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Offshore well costs and risks have proven the need for research in deep water drilling and well control. This paper presents the experimental research that is ongoing at the Dual Gradient Drilling Laboratory to understand the physics involved in gas migration.
A scaled prototype of an offshore well was built to mimic the Controlled Mud Level (CML) Drilling method. Instrumentation and cameras were used to monitor flow rate and gas liquid ratio. Set rates for the mud outflow and inflow were determined and tested at constant gas influx, by increasing the mud inflow in narrow increments.
The prototype was successfully tested and removed a large amount of gas from the system before the gas migrated into the voided section. The results of these tests show that using high mud inflow rates can eliminate the single bubble system, leading to a dispersed bubble system. With dispersed gas bubbles the removal of the gas is a process of determining an optimal outflow rate that would also accommodate the fluid level requirement. Also, it was observed that the geometry of the outflow line have a major effect on the gas flow behavior. With the reduced outlet diameter, a Bernoulli flow was observed which increased bubble dispersion.
This paper provides information regarding the research on gas behavior and migration in a scaled riser system during circulation. These experiments can be used to study the effects of flow rates on gas dispersion and elimination from the CML Drilling method.
Manikonda, Kaushik (Texas A & M University) | Hasan, Abu Rashid (Texas A & M University) | Barooah, Abinash (Texas A & M University) | El-Naas, Muftah (Qatar University) | Sleiti, Ahmad Khalaf (Qatar University) | Rahman, Mohammad Azizur (Texas A & M University)
This paper presents a simple mechanistic model to describe a gas kick in a drilling riser with water- based mud (WBM) and synthetic-based mud (SBM). This model can estimate key kick parameters such as the change in the wellhead pressure, kick ascent time, and pit gain. In addition, this model also predicts the solubility of the gas kick in SBM at various depths in the annulus. We used the commercial chemical process simulation software, HYSYS, to validate the results of this solubility model. This paper also presents the gas kick experimental results from a 20-ft. tall vertical flow loop at Texas A&M University, Qatar.
The base case investigates a gas kick in a vertical 10,000 ft. deep, 12.415 in. drilling riser with WBM. Our analytical model uses the Hasan-Kabir two-phase flow model and develops a set of equations that describe the pressure variation in the annulus. This computed pressure change allows estimates of pit- gain. Our experimental data comes from a 20-ft. tall flow loop with a 2.5 in. steel tube, inside a 4.5 in. Acrylic pipe, that simulates a riser. For these gas kick experiments, we injected specific amounts of gas at the bottom of the setup and recorded the bubble's expansion and migration.
The mechanistic model predicted explosive unloading of the riser near the wellhead. A comparison between our model results and HYSYS values for methane liquid-phase mole fraction showed a maximum 8% deviation with complete agreement on bubble point (Pb) pressure and location estimates. Similarly, our model calculated the solution gas-oil ratio (Rs), with a maximum divergence of 3% from HYSYS estimates. From the comparison studies with other empirical Bo & Rs correlations, we note that the estimates of our model agreed best with those of O'Bryan's (Patrick Leon O'Bryan, 1988) correlations.
Numerical kick simulators that exist today are notoriously time and power-intensive, limiting their on field utility. Our mechanistic model minimizes computation time through its simple, analytical form to describe kick migration. Our model offers another layer of novelty through the analytical, thermodynamic solubility modeling as opposed to empirical modeling sused by most of the current gas kick simulators.
Gas influxes are common during drilling operations. If not handled properly, in open loop systems gas can expand and unload the riser, however in closed loop systems, unless gas is depressurized, the top section of the riser will become over pressured. Riser unloading has not been accurately modeled due to uncertainties in gas expansion. This paper presents studies on gas expansion and unloading in a lab scale model at Texas A&M University.
We have performed experiments in a scaled riser at the Dual Gradient Drilling laboratory. We ran experiments using water and air as the gas phase. We recorded the change in volume as gas expands and liquid height changes. We mitigated the disproportion of the atmospheric pressure and the scale of the model by using vacuum pumps. We also measured overflow by allowing liquid level to rise to determine the final gas volume.
The experiments provide information on gas expansion and riser unloading. During the expansion process the top of the bubble travels rapidly building momentum, which carries a large volume of mud to the top of the riser model. This occurs through the rapid change in gas volume and the energy released as the pressure decreases. By allowing gas to expand in a controlled environment we measured the impact of gas expansion.
This paper provides information on controlled and uncontrolled gas expansion, impact on riser unloading, and benefits of a closed system.
A transient flow model capable of modeling gas solubility will be used to perform a sensitivity analysis of kick behavior in a subsea backpressure MPD system when using oil based mud. The parameters of interest are choke pressures, pit volume, and return rates. At HPHT conditions, gas kicks can be entirely dissolved in oil based mud. However, when being circulated upwards, free gas will emerge at a certain depth. The required choke pressure to maintain a constant bottomhole pressure depends on the amount of gas released from the mud and where this occurs. Another parameter that impacts both choke pressure and return flow is the geometry, whether we have a wide riser for subsea MPD or are considering an MPD operation from a fixed installation with a narrower geometry. In this paper, the riser geometry will be varied. The paper will contribute in showing how transient models can assist in the planning of MPD operations. It will also provide insight into influx behavior and its impact on surface parameters with focus on oil based mud.
Gu, Qifan (The University of Texas at Austin) | Fallah, AmirHossein (The University of Texas at Austin) | Ambrus, Adrian (The University of Texas at Austin, Norwegian Research Centre) | Ma, Zheren (The University of Texas at Austin, Quantum Reservoir Impact) | Chen, Dongmei (The University of Texas at Austin) | Ashok, Pradeepkumar (The University of Texas at Austin) | Oort, Eric van (The University of Texas at Austin)
Riser gas unloading events in deepwater subsea well construction operations are very hazardous and difficult to control. Gas kicks may go unnoticed when gas dissolves in non-aqueous fluids (NAF), with dissolved gas volumes not showing significant pit gains on surface when conventional well control indicators are used. Once dissolved gas is circulated above subsea blow-out preventers (BOP) and comes out of solution at pressure and temperature conditions below the bubble point, it can quickly displace a large mud volume in the riser at relatively shallow depths, leaving little time to react for diverting the gas at surface.
In this paper, we present a variant of the constant bottom-hole pressure (CBHP) approach to managed pressure drilling (MPD) that offers a comprehensive way to manage and control riser gas unloading behavior. The technique enables early kick detection, thereby allowing more sensitive diagnosis of gas kicks going into solution, and preventing them from creating a hazardous situation in the first place. It also allows for the application of back-pressure on the dissolved gas-NAF fluid mixture in the riser, in order to delay, minimize or even prevent gas breaking out close to surface (in case dissolved gas is allowed above the subsea BOPs). Complicating factors include the pressure integrity limits of the marine riser, and the fact that the nature of any kick (whether it is gas, fluid or a combination, its detailed composition, volume, etc.) is generally unknown. It is argued here that such challenges are best met in the field by using automated MPD control methodology.
The designed controller (triple-mode constant bottom-hole pressure (TCBHP)) combines three modes of operation: pressure control, flow control and solubility control, each with their advantages and limitations. The controller can automatically switch between control modes based on observed kick behavior, thereby gaining the ability to compensate for the limitations of the individual control modes and, more importantly, deal with kicks agnostically, i.e. independent of their nature. We describe the architecture and underlying details of the new controller, and show its functionality by simulating several riser gas unloading scenarios. Two distinct cases that are given special consideration are: (1) the case where the subsea BOPs remain open after the kick has passed them and the controller regulates pressure using maximum allowable surface pressure (MASP) / downhole fracture gradient information as an upper limit; (2) the case where the subsea BOPs are closed after the kick has passed them and the controller regulates pressure within riser pressure limits when the kick is circulated to surface using the riser booster pump. Simulation results show that the new TCBHP controller can quickly and robustly control riser gas unloading situations with complicated transient conditions, without fracturing downhole formations and/or jeopardizing the pressure integrity of the riser. Its development aims to help mitigate some of the fears and concerns around riser gas unloading, and making automated subsea well control using CBHP- MPD technology a reality in the near-future.