure. It is especially useful in producing hydrocarbon at high GOR.As the gas is re-combined with the liquid above the pump installation depth, the hydrostatic pressure gradient is reduced consequently. However, if the gas reinjection valve is placed at a shallow depth, the well segment at reduced fluid density is subsequently short, so the contribution of gas lift is restricted. Vice versa, if the gas reinjection valve is placed at the depth close to the pump, it requires high pressure to open the gas injection valve, so the gas reinjection can happen infrequently and the production rate is unsfplease. This paper has proposed a genetic optimization method to maximize the overall production system efficiency. A multi-variable vector has been defined, which includes pumping speed and depth, mechanical power, rod string diameter and length, surface stroke length, downhole separator efficiency, as well as gas reinjection valve depth. The optimized object can be the system lifting efficiency or Net Present Value, which must be a function of this vector in the constraint of mass and momentum conservations.This work has been applied as the primary guide for four oil producers with rod pump installed in Jilin field, China. The average system lifting efficiency and production rate have been increased by 20% and 15% respectively. This analytical model has enhanced the field performance. Most importantly, the same concept can be applied for other pump-assisted wells.

Evaluation and Performance of Packer-Type Downhole Gas Separators

Summary Advances in horizontal drilling and large fracturing technology have resulted in wells that produce larger volumes of oil and gas than have been common domestically. Artificially lifting large volumes of oil and associated gas to the surface has always been a problem because of the difficulty of separating liquid from large volumes of gas downhole, especially in rod-pumped wells. This paper describes a separation technique that uses a packing element to divert the formation fluids through the separator and into the casing annulus at the top of the separator, above the pump inlet at the bottom of the separator, so that the liquids and gas can separate by gravity. The pump seating nipple is located at the bottom of the separator so that the pressure drop is less in the liquids moving from the casing annulus to the pump intake. Better pump fillage is obtained with the technique of setting the pump intake at the bottom of the separator rather than above the long separator, as show by the field data. This paper also describes a quantitative technique for evaluating the effectiveness of downhole gas separators. Often, the evaluation of separator performance is based only on pump fillage and the total gas production from the well, instead of a comparison of the liquid fillage in the pump in relation to the percentage of liquid that exists in the casing annulus surrounding the pump.

Key techniques of reservoir engineering and injection–production process for CO2 flooding in China's SINOPEC Shengli Oilfield

This paper addresses the geological problems and engineering hot points of the CO2 flooding, such as the big vertical span of the beach-bar sand, the strong reservoir heterogeneity, the distribution of residual oil, and the problem of gas channeling. The core identification, log analysis, seismic interpretation, laboratory test and numerical simulation of reservoir engineering are integrated to investigate the geological characteristics of the reservoir in the demonstration zone of SINOPEC Shengli Oilfield. It demonstrates the reservoir is large but it has thin thickness, low porosity and super-low permeability. Due to some great differences between the beach sand and the bar sand, the oil reservoirs of demonstration zone are divided into 2 sand groups, 8 small layers, and 17 sand bodies in total. Then, a 3D geological model and qualitative evaluation system of safety of vertical faults are built. The optimal evaluation method of CO2 flooding and sequestration is established. According to the engineering optimization of the CO2 flooding, the results of a recommendation scheme indicate that the enhance oil recovery can increase by 6.7%, the total injection volume is expected to reach to 563×104t, and CO2 sequestration rate is 60.5%. Finally, the multi-level umbrella downhole gas separator is designed, and the high gas–oil ratio (GOR) production string and free kill gas injection string are also successfully developed for the CO2-EOR.

Multi-Cup Isoflux Gas Anchor Numerical Simulation of Oil-Water Separating

Gas anchor is also known as downhole oil and gas separator. It plays a role of separating the gas and oil before the fluid flows into the pump, thus to eliminate the influence of gas and improve pump efficiency. It can also extend the lifetime of the equipment and increase the oil well output. On the analysis of factors affecting the efficiency of gas anchor, I built 3D models in Gambit on the basis of the blueprint of multi-cup isoflux gas anchor. I used Fluent to calculate the multi-cup isoflux gas anchor separation of oil and gas with laminar flow model. Then I got the distribution of speed and pressure in the multi-cup isoflux gas anchor and derived the formula of local resistance ratio. Learned that on the application of the multi-cup isoflux gas anchor the pumping wells pump efficiency enhances on an average of more than 25%.

Laboratory Testing of Downhole Gas Separators

Summary This paper provides insights on the design of downhole gas separators based on the laboratory study of various separator designs. A downhole gas separator, also known as a gas anchor, may be installed below the pump to separate free gas from the produced liquid. The free gas-produced downhole is usually separated through the casing-tubing annulus (the casing-tubing annulus acts as a natural gravity separator) while the liquid is produced through the tubing. However, inefficient gas anchor designs are widespread and an acceptable guide for their optimum design does not currently exist. Laboratory testing of downhole gas separators has been ongoing since January 2005 at The University of Texas at Austin Petroleum Production Engineering Facility (UTAPPEF) using an instrumented full-scale model of a wellbore and separator constructed with clear acrylic pipe to visualize the fluid mechanics of the separation process. An air and water mixture is injected through the well's perforations. The air and water flow rate measurements are used to measure and define a performance plot of each separator design. The separator designs tested differed in entry-port configuration, size of dip tube, and the relative position of the separator-fluid entry ports with respect to the well's perforations. Based on the results of the tests, a new separator design that includes the effect of centrifugal forces to separate the gas and liquid phases was developed. The results show that, for the conditions in the laboratory, 100% separation was achieved whenever the entry ports were located 1- to 2-feet (ft) below the bottom-most casing perforation thereby dispelling the predominant industry-held opinion that more distance is required between the separator-fluid entry ports and the bottom-most casing perforation. Similarly, laboratory results equally show that an optimum dip tube length of 5.5 ft is sufficient for the optimal separation of free gas from the produced liquid by the separator. This clearly runs contrary to the accepted industry practice, as many industry models employed for the estimation of dip-tube length were found to over-estimate the dip-tube length, and this subsequently results in the undesirable increased pressure drop across the separator. Lastly, the entry port geometry does not appear to have a significant impact on the separator performance as long as sufficient flow area is present. The efficiency of all gravity-driven separators was limited by the liquid velocity inside the separator annulus. When the liquid velocity inside the separator averaged approximately 6 in. per second or less, an almost-to-complete gas separation was achieved. On the other hand, the centrifugal separator had a liquid capacity 70% greater than any of the gravity-driven, static-downhole gas separators.

Downhole Gas Separators-A Laboratory and Field Study

Abstract Downhole gas separators are often the most inefficient part of a sucker rod pump system. This paper presents laboratory data on the performance of five different gas separator designs. Only continuous flow was studied. Field data is presented on one of the designs. The field data indicates that success or failure of the gas separator is dependent upon the fluids and wellbore pressures, as well as the mechanical design of the gas separator. Successful and unsuccessful examples of gas separator performance in the field are shown along with field fluid data properties. While the study is not complete, this is the first of hopefully several papers that will present the results of this investigation. Introduction Patterson and Leonard(1) studied different downhole gas sep-aration designs for coalbed methane operations in Wyoming. In these designs, the inlet to the gas separators were smaller than normally used and, along with some baffles (thought to allow gas to vent from inside the gas separator), obtained good gas separation in the field installation. It was these installations which prompted the laboratory study of the gas separator geometry to understand if the ‘rules-of-thumb’ used by the industry for gas separator design were valid. The objectives of this paper are to give a clearer insight into the mechanisms of gas interference in pumping wells and to present the results of recent laboratory and field studies on the flow characteristics and performance of some downhole gas separators. In these applications, the separation of gas from liquid is achieved through gravity separation without the introduction of other mechanisms (centrifugal forces, nozzles, etc.). Thus, the difference in density between the gas and liquid is the main driving force for separation. This implies that forces that oppose the effect of gravity, such as viscous drag caused by high fluid velocity and turbulence, will be detrimental to the separation process. Thus, high velocity of liquid or gas should be avoided, if possible. The Pumping Wellbore as a Gas-Liquid Separator The preferred pumping installation for maximum pump efficiency requires installing the pump intake below the lowest point of fluid entry into the wellbore and requires having an open casing-tubing annulus from the bottom to the wellhead. Gas and liquid enter the wellbore through the perforations and liquid flows to the bottom of the well under the action of gravity. The lighter gas bubbles rise through the liquid, forming a gaseous liquid column from the bottom of the perforated interval to the fluid level. Then, gas flows through the casing-tubing annulus to the wellhead where it exits to the flow line. Practically 100% liquid accumulates at the bottom of the well and enters the pump intake to be discharged by the pump into the tubing. In a large number of wells, it is not possible to install the pump intake below the lowermost fluid entry point in the wellbore. Typically, this is caused by an insufficient rathole, the presence of liners or gravel packs, concerns regarding sanding up and sticking the pump or just the operator's preference.

A Laboratory Study With Field Data of Downhole Gas Separators

Summary Downhole gas separators are often the most inefficient part of a sucker-rod pump system. This paper presents laboratory data on the performance of five different gas-separator designs. Only continuous flow was studied. Field data are presented on two of the designs. The field data indicate that success or failure of the gas separator is dependent upon the fluids and wellbore pressures as well as the mechanical design of the gas separator. Successful and unsuccessful examples of gas-separator performance in the field are shown along with field fluid data properties.

New Well-Testing Methods for Rod-Pumping Oil Wells—Case Studies

Summary A new well-testing method that provides high-resolution bottom-hole pressure (BHP) measurement has been developed for use with rod-pumping oilwell completions. This new well-testing technique is conducted by running a high-resolution downhole pressure gauge in a specially designed carrier assembly with the downhole pump. The gauge assembly is run on the rod string without pulling the completion packers or tubulars. With the new method, a down-hole pressure system is in place while the well is flowing during the entire well test. High-resolution downhole pressure data can then be recorded during all flow and shut-in periods of the well test, providing more accurate pressure data and enhanced capability for reservoir analysis. Traditional techniques use wireline-conveyed pressure gauges, tubing-conveyed pressure-recording devices, and surface pressure-monitoring systems with an advanced fluid-level measurement to record data from pumping well completions. These methods are limited in their ability to provide accurate test results when low-pressure conditions prevail. The new method allows operators of low-pressure oilwell completions to use advanced well-testanalysis software to analyze actual pressure data to obtain reservoir properties and quantify skin damage. Data can be acquired from many types of rod-pump completions, including wells with downhole gas separators. Excellent pressure resolution has been obtained from this new testing technique with either of two downhole assembly designs depending upon the completion configuration. The results and the ensuing well-test analyses in several pilot pumping wells will illustrate that the new process permits more flexibility in well testing and allows for greater accuracy and better interpretation of data in low-pressure rod-pumping oil wells.

Decentralized Continuous-flow Gas Anchor

Abstract A new gas separator has been developed and successfully field-tested in more than 40 beam pumped wells which were subject to severe gas interference. The new design is based on several innovations:Decentralization of the gas separator in the casing ensures that a maximum amount of liquid enters the separator.The presence of two or more ports on the gas separator near where it touches the casing wall.Construction of the gas separator so that the outer tube of the gas separator is pressed against the casing wall to prevent cross-flow of liquid and gas.The addition of a diverter at the bottom of the separator to direct most of the gas flow, towards the wide side of the annulus. These innovations have resulted in a gas separator efficiency much greater than that of conventional designs. Introduction Numerous studies have been performed in recent years in an attempt to improve the performance and efficiency of a beam pumped system [see References (1) through (8)]. The studies include the motor or power unit, the surface pumping system, the rod string and the downhole pump. In wells which produce gas, the inefficient separation of gas from liquid at the pump inlet often results in very poor pump performance and low overall efficiency of the entire pumping system. This can be verified by obtaining a dynamometer pump card which indicates considerable free gas in the pump even though liquid is present in the casing annulus in the form of a high gaseous liquid column. Terminology When discussing downhole gas separators in pumped wells, there appear to be various terms which are used indiscriminately: gas anchor, mud anchor, dip tube, perforated sub, etc. In order to avoid confusion, the terminology adopted in this paper is as follows:Gas Separator: The complete assembly of elements installed below the seating nipple and the pump intake with the purpose of maximizing the amount of liquid entering the pump.Outer Barrel: The outermost element of the gas separator which is connected to the tubing (often called mud anchor).Dip Tube: The innermost element of the gas separator which is attached to the pump intake (often called gas anchor). Natural Gas Separator The preferred technique for separating gas from the pump is to place the tubing fluid entrance ports (or gas separator ports) below the bottom of the formation's perforated interval as shown in Figure 1. The natural separation of gas and liquid by gravity will result in relatively gas-free liquid entering the pump intake if the downward liquid velocity in the casing/tubing annulus is less than approximately 6 in./sec. The gas separator inlet should be placed at least l0ft. below the formation gas entry zone to allow free gas separation from liquid between the tubing outer diameter and the casing inner diameter See Table 1 for natural gas separator capacities. For greater capacity, it is necessary to use a modified gas separator with an outer diameter smaller than the tubing (see Figure 2).

Development and Field Test Results of an Efficient Downhole Centrifugal Gas Separator

Summary Performance degradation of a centrifugal pump caused by ingested Performance degradation of a centrifugal pump caused by ingested free gas is well understood. High-GOR wells typically have a very high free-gas/liquid ratio at the pump intake, making the efficiency of the downhole gas separator crucially important in preventing free gas from entering the pump and hence producing the preventing free gas from entering the pump and hence producing the well successfully. This paper describes the design, development, and laboratory testing of a new rotary gas separator that is currently being installed in high-GOR wells. Laboratory tests were conducted using air in water at low intake pressures. Free-gas and liquid separation theory is reviewed briefly to show that test conditions in many respects represent the worst case of a downhole condition from which valid predictions of downhole performance can be made. Actual oil and production rate data from several oil wells showing before and after performance of the rotary design and conventional reverse flow designs are presented also. An average improvement in production of 95% was observed. Free-gas separation efficiencies at pump intake conditions of pressure, temperature, and viscosity are compared also. Introduction The practical results of excessive free-gas ingestion in a downhole electrical submersible pump (ESP) are excessive on/off cycling, rough pump operation as evidenced by a "gassy" amperage chart, high dynamic fluid level, and reduced equipment operating life. The result is reduced oil and gas production and higher operating costs. Equally important is that an operator may select gas lift, hydraulic, or sucker rod pumps to produce a high-GOR field when ESP's would be the best economic choice if free gas could be separated from the oil more efficiently at the intake of the downhole pump. This paper describes the development and field testing of an efficient centrifugal gas separator. Extensive laboratory tests were conducted to evaluate different design concepts and are reported briefly. Because of the uncertainty of downhole conditions such as fluid velocity, viscosity, water cut, and temperature, laboratory test results cannot predict downhole performance accurately. For this reason the primary emphasis in performance accurately. For this reason the primary emphasis in this paper is placed on actual field results. Approximately 1,500 centrifugal separators of the type we describe have been installed worldwide, and the reported results consistently show reduced dynamic or significant reduction of cyclic operation. The field results we present are from a number of Sun E and P Co. wells offshore California that were particularly well instrumented and recorded. Theory Many gas separators have been devised that use various physical mechanisms to obtain separation. Among these are gravity, agitation, and centrifugal force. The oldest and simplest bottomhole gas separator is the natural gas anchor. The natural gas anchor consists of simply setting the pump intake below the casing perforations. As the oil and entrained gas bubbles flow through the casing and experience a pressure drop, the bubbles grow in size and begin to rise. As they pressure drop, the bubbles grow in size and begin to rise. As they join other bubbles, they grow even larger. The rate of bubble rise increases with bubble size, with larger bubbles rising at about 0.5 ft/sec [0.15 m/s]. Gravity separation occurs when the fluid downhole velocity is lower than the bubble rise velocity. This is accomplished by maximizing the downflow area. This area is limited by casing size, equipment diameter, and fluid viscosity. The reverse flow gas separator, shown in Fig. 1, is a modified gas anchor that permits placing the pump intake above the casing perforations by locating the housing intake ports above inner intake passages. This causes the flow to make a 180 turn, which causes free gas separation to occur. A low downward flow velocity also may permit some additional natural separation. This type of separator has been used in centrifugal pumping applications for 50 years, but its efficiency is limited in applications where moderate to high flow rates are encountered. Screen-type gas/liquid separators using a fine screen have proved to be effective if the two-phase fluid is clean. proved to be effective if the two-phase fluid is clean. Experiments have shown, however, that the screen may clog quickly when used in an oilwell environment. For a gas separator to operate efficiently, it must ingest the two-phase mixture with minimal pressure drop. This is necessary to prevent additional gas breakout inside the separator. The goal is to induce inflow and then to boost the pressure by means of an inducer, to separate the phases, and then to vent the free gas back into the wellbore while the liquid-phase fluid enters the first stage of the centrifugal pump. JPT p. 1295

A downhole gas separator for methane production from geopressured aquifers

The downhole gas separator is a semipermeable device that produces methane gas from solution in brine by a diffusion process. It is in the form of a 6 in (15·2 cm) diameter, 1000 ft (303 m) long cylinder located downhole near the brine/methane-producing formation. As brine flows upward past the separator, 90 percent of the brine methane is extracted while 60 percent and 19 percent of the brine carbon dioxide and hydrogen sulfide, respectively, are extracted. The preferential extraction of methane over carbon dioxide and hydrogen sulfide, achieves a purification of the product gas. Subsequently, the brine flows further upward and into a shallower disposal reservoir penetrated by the same wellbore; surface brine handling is entirely eliminated. The separator is assembled at the wellhead from one hundred 10 ft (3 m) long segments in the fashion of a production string. Because of operations at near bottomhole pressures, maintenance requirements due to calcite scale are reduced substantially over surface-type equipment.