Underbalanced Drilling Operations Research Articles

Foam Stability Analysis at High pH and Saline Environments for Underbalanced Drilling Operations

Foam Stability Analysis at High pH and Saline Environments for Underbalanced Drilling Operations

Impact of Pressure and Temperature on Foam Behavior for Enhanced Underbalanced Drilling Operations.

Foam, a versatile underbalanced drilling fluid, shows potential for improving the drilling efficiency and reducing formation damage. However, the existing literature lacks insight into foam behavior under high-pH drilling conditions. This study introduces a novel approach using synthesized seawater, replacing the conventional use of freshwater on-site for the foaming system's liquid base. This approach is in line with sustainability objectives and offers novel perspectives on foam stability under high-pH conditions. Experiments, conducted with a high-pressure, high-temperature (HPHT) foam analyzer, investigate how pressure and temperature affect foam properties. The biodegradable foaming agent ammonium alcohol ether sulfate (AAES) is employed. Results demonstrate that the pressure significantly impacts foam stability. Increasing pressure enhances stability, reducing decay rates and promoting uniform bubble sizes, especially at lower temperatures. This highlights foam's capacity to withstand high-pressure conditions. Conversely, the temperature plays a substantial role in foam decay, particularly at elevated temperatures (75 and 90 °C). Decreased liquid viscosity accelerates the liquid drainage and foam decay. While pressure mainly influences the AAES foam stability at temperatures up to 50 °C, temperature becomes the dominant factor at higher temperatures. Temperature's impact on foamability is minimal under constant pressure, maintaining consistent gas volume for maximum foam height. However, foam stability is sensitive to temperature variations, with increasing temperature leading to a more significant bubble size increase gradient. These findings stress the importance of considering temperature effects in foam drilling, particularly in deep and high-temperature environments. AAES foam exhibits stability at lower temperatures, making it suitable for surface and intermediate drilling. Understanding temperature-induced changes in foam structure and bubble size is essential for optimizing performance in high-temperature and deep drilling scenarios.

Effect of two-phase gas-liquid flow patterns on cuttings transport efficiency

Effective cuttings transport and accurate drilling hydraulics prediction remain issues of concern during drilling operations of horizontal, extended reach and multilateral wells. While several studies have adopted a two- or three-layered modelling approach to evaluate cuttings transport efficiency, they have neglected the effect of the gas-liquid fluid flow pattern within the annulus on cuttings transport. An experimental and theoretical study was carried out to evaluate the interplay between the two-phase gas-liquid flow patterns and the major drilling parameters and investigate its influence on the cuttings and fluid flow dynamics in a horizontal and inclined drilling wellbore. Several mathematical flow pattern dependent multi-layered models valid for any level of wellbore eccentricity were developed for the different cuttings transport mechanisms in the bubble, dispersed bubble, stratified and slug gas-liquid flow patterns, thereby providing a method to evaluate cuttings transport efficiency and perform wellbore hydraulics calculations for underbalanced drilling operations. Experimental results show that both fluid flow pattern and the drilling fluid flowrate are the most influential controllable parameters that affect the cuttings transport efficiency. Moreover, the hole cleaning requirements for an eccentric annulus is higher than that required for the concentric annulus of both single-phase and two-phase Newtonian or non-Newtonian fluids. Inclination angle was also found to influence hole cleaning and the degree of its effect is highly dependent on the fluid properties, the cutting transport mechanism and prevailing gas-liquid flow pattern. In the horizontal and inclined eccentric annuli, drillpipe rotation can improve cuttings transport for both single-phase and two-phase flows, but generally the effect of the drillpipe rotation on two-phase flow for cutting transport is much less than that of the single-phase flow. Overall, a good match was found between the mathematical flow pattern dependent multi-layered models and the experimental data. The findings of this study serve as a guide in the prediction of the wellbore dynamics for underbalanced drilling operations and provides a tool that can be applied for wellbore pressure management and the evaluation of hole cleaning based upon the specified flow conditions.

Effects of Annulus Geometry and Liquid Properties on the Well Conditions during UBD Operation

Well conditions during drilling operation can be predicted using numerical simulation. During under-balanced drilling (UBD) operation, controlling the bottom-hole pressure (BHP) in a suitable range and also appropriate hole-cleaning is essential. In this paper, numerical simulation of gas-liquid-solid three-phase flow in the annulus is used to study the effects of annulus geometry and also liquid properties on the BHP and hole-cleaning during UBD operation. To validate the numerical simulation, the results are compared with the experimental data from a laboratory study. Also, the gain results from developed code are compared with the actual field data from a real well, several mechanistic models from WellFlo software, and gas- liquid two- fluid numerical simulation. Due to the significance of controlling the BHP and hole-cleaning during UBD operation, the effects of annulus geometry and liquid phase properties on BHP and the solid volume fraction distribution are investigated. According to the results, changing the hydraulic diameter and cross-sectional area of the annulus can affect BHP and hole- cleaning in UBD operation. In other words, increasing the hydraulic diameter at a constant cross- sectional area improves hole-cleaning and decrease BHP. Also, decreasing the cross-sectional area at a constant hydraulic diameter improves hole-cleaning and increase BHP. The results show that the liquid viscosity affects hole-cleaning through two contrary mechanisms. In fact, by increasing the liquid viscosity, carrying capacity of the liquid phase is increased and cutting transfer velocity is decreased.

Real-Time Interpretation Model of Reservoir Characteristics While Underbalanced Drilling Based on UKF

This study presents a novel interpretation model for reservoir characteristics while underbalanced drilling (UBD), by incorporating an unscented Kalman filter (UKF) algorithm in a three-phase variable mass flow model of oil, gas, and liquid. In the model, the measurement parameters are simplified to bottomhole pressure and liquid outlet flow, for decreasing the amount of the computation and time. By taking into account real-time measurements, the permeability and reservoir pressure along the well can be continuously updated. Three cases including single-parameter and double-parameter estimations have been simulated, and the performance is tested against the extended Kalman filter (EKF). The results show that single-parameter estimation of reservoir permeability or pressure achieves superior performance. The filtered values of bottomhole pressure and outlet flow trace the measured values in real time. When a new section of a reservoir is opened, the estimated reservoir permeability or pressure can always be quickly and accurately returned to its true value. However, it is not possible for the double-parameter estimation to obtain good results; its interpretation accuracy is low. UKF is superior to EKF in both estimation accuracy and convergence speed, which further illustrates the superiority and accuracy of the novel interpretation model based on UKF. Benefits from this model are seen in accurate bottomhole pressure and reservoir characteristic predictions, which are of major importance for safety and economic reasons during UBD and follow-up completion operations.

Reservoir characterization in under-balanced drilling with nonlinear Moving Horizon Estimation with manual and automatic control conditions

This paper discusses the state estimation of the Under-Balanced Drilling (UBD) system in the presence of parametric uncertainties. During this process, the production indices of oil and gas from the reservoir into the well in the manual and automatic control conditions are estimated by employing the nonlinear Moving Horizon Estimation (MHE) based on a low-order lumped (LOL) model. The LOL model has a low computational load and is suitable for reservoir characterization during UBD operations. The estimation algorithms are tested by using a difficult scenario which is created by the OLGA multiphase flow simulator. The simulation results indicate that the nonlinear MHE has a high performance and can identify the production indices of gas and oil. Moreover, the method has the capability to diagnose the rapid variation of the production constant in different conditions, such as working with a manual or automatic controller. The results of the scenario with the swift change in the production index of gas demonstrate that the nonlinear MHE has a higher performance than the Unscented Kalman Filter (UKF). The effect of uncertainties and errors in the reservoir and well parameters on the nonlinear MHE is evaluated. It is revealed that the presence of parametric uncertainty in the rdeservoir pore pressure can significantly affect the estimators' performance.

Underbalanced Drilling Optimum Methodology

A methodology to properly screen all UBD techniques to reduce failure/misapplication and align objectives with expectations had been absent. The paper addresses the latest enhancements to better understand and screen options for UBD operations according to two aspects: formation applicability and engineering applicability. The screening tool considers a range of economic and technical parameter to provide a relative ranking for each candidate of the Underbalanced drilling (UBD) technique. As a case, the methodology was available to optimize the UBD techniques in a risky exploration well in Xinjiang oilfield, which provided guidance and technical support to screen the UBD candidate, design the drilling modeconversion, and establish the emergency schedule.

Automated Pressure Control for UBD Operations: Case Study and Field Validation

Summary In this paper, we present field results from a series of drilling operations where advances in sensor and control-system technology enable a high degree of automated pressure control in underbalanced-drilling (UBD) operations in Siberia, Russia. Traditionally, this fractured carbonate field was drilled conventionally using water as drilling fluid, accepting losses after hitting the first fractures, resulting in wells with access to only two or three fractures and not reaching target depth (TD) of the productive section. For these operations, a combination of nitrified water and crude oil was used. Applying automated pressure control with UBD equipment enables drilling with a controlled drawdown from the fractures, enabling the drilling of significantly longer wells, maximizing discovered fractures per well. In this paper, we show how the drilling of these wells has been achieved with the combination of sensors for managed-pressure drilling (MPD) and UBD and an automated pressure control system that maintains a constant drawdown throughout operations.

Mathematical modeling of formation damage effect during underbalanced drilling operations

Formation damage during underbalanced drilling is created due to the spontaneous counter-current imbibition when drilling mud is in contact with saturated formations. Therefore, it should be appropriately recognized and investigated. The objective of this study is to develop a numerical model to simulate the penetration rate of drilling mud in the formation during underbalanced drilling. Modeling of this phenomenon is done by implementing its formulation through implicit pressure–explicit saturation (henceforth, IMPES) method. To validate the results of the developed model with a pilot study in this field, it is demonstrated that the model performs effectively in determining the penetration range of muds and finding the distribution of drilling mud saturation across the core. Consequently, the degree of damage to the formation through laboratory studies is usually time-consuming and therefore, the developed model can be employed for initial studies. Increasing the unbalanced pressure difference, oil viscosity and porosity will reduce the amount of drilling mud penetration in the formation during underbalanced drilling operations.

Thermal simulation of two-phase flow in under-balanced drilling operation with oil and gas production

The accurate prediction of wellbore temperature distribution helps to accurately estimate well pressure profile and bottom-hole pressure (BHP) which is important in the under-balanced drilling (UBD) operation. In this paper effect of temperature variation due to heat transfer of drilling fluid with the formation and also oil and gas production from the reservoir into the annulus in under-balanced drilling condition were investigated. Gas-liquid two-phase flow model considering thermal interaction with the formation is used to numerically simulate a well with real dimensions. Based on drilling fluids flow and heat transfer characteristics in wells, conservations of mass and momentum and energy equations have been developed to compute BHP and wellbore temperature and pressure profile. After temperature and pressure validation of the numerical model, the effect of heat transfer between drilling fluid inside the well and the formation was considered on the pressure distribution and bottom-hole pressure. The results of two-phase flow, considering thermal effect gives better results compared to two-phase flow with geothermal temperature distribution analysis and better accuracy in comparison with other models.

Pressure Profile Estimation through CFD in UBD Operation Considering with Influx to Wellbore

Nowadays, UnderBalanced Drilling (UBD) technology is widely applicable in the petroleum industry due to its advantages to an overbalanced drilling operation. UBD success depends on maintaining the drilling fluid circulating pressure below the reservoir pore pressure during operations. One of the main prerequisites of a successful UBD operation is the correct estimation of the pressure profile. In this investigation, the pressure profile was obtained with consideration of the influx to the wellbore. A spreadsheet was developed to obtain the pressure profile using an analytical solution for aerated mud in UBD operation. Moreover, a numerical simulation was employed to simulate the three-phase flow in annulus through the UBD operation and the transient Eulerian model flow via the turbulence k-e model. The effects of solid particle size and rotation of the inner pipe were considered on the pressure drop. It was observed that pressure drop was significantly increased with increasing solid particle size while it remained almost constant with increasing of the inner pipe rotation. The analytical and numerical results were compared with published experimental results and showed a good agreement.

A simplified framework and performance analysis for while-drilling telemetry systems in underbalanced drilling operations

As it stands, underbalanced drilling (UBD) operation has been beneficial in the exploration of hydrocarbons and production, with advantages like faster drilling control of lost circulation, stuck pipe situations, minimal or less formation damages and an enhanced ability to expedite development of brown fields. In spite of its edge over conventional drilling, the techniques can be more complex as well. Thus, a telemetry system is required to link downhole and surface while-drilling components transmitting and providing useful information under the real-time condition to the driller during drilling operations for more precise wellbore placement to ensure maximum reservoir exposure, ultimate production increment and monitoring conditions. A novel and systematic decision support system model based on distance-based approach has been developed for performance evaluation and optimal selection of while-drilling telemetry systems for the potential development of a depleted reservoir field. The proposed methodology attests to the need for the relative importance of criteria for a given application, without which inter-criterion comparison could not be accomplished. It requires a set of model selection criteria, along with available telemetry techniques and their level of criteria for optimal selection; and it successfully presents the results in terms of a merit value which is used to rank the while-drilling telemetry systems. This aims at delivering reliable drilling performance resulting to deliverable of real-time downhole data collection and most importantly places the matured well with minimal non-productive time during UBD operations. The scientific value of this study is the development of a simplified user-friendly decision support system for telemetry systems performance evaluation and selection optimization. The proposed framework was being validated and applied to a matured reservoir for an optimum selection of telemetry techniques in the UBD operation. The result of computations displayed similar results obtained by expert judgment demonstrating the significance effect and need for telemetry systems.

Flow and pressure control of underbalanced drilling operations using NMPC

This work evaluates the use of non-linear model predictive control (NMPC) for multivariate control of pressure and flow during underbalanced drilling (UBD) using a fit-for-purpose mechanistic model. Flow and pressure control are instrumental to the stability, safety, and successful drilling of a well. Failure to keep within the pressure envelope may have severe economic and environmental consequences. A relevant drilling scenario with different control objectives is evaluated to show the impact of the tuning and configuration of the control system. It is shown that with a sufficiently good model match, it is possible to achieve good performance with small deviations from the selected operational criteria, even when there are large changes in operational area. The multivariate control system manipulates choke pressure, pump flow, and separator set-points to simultaneously achieve both downhole and topside objectives. The optimization problem is solved within the relevant time dynamics of the system.

Reservoir characterization in under-balanced drilling using low-order lumped model

Estimation of the production index of oil and gas from the reservoir into the well during under-balanced drilling (UBD) is studied. This paper compares a Lyapunov-based adaptive observer and a joint unscented Kalman filter (UKF) based on a low order lumped (LOL) model and the joint UKF based on the distributed drift-flux model by using real-time measurements of the choke and the bottom-hole pressures. Using the OLGA simulator, it is found that all adaptive observers are capable of identifying the production constants of gas and liquid from the reservoir into the well, with some differences in performance. The results show that the LOL model is sufficient for the purpose of reservoir characterization during UBD operations. Robustness of the adaptive observers is investigated in case of uncertainties and errors in the reservoir and well parameters of the models.

State and Parameter Estimation of a Drift-Flux Model for Underbalanced Drilling Operations

We consider a drift-flux model (DFM) describing multiphase (gas–liquid) flow during drilling. The DFM uses a specific slip law, which allows for transition between single and two phase flows. With this model, we design unscented Kalman filter (UKF) and extended Kalman filter (EKF) for the estimation of unmeasured state, production, and slip parameters using real-time measurements of the bottom-hole pressure, outlet pressure, and outlet flow rate. The OLGA high-fidelity simulator is used to create two scenarios from underbalanced drilling on which the estimators are tested: a pipe connection scenario and a scenario with a changing production index (PI). A performance comparison reveals that both UKF and EKF are capable of identifying the PIs of gas and oil from the reservoir into the well with acceptable accuracy, while the UKF is more accurate than the EKF. Robustness of the UKF and EKF for the pipe connection scenario is studied in case of uncertainties and errors in the reservoir and well parameters of the model. It is found that these methods are very sensitive to errors in the reservoir pore pressure value. However, they are robust in the presence of error in the liquid density value of the model.

Modeling and simulation of gas-liquid-solid three-phase flow in under-balanced drilling operation

In this study, gas-liquid-solid three-phase flow in the annulus of a well with actual dimensions is simulated numerically considering under-balanced drilling (UBD) operations. The multi-fluid approach which is the most accurate and sophisticated method is used for the simulation. For this purpose, one-dimensional form of steady-state three-fluid model in the Eulerian frame of reference is used to simulate the three-phase flow in the UBD operation. The results have been validated with actual field data and also with the result of gas-liquid two-phase flow model. The comparisons show that three-phase numerical simulation gives better accuracy compared to two-phase numerical simulation and most of other models which are using mechanistic approach. Given the importance of the prediction and control of the bottom-hole pressure (BHP) during drilling operations, the effects of controlling parameters such as liquid and gas flow rate, drilling velocity, size of cuttings and choke pressure on the BHP are investigated. Also, due to the importance of the hole-cleaning during drilling operations, the effects of different controlling parameters on the solid volume fraction distribution during annulus are discussed. Results show that the increasing of the gas flow rate and decreasing of the cutting size can be used as a tool for improved hole-cleaning in the UBD operation.

Simulation of Wellbore Stability during Underbalanced Drilling Operation

The wellbore stability analysis during underbalance drilling operation leads to avoiding risky problems. These problems include (1) rock failure due to stresses changes (concentration) as a result of losing the original support of removed rocks and (2) wellbore collapse due to lack of support of hydrostatic fluid column. Therefore, this paper presents an approach to simulate the wellbore stability by incorporating finite element modelling and thermoporoelastic environment to predict the instability conditions. Analytical solutions for stress distribution for isotropic and anisotropic rocks are presented to validate the presented model. Moreover, distribution of time dependent shear stresses around the wellbore is presented to be compared with rock shear strength to select appropriate weight of mud for safe underbalance drilling.

Prediction of frictional pressure loss for multiphase flow in inclined annuli during Underbalanced Drilling operations

In Underbalanced Drilling (UBD) method, it is difficult to predict the equivalent circulation density due to co-existence of three phases which are air, cuttings and drilling fluid. This study presents the application of a developed model inspired from a novel intelligent algorithm namely radial basis function optimized by genetic algorithm (GA-RBF) algorithm to calculate frictional pressure loss of two-phase gasified drilling fluid flow along with cutting as the third phase in inclined wellbore portions. The suggested approach was conducted to extensive data reported in literature and was based on Rate of Penetration (ROP), wellbore inclination, pipe rotation and in situ flow rate of each phase. The results of this study show that the proposed model could reproduce the experimental frictional pressure loss data to an acceptable accuracy due to high correlation coefficient (R2 > 0.99) and very small values of average absolute relative deviation (AARD) (2.166726), standard deviation (STD) (0.038222) and root mean square error (RMSE) (0.008783). Results of this study could couple with commercial drilling simulators to accurately predict the frictional pressure loss of three phase flow.

A Methodology for Classifying Operating Regimes in Underbalanced-Drilling Operations

Summary This paper proposes an extension to an existing operating-envelope technique used for underbalanced drilling (UBD) to enhance control of bottomhole pressure and inflow parameters. With the use of an implementation of the drift-flux model (DFM) with boundary conditions typically encountered in underbalanced operations (UBO), a steady-state analysis of the system is performed. Through this analysis, four distinct operating regimes are identified, and the behavior in each of them is investigated through steady-state calculations and transient simulations. In particular, the analysis reveals that a section of the operating envelope previously believed to be unstable/transient is, in fact, stable/steady when a fixed choke opening is used as an independent variable in place of a fixed wellhead pressure (WHP). This results in the steady-state operating envelope being extended, and gives an increased understanding of the well behavior encountered in UBO toward enabling the introduction of automated control. Finally, we investigate the mechanism behind severe slugging in UBO and argue that the cause is different from that of the slugging encountered in production and multiphase transport.

Risk analysis of deepwater drilling operations using Bayesian network

Deepwater drilling is one of the high-risk operations in the oil and gas sector due to large uncertainties and extreme operating conditions. In the last few decades Managed Pressure Drilling Operations (MPD) and Underbalanced Drilling (UBD) have become increasingly used as alternatives to conventional drilling operations such as Overbalanced Drilling (OVD) technology. These newer techniques provide several advantages however the blowout risk during these operations is still not fully understood. Blowout is regarded as one of the most catastrophic events in offshore drilling operations; therefore implementation and maintenance of safety measures is essential to maintain risk below the acceptance criteria. This study is aimed at applying the Bayesian Network (BN) to conduct a dynamic safety analysis of deepwater MPD and UBD operations. It investigates different risk factors associated with MPD and UBD technologies, which could lead to a blowout accident. Blowout accident scenarios are investigated and the BNs are developed for MPD and UBD technologies in order to predict the probability of blowout occurrence. The main objective of this paper is to understand MPD and UBD technologies, to identify hazardous events during MPD and UBD operations, to perform failure analysis (modelling) of blowout events and to evaluate plus compare risk. Importance factor analysis in drilling operations is performed to assess contribution of each root cause to the potential accident; the results show that UBD has a higher occurrence probability of kick and blowout compared to MPD technology. The Rotating Control Devices (RCD) failure in MPD technology and increase in flow-through annulus in UBD technology are the most critical situations for kick and blowout.