Effect Of Free Gas Research Articles

New correlations for predicting two-phase electrical submersible pump performance under downhole conditions using field data

AbstractElectrical Submersible Pump (ESP) is one of the major Artificial Lift methods that is reliable and effective for pumping high volume of fluids from wellbores. However, ESP is not recommended for applications with high gas liquid ratio. The presence of free gas inside the pump causes pump performance degradation which may lead to problems or even failure during operations. Thus, it is important to investigate effect of free gas on ESP performance under downhole conditions. At present, existing models or correlations are based on/verified with experimental data. This study is one of the first attempts to develop correlations for predicting two-phase gas–liquid pump performance under downhole conditions by using field data and laboratory data. Field data from three oil producing wells provided by Strata Production Company and Perdure Petroleum LLC. as well as experimental data obtained from experimental facility at Production and Drilling Research Project—New Mexico Tech were used in this study. Actual two-phase pump differential pressure per stage is obtained from experiments or estimated from field data and was normalized using pump performance curve. The values are compared to pump performance curve to study the relationships between pump performance and free gas percentage at pump intake. Correlations to predict ESP performance in two-phase flow under downhole and experimental conditions was derived from the results using regression technique. The correlation developed from field data presented in this study can be used to predict two-phase ESP performance under downhole conditions and under high gas fraction. The results from the experimental data confirm the reliability of the developed correlation using field data to predict two-phase ESP performance under downhole conditions. The developed correlation using the laboratory data predicts quite well the two-phase pump performance at the gas fraction of less than 15% while it is no longer reliable when free gas fraction is more than 15%. The findings from this study will help operating companies as well as ESP manufacturers to operate ESPs within the recommended range under downhole conditions. However, it is recommended to use the proposed correlation on reservoirs with conditions similar to those of the three presented wells.

A state-dependent constitutive model for coarse-grained gassy soil and its application in slope instability modelling

Free gas in sandy marine sediments is a common occurrence worldwide. A distinct feature of gassy sand is that, under undrained shearing, presence of occluded gas bubbles in the pore fluid can increase the undrained strength of sand at a relatively loose state, while reduce the strength of a relatively dense sand. Previous theoretical analyses have primarily focused on modelling the ‘beneficial’ effect of free gas on loose sand in migrating static liquefaction, with few attempts to describe the ‘detrimental’ effect of gas on dense sand under undrained loading. This study presents a state-dependent critical state model, which describes the distinct behavior of gassy sand with various states in a unified way. Comparison between the model predictions and test data of three gassy sands shows that the new model can capture the constitutive behavior of gassy marine sand over a wide range of initial states and degrees of saturation (typically between 85% and 100% for unsaturated marine sediments) using a single set of parameters. Parametric studies were performed to quantify the effects of gas (either ‘detrimental’ or ‘beneficial’) on sand with various initial states. The new model has been implemented in ABAQUS and used to simulate the stability of submarine slopes under undrained loading condition. It is found that free gas can improve and weaken the slope stability for loose and dense sand, respectively.

Evaluation of the effect of adsorbed gas and free gas on mechanical properties of coal

The mechanical properties of coal are important parameters for coalbed methane (CBM) extraction and gas outburst control. However, the effect of adsorbed gas on strength cannot be evaluated quantitatively yet. To better understand the weakening mechanisms of free and adsorbed gas on the strength of coal, normal coal, and deformed coal are chosen to test their mechanical properties of CH4-saturated and non-gas-saturated specimens under different conditions. Under the same effective stresses, the peaks of strength of CH4-saturated specimens with high-pressure gas are lower than those with low-pressure gas, implying that the adsorbed gas can also weaken the peak strength of coal. Then, a new model for the weakening values of strength induced by free and adsorbed gas was developed, and the effects of free gas and adsorbed gas on the strength of coal were assessed by our model. The results show that the free gas and adsorbed gas can weaken the strength of coal for different weakening mechanisms. The ratio of weakening value of strength due to free gas to that of adsorbed gas of the normal coal is 1.3–3.4, and that of the deformed coal is 8.4–19.8. These results can help us to better understand why the weakening effect of adsorbed CH4 in the laboratory is ignored in earlier studies.

Cone penetration testing to assess slope stability in the 1979 Nice landslide area (Ligurian Margin, SE France)

In the landslide-prone area near the Nice international airport, southeastern France, an interdisciplinary approach is applied to develop realistic lithological/geometrical profiles and geotechnical/strength sub-seafloor models. Such models are indispensable for slope stability assessments using limit equilibrium or finite element methods. Regression analyses, based on the undrained shear strength (su) of intact gassy sediments are used to generate a sub-seafloor strength model based on 37 short dynamic and eight long static piezocone penetration tests, and laboratory experiments on one Calypso piston and 10 gravity cores. Significant strength variations were detected when comparing measurements from the shelf and the shelf break, with a significant drop in su to 5.5kPa being interpreted as a weak zone at a depth between 6.5 and 8.5m below seafloor (mbsf). Here, a 10% reduction of the in situ total unit weight compared to the surrounding sediments is found to coincide with coarse-grained layers that turn into a weak zone and detachment plane for former and present-day gravitational, retrogressive slide events, as seen in 2D chirp profiles. The combination of high-resolution chirp profiles and comprehensive geotechnical information allows us to compute enhanced 2D finite element slope stability analysis with undrained sediment response compared to previous 2D numerical and 3D limit equilibrium assessments. Those models suggest that significant portions (detachment planes at 20m or even 55mbsf) of the Quaternary delta and slope apron deposits may be mobilized. Given that factors of safety are equal or less than 1 when further considering the effect of free gas, a high risk for a landslide event of considerable size off Nice international airport is identified.

Modeling Oscillatory Behavior of Electrical Submersible Pump Wells Under Two-Phase Flow Conditions

The effect of free gas on electrical submersible pump (ESP) performance is well known. At a constant rotational speed and constant liquid flow rate, a small amount of gas causes a mild head reduction when compared to the single phase liquid head. However, at higher gas rates, a drastic reduction in the head is observed. This critical condition, known as the surging point, is a combination of liquid and gas flow rates that cause a maximum in the head performance curve. The first derivative of the head with respect to the liquid flow rate changes sign as the liquid flow rate crosses the surging point. In several works on ESP two-phase flow performance, production conditions to the left of the surging region are described or reported as unstable operational conditions. This paper reviews basic concepts on stability of dynamical systems and shows through simulation that ESP oscillatory behavior may result from two-phase flow conditions. A specific drift flux computation code was developed to simulate the dynamic behavior of ESP wells producing without packers.

Evaluation of the effect of free gas on the measurement error for crude oil mass by coriolis flowmeters and dissolved gas on the error of measuring well production rate

Results are presented for experimental studies of estimating the effect of free gas on the metrological properties of coriolis flowmeters. The requirement is demonstrated for considering gas dissolved in oil during measurement of well production rate.

In situ pore-pressure evolution during dynamic CPT measurements in soft sediments of the western Baltic Sea

We present in situ strength and pore-pressure measurements from 57 dynamic cone penetration tests in sediments of Mecklenburg (n = 51), Eckernforde (n = 2) and Gelting (n = 4) bays, western Baltic Sea, characterised by thick mud layers and partially free microbial gas resulting from the degradation of organic material. In Mecklenburg and Eckernforde bays, sediment sampling by nine gravity cores served sedimentological characterisation, analyses of geotechnical properties, and laboratory shear tests. At selected localities, high-resolution echo-sounder profiles were acquired. Our aim was to deploy a dynamic cone penetrometer (CPT) to infer sediment shear strength and cohesion of the sea bottom as a function of fluid saturation. The results show very variable changes in pore pressure and sediment strength during the CPT deployments. The majority of the CPT measurements (n = 54) show initially negative pore-pressure values during penetration, and a delayed response towards positive pressures thereafter. This so-called type B pore-pressure signal was recorded in all three bays, and is typically found in soft muds with high water contents and undrained shear strengths of 1.6–6.4 kPa. The type B signal is further affected by displacement of sediment and fluid upon penetration of the lance, skin effects during dynamic profiling, enhanced consolidation and strength of individual horizons, the presence of free gas, and a dilatory response of the sediment. In Mecklenburg Bay, the remaining small number of CPT measurements (n = 3) show a well-defined peak in both pore pressure and cone resistance during penetration, i.e. an initial marked increase which is followed by exponential pore-pressure decay during dissipation. This so-called type A pore-pressure signal is associated with normally consolidated mud, with indurated clay layers showing significantly higher undrained shear strength (up to 19 kPa). In Eckernforde and Gelting bays pore-pressure response type B is exclusively found, while in Mecklenburg Bay types A and B were detected. Despite the striking similarities in incremental density increase and shear strength behaviour with depth, gas occurrence and subtle variations in the coarse-grained fraction cause distinct pore-pressure curves. Gaseous muds interbedded with silty and sandy layers are most common in the three bays, and the potential effect of free gas (i.e. undersaturated pore space) on in situ strength has to be explored further.

Application of a Bubble Tracking Technique for Estimating Downhole Natural Separation Efficiency

Abstract Liquid velocity and pump intake pressure are known to have strong effects on the separation efficiency of gas and liquid which may impact the flow efficiency of downhole pumps. This paper describes a new model to evaluate the effectiveness of this separation. The model correctly predicts the decline in separation efficiency with higher liquid flowrates, which is shown to be in agreement with presented experimental data. A calibration procedure is conducted which shows a descending trend of bubble size when the pump intake pressure is increased. The new model shows good agreement with the experimental data in the bubble flow regime, since it handles the two-phase natural separation problem by combining the single-phase flow field solution with a bubble tracking method. It is the first CFD-based simplified model which incorporates the multiple effects that govern the natural gas-liquid separation process. If a correlation for bubble size is properly obtained, the model will cover the omprehensive effects of operational and geometric conditions and provide an accurate prediction of gas-liquid separation efficiency over the full spectrum of normal bottomhole pump operating conditions. Finally, the single-phase flow field is solved in a 2D Cartesian coordinate system. The acceleration of the liquid phase, a nature of fluid behaviour in a cylindrical system, is not considered. This is a necessary improvement in the model to properly calculate the slip velocity in the radial direction. Introduction In pumping wells, the presence of free gas can cause several operational problems. The gas ingested by the pump can lead to poor pump performance and low overall efficiency of the entire pumping system. For any pumping lift system, estimating natural separation efficiency is a very important task whether the well is equipped with a bottomhole gas separator or not. A sketch of the natural separation process is shown in Figure 1. The movement of the free gas in the annulus space in front of the pump intake is the result of gravitational, inertial, and interfacial effects. The gas phase being lighter than the liquid tends to continue flowing vertically due to buoyancy effects. On the other hand, the gas phase is also being dragged due to interfacial forces and pressure gradient effects toward the pump intake by the liquid phase. As a consequence, part of the gas stream will eventually follow the liquid hase entering the pump and the rest of the gas will continue its vertical movement. The fraction of gas that is vented through the annulus is known as natural separation efficiency. FIGURE 1: Sketch of the natural separation process. Available in Full Paper. Lea(1) was the first investigator to conduct test programs to evaluate the effects of free gas on the performance of submersible pumps; his results included a graph of natural separation efficiency as a function of liquid flow rate and gas fraction at the pump intake. Alhanati(2) derived the first mechanistic model to estimate the natural separation efficiency for vertical wells.

Analysis of Electric-Submersible-Pumping Systems

Summary This paper presents a field-proven analytical method of evaluating electric-submersible-pumping equipment and well performance jointly. A computerized mathematical model that considers the effects of free gas, pump speed (variable-frequency drives), and pump tapering is described. The method allows accurate calculations of important downhole parameters, including pump intake pressure, pump intake volume (including free gas), pump pressure, and fluid density profile. Lifting cost parameters—such as pump and motor power requirements, monthly power consumption, pump and motor performance, and overall system efficiency—are also determined. Once operating conditions are known, decisions can be made on ways to increase production (if additional potential exists) or to reduce operating costs. Thus, the same basic goals that often justify frequent analysis of rod-pumping systems can be applied to submersible pumping.