Barite Sag Research Articles

Effect of Oxidation On Explosion Potential And Barite Sag Reduction In Oil- And Synthetic-Based Drilling Fluids

Abstract Oil and synthetic-based drilling fluids are the drilling muds of choice in high pressure and high temperature wells. The hot drilling fluid is returned to the surface and is exposed to air in the mud tanks. Since the base liquids of these drilling fluid systems are expensive, they are used in many wells before disposal. The long exposure of the oil to air can result in a substantial oxidation of the oil; especially in hot reservoirs in summer temperatures. To simulate the oxidation of these drilling fluids, an experimental study was conducted in which 20 oil-based and synthetic-based drilling fluid systems which are used in Western Canada, were examined. The drilling fluid systems were aged in stationary reactors in the presence of one charge of air for 52 hours at 150 ºC and 13.5 MPa initial pressure. While rapid heat releases or explosions in the synthetic-based drilling fluids were observed, oil-based systems were safe under the conditions tested. The rheological properties of the drilling fluid samples after aging were compared with the original (before aging) samples. The results at an ultra low shear rate range of 0.0 – 0.2 s−1 and a high shear rate range of 0 – 4,000 s−1 are presented. It is shown that the aged drilling fluid systems exhibited a tremendous increase in viscosity at ultra-low shear rates. Furthermore, relatively low increases in viscosity at medium and high shear rates were observed. Introduction Demand for energy sources is increasing throughout the world at a rapid rate due to industrial development and population growth. The search for new sources of oil and gas on land and offshore, and also developing current resources, has intensified due to this increasing demand. Oil production involves two main phases of drilling operations; namely, exploration and development. The exploration phase operations determine the potential hydrocarbon reserves and include drilling of exploratory wells, whereas, the development phase operations include drilling of production wells once a hydrocarbon reserve has already been discovered. Drilling fluids are one of the essential components of the drilling operation. Inappropriate design of a drilling fluid system can lead to numerous problems such as stuck pipe, lost circulation, wellbore instability, gellation, insufficient hole cleaning, bit balling, barite sag, excessive torque and drag, corrosion, H2S contamination and plugging due to gas hydrates. Due to some underbalanced drilling applications that employ gasified liquids, oxidation of oil and synthetic-based drilling fluids has recently received some attention. Historically, in situ combustion(1) and high pressure air injection (HPAI)(2, 3) are two major areas in thermal enhanced oil recovery techniques where the oxidation of oils are of special importance. In these processes, it is of vital importance to know what effects the oxidation has on rheological properties of the oil, because it affects the mobility of the oil and, therefore, the recovery factor. In drilling fluids, in addition to changes in rheological properties, safety considerations and explosion potential require investigation. In the first phase of this research, the oxidation of different base oils was investigated.

The Effect of Oxidation on Viscosity of Oil-Based Drilling Fluids

Abstract True downhole rheological properties affect equivalent circulating density, hole cleaning, barite sag, surge/swab pressures during tripping, pump pressure, and bit hydraulics. Gelation and excessive viscosity are major concerns at high temperatures. When using oil-based drilling fluids lightened by an injection of de-oxygenated air (containing small amounts of oxygen, usually around five per cent) in underbalanced drilling (UBD) operations, there is a need to be able to predict the effect of oxidation on the viscosity of the oil-based muds as a function of temperature and pressure. This paper presents the results of an experimental investigation that was aimed at establishing the effect of oxidation on the viscosity of an oil-based drilling fluid. The drilling fluid was aged for 2.5 days and for 7 days in the presence of air at temperatures ranging from 100 to 150 °C and at pressures ranging from 14 to 44 MPa. The viscosity of the drilling fluid samples after aging is compared with the corresponding fresh samples (before aging). The results show that oxidation causes an increase in viscosity. The amount of the increase depends on the amount of oxygen reacted, which is a function of temperature, pressure, and time. The higher temperatures of the reactors lead to the higher increases in viscosity. Furthermore, at higher temperatures, solid (mostly coke) formation was observed. Introduction Underbalanced drilling (UBD), in which drilling fluid pressure is lower than reservoir pore pressure, has been studied by many researchers since the late 1980s. This method can improve the financial returns on drilling a well by reducing formation damage (especially in horizontal wells and depleted reservoirs), minimizing lost circulation (especially in fractured reservoirs and depleted reservoirs), increasing the penetration rate of drilling, increasing bit life, minimizing differential sticking, reducing stimulation, and improving formation evaluation(1). The underbalanced condition is created by using different drilling fluids such as dry air, nitrogen, natural gas, mist, stable foam, stiff foam, gasified liquids, glass bubbles, and liquids. In gasified liquids, a gas is injected into the liquid to reduce the density of the drilling fluid. In these mixtures, since the pressure difference between the formation and the wellbore is relatively low (1.72 - 3.45 MPa), these drilling fluids have fewer UBD related problems such as wellbore instability and formation fluid inflows(1). These specifications have made them the most appropriate drilling fluid in UBD. The unique performance of oils makes them preferable to water as the liquid phase in gasified liquids. Oils prevent clay swelling and they are resistant to contaminants. Their rheological properties are stable at high pressures and temperatures. Also, they are highly lubricious and non-corrosive. On the other hand, to prevent downhole fires, de-oxygenated air from membrane units with maximum five per cent oxygen is usually used as the gas phase in gasified liquids(2). The presence of oxygen causes the oil in the gasified liquid to be oxidized. In this paper, the effect of oxidation on the viscosity of the gasified liquids is investigated.

Correlation of Ultralow-Shear-Rate Viscosity and Dynamic Barite Sag

SummaryThe occurrence of barite sag has been a well recognized but poorly understood phenomenon in the drilling industry resulting in problems such as lost circulation, well control and stuck pipe. The financial impact on drilling costs, usually resulting from rig-time lost while circulating and conditioning the drilling fluid system, is not trivial. Recurring barite sag problems reportedly have resulted in the loss of drilling projects.Originally thought to occur under static conditions, barite sag is recognized now to occur more readily under dynamic, low-shear-rate conditions. Industry experts have offered a variety of measuring parameters, based upon empirical data, that only partially correlate with the occurrence of barite sag. Prediction of barite sag in dynamic flow has created an engineering challenge.The effect of shear rate on dynamic barite sag, for invertemulsion drilling fluids, has been studied and quantified using new and advanced technology. A new field viscometer capable of measuring viscosity at shear rates of 0.0017 sec−1 and an eccentric wellbore-hydraulics model were used to develop and understand this relationship. Changes in mud weight as a function of shear rate, hole angle, annular velocity (AV), and eccentricity correlate with ultralow-shear-rate viscosity.Based upon experimental results, field technology has been developed to predict the potential for barite sag of invert-emulsion drilling fluids and to provide remedial measures through ultralow-shear-rate-viscosity modification. The efficacy of using traditional rheological measurements as indicators of barite sag potential is addressed.

Barite Sag: Measurement, Modeling, and Management

Summary A joint industry project was established to study barite sag mechanisms and to develop field guidelines to manage the consequences. A simple empirical model was developed to compare sag potential for a wide range of fluid types. In the study, physical properties of the mud, wellbore conditions, and characteristics of the weighting material were shown to have a large influence on sag behavior. The study also included direct measurements of the properties of settled weight-material beds. These results provide new insight into the mechanisms of barite sag and how best to manage problems in the field. Data from the tests clearly demonstrate that the parameters affecting sag are interrelated and seldom act in isolation. For all muds tested, the highest sag occurred at low annular velocities over angles from 60 to 75°. Drillpipe rotation was particularly beneficial in minimizing barite settlement. Rotation also assisted in re-distributing barite deposits formed on the low side of the hole. The improved understanding of the mechanisms of barite sag enabled development of practical field guidelines. Case history studies presented in the paper demonstrate how the results of the work together with better field monitoring have been successfully applied to manage the effects of barite sag in high-pressure/high-temperature and extended-reach drilling operations.

Dynamic Barite Sag in Oil Based Drilling Fluids

A rheological technique is described which utilises flat parallel plate geometry. The technique is sensitive enough to pick up the effect of syneresis and resulting movement of denser materials from the surface layers into the bulk of the drilling mud samples over time. Examples of down flow curves are given which demonstrate that dynamic sag while in flow is represented by a dip in this curve depending flow on particular samples, when expressed as log/log plots of viscosity vsshear rate. The up curves illustrate the degree of syneresis and static sag in the samples. Log/log plots of viscosity vs shear rate for both up and down flow curves exhibit one or two crossover points. The lower crossover points can be plotted on a 3D graph for easy visualisation of the sagging tendencies of various oil mud samples.